Reverse engineering a counterfeit 7805 voltage regulator

Under a microscope, a silicon chip is a mysterious world with puzzling shapes and meandering lines zigzagging around, as in the magnified image of a 7805 voltage regulator below. But if you study the chip closely, you can identify the transistors, resistors, diodes, and capacitors that make it work and even understand how these components function together. This article explains how the 7805 voltage regulator works, all the way down to how the transistors on the silicon operate. And while exploring the chip, I discovered that it is probably counterfeit.

Die photograph of a 7805 voltage regulator.

Die photograph of a 7805 voltage regulator. Click to enlarge.

A voltage regulator takes an unregulated input voltage and converts it to the exact regulated voltage an electronic circuit requires. Voltage regulators are used in almost every electronic circuit, and the popular 7805 has been used everywhere from computers[1] to satellites, from DVD player and video games to Arduinos[2]. and robots. Even though it was introduced in 1972 and more advanced regulators[3] are now available, the 7805 is still in use, especially with hobbyists.

The 7805 is a common type of regulator known as a linear regulator. (As its name hints, the 7805 produces 5 volts.) A linear regulator is built around a large transistor that controls the amount of power flowing to the output, acting similar to a variable resistor. (This transistor is visible in the right half of the die photo above.) A drawback of a linear regulator is that all the "extra" voltage gets converted into heat. If you put 9 volts into a linear regulator and get 5 volts out, the extra 4 volts gets turned into heat in the regulator, so the regulator is only about 56% efficient. (The main competitor to a linear regulator is a switching power supply - a much more efficient, but much more complicated way to produce regulated voltage. Switching power supplies have replaced linear regulators in many applications, such as phone chargers and computer power supplies.)

A 7805 voltage regulator in a metal TO-3 package.

A 7805 voltage regulator in a metal TO-3 package. The 7805 is more commonly found in a smaller plastic package.

Linear regulators such as the 7805 became very popular because they are extremely easy to use: just feed the unregulated voltage into one pin, ground the second pin, and get regulated voltage out the third pin[4]. Another feature that made the 7805 popular is it is almost indestructible - if you short-circuit it, put too much voltage in, or run it too hot, it will shut down before getting damaged, due to internal protection circuits.

The components of the integrated circuit

Like most chips, the 7805 is built from a tiny piece of silicon. To make the chip function, a process called doping treats regions of the silicon with elements such as phosphorus or boron. In the die photo, these regions have a slightly different color, which makes the structure of the chip visible. Phosphorus gives the region excess electrons (i.e. negative), so it is known as N silicon. Boron has the opposite effect, creating positive P silicon. The amount of doping in a silicon chip is surprisingly small, varying from 1 foreign atom for every thousand atoms of silicon down to one foreign atom per billion atoms of silicon. Because silicon is so sensitive to impurities, the original silicon wafer must be an insanely pure crystal, up to 99.999999999% pure - a level known as eleven nines.

On top of the silicon, a thin layer of metal connects different parts of the chip. This metal is clearly visible in the die photo as white traces and regions.[5] A thin, glassy silicon dioxide layer provides insulation between the metal and the silicon, except where rectangular contact holes in the silicon dioxide allow the metal to connect to the silicon. Around the edge of the chip, thin wires connect the metal pads to the chip's external pins - the black blobs in the photo show where the wires were attached.

Transistors inside the IC

Transistors are the key components in the chip. The 7805 uses NPN and PNP bipolar transistors (unlike digital chips which usually have CMOS transistors). If you've studied electronics, you've probably seen a diagram of a NPN transistor like the one below, showing the collector (C), base (B), and emitter (E) of the transistor, The transistor is illustrated as a sandwich of P silicon in between two symmetric layers of N silicon; the N-P-N layers make a NPN transistor. It turns out that transistors on a chip look nothing like this, and the base often isn't even in the middle!

An NPN transistor and its oversimplified structure.

An NPN transistor and its oversimplified structure.

The photo below shows one of the transistors in the 7805 as it appears on the chip.[6] The different brown and purple colors are regions of silicon that has been doped differently, forming N and P regions. The gray areas are the metal layer of the chip on top of the silicon - these form the wires connecting to the collector, emitter, and base.

Structure of a NPN transistor inside the 7805 voltage regulator.

Structure of a NPN transistor inside the 7805 voltage regulator.

Underneath the photo is a cross-section drawing showing approximately how the transistor is constructed. There's a lot more than just the N-P-N sandwich you see in books, but if you look carefully at the vertical cross section below the 'E', you can find the N-P-N that forms the transistor. The emitter (E) wire is connected to N+ silicon. Below that is a P layer connected to the base contact (B). And below that is a N+ layer connected (indirectly) to the collector (C).[7] The transistor is surrounded by a P+ ring that isolates it from neighboring components.

Resistors inside the IC

Resistors are a key component of analog chips and are formed from strips of silicon doped to have high resistance. The photo below shows two resistors in the 7805 voltage regulator, formed from greenish-purple strips of P silicon. (The gray metals strips connect to the resistors at the square contacts and wire the resistors to other parts of the chip.) The value of the resistor is proportional to its length[8], so the short resistor on the right (850Ω) is smaller than the meandering resistor on the left (4000Ω). Resistors with large values take up an inconveniently large area on the chip - in the top left of the die photo you can see the serpentine path of an 80KΩ resistor.

Two resistors on the 7805 voltage regulator's silicon die.

Two resistors on the 7805 voltage regulator's silicon die.

How the 7805 works

I've colored the following schematic[9] to indicate the main blocks of the 7805 regulator. The heart of the 7805 chip is a large transistor that controls the current between the input and output, and thus controls the output voltage. This transistor (Q16) is red on the diagram below. On the die, it takes up most of the right half of the chip because it needs to handle over 1 amp of current.

Components of the 7805 regulator: bandgap (yellow), error amp (orange), output transistor (red), protection (purple), startup (green).

Components of the 7805 regulator: bandgap (yellow), error amp (orange), output transistor (red), protection (purple), startup (green).

The bandgap reference (yellow) is what keeps the voltage stable. It takes the scaled output voltage as input (Q1 and Q6), and provides an error signal (to Q7) indicating if the voltage is too high or too low. The key feature of the bandgap is it provides a stable and accurate reference, even as the chip's temperate changes. The next section will discuss the bandgap in detail.

The error signal from the bandgap reference is amplified by the error amplifier (orange). The amplified signal controls the output transistor through large driver Q15. This closes the negative feedback loop that controls the output voltage. The startup circuit (green) provides initial current to the bandgap circuit, so it doesn't get stuck in an off state.[10] The circuits in purple provide protection against overheating (Q13), excessive input voltage (Q19), and excessive output current (Q14). If there is a fault, these circuits reduce the output current or shut down the regulator, protecting it from damage.

The voltage divider (blue) scales down the voltage on the output pin for use by the bandgap reference. It has an interesting implementation that allows different chips in the 78XX family to produce different voltages. (For instance 12 volts from the 7812 and 24 volts from the 7824.) The image below shows the square contacts between the metal (white) and the resistor (turquoise) that control the values of R20 and R21. For a different regulator, a simple change to the position of the variable contact increases the resistance of R20 and thus the output voltage of the chip.

The feedback voltage divider inside the 7805 voltage regulator consists of two resistors.

The feedback voltage divider inside the 7805 voltage regulator consists of two resistors.

How a bandgap reference works

The main problem with producing a stable voltage from an IC is the chip's parameters change as temperature changes: it's no good if your 5 volt phone charger starts producing 3 or 7 volts on a hot day. The trick to building a stable voltage reference is to create one voltage that goes down with temperature and another than goes up with temperature. If you add them together correctly, you get a voltage that is stable with temperature. This circuit is called a "bandgap reference".

To create a voltage that goes down with temperature, you put a constant current through the transistor and look at the voltage between the base and emitter, called VBE. The graph below shows how this voltage drops as the temperature increases. At the left, the line hits the bandgap voltage of silicon, about 1.2 volts; this will be important later.

Vbe vs temperature for a transistor

Vbe vs temperature for a transistor

If you set up a second transistor this way but with a lower current[11], you get the same effect but the voltage VBE curve drops faster. This may not seem helpful since we need a voltage that goes up with temperature. But here's the trick: if you subtract the two VBE voltages, the difference increases as temperature increases, since the lines get farther apart. The difference is called ΔVBE. The graph below shows the VBE curves for two different transistors, and you can see how the difference ΔVBE between the curves increases with temperature, even though both curves decrease with temperature.

Voltages in a bandgap reference: Vbe for two transistors as temperature changes.

Voltages in a bandgap reference: Vbe for two transistors as temperature changes.

The final step to a bandgap reference is to combine VBE and ΔVBE in the right ratio so the result is constant with temperature. It turns out that if the values sum to the bandgap voltage, the drop in VBE and the increase in ΔVBE cancel out. In the graph below, adding 10 copies of ΔVBE is the right ratio; the exact ratio depends on the particular transistors. The important thing to notice in the graph below is that as the temperature changes, VBE+nΔVBE remains constant - the top of the of purple ΔVBEs remains at the bandgap voltage.

By adding multiples of ΔVbe to Vbe, the bandgap voltage is reached regardless of temperature.

By adding multiples of ΔVbe to Vbe, the bandgap voltage is reached regardless of temperature.[12]

How the 7805's bandgap reference works

The 7805's bandgap reference uses the above bandgap principles, but there are several important differences. First, the bandgap voltage in practice turns out to be about 1.25 volts instead of 1.2. Second, the 7805's bandgap creates a larger (and thus more accurate) 2ΔVBE by taking the difference between two high-current VBEs and two low-current VBEs. Finally, 2ΔVBE is scaled and added to three VBEs to form three times the bandgap voltage, or about 3.75V.

The diagram below shows the 7805's bandgap circuit with arrows showing voltage changes (not currents). Starting at ground, the red arrow shows an increase of (large) VBE across Q3, and another (large)VBE across Q2. The green arrows show drops of (small) VBE across Q4 and Q5. The result is the difference 2ΔVBE ends up across R6.

The next step is very important as it scales up the voltage. The current through R7 will be the same as the current through R6 (ignoring small base currents). But R7 is 16.5 times as large as R6, so by Ohm's law, the voltage across R7 will be 16.5 times as large, i.e. 33ΔVBE.

Finally, we can see the bandgap's voltage by looking at the purple lines. Starting at ground, the voltage goes up by VBE across Q8, another VBE across Q7, then the R7 voltage, and finally a third VBE across Q6. Assuming the chip designers picked the scale factor of 33 correctly, the final voltage will be three bandgap voltages, or 3.75V.[13] (Vin here is the voltage input to the bandgap, not the voltage input to the 7805.)

How the bandgap voltage is generated in the 7805 voltage regulator.

How the bandgap voltage is generated in the 7805 voltage regulator.
A traditional bandgap circuit generates a stable reference voltage, but discussions of bandgaps usually ignore a big issue: in devices such as the 7805 or the TL431, the bandgap circuit does not generating a stable reference voltage. Instead, the 7805's bandgap works "backwards". The 7805's scaled output voltage provides the input voltage (Vin) to the bandgap reference, and the bandgap provides an error signal as output. The 7805's bandgap circuit removes the feedback loop that exists inside a traditional bandgap reference. Instead, the entire chip becomes the feedback loop.

In more detail, if the output voltage is correct (5V), then the voltage divider provides 3.75V at Vin, and the VBE and ΔVBE voltages are as described above. If the output voltage rises or falls slightly, this change propagates through Q6 and R7, causing the voltage at the base of Q7 to rise or fall accordingly. This change is amplified by Q7 and Q8, generating the error output.[14] The error output, in turn, decreases or increases the current through the output transistor, and this negative feedback loop adjusts the output voltage until it is correct.

Interactive chip viewer

The image and schematic[9] below are an interactive exploration of the 7805. Click a component to see its location on the die and in the schematic highlighted. The box below will give an explanation of the component. For transistors, the emitter, base, and collector will be indicated on the die.

Why I think this chip is counterfeit

The outside of the package has the ST Microelectronics logo, but for several reasons I think the chip is counterfeit and manufactured by someone else. First, on the die itself (below) there is no ST logo, no mask copyright, and no manufacturer information at all. (I have no explanation for why the die is labeled 2805 and not 7805, or what P414 means.) In addition, the circuit on the die is totally different from the internal circuit in the ST Microelectronics 7805 datasheet. The metal of the package looks grainy and low quality. Finally, I bought the part off eBay, not from a reputable supplier, so it could have come from anywhere. For these reasons, I conclude that the part I got is counterfeit and not a genuine ST Microelectronics LM7805. From what I hear, there's a lot of semiconductor counterfeiting happening so I'm not surprised to get a counterfeit part. (But see a dissenting opinion.)

Label on the die of a 7805 voltage regulator.

Label on the die of a 7805 voltage regulator.

7805 history, and a look at some other designs

I had assumed that all 7805 chips were pretty much the same. But one surprise from studying datasheets is that different manufacturers use totally different internal circuitry for the same 7805 chip and the name "7805" doesn't mean much more than "some sort of 5 volt regulator."

To explain this, I'll start with a brief history of voltage regulators. Simple IC voltage regulators got their start way back in 1968 when Fairchild introduced the µA723 voltage regulator, which used a temperature-compensated Zener diode to provide an adjustable voltage. In 1969 analog design genius Robert Widlar[15] developed the National LM109 5-volt regulator, which was much simpler to use. It was followed in 1972 by Fairchild's 7800 series of voltage regulators, ranging from 5 volts to 24 volts. In 1973 National came out with an improved regulator series, the LM340-XX.

From this history, you'd expect that there's a LM109 design, a 7805 design, and a LM340 design. However, it turns out that the part numbers are really just marketing, and have little to do with what's inside the chip. Some 7805s are closer to the LM109 than to other 7805s, and some LM340s are closer to 7805s than to other LM340s.

For instance, the Fairchild µA109 uses the common Fairchild 7800 series design. On the other hand, the National LM7805 is very different from the Fairchild 7805, but is identical to the National LM340, even sharing the same datasheet. This design is very close to the original National LM109, so in effect National sold the same design under three different names.[16] Thus, it looks like companies reuse the same voltage regulator design, changing little more than the part number between devices. I suspect manufacturers are constrained by patents[17], so they use the part numbers they want on the devices they can make.

How a different, more popular 7805 design works

It turns out that 7805 design I reverse-engineered above is fairly rare, and most 7805 chips use a different design, shown below.[16] While the overall architecture of this design is similar to the LM109-derived 7805 chip I examined, most of the pieces have substantial changes. The current mirror[18], the startup circuit, the bandgap regulator, and the protection circuitry are all different.

Internal schematic of the Signetics µ¼A7805 regulator from the datasheet.

Internal schematic of the Signetics µA7805 regulator from the datasheet.
Since this design is so popular, I'll give a brief explanation of how its bandgap circuit works.[19] In the figure below, there's a large VBE (red arrow) across high-current transistor Q1, and a small VBE (green arrow) across low-current transistor Q2. Thus, ΔVBE appears across R3, generating a current through R3, Q2, and R2. Since R2 has 20 times the resistance as R3, 20ΔVBE appears across R2, by Ohm's law.

Now, to find the temperature-compensated stable voltage for this circuit, follow the blue arrows up from ground. (As before, the arrows do not indicate current flow, and Vin is the input to the bandgap not the chip.) Going through Q3, Q4, R2, Q5 and Q6, the voltages sum to 4VBE+20ΔVBE. Since there are four VBEs, the circuit must be designed for four times the bandgap voltage, or approximately 5V. Thus, this circuit's stable point is 5V. At this voltage, the error amplifying transistors (Q4/Q3) will be in the active region and will respond to any variation away from it.[20]

How the bandgap voltage is generated in the Signetics 7805 regulator.

How the bandgap voltage is generated in the Signetics 7805 regulator.

How I looked at the 7805 die, and how you can too

Usually getting the die out of an IC requires concentrated acid to dissolve the epoxy package. But some ICs, such as the 7805, are available in metal cans which can be easily opened with a hacksaw. I used a metallurgical microscope for my die photos, but even a basic middle-school microscope shows you the metal layer at at low magnification. If you're at all interested in IC structure, or want to show kids what ICs look like inside, you should get an IC in a metal can, saw it open yourself, and take a look. Many different ICs in metal cans are available for under $5 on eBay; search for "TO-99 IC". I find older chips such as the 7805 are better for this than modern chips: the simpler circuits and larger features make it easier to see the internals.

Inside a 7805 voltage regulator. The tiny silicon die is visible in the middle of the TO-5 package.

Inside a 7805 voltage regulator. The tiny silicon die is visible in the middle of the TO-5 package.

The photo above shows the 7805 regulator after removing the top with a hacksaw. The metal package is almost entirely empty inside - the silicon die is very small compared to the space available. The metal acts as an effective heat sink to cool the chip under high load. Even without magnification, the large output transistor is visible at the right side of the die. The thin wires between the pins and die are visible, including the two separate wires to the output pin.

Conclusion

I hope this article has given you a better understanding of how a voltage regulator works and what's inside a silicon chip. Perhaps it has even inspired you to saw open some chips of your own to explore the tiny world on a silicon chip for yourself. And while you sit at your computer, think of the many voltage regulators around you quietly keeping your electronics working smoothly, whether made by their supposed manufacturer or not.

Notes and references

[1] Computers usually get most of their power from switching power supplies for efficiency, but linear regulators still have their place. OlderATXpowersuppliesused the 7805 for the 5V standby power, while othersusedthe related 7905 and 7912 regulators for -5V and -12V. Modern computers still use linear regulators in surprising numbers. For instance the MacBook Pro (A1278) uses a low-dropout regulator to generate 1.8 volts, a switching controller with 3.3 and 5V linear regulators inside, a main switching controller with a 5V regulator inside, a low-noise 4.6V regulator for audio and another regulator to generate 3.3V for the keyboard.

[2] Earlier Arduinos such as the Arduino USB, NG and Severino were powered through a 7805 regulator. Recent Arduino models, however, use a switching step-down converter and an ultra-low-dropout 3.3V regulator. This regulator uses the same principles as the 7805, but is much more advanced.

[3] A big advantage of more modern voltage regulators is they don't require as large an input voltage. The 7805 requires at least two extra volts input (i.e. 7 volts in to produce 5 volts out) - this is the dropout voltage. Newer low-dropout (LDO) regulators can require as little as 0.1 extra volts. Modern regulators (such as the TPS796xx) also have much less noise in the output. Despite this, the 7805 is still popular, especially withhobbyists. Adafruit has a nice comparison of regulators.

[4] Depending on the application, you'd probably want to add input and output capacitors to the 7805 regulator to filter out transients due to fluctuations in the input voltage or output load.

[5] While the 7805 chip has a single layer of metal over the silicon to interconnect the circuitry, modern CPUs use many more layers of metal due to their complexity. For example, Haswell uses 11 layers while IBM's POWER8 uses an astounding 15 metal layers. Needless to say, I'm not going to figure out how those chips work with my microscope.

[6] The 7805 uses a wide variety of transistor layouts, as you can see from the labeled die photo. Several transistors in the bandgap use two emitters for one transistor (e.g. Q2, Q3, Q4, Q5) to improve matching between transistors; the PNP current mirror transistors Q11 and Q11-1 also have multiple emitters. Pairs of transistors can share a single base (e.g. Q11 and Q11-1), share a single collector (Q17 and Q18), or share both (Q14 and Q19). Some transistors move the base to the middle (e.g. Q6). To support high current, the output transistors (Q15, Q16) have a totally different, much larger structure.

[7] You might have wondered why there is a distinction between the collector and emitter of a transistor, when the simple picture of a transistor is totally symmetrical. As you can see from the die photo, the collector and emitter are very different in a real transistor. In addition to the very large size difference, the silicon doping is different. The result is a transistor will have poor gain if the collector and emitter are swapped.

[8] The resistance of a resistor in silicon is proportional to its length divided by its width. If you double the length, it's like two resistors in series, so the resistance doubles. If you double the width, it's like two resistors in parallel, so the resistance is cut in half. One convenient consequence is if the chip is scaled down (Moore's law), the resistors keep the same values, since the width and length scale equally.

Silicon resistance is measured with the unusual unit ohms per square (Ω/□). Note that there's no distance unit - it doesn't matter if you have a square millimeter or square inch of material; the resistance is the same because the dimensions cancel out. For the 7805, I estimate 140 ohms/square for the resistors.

[9] I looked at dozens of datasheets and the chip I examined almost exactly matches the schematic for the Korean Electronics KIA7805. The National LM340/LM78XX schematic is very similar

[10] Bandgap circuits usually have two stable voltages - the desired voltage and 0 volts. To keep the bandgap from getting stuck at 0 volts, a startup circuit will "push" the bandgap away from 0 volts so it will settle at the desired voltage. The startup circuit is discussed in Widlar's application note AN-42 for the similar LM109 (page 5).

[11] When building a bandgap reference, what really matters for VBE is the current density through the transistors - the current divided by the area of the emitter. Decreasing the current through the transistor decreases the current density. The second way to decrease current density is to use a larger transistor with a larger emitter. Often five or ten identical transistors in parallel will be combined to form this large transistor to ensure the large transistor and the small transistor are exactly matched.

[12] The VBE line for a bandgap reference is only perfectly straight in theory, so the resulting bandgap voltage will vary slightly with temperature. To increase stability, some more complex bandgap references compensate for second-order effects.

[13] Bandgap reference references: How to make a bandgap voltage reference in one easy lesson by Paul Brokaw, inventor of the Brokaw bandgap reference. A presentation on the bandgap reference is here. The Design of Band-Gap Reference Circuits: Trials and Tribulations by analog chip design legend Bob Pease discusses real-world bandgap designs.

[14] You might wonder how the error output knows what voltage to switch at. For a Darlington pair (Q7/Q8) to be active, the base voltage must go above 2VBE (Wikipedia). The bandgap reference was constructed assuming that at the reference voltage, there will be VBE drops across Q7 and Q8. Thus, it's not a coincidence that Darlington pair Q7/Q8 is right in the active region (2VBE) at the bandgap voltage making the error output very sensitive to any moves away from the reference voltage. If the output voltage rises or falls, the voltage at the base of Q7 rises or falls accordingly, and the transistors greatly amplify this change. Also note that an increase in output voltage causes a decrease in the error output, yielding negative feedback for the whole chip.

[15] By all reports, Robert Widlar was an amazing analog engineer, as well as an alcoholic crazy guy. Widlar invented key analog IC circuits such as the Widlar current source as well as groundbreaking ICs such as the µA702 and µA723. In 1970 he sold his stock options for a million dollars (about 6 million adjusted for inflation) and retired to Mexico at 33. Some entertaining stories about him are here, on Wikipedia, and pictures of his sheep.

[16] Most 7805 datasheets show the same internal schematic. Some chips using the common design are Fairchild 7800 series, Hi-Sincerity H78XX, FCI LM7800, MCC MC7805, Microelectronics ML7800, Motorola MCT7800, uPC7800H, JRC NJM7800, TI uA7800, Signetics uA7800, and ST L7805. Other chips use variants of the common design: AS78XXA, UTC LM78XX, L78L05 and Motorola MC7800.

The LM109-based design of the 7805 that I looked at is very different from the common design and appears to be fairly rare; it is used by National LM340/LM7800 and KEC KIA7805AF. There are a few differences to note between this design and the original National LM109. In order to support multiple output voltages, the 7805 design uses a resistor divider and a different circuit feeding the bandgap reference. This probably also motivated the removal of a couple transistors from the bandgap circuit so its voltage is one VBE drop lower. The startup circuit is also slightly changed.

[17] Widlar's patent on the bandgap reference is 3617859. A later patent with a bandgap reference very similar to the LM109's is 4249122.

[18] A current mirror is a very useful way of connecting transistors so the current through the second transistor matches the current through the first transistor. For more information about current mirrors, you can check Wikipedia or any analog IC book such as chapter 3 of Designing Analog Chips.

[19] Several sources give an explanation of the common 7805 design that is plausible but wrong. The faulty explanation is that Zener D1 provides the reference voltage. It feeds into a comparator built from Q13 and Q10 (or Q6) as a differential pair and Q1, Q7, and Q2 forming a current mirror active load. The most obvious problem with this is Q13, Q6, R1, and R2 are all tied together which would short out the two sides of the supposed differential pair / current mirror.

Ironically, the design of the 7905 (the negative-voltage version of the 7805) is similar to the erroneous 7805 explanation. The 7905 uses a Zener diode to provide the reference voltage. A comparator with a current mirror active load generates the error signal by comparing the reference voltage with the feedback voltage. Meanwhile another current mirror ensures a constant (probably temperature-compensated) current flows through the Zener diode. I had expected the 79XX chips would be mirror-images of the 78XX chips, but the internal design turns out to be fundamentally different. This explains why the block diagrams in 7905 datasheets show a comparator and 7805 datasheets just show an "error amplifier" box.

[20] In the common 7805 design, I believe the purpose of Q7 and R10 is to pull the same current from Q1's base that Q4 and R14 pull from Q2's base, to keep both sides balanced. Because R1 is 1KΩ and R2+R3 is 21kΩ, 21 times the current should flow through Q1 as through Q2.

Reverse-engineering the TL431: the most common chip you've never heard of

A die photo of the interesting but little-known TL431 power supply IC provides an opportunity to explore how analog circuits are implemented in silicon. While the circuit below may look like a maze, the chip is actually relatively simple and can be reverse-engineered with a bit of examination. This article explains how transistors, resistors, and other components are implemented in silicon to form the chip below.

Thumbnail of the TL431 die.

Die photo of the TL431. Original photo by Zeptobars.

The TL431 is a "programmable precision reference"[1] and is commonly used in switching power supplies, where it provides feedback indicating if the output voltage is too high or too low. By using a special circuit called a bandgap, the TL431 provides a stable voltage reference across a wide temperature range. The block diagram of the TL431 below shows that it has a 2.5 volt reference and a comparator[1], but looking at the die shows that internally it is quite different from the block diagram.

TL431 block diagram from Fairchild datasheet

TL431 block diagram from the datasheet

The TL431 has a long history; it was introduced in 1978[2] and has been a key part of many devices since then. It helped regulate the Apple II power supply, and is now used in most ATX power supplies[3] as well as the the iPhone charger and other chargers. The MagSafe adapter and other laptop adapters use it, as well as minicomputers, LEDdrivers, audio power supplies, video games and televisions.[4]

The photos below show the TL431 inside six different power supplies. The TL431 comes in many different shapes and sizes; the two most common are shown below.[5] Perhaps a reason the TL431 doesn't get much attention because it looks like a simple transistor, not an IC.

Six examples of power supplies using the TL431. Top row: cheap 5 volt power supply, cheap phone charger, Apple iPhone charger (uses TL431 and 'GB9' variant). Bottom row: MagSafe power adapter, KMS USB charger, Dell ATX power supply (with optoisolators in front)

Six examples of power supplies using the TL431. Top row: cheap 5 volt power supply, cheap phone charger, Apple iPhone charger (also 'GB9' variant in lower left). Bottom row: MagSafe power adapter, KMS USB charger, Dell ATX power supply (with optoisolators in front)

How components are implemented in the TL431's silicon

Since the TL431 is a fairly simple IC, it's possible to understand what's going on with the silicon layout by examining it closely. I'll show how the transistors, resistors, fuses, and capacitors are implemented, followed by a reverse-engineering of the full chip.

Implementing different transistor types in the IC

The chip uses NPN and PNP bijunction transistors (in contrast to chips like the 6502 that use MOSFET transistors). If you've studied electronics, you've probably seen a diagram of a NPN transistor like the one below, showing the collector (C), base (B), and emitter (E) of the transistor, The transistor is illustrated as a sandwich of P silicon in between two symmetric layers of N silicon; the N-P-N layers make a NPN transistor. It turns out that on the chip, the transistors look nothing like this. The base isn't even in the middle!

Symbol and structure of an NPN transistor.

Symbol and structure of an NPN transistor.

The photo below shows one of the transistors in the TL431 as it appears on the chip. The different pink and purple colors are regions of silicon that has been doped differently, forming N and P regions. The whitish-yellow areas are the metal layer of the chip on top of the silicon - these form the wires connecting to the collector, emitter, and base.

Underneath the photo is a cross-section drawing showing approximately how the transistor is constructed.[6] There's a lot more than just the N-P-N sandwich you see in books, but if you look carefully at the vertical cross section below the 'E', you can find the N-P-N that forms the transistor. The emitter (E) wire is connected to N+ silicon. Below that is a P layer connected to the base contact (B). And below that is a N+ layer connected (indirectly) to the collector (C).[7] The transistor is surrounded by a P+ ring that isolates it from neighboring components. Since most of the transistor in the TL431 are NPN transistors with this structure, it's straightforward to pick out the transistors and find the collector, base, and emitter, once you know what to look for.

An NPN transistor from the TL431 die, and its silicon structure.

An NPN transistor from the TL431 die, and its silicon structure.

The NPN output transistor in the TL431 is much larger than the other transistors since it needs to handle the full current load of the device. While most of the transistors are operating on microamps, this transistor supports up to 100 mA. To support this current, it is large (taking up more than 6% of the entire die), and has wide metal connections to the emitter and collector.

The layout of the output transistor is very different from the other NPN transistors. This transistor is built laterally, with the base between the emitter and collector. The metal on the left connects to the 10 emitters (bluish N silicon), each surrounded by pinkish P silicon for the base (middle wire). The collector (right) has one large contact. The emitter and base wires form nested "fingers". Notice how the metal for the collector gets wider from top to bottom to support the higher current at the bottom of the transistor. The image below shows a detail of the transistor, and the die photo shows the entire transistor.

Closeup of the high-current output transistor in the TL431 chip.

Closeup of the high-current output transistor in the TL431 chip.

The PNP transistors have an entirely different layout from the NPN transistors. They consist of a circular emitter (P), surrounded by a ring shaped base (N), which is surrounded by the collector (P). This forms a P-N-P sandwich horizontally (laterally), unlike the vertical structure of the NPN transistors.[8]

The diagram below shows one of the PNP transistors in the TL431, along with a cross-section showing the silicon structure. Note that although the metal contact for the base is on the edge of the transistor, it is electrically connected through the N and N+ regions to its active ring in between the collector and emitter.

Structure of a PNP transistor in the TL431 chip.

Structure of a PNP transistor in the TL431 chip.

How resistors are implemented in silicon

Resistors are a key component in an analog chip such as the TL431. They are implemented as a long strip of doped silicon. (In this chip, it looks like P-silicon is used for the resistors.) Different resistances are obtained by using different lengths of resistive material: the resistance is proportional to the length-to-width ratio.

The photo below shows three resistors on the die. The three long horizontal strips are the resistive silicon that forms the resistors. Yellowish-white metal conductors pass over the resistors. Note the square contacts where the metal layer is connected to the resistor. The positions of these contacts control the active length of the resistor and thus the resistance. The resistance of the resistor on the bottom is slightly larger because the contacts are slightly farther apart. The top two resistors are connected in series by the metal on the upper left.

Resistors in the TL431.

Resistors in the TL431.

Resistors in ICs have very poor tolerance - the resistance can vary 20% from chip to chip due to variations in the manufacturing process. This is obviously a problem for a precision chip like the TL431. For this reason, the TL431 is designed so the important parameter is the ratio of resistances, especially R1, R2, R3, and R4. As long as the resistances all vary in the same ratio, their exact values don't matter too much. The second way the chip reduces the effect of variation is in the chip layout. The resistors are laid out in parallel bands of the same width to reduce the effect of any asymmetry in the silicon's resistance. The resistors are also placed close together to minimize any variation in silicon properties between different parts of the chip. Finally, the next section shows how the resistances can be adjusted before the chip is packaged, to fine-tune the chip's performance.

Silicon fuses to trim the resistors

One feature of the TL431 that I didn't expect is fuses for trimming the resistances. During manufacture of the chips, these fuses can be blown to adjust the resistances to increase the accuracy of the chip. Some more expensive chips have laser-trimmed resistors, where a laser burns away part of the resistor before the chip is packaged, providing more control than a fuse.

The die photo below shows one of the fuse circuits. There is a small resistor (actually two parallel resistors) in parallel with a fuse. Normally, the fuse causes the resistor to be bypassed. During manufacture, the characteristics of the chip can be measured. If more resistance is required, two probes contact the pads and apply a high current. This will blow the fuse, adding the small resistance to the circuit. Thus, the resistance in the final circuit can be slightly adjusted to improve the chip's accuracy.

A trimming fuse in the TL431.

A trimming fuse in the TL431.

Capacitors

The TL431 contains two capacitors internally, and they are implemented in very different ways.

The first capacitor (under the TLR431A text) is a is formed from a reverse-biased diode (the reddish and purple stripes). The junction of a reverse-biased diode has capacitance, which can be used to form a capacitor (details). One limitation of this type of capacitor is the capacitance varies with voltage because the junction width changes.

A junction capacitor in the TL431 chip with interdigitated PN junctions. The die id is written in metal on top.

A junction capacitor in the TL431 chip with interdigitated PN junctions. The die id is written in metal on top.

The second capacitor is formed in an entirely different manner, and is more like a traditional capacitor with two plates. There's not much to see: it has a large metal plate with the N+ silicon underneath acting as the second plate. The shape is irregular, to fit around other parts of the circuit. This capacitor takes up about 14% of the die, illustrating that capacitors use space very inefficiently in integrated circuits. The datasheet indicates these capacitors are each 20 pF; I don't know if this is the real value or not.

A capacitor in the TL431 chip.

A capacitor in the TL431 chip.

The TL431 chip reverse-engineered

The components of the TL431, shown on the silicon die.

The TL431 die, labeled.

The diagram above indicates the components on the die of the TL431, labeled to correspond to the schematic below. From the earlier discussion, the structure of each component should be clear. The three pins of the chip are connected to the "ref", "anode", and "cathode" pads. The chip has a single layer of metal (yellowish-white) that connects the components. The schematic shows resistances in terms of an unknown scale factor R; 100 Ω is probably a reasonable value for R, but I don't know the exact value. One big surprise from looking at the die is the component values are very different from the values in previously-published schematics. These values fundamentally affect how the bandgap voltage reference work.[9]

Internal schematic of the TL431

Internal schematic of the TL431

How the chip works

Externally, the TL431's operation is straightforward. If the voltage on the ref pin input goes above 2.5 volts, the output transistor conducts, causing current flow between the cathode and anode pins. In a power supply, this increase in current flow signals the power supply control chip (indirectly), causing it to reduce the power which will bring the voltage back to the desired level. Thus, the power supply uses the TL431 to keep the output voltage stable.

I'll give a brief summary of the chip's internal operation here, and write up a detailed explanation later. The most interesting part of the chip is the temperature-compensated bandgap voltage reference.[10] The key to this is seen by looking at the die: transistor Q5 has 8 times the emitter area as Q4, so the two transistors are affected differently by temperature. The outputs of these transistors are combined by R2, R3, and R4 in the right ratio to cancel out the effects of temperature, forming a stable reference.[11][12]

The voltages from the temperature-stabilized bandgap are sent into the comparator, which has inputs Q6 and Q1; Q8 and Q9 drive the comparator. Finally, the output of the comparator goes through Q10 to drive the output transistor Q11.

Decapping the TL431 the low tech way

Getting an IC die photo usually involves dissolving the chip in dangerous acids and then photographing the die with an expensive metallurgical microscope. (Zeptobars describes their process here). I wondered what I'd end up with if I just smashed a TL431 open with Vise-Grip pliers and took a look with a cheap microscope. I broke the die in half in the process, but still got some interesting results. The picture below shows the large copper anode inside the package, which acts as a heat sink. Next to this is (most of ) the die, which is normally mounted on the copper anode where the white circle is. Note how much smaller the die is than the package.

The TL431 package, the internal anode, and most of the die.

The TL431 package, the internal anode, and most of the die.

Using a basic microscope, I obtained the photo below. While the picture doesn't have the same quality as Zeptobars', it shows the structure of the chip better than I expected. This experiment shows that you can do a basic level of chip decapping and die photography without messing around with dangerous acids. From this photo I can see that the cheap TL431s I ordered off eBay are identical to the one Zeptobars decapped. Since the Zeptobars chip didn't match published schematics, I wondered if they ended up with a strange variant chip variant, but apparently not.

Piece of the TL431 die, photographed through a microscope.

Piece of the TL431 die, photographed through a microscope.

Conclusion

Is the TL431 really the most popular IC people haven't heard of? There's no way to know for sure, but I think it's a good candidate. Nobody seems to publish data on which ICs are produced in largest quantities. Some sources say the 555 timer is the most popular chip with a billion produced every year (which seems improbably high to me). The TL431 must be high up the popularity list - you probably have a TL431 within arms-reach right now (in your phone charger, laptop power adapter, PC power supply, or monitor). The difference is that chips such as the 555 and 741 are so well-known that they are almost part of pop culture with books, T-shirts and even mugs. But unless you've worked on power supplies, chances are you've never heard of the TL431. Thus, the TL431 gets my vote for the most common IC that people are unaware of. If you have other suggestions for ICs that don't get the attention they deserve, leave a comment.

Acknowledgments

The die photos are by Zeptobars (except the photo I took). The schematic and analysis are heavily based on Cristophe Basso's work.[12] The analysis benefited from discussion with Mikhail of Zeptobars, and the Visual 6502 group, in particular B. Engl.

Notes and references

[1] Because the TL431 has an unusual function, there's no standard name for its function. Different datasheets describe it as a "adjustable shunt regulator", a "programmable precision reference", a programmable shunt voltage reference", and a "programmable zener".

[2] I dug up some history on the origins of the TL431 from Texas Instruments' Voltage Regulator Handbook (1977). The precursor chip, the TL430, was introduced as an adjustable shunt regulator in 1976 The TL431 was created as an improvement to TL430 with better accuracy and stability and was called a precision adjustable shunt regulator. The TL431 was announced as a future product in 1977 and launched in 1978. Another future product that TI announced in 1977 was the TL432, which was going to be "Timer/Regulator/Comparator Building Blocks", containing a voltage reference, comparator, and booster transistor in one package. preliminary datasheet. But when the TL432 came out, the "building block" plan had been abandoned. The TL432 ended up being merely a TL431 with the pins in a different order, to help PC board layout. datasheet.

[3] Modern ATX power supplies (example, example) often contain three TL431s. One provides feedback for the standby power supply, another provides feedback for the main power supply, and a third is used as a linear regulator for the 3.3V output.

[4] It's interesting to look at the switching power supplies that don't use the TL431. Earlier switching power supplies typically used a Zener diode as a voltage reference. The earliest Apple II power supplies used a Zener diode as the voltage reference (Astec AA11040), but this was soon replaced by a TL431 in the Astec AA11040-B revision. The Commodore CBM-II model B used a TL430 instead of TL431, which is an unusual choice. The original IBM PC power supply used a Zener diode for reference (along with many op amps). Later PC power supplies often used the TL494 PWM controller, which contained its own voltage reference and operated on the secondary side. Other ATX power supplies used the SG6105 which included two TL431s internally.

Phone chargers usually use the TL431. Inexpensive knockoffs are an exception; they often use a Zener diode instead to save a few cents. Another exception is chargers such as the iPad charger, which use primary-side regulation and don't use any voltage feedback from the output at all. See my article on power supply history for more information.

[5] The TL431 is available in a larger variety of packages than I'd expect. Two of the photos show the TL431 in a transistor-like package with three leads (TO-92). The remaining photos show the surface-mounted SOT23-3 package. The TL431 also comes in 4-pin, 5-pin, 6-pin, or 8-pin surface-mounted packages (SOT-89, SOT23-5, SOT323-6, SO-8 or MSOP-8), as well as a larger package like a power transistor (TO-252) or an 8-pin IC package (DIP-8). (pictures).

[6] For more information on how bipolar transistors are implemented in silicon, there are many sources. Semiconductor Technology gives a good overview of NPN transistor construction. Basic Integrated Circuit Processing is a presentation that describes transistor fabrication in great detail. The Wikipedia diagram is also useful.

[7] You might have wondered why there is a distinction between the collector and emitter of a transistor, when the simple picture of a transistor is totally symmetrical. Both connect to an N layer, so why does it matter? As you can see from the die photo, the collector and emitter are very different in a real transistor. In addition to the very large size difference, the silicon doping is different. The result is a transistor will have poor gain if the collector and emitter are swapped.

[8] The PNP transistors in the TL431 have a circular structure that gives them a very different appearance from the NPN transistors. The circular structure used for PNP transistors in the TL431 is illustrated in Designing Analog Chips by Hans Camenzind, who was the designer of the 555 timer. If you want to know more about analog chips work, I strongly recommend Camenzind's book, which explains analog circuits in detail with a minimum of mathematics. Download the free PDF or get the printed version.

The structure of a PNP transistor is also explained in Principles of Semiconductor Devices. Analysis and Design of Analog Integrated Circuits provides detailed models of bipolar transistors and how they are fabricated in ICs.

[9] The transistors and resistors in the die I examined have very different values from values others have published. These values fundamentally affect the operation of the bandgap voltage reference. Specifically, previous schematics show R2 and R3 in a 1:3 ratio, and Q5 has 2 times the emitter area as Q6. Looking at the die photo, R2 and R3 are equal, and Q5 has 8 times the emitter area as Q4. These ratios result in a different ΔVbe. To compensate for this, R1 and R4 are different between previous schematics and the die photo. I will explain this in detail in a later article, but to summarize Vref = 2*Vbe + (2*R1+R2)/R4 * ΔVbe, which works out to about 2.5 volts. Note that the ratio of the resistances matters, not the values; this helps counteract the poor resistor tolerances in a chip.

In the die, Q8 is formed from two transistors in parallel. I would expect Q8 and Q9 to be identical to form a balanced comparator, so I don't understand the motivation behind this. My leading theory is this adjusts the reference voltage up slightly to hit 2.5V. B. Engl suggests this may help the device operate better at low voltage.

[10] I won't go into the details of a bandgap reference here, except to mention that it sounds like some crazy quantum device, but it's really just a couple transistors. For more information on how a bandgap reference works, see How to make a bandgap voltage reference in one easy lesson by Paul Brokaw, inventor of the Brokaw bandgap reference. A presentation on the bandgap reference is here.

[11] In a sense, the bandgap circuit in the TL431 operates "backwards" to a regular bandgap voltage reference. A normal bandgap circuit provides the necessary emitter voltages to produce the desired voltage as output. The TL431's circuit takes the reference voltage as input, and the emitter voltages are used as outputs to the comparator. In other words, contrary to the block diagram, there is not a stable voltage reference inside the TL431 that is compared to the ref input. Instead, the ref input generates two signals to the comparator that match when the input is 2.5 volts.

[12] There are many articles about the TL431, but they tend to be very technical, expecting a background in control theory, Bode plots, etc. The TL431 in Switch-Mode Power Supplies loops is a classic TL431 paper by Christophe Basso and Petr Kadanka. This explains the TL431 from the internals through loop compensation to an actual power supply. It includes a detailed schematic and description of how the TL431 operates internally. Other related articles are at powerelectronics.com. Designing with the TL431, Ray Ridley, Switching Power Magazine is a detailed explanation of how to use the TL431 for power supply feedback, and the details of loop compensation. The TL431 in the Control of Switching Power Supplies is a detailed presentation from ON Semiconductor. The TL431 datasheet includes a schematic of the chip's internals. Strangely, the resistances on this schematic are very different from what can be seen from the die.

iPad charger teardown: inside Apple's charger and a risky phony

This article is now available in Chinese: [中文翻译版本] Apple iPad原装充电器拆解.
Apple sells their iPad charger for $19, while you can buy an iPad charger on eBay for about $3. From the outside, the chargers look the same. Is there a difference besides the price? In this article, I look inside real and counterfeit chargers and find that the genuine charger has much better construction, power quality, and most importantly safety. The counterfeit turns out to be a 5 watt charger in disguise, half the power of a genuine charger.

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Counterfeit
A real Apple iPad charger (left) and a counterfeit charger (right

From the outside, the real charger (left) and counterfeit charger (right) are almost identical. If you look very closely, you can spot are a few differences in the text: The counterfeit removed "Designed by Apple in California. Assembled in China" and the manufacturer "Foxlink"[1], probably for legal reasons. (But strangely, the counterfeit still says "TM and © 2010 Apple Inc.") The counterfeit charger displays a bunch of certifications (such as UL) that it doesn't actually have. As you will see below, there is no way it could pass safety testing.

Opening up the chargers reveals big differences between them. The genuine charger on the left is crammed full of components, fitting as much as possible into the case. The counterfeit charger on the right is much simpler with fewer components and much more empty space. The Apple charger uses larger, higher-quality components (in particular the capacitors and the transformer); below you will see that these have a big effect on power quality and safety.

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Counterfeit
The components inside a real iPad charger (left) and a counterfeit charger (right).

The components inside a real iPad charger (left) and a counterfeit charger (right).

One safety difference is obvious: the Apple charger has much more insulation. The upper (high-voltage) half is wrapped in yellow insulating tape. Some components are encased in shrink tubing, there are plastic insulators between some components, and some wires have extra insulation. The counterfeit charger only has minimal insulation.

The build quality of the Apple charger is much higher. In the counterfeit charger, some components are visibly crooked or askew. While this doesn't affect the circuit electrically, it indicates a lack of care in construction.

Flipping the boards over reveals that the circuitry of the genuine Apple charger is much more complex than the counterfeit. The Apple board is crammed with tiny surface-mounted components in every available spot. The counterfeit board has a lot of empty space, with just a few components. Note the reddish insulating tape in the lower center of the Apple board, another safety feature of the genuine charger.

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Counterfeit
The circuit board of a real iPad charger (left) and a counterfeit charger (right).

The circuit board of a real iPad charger (left) and a counterfeit charger (right).

How the chargers work

Both the real and counterfeit chargers use similar flyback[2] switching power supply circuits. The switching power supply is the innovation that allows these chargers to be so compact, unlike the heavy "wall warts" powering older consumer electronics. The principle of a switching power supply is the power is switched on and off tens of thousands of times a second, allowing it to provide the exact amount of power required with very little power wasted as heat. In addition, the high frequencies allow the charger to use a small transformer, unlike the bulky transformers used for 60 Hz AC.

Since the counterfeit charger is much simpler, it is easier to understand how it works and I'll explain it first in reference to the picture below. The AC power enters through the white wires in the upper left. It passes through a fusible resistor, which acts as a safety fuse. Below this, the bridge rectifier contains four diodes which convert the AC into DC (at about 170 to 340 volts[3]). The input capacitor smooths out this power. The 4-pin control IC[4] monitors the charger and uses the switching transistor to turn the high-voltage DC on and off 41,000 times per second. This chopped DC is fed into the primary winding of the flyback transformer. The transformer converts this to the desired high-current 5 volts. The output diode produces DC, and the output capacitor smooths it out. Finally, the output voltage is available at the USB connector to power your iPad. A few components round out the circuit. A feedback winding on the transformer provides voltage feedback to the control IC. This winding also powers the IC; the IC power capacitor smooths out this power. Finally, the blue snubber[5] capacitor absorbs current spikes when the transistor is switched off.[6]

Counterfeit
Inside a counterfeit iPad charger

Inside a counterfeit iPad charger

The genuine iPad charger below operates on similar principles, although the circuit is more advanced. The AC input is on the lower right, and goes through a 2A fuse (in black insulation for safety). The primary has much more filtering than in the counterfeit charger with a filter coil (common mode choke), inductor, and two large electrolytic capacitors. This increases the cost, but improves the power quality. On the output side (left), the charger has two filter capacitors, including a high-quality aluminum polymer capacitor (with the magenta stripe). The Y capacitors help reduce interference.[7] The tiny NTC temperature sensor lets the charger shut down if it overheats. (I removed some of the charger's insulation to make the components visible in this photo.)

iPad
Inside a genuine iPad charger.

Inside a genuine iPad charger.

On the other side of the circuit board, things get complicated in the Apple charger. Starting with the AC input in the upper right, the charger includes additional input filters as well as spark gaps.[8] The latch release circuit[9] lets the charger reset quickly from faults. The control IC[10] provides advanced control of the charger under varying conditions. (This IC is much more complex than the control IC in the counterfeit charger.) The current sense resistor lets the IC monitor the current through the transformer and the line voltage resistors let the IC monitor the input voltage (as well as initially powering-up the IC[9]). The protection circuit uses the temperature sensor on the other side of the board to shut down if there is an over-voltage or over-temperature problem. [20]

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The circuit board inside a genuine iPad charger showing the components.

The circuit board inside a genuine iPad charger showing the components.

The secondary side includes some special features for power quality. The Y-capacitor filter works with the Y capacitors to filter out noise. The output filter circuitry is more complex than in the counterfeit. Note that the real charger has a ground connection, unlike the counterfeit charger which has a plastic pin here.[11]

Both chargers use resistors to put special voltages on the USB data lines[12] to indicate the charger type, using Apple's proprietary system (details). (This is why iPads say "Charging is not supported with this accessory" with some chargers.) Through these resistors the genuine charger indicates that it is an Apple 2A charger, while the counterfeit indicates that it is an Apple 1A charger. This shows that the counterfeit is really a 5W charger packaged as a 10W charger.

When looking at these circuits up close, it's easy to forget just how small the components are. The picture below shows one of the surface-mount components (a 0-ohm resistor[13]) from the iPad charger. It is just to the left of Roosevelt's chin on the dime.

A zero-ohm resistor from the iPad charger, on top of a dime

A zero-ohm resistor

Safety, or lack thereof

Safety probably isn't something you think about when you plug in your charger, but it's important. Inside the charger is 170 volts or more with very little separating it from your iPad and you. If something goes wrong, the charger can burn up (below), injure you, or even killyou. Devices such as chargers have strict safety standards[14] - if you get a charger from a reputable manufacturer. If you buy a cheap counterfeit charger, these safety standards are ignored. You can't see the safety risks from the outside, but by taking the chargers apart, I can show you the dangers of the counterfeit.

Counterfeit iPhone
Counterfeit iPhone charger that burned up
A Counterfeit iPhone charger that burned up. Photo by Anool Mahidharia. Used with permission

Creepage and clearance

The UL regulations[14] require safe separation between the high voltage and the low voltage. This is measured by creepage - the distance between them along the circuit board, and clearance - the distance between them through air. The regulations are complex, but in general there should be at least 4mm between high-voltage circuitry and low-voltage circuitry.

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The iPad charger provides safe creepage and clearance distances between the high-voltage side (bottom) and low-voltage side (top).

The iPad charger provides safe creepage and clearance distances between the primary high-voltage side (bottom) and secondary low-voltage side (top).

The image above shows how the genuine iPad charger's circuit board separates the high voltage (bottom) from the low voltage (top). The happy face on the right marks an empty region that provides a safety gap between the primary and secondary. (This is a contrast with the rest of the circuit board, which is crammed full of components.) This gap of 5.6mm provides a comfortable safety margin. The happy face on the left marks a slot in the board that separates the low voltage and high voltage. The photo below shows how an insulating fin is built into the case and through this slot to protect the USB connector. Additional reddish-brown insulating tape goes through this slot, and the whole high-voltage section is wrapped in yellow insulating tape. The result is multiple layers of protection.

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The iPad charger case has a plastic fin that slides around the USB port to provide extra insulation.

The iPad charger case has a plastic fin that slides around the USB port to provide extra insulation.

The creepage distance on the counterfeit charger board below is scary - only 0.6 mm separation between low and high voltage. The sad face on the right shows where a low-voltage trace is nearly touching the high-voltage trace below. (The ruler on the right indicates millimeters.) The board isn't as bad as it could be: the happy face on the left marks a slot cut in the circuit board under the transformer to increase the creepage distance. But overall, this board is unsafe. If you use the charger in a humid bathroom and a drop of water condenses across the 0.6 mm gap, then zap!

Counterfeit
Dangerous creepage in a counterfeit iPad charger.

Dangerous creepage in a counterfeit iPad charger.

Safety in the transformer

For safety, the high-voltage and low-voltage sides of the charger must be electrically isolated.[15] But obviously the electrical power needs to get through somehow. The flyback transformer accomplishes this task by using magnetic fields to transfer the power without a dangerous direct connection. Because the transformer is a large and relatively expensive component, it is tempting to take safety and quality short cuts here. The genuine transformer (left) is considerably larger than the counterfeit (right), which is a hint of better quality and more power capacity. Disassembling the transformers shows that this is the case.

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Counterfeit
The flyback transformers from an iPad charger (left) and a counterfeit charger (right). Dime and banana are for scale.

The flyback transformers from an iPad charger (left) and a counterfeit charger (right). Dime and banana are for scale.

The key safety requirement of the transformer is to separate the high-voltage windings from the low-voltage secondary winding, and the counterfeit charger fails here. The pictures below show the transformers after removing primary windings and insulating tape, revealing the secondary winding. The wires look similar at first glance, but the the genuine charger (left) has triple-insulated wire while the counterfeit (right) is uninsulated except for a thin varnish. The triple-insulated wire is an important safety feature that keeps the high voltage out even if there is a flaw in the insulating tape and in the wire's insulation. Also note the additional black and white insulation on the wires where they leave the transformer. In the counterfeit charger, the only thing separating the secondary winding from high voltage is the insulating tape. If there is a flaw in the tape or the wires shift too far, then zap!

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Inside the transformer of an iPad charger, This is the triple-insulated secondary winding.
Counterfeit
The secondary winding does not have triple-insulated wiring. This is a major safety flaw in the counterfeit iPad charger.

The real charger provides much more power with much less noise

Lab measurements of the output from the chargers shows a couple problems with the counterfeit. First, the counterfeit turns out to provide at most 5.9W, not 10W. Second, the output voltage is extremely noisy and full of spikes.

The following voltage-vs-current graphs show the performance of the iPad charger (left) and counterfeit charger (right) under increasing load. The line for the real charger goes much farther to the right, showing that the real charger provides much more current. By my measurements, the real charger provides a maximum of 10.1 watts, while the counterfeit charger provides only 5.9 watts. The consequence is the real charger will charge your iPad almost twice as fast. (For details on these graphs, see my article testing a dozen chargers.) The other thing to note is the line for the Apple charger is smooth and thin, while the counterfeit charger's line is all over the place. This indicates that the power provided by the counterfeit charger is noisy and low quality.

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Voltage vs current graph for iPad charger
Counterfeit
Voltage vs current graph for fake iPad charger

The next pair of graphs shows the power quality. The yellow line shows the voltage. The real charger has a stable yellow thin line, while the counterfeit charger's output has large voltage spikes. (I had to change the scale to get the output to fit on the screen, so the counterfeit charger is actually twice as bad as it appears here.) The bottom of the counterfeit charger's yellow line is wavy, due to 120 Hz ripple appearing in the output voltage.

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High frequency oscilloscope trace from Apple iPad charger
Counterfeit
High frequency oscilloscope trace from counterfeit iPad charger

The orange line shows the frequency spectrum of the output: lower is better, and higher is exponentially worse. The counterfeit spectrum is much higher in general, with a large spike at the switching frequency. This shows that the counterfeit charger's power is worse across the frequency spectrum.

You might wonder if the power quality actually matters. The biggest impact it has is on touchscreen performance. The interference from bad power supplies is known to cause the touchscreen to behave erratically.[16] If your screen malfunctions when plugged into a charger, this is probably the cause.

Inside the real charger's transformer

There's more inside the transformer that you'd expect. This section does a full teardown of the transformer from the genuine charger.

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A copper band surrounds the ferrite core in the flyback transformer from an iPad charger.
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Removing the ferrite core and insulation reveals the double-stranded primary winding.

The first photo above shows that underneath the the first layer of yellow insulating tape, a layer of copper foil is attached to the transformer's ferrite core to ground it. Next, removing the ferrite core and more insulation reveals the double-stranded primary winding. The high-voltage input is fed into this winding.

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After removing the triple-stranded bias winding and insulating tape, the secondary winding of the transformer is visible. Note the triple-insulated wires used for the secondary winding.
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The next layer of insulation contains copper foil.

Underneath the primary winding and more insulating tape is the triple-stranded bias winding, which provides feedback and power to the control IC. (In the photo, this winding has been removed and is surrounding the transformer.) After removing more insulating tape, the secondary winding of the transformer is visible. As discussed in the safety section, the secondary winding has triple-insulated wires and extra insulation where the wires leave the transformer. The next layer of insulation (right) contains copper foil. This helps reduce interference.

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The innermost layer of the iPad charger flyback transformer is the primary winding.

Finally, the innermost layer of the iPad charger flyback transformer is the second half of the primary winding (above). Splitting the primary winding into two layers is more expensive, but results in a better transformer due to better coupling of the magnetic fields.

In comparison, the transformer of the counterfeit charger is much lower quality. (I haven't included the pictures for reasons of space; click through to see them.) It simply has the bias winding (pic), secondary winding (pic), and primary winding (pic) , separated by insulating tape. Unlike the genuine transformer, the counterfeit saves cost by omitting the copper foil layers. The counterfeit also doesn't use the more expensive split, multi-stranded windings that the genuine charger uses. As discussed earlier, the secondary winding is plain copper wire, not triple-insulated wire, which is a significant safety flaw.

How does the iPad charger compare to the iPhone charger?

The iPad charger is considerably larger than the iPhone charger and provides twice the power. In my detailed iPhone charger teardown I looked at the internals of the iPhone charger. The iPhone charger (below) uses two circuit boards that combine to form a one inch cube, which is impressive engineering. The iPhone and iPad chargers are both flyback switching power supplies, but the feedback mechanisms are very different.[17] Overall, I like the iPhone charger more than the iPad charger from a design standpoint, mainly because of the harder engineering challenge of cramming everything into a much smaller space.

iPhone
Inside the iPhone charger: input inductor (green), Y capacitor (blue), flyback transformer (yellow), USB connector (silver). The primary circuit board is on top and the secondary board on the bottom.

Schematics

In my iPhone charger teardown, I drew up a schematic of the charger, but for the iPad chargers I didn't need to do this. The genuine iPad charger is almost identical[18] to the reference design schematic provided by iWatt. The counterfeit charger is almost identical to the schematic in the DB02A controller datasheet. You can see from the schematics that the genuine charger has a much more complex circuit than the counterfeit. (Click the thumbnails below to get to the datasheets.)

iPad
Thumbnail: Click for schematic of iPad charger based on iWatt 1691 controller.
Counterfeit
Thumbnail: Click for schematic of counterfeit iPad charger based on DB02A controller.

Is the Apple charger worth the price?

Apple's charger is expensive compared to other chargers, but is a high quality product. You should definitely stay away from the cheap counterfeit chargers, as they are low quality and dangerous. Non-Apple name brand chargers are generally good quality according to my tests, with some better than Apple. If you want to get an Apple charger without the high price, the best way I've found is to buy a used one on eBay from a US source. I've bought several for testing, and they have always been genuine.

I wrote earlier about Apple's huge profit margins on chargers. Apple has since dropped their charger prices from $29 to $19, which is more reasonable, but looking at the price of similar chargers from other manufacturers and the cost of components, I think Apple has a huge profit margin even at $19.[19]

In any case, the iPad charger is an impressive piece of engineering with a lot of interesting circuitry inside. The counterfeit charger is also impressive in its own way - it's amazing that a charger can be manufactured and sold for such a low price (if you don't care about safety and quality). Overall, you mostly get what you pay for; even if you can't tell from the outside, there are big differences inside the case.

Notes and references

[1] Foxlink (Taiwan), Foxconn (Taiwan), and Flextronics (Singapore) are all manufacturers for Apple with confusingly similar names. Foxconn is the company with controversy over employee treatment; this charger is made by Foxlink, a different company. Interestingly, the chairmen of both companies are brothers and the companies do a lot of business with each other. The companies state that they are entirely independent, though (statement, Foxlink annual report). Foxconn and Flextronics are the world's #1 and #3 largest electronics manufacturing companies according to the MMI top 50 for 2013, while Foxlink is smaller.

[2] The chargers uses a flyback design, where the transformer operates "backwards" from how you might expect. When a voltage pulse is sent into the transformer, the output diode blocks the output so there is no output - instead a magnetic field builds up in the transformer. The transformer core has a tiny air gap to help store this field. When the voltage input stops, the magnetic field collapses, transferring power to the output winding. Flyback power supplies are very common for low-wattage power supplies.

[3] You might wonder why the DC voltage inside the power supply is so much higher than the line voltage. The DC voltage is approximately sqrt(2) times the AC voltage, since the diode charges the capacitor to the peak of the AC signal. Thus, the input of 100 to 240 volts AC is converted to a DC voltage of 145 to 345 volts internally. This isn't enough to be officially high voltage but I'll call it high voltage for convenience. According to standards, anything under 50 volts AC or 120 V dc is considered extra-low voltage and is considered safe under normal conditions. But I'll refer to the 5V output as low voltage for convenience.

[4] The counterfeit charger uses a DB02A controller IC. This controller only has four pins and is in a TO-94 (SIP-4) package. (According to the official JEDEC standard, TO-94 is a bolt-like package for large SCRs. It's a puzzle why some companies use TO-94 to describe 4-pin inline packages.) According to the datasheet (Chinese), the chip is for 500mA-1000mA chargers, which explains why the counterfeit charger only produces 5 watts, instead of the 10 watts an iPad charger is supposed to produce. This controller is very inexpensive, available for ¥ 0.35 (about 6 cents).

I couldn't find any US chips similar to this chip, even after a lot of searching; it appears to be a Chinese design with datasheets only in Chinese, manufactured by "Fine Made" Shenzhen Fuman Electronics. Since the chip only has four pins, I expected it to be a trivial Ringing Choke Converter (RCC) circuit with just a couple transistors inside the chip - but I cracked it open with Vise-Grips and it turns out to be a fairly complex chip. I took a picture through a microscope of the IC die, which is about 1 mm across. One interesting feature is the many white pads around the outside of the die, which are used to blow fuses to trim various resistances in the chip. I wasn't expecting to see this level of quality and sophistication. The die has the label "N7113 802" at the right; I don't know what this indicates. Three of the four wires connect in the lower left, and the fourth in the lower right.

Die photo of the DB02A SMPS controller chip.

Die photo of the DB02A SMPS controller chip used in the counterfeit charger.

[5] When a diode or transistor switches, it creates a voltage spike, which can be controlled by a special snubber or clamp circuit. For a lot of information on snubbers and clamps, see Passive Lossless Snubbers for High Frequency PWM Conversion and Switchmode Power Supply Reference Manual.

[6] In the counterfeit charger, the switching transistor is a ALJ 13003 NPN power transistor (datasheet), apparently made by Shenzhen LongJing Microelectronics Co. This transistor is a version of Motorola's MJE 13003 switchmode transistor which was introduced in 1976 (MJE indicates power device in a plastic package). The bridge rectifier is a B6M (datasheet). The output diode is a SR260 Schottky barrier rectifier.

[7] The iPad charger uses special Y-capacitors to bridge the high-voltage and low-voltage sides of the charger. This capacitor helps reduce EMI interference, and is specially designed to avoid any safety hazard. It does, however, pass a tiny amount of electricity - if you feel a tingle from your charger, these capacitors are probably the cause. For more information on X and Y capacitors, see Kemet's presentation and Designing low leakage current power supplies.

[8] The iPad has two spark gaps next to inductor L1 (the input AC common mode choke). I couldn't find a lot of information on this sort of spark gap, but one example of it is an Infineon SMPS design, where similar spark gaps are designed to discharge accumulated charge for a 3KV lightning surge test.

[9] The Apple charger includes a "latch release circuit". If there is a fault, the control IC will shut down the charger until power is removed. However, after unplugging a charger, the input capacitors may store power for many seconds. (You may have seen LEDs remain illuminated for several seconds after unplugging devices.) The latch release circuit ensures that the charger will reset properly even if you plug it back in quickly. It does this by providing a separate diode bridge for the charger's power - this circuit has a much smaller capacitor, so it will power off quickly. (See the schematic for details.) This seems like over-engineering to me, adding extra circuitry for this rare case.

In normal use, by the way, the control IC is powered by the transformer's feedback winding. But if the control IC isn't running, the transformer won't work, leading to a chicken-and-egg situation. The solution is a startup power path where the control IC gets enough power from the AC input to start up, and then switches to the transformer.

[10] The genuine charger uses a complex control chip manufactured by iWatt, the 1691. This chip monitors the input line voltage, the current through the transformer, and the voltage feedback from the transformer. It controls the switching frequency and length of time the power is switched on, with different behavior under no load, low load, and high load, as well as constant monitoring for faults. A detailed presentation on the iW1691 is here. This chip sells for about 30 cents, but I expect Apple gets a better price.

[11] The real charger has a metal ground pin that connects to the power plug, while the counterfeit has a plastic pin. This is one difference between the chargers that is visible externally if you slide the power plug off the charger. Ironically, the US plug doesn't use the ground connection, so this is one safety issue that doesn't make any difference in practice.

[12] Apple uses a proprietary technique for the charger to indicate to the device what kind of charger it is. Different types of Apple chargers use resistances to put different voltages on the USB D+ and D- pins. For details on USB charging protocols, see my earlier references.

[13] While it would be nice to find superconductors inside the charger, unfortunately the zero-ohm resistor is a bit more than 0 ohms. While this resistor may seem pointless, it allows the manufacturers to substitute a resistor later if different transistors require it.

[14] The outside of the charger has the slightly mysterious text: "For use with information technology equipment". This indicates that the charger is covered by the safety standard UL 60950-1, which specifies the various isolation distances required. For a brief overview of isolation distances, see i-Spec Circuit Separation and some of my earlier references.

[15] Only a few special components can safely bridge the gap between the high voltage side of the charger and the low voltage side. The most obvious is the transformer. Y-capacitors can also bridge the primary and secondary side because they are designed not to pass dangerous currents, and not to short out if they fail. Optoisolators use a light signal to provide feedback between the circuits in an iPhone charger, but are not used in the iPad charger.

[16] For an explanation of why the noisy output from cheap chargers messes up touchscreens, see Noise Wars: Projected Capacitance Strikes Back. The article discusses how capacitive touchscreen ICs need to sense pico-Coulombs of charge, which is very difficult when AC noise is present. The article blames touchscreen problems on aftermarket low cost chargers.

[17] The biggest difference between the iPhone charger and the iPad charger is the feedback used to regulate the voltage. The iPhone charger measures the output voltage with a TL431 chip and sends a feedback signal to the control IC via an optoisolator. The iPad charger avoids these components by using primary-side regulation. Instead of measuring the actual output voltage, the iPad control IC looks at the voltage in the feedback winding, which should approximately match the output voltage.

[18] I noticed only a few significant differences between the iPad charger and iWatt's published 1691 charger reference design. This probably means iWatt did most of the design work for Apple.

Comparing the actual charger with the reference design shows a few filtering improvements. The charger has RC snubbers the input bridge rectifier (a rare feature also in the iPhone charger). The charger has an extra diode on the secondary for filtering, as well as a (zener?) diode in the switching transistor drive circuit. The iPad charger uses two Y-capacitors instead of one, and a R/C filter attached to the Y-capacitor on the secondary side. The charger connects line ground to secondary ground through a resistor. The reference design doesn't show the USB data resistors[12].

[19] Some people think that I'm ignoring Apple's cost of designing chargers when figuring their large profit margin. First, if you spend $2 million on design and manufacture 200 million chargers, then design adds only one cent to the cost per charger. Second, iWatt's designers deserve credit for the complex control chip and the reference design, which is most of the design work.

[20] For those interested in the components, the iPad charger's primary diodes (F6w) are 1.5A 60V Schottky Barrier Diodes (datasheet). The "T3" diodes are fast switching diodes (datasheet). The switching transistor is an Infineon SPA04N60C Cool MOS® 650V power transistor (datasheet). The bridge rectifier is a bridge: MB10S CD 0.5A bridge rectifier with high surge capacity (datasheet). The component in the protection circuit that looks like a transistor is a BAV70 dual high-speed switching diode (datasheet). The output diode is a SBR10U45SP5 10A super barrier rectifier (datasheet). The Y capacitors are 220pF 250V. The input capacitors are Samxon 10µFand 4.7µF 400v electrolytics. The output capacitors are a Koshin KLH 820µF 6.3V aluminum electrolytic, and a 820 µF 6.3V X-CON ULR aluminum polymer capacitor (which is more expensive than a regular electrolytic, but filters better because of its lower ESR).