Teardown and exploration of Apple's Magsafe connector

Have you ever wondered what's inside a Mac's Magsafe connector? What controls the light? How does the Mac know what kind of charger it is? This article looks inside the Magsafe connector and answers those questions.

The Magsafe connector (introduced by Apple in 2006) is very convenient. It snaps on magnetically and disconnects if you pull on it. In addition it is symmetrical so you don't need to worry about what side is up. A small LED on the connector changes color to indicate the charging status.

The picture below shows the newer Magsafe 2 connector, which is slimmer. Note how the pins are arranged symmetrically; this allows the connector to be plugged in with either side on top. The charger and computer communicate through the adapter sense pin (also called the charge control pin), which this article will explain in detail below. The two ground pins are slightly longer than the others so they make contact first when you plug in the connector (the same as USB).

The pins of a Magsafe 2 connector. The pins are arranged symmetrically, so the connector can be plugged in either way.

Magsafe connector teardown

I had a Magsafe cable that malfunctioned, burning the power pins as you can see in the photo below, so I figured I'd tear it down and see what's inside. The connector below is an older Magsafe; notice the slightly different shape compared to the Magsafe 2 above. Also note that the middle adapter sense pin is much smaller than the pins, unlike the Magsafe 2.

Removing the outer plastic shell reveals a block of soft waxy plastic, maybe polyethylene, that helps diffuse the light from the LEDs and protects the circuit underneath.

Cutting through the soft plastic block reveals a circuit board, protected by a thin clear plastic coating. The charger wires are soldered onto the back of this board. Only two wires - power and ground - go to the charger unit. There is no data communication via the adapter sense pin with the charger unit itself.

Disassembling the connector shows the spring-loaded "Pogo pins" that form the physical connection to the Mac. The plastic pieces hold the pins in place. The block of metal on the left is not magnetized, but is attracted by the strong magnet in the Mac's connector.

The circuit board inside the Magsafe connector is very small, as you can see below. In the middle are two LEDs, orange/red and green. Two identical LEDs are on the other side. The tiny chip on the left is a DS2413 1-Wire Dual Channel Addressable Switch. This chip has two functions. It switches the status LEDs on and off (that's the "dual channel switch" part). It also provides the ID value to the Mac indicating the charger specifications and serial number.

The chip uses the 1-Wire protocol, which is a clever system for connecting low-speed devices through a single wire (plus ground). The 1-Wire system is convenient here since the Mac can communicate with the Magsafe through the single adapter sense pin.

Understanding the charger's ID code

You can easily pull up the charger information on a Mac (Go to "About this Mac", "More Info...", "System Report...", "Power"), but much of the information is puzzling. The wattage and serial number make sense, but what about the ID, Revision, and Family? It turns out that these are part of the 1-Wire protocol used by the chip inside the connector.

Every chip in the 1-Wire family has a unique 64-bit ID that is individually laser-programmed into the chip. In the 1-Wire standard, the 64-bit ID consists of an 8-bit family code identifying the type of 1-Wire device, a 48-bit unique serial number, and an 8-bit non-cryptographic CRC checksum that verifies the ID number is correct. Companies (such as Apple) can customize the ID numbers: the top 12 bits of the serial number are used as a customer ID, the next 12 bits are data specified by the customer, and the remaining 24 bits are the serial number.

With this information, the Mac's AC charger information now makes sense and the diagram below shows how the 64-bit ID maps onto the charger information. The ID field 100 is the customer ID indicating Apple. The wattage and revision are in the 12 bits of customer data (hex 3C is 60 decimal, indicating 60 watts). The Family code BA is the 1-Wire family code for the DS2413 chip. Thus, much of the AC charger information presented by the Mac is actually low-level information about the 1-Wire chip.

The 1-Wire chip inside a Magsafe connector has a 64-bit ID code. This ID maps directly onto the charger properties displayed under 'About this Mac'.

There are a few complications as the diagram below shows. Later chargers use the family code 85 for some reason. This doesn't indicate an 85 watt charger. It also doesn't indicate the family of the 1-Wire device, so it may be an arbitrary number. For Magsafe 2 chargers, the customer ID is 7A1 for a 45 watt charger, 921 for a 60 watt charger, and AA1 for an 85 watt charger. It's strange to use separate customer IDs for the different models. Even stranger, for an 85 watt charger the wattage field in the ID contains 60 (3C hex) not 85, even though 85 watts shows up on the info screen. The Revision is also dropped from the info screen for later chargers.

In a Magsafe 2 connector, the 64-bit ID maps onto the charger properties displayed under 'About this Mac'. For some reason, the 'Customer data' gives a lower wattage.

How to read the ID number

It's very easy to read the ID number from a Magsafe connector using an Arduino board and a single 2K pullup resistor, along with Paul Stoffregen's Arduino 1-Wire library and a simple Arduino program.

The circuit to access a 1-Wire chip from an Arduino is trivial - just a 2K pullup resistor.

Touching the ground wire to an outer ground pin of the Magsafe connector and the data wire to the inner adapter sense pin will let the Arduino immediately read and display the 64-bit ID number. The charger does not need to be plugged in to the wall - and in fact I recommend not plugging it in - since one interesting feature of the 1-Wire protocol is the device can power itself parasitically off the data wire, without a separate power source.

The 64-bit ID can be read out of a Magsafe connector by probing the outer pin with ground, and the middle pin with the 1-Wire data line.

To make things more convenient, the serial number can be displayed on an LCD display. The circuit looks complicated, but it's just a tangle of wires connecting the LCD display. Using a simple program, the 64-bit ID number is displayed on the bottom line of the display. The top line is a legend indicating the components of the code: "cc" CRC check, "id." customer id, "ww" wattage, "r" revision, "serial" serial number, and "ff" family. The number below corresponds to an 85 watt charger (55 hex = 85 decimal).

A 1-Wire ID reader with LCD display. Touching the wires to the contacts of the Magsafe connector displays the ID code on the bottom line of the display. The top line indicates the components of the code: CRC check, customer id, wattage, revision, serial number, and family.

Controlling the Magsafe status light

The Mac controls the status light in the Magsafe connector by sending commands through the adapter sense pin to the 1-Wire DS2413 switch IC to turn the two pairs of LEDs on or off. By sending the appropriate commands to the IC through the adapter sense pin, an Arduino can control the LEDs as desired.

The picture below demonstrates the setup. The same simple resistor circuit as before is used to communicate with the chip, along with a simple Arduino program that sends commands via the 1-Wire protocol. These commands are described in the DS2413 datasheet but should be obvious from the program code.

I used a cable removed from a dead charger for simplicity. The LEDs are normally powered by the charger's voltage, which I simulated with two 9-volt batteries. To hook the Arduino to the connector, this time I used a Mac DC input board that I got on eBay; this is the board in a Mac that the Magsafe connector plugs into. The only purpose of the board here is to give me a safer way to attach the wires than poking at the pins.

The connector contains a pair of orange/red LEDs and a pair of green LEDs, which can be switched on and off independently. When both pairs are lit, the resulting color is yellow. Thus, the connector can display three colors. The Arduino program cycles through the three colors and off, as you can see from the pictures above.

The charger startup process

When the Magsafe connector is plugged into a Mac, a lot more happens than you might expect. I believe the following steps take place:

1. The charger provides a very low current (about 100 µA) 6 volt signal on the power pins (3 volts for Magsafe 2).
2. When the Magsafe connector is plugged into the Mac, the Mac applies a resistive load (e.g. 39.41KΩ), pulling the power input low to about 1.7 volts.
3. The charger detects the power input has been pulled low, but not too low. (A short or a significant load will not enable the charger.) After exactly one second, the charger switches to full voltage (14.85 to 20 volts depending on model and wattage). There's a 16-bit microprocessor inside the charger to control this and other charger functions.
4. The Mac detects the full voltage on the power input and reads the charger ID using the 1-Wire protocol.
5. If the Mac is happy with the charger ID, it switches the power input to the internal power conversion circuit and starts using the input power. The Mac switches on the appropriate LED on the connector using the 1-Wire protocol.

This process explains why there is a delay of a second after you connect the charger before the light turns on and the computer indicates the battery is charging. It also explains why if you measure the charger output with a voltmeter, you don't find much voltage.

The complex sequence of steps provides more safety than a typical charger. Because the charger is providing extremeley low current at first, there is less risk of shorting something out while attaching the connector. Since the charger waits a full second before powering up, the Magsafe connector is likely to be firmly attached by the time full power is applied. The safety feature are not foolproof, though, as the burnt-up connector I tore apart shows.

Don't try this at home

Warning: I recommend you don't try any of these experiments. 85 watts is enough to do lots of damage: blow out your Mac's DC input board, send flames out of a component, blow fuses, or vaporize PC traces, and that's just the things I've had happen to me. The Mac and charger both have various protection mechanisms, but they won't take care of everything. Poking at your charger while it's plugged in is a high-risk activity.

Reading your charger's ID by probing the pins while it's not plugged in is considerably safer, but I can't guarantee it. If you mess up your charger, computer or Arduino you're on your own.

Conclusions

There's more to the Magsafe charger connector than you might expect. The center pin of the connector - the adapter sense pin - controls a tiny chip that both identifies the charger and controls the status LED. It is part of a complex interaction between the charger and the Mac. Using an Arduino microcontroller, this chip can be accessed and controlled using the 1-Wire protocol. Is this useful? Not really, but hopefully you found it interesting.

Teardown of the mysterious KMS 4-port USB charger

In this article I tear down a 4-port USB charger of puzzling origin. This charger is a huge step above the $2 counterfeit chargers I examined earlier in design and manufacture, but considerably below the quality of name-brand chargers. Likewise with safety - the charger was built with some attention to safety, but appears to fall short of UL standards.

One puzzle about this charger is it's unclear who makes it and what model it is. The case says it's the KMS AC-09 but the circuit board says "TC09-new-V4.2". Amazon lists the brand as "Cosmos®", but I couldn't find any sign that KMS or Cosmos are actual companies. After some web searches, I think the charger is built by Guangzhou Panyu Qiaonan Saidi Electronic Factory (more) as the TC09 charger for $5.30 wholesale, or maybe HK Yingjia International, a consumer electronics manufacturer in Shenzhen (more). In any case, I'll call this the "KMS charger" since I need to call it something.

In my previous lab analysis of 12 chargers, I compared a dozen different chargers in 9 different categories, rating them from 1 to 5 'bolts' and the KMS charger came in about average in terms of performance. The results for the KMS charger are summarized below. For details on these measurements, see my previous article A dozen USB chargers in the lab).

Overall rating
Vampire (idle) power usage
Efficiency under load
Achieves power rating
Spikes in output
High-frequency noise in output
Ripple in output
Voltage sag
Current sag
Regulation quality

The good and the bad

Overall, this charger is much higher quality than the $2 counterfeit chargers, but considerably lower quality than name-brand chargers.

The charger provides more filtering than basic chargers, from the large input choke to the multiple output inductors. It includes X and Y capacitors for filtering.

The charger looks mostly safe, although it doesn't have UL certification and I suspect it would fail certification. The 6mm clearance between the primary and secondary looks solid. However, the transformer windings are only separated by 3mm, rather than 6mm, as I show below. (This is still much superior to the $2 chargers that have almost no separation.)

One interesting feature of the power supply is the power plug can be interchanged for use in different countries. (Some other chargers such as the HP TouchPad and Apple iPad are similar.)

The charger has some quality issues. The power quality measurements I did in my previous article show the KMS charger has fairly poor quality output, with a lot of noise in the output.

The IC datasheet recommends 200 mm² of foil on the IC output pins to provide cooling. I measured about 18 mm² (less than 10% of recommended), which suggests the charger may overheat under full load.

The above photo shows that the build quality of the charger is not extremely high. The inductor at the front right is very crooked, and the optocoupler at the left is somewhat crooked. While this doesn't affect the performance, it shows the assembly was rapid rather than careful. More concerning, some of the solder joints appear to be almost bridged, which could cause catastrophic failure of the charger. I also found a government report of a KMS charger catching fire, apparently due to a loose wire in the power plug.

One unique feature of the charger is the blue LEDs which cause it to emit an eerie blue glow when in use. A lot of users dislike this though (according to reviews), because the light is distracting at night.

The circuit

For readers interested in circuits, I have prepared the above approximate schematic (click for a larger view). The circuit is pretty straightforward compared to other chargers (look at my iPhone charger schematic for comparison). Starting at the upper left, the input AC is converted to DC by the diode bridge, and then filtered by a simple inductor-capacitor filter. This high-voltage DC is connected to the flyback transformer primary. The THX203H control IC switches the other side of the flyback transformer to ground through the current-sense resistors R12A and R12B and inductor L3. (Most chargers use a separate switching transistor, but in this charger, the transistor is inside the control IC.) The snubber circuit R2, C3, and D6 absorbs some of the high-frequency switching spikes (although looking at the output below, this circuit isn't entirely successful). The auxiliary transformer winding and D7 and C4 provide the DC power to the control IC. The optocoupler provides feedback to the IC, indicating the output voltage level.

On the secondary side, the high-speed Schottky diodes (D5) convert the transformer output to DC. This is then filtered through an inductor-capacitor filter that smooths it out. The output voltage feedback is generated by the TL431A regulator and fed into the optocoupler.[1]

Finally, the actual USB output circuitry has more components than you'd expect. For each pair of ports, four resistors set the D+ and D- voltages to indicate to devices that the charger is (pretending to be) an Apple 2A charger. Each port has a small bypass capacitor to smooth out power transients. Finally there are two blue LEDs with current-limiting resistors to provide the blue glow.

The controller IC poses a bit of a mystery. It's labeled as the THX 203H controller, which turns out to be manufactured by NanJing TongHuaXin Electronic Co, Ltd., a Chinese switching power supply chip company (details). The datasheet for this part is very hard to understand, as it is machine-translated from Chinese, for example:

The startup circuit inside IC is designed as a particular current inhalation way, so it can start up with the magnification function of the power switch tube itself.

After some more investigation, this chip seems to be the SDC603 Current Mode PWM Controller designed by SDC Semi (Shaoxing Devechip Microelectronics Co., Ltd.). This is a Chinese state-level R&D center that is part of China's Torch Plan Project to develop high-tech industries. (Also check out the SDC company song.)

The controller chip is a basic 8-pin current-mode PWM controller chip. It includes a built-in NPN power transistor, which reduces the charger part count. The chip can produce 12 watts output power.

Circuit board

The above picture shows the KMS charger circuit board on the left and a circuit board from the HP TouchPad charger on the right. Compact phone chargers such as the iPhone or TouchPad chargers go to amazing effort to pack the components as tightly as possible. The KMS charger on the other hand has a much more spacious design with a lot of wasted space. Since any charger with 4 USB ports is going to be fairly large, they probably figured it's not worth the effort to make the rest of the circuitry compact. The difference in density between the two circuit boards is striking, though.

A key safety feature of the KMS charger is visible in the middle of the circuit board - note the angular cut-out slot, and the empty vertical region with no circuitry. This isolates the high-voltage circuits on the right from the low-voltage output circuits on the left. The KMS charger has a safe 6mm gap and the cut-out provides additional creepage distance. Counterfeit chargers usually skip this critical safety feature, with only a millimeter or two keeping the high voltage from reaching the output and shocking the user.

You might wonder how the charger works if the high voltage and low voltage circuits are separated by a gap. The key is that any components that cross this gap must be specially designed to avoid electrical hazards. The key component is the flyback transformer, which transfers the power through magnetic fields, avoiding any direct electrical connection between the two sides. The feedback signal passes from the secondary to the primary through an optocoupler, which transmits the feedback through a light signal, again avoiding an electrical connection. Finally, a Y safety capacitor connects the primary and secondary grounds to reduce electrical noise. The design of a Y capacitor ensures it won't pass dangerous electrical currents, and won't short out even under fault conditions.

Transformer teardown

The flyback transformer is the key component of a charger and usually the largest and most expensive. The transformer is where the high input voltage is converted to the output voltage, and the two voltages are in extremely close proximity, so the safety of the transformer is critical. From the outside, you can't tell if the manufacturer saved a few cents by leaving out most of the insulation, as happens with $2 chargers. I tore apart the transformer of the KMS charger to see what's inside.

The black circle on top of the transformer seen earlier is simply a foam disk, which helps reduce transformer noise by padding the transformer against the case. If a charger makes a high-pitched noise, it's usually coming from the transformer. Power supplies are usually designed with switching frequencies higher than people can hear, but in some circumstances it's still audible, especially if you are young and haven't lost high frequency hearing.

Under the first layers of insulating tape is a copper 'belly band' which surrounds the transformer to provide noise shielding from eddy currents in the transformer.[2] This copper shielding is omitted from super-cheap transformers, showing that this charger goes beyond the minimum.

The windings are all separated by insulating tape. Under the belly band and insulating tape is the auxiliary winding, which provides power to the control IC. You might wonder why the IC needs a separate power supply instead of using the USB power output, but this wouldn't be safe because the USB output would no longer be isolated from the input. This winding is 9 turns of wire; since the IC requires low current, the wire is fairly thin.

Above you can see half of the primary winding, which is fed by the input power. This winding has 40 turns of wire.

An interesting safety feature is the 3 mm "margin tape"[3] to the lower right of the winding, which ensures that the primary winding stays 3 mm away from the edge. I was interested to see this, since other transformers I've disassembled use triple-insulated wire instead of boundary tape. To ensure safe electrical isolation between the primary and secondary windings, either the secondary wires need to be triple-insulated, or there needs to be at least 6mm of distance between the windings. Super-small chargers don't have 3mm of extra room, so they use the more expensive triple-insulated wire. But since the KMS is larger, it uses the 3mm margin tape. I'm not an expert on safety requirements, but it looks like this transformer doesn't quite meet the requirements. Normally, the margin tape is put on both sides, so there's a total of 6mm creepage distance between the windings.[4][5] But since the tape is only on one side, the windings only have half of the required distance.

The secondary winding provides the low-voltage high-current output with 8 turns of wire. In order to support 2 amps, this winding has thick wire with four strands in parallel. I haven't seen parallel strands like this before, probably because the KMS charger supplies higher power. Note the 3mm margin tape keeping the winding away from the edge.

Finally, the second half of the primary winding forms the innermost layer of the transformer; this is also 40 turns of wire. The primary winding is split into two layers that surround the secondary winding for better electrical properties. Note that the primary winding is 80 turns, while the secondary output winding is 8 turns. To oversimplify a bit, this means the output will be 10 times the current of the input at 1/10 the voltage, which is how the high voltage low current input results in the low voltage high current output. The above picture gives a good view of the 3mm margin tape at the right that keeps the wire away from the edge of the core.

Measuring the charger in use

The charger is a switching power supply using a flyback transformer. How this works is the high voltage DC is switched on and off tens of thousands of times a second by the control IC. These pulses of DC are sent into the flyback transformer. A flyback transformer is different from normal transformers in that the output diode blocks power from flowing out of the transformer while power is flowing in. Instead, as the current increases, power is stored in the transformer as a magnetic field. When the input current switches off, the stored power then flows out of the transformer, providing the desired output.

By looking at the output voltage and frequency spectrum, we can determine a fair bit about how the device operates. I measured a constant 60 kHz switching frequency above 1 amp output load, but a dropping frequency for lower loads. The datasheet gives some clues to this behavior. The power supply normally operates using PWM (pulse width modulation). The switching frequency is constant, but the amount of time the power transistor is on varies. The longer it is on, the more power into the transformer and the more output power. This matches the observed behavior from 1 amp to 3.5 amps. The datasheet also describes how the switching frequency drops under low power, which matches what I observed below 1 amp.

The above oscilloscope trace illustrates the behavior when producing 2 amps. The frequency spectrum shows narrow peaks (orange) at the 60 kHz switching frequency and harmonics. The yellow output voltage shows a bunch of large spikes due to the power switching on and off - this indicates that the charger isn't filtering the output very well, letting these spikes get into the connected device.

The diagram below zooms in to show the output in more detail. Each spike is when the switching transistor turns on at 60 kHz. The output power drops as the current through the flyback transformer increases (since the transformer secondary is blocked by the diode at this time). The output then climbs when the transistor switches off and the power is transferred to the secondary.

As the charger load increases above 3 amps, the quality of the output significantly decreases, and large 120 Hz ripple appears in the output (yellow). This is probably because the input capacitors can't store enough power to provide a constant output at this high load. Since the charger is only rated to provide 2.1 amps of output, I don't consider this a design flaw, but it's interesting to see this behavior in the output. The key result here is not to overload the charger, because the power quality gets much worse.

The charger is designed to reduce the switching frequency under low load for efficiency. I found this feature kicks in at loads under 1 amp, with the switching frequency smoothly dropping from 60 kHz to 29 kHz at 250 mA load and even lower under no load. The graph below shows the frequency spectrum at 250 mA load. Note that the spikes are wider than the previous case since the frequency becomes more unstable when it is reduced.

The output waveform below at 250 mA is similar to the previous (2A) case, except at a lower frequency. Note that the output still has large spikes when the transistor switches on. The output voltage drops while the switching transistor is on and then rises while the transistor is off (due to the flyback design), so you can see below that the transistor is off most of the time at low power.

Power consumption

Measuring the power consumption of a charger is tricky because the charger doesn't use power like a normal resistive load, but uses a nonlinear part of the input current. This results in a power factor lower than unity. (You might expect that the poor power factor is because the charger switches on and off thousands of times a second, but actually it's the fault of the diode bridge.) I measured the power consumption of the charger under load by measuring the instantaneous line voltage and current, computing the instantaneous power, and then computing the real power from this.[6] In the following diagrams, the input line voltage is shown in yellow, and the input current is in cyan. The instantaneous power is graphed in orange at the bottom - simply the product of the voltage and current.[7]

The oscilloscope output below shows the power usage of the charger under no load. The line input voltage (yellow) is a nice sine wave, but the current (cyan) is very irregular. There is a bump corresponding to the voltage peaks as the input diodes conduct and re-charge the filter capacitors. The remaining current oscillations are unusual - I haven't seen them in other chargers, and I expect they are due to the large input choke. From the orange line you can see that the power usage has small spikes at 120 Hz. Taking the power factor into account and computing real power shows the charger uses 180 mW when idle which is fairly high, but actually lower than the Apple iPhone charger.

With load applied to the charger, the power usage shoots up as shown below. I compute the power usage as 6.4 watts, while the charger is supplying 4.4 watts to the output, for an efficiency of 69%. The shape of the current curve (cyan) and power curve (orange) shows that the charger is taking line power about half the time (the big curved peaks), and not for the other half (the flat oscillations in between). This illustrates the bad power factor that switching power supplies have. (PC power supplies often use power factor correction (PFC) circuits to improve the power factor.)The yellow input voltage curve is somewhat distorted, probably due to the lame isolation transformer I used.

You might wonder what happens if you short-circuit the output of the charger. It is designed to shut down before damage occurs, rather than self-destruct. After the internal voltage drops, the charger will start up again, and repeat this cycle until the problem goes away. This is called "hiccup mode", since the charger generates hiccups of power. The oscilloscope trace below shows the power consumption of the KMS charger when shorted. Note the pulses as it start up and shuts down every 250 milliseconds.

Components

For those who are interested in the components, I have some details. The two 6.8uF 400V electrolytic capacitors in the primary are made by ChengX. The two 470uF capacitors in the secondary are made by JWCO. The X capacitor is a .1uF K 275V X2 made by Dain Electronics, a Chinese manufacturer of plastic metal film capacitors, now merged with WINDAY Electronic Industrial Co Ltd. The Y1 capacitor is a JN222M 2200pF disk ceramic suppression capacitor manufactured by Jya-Nay, a Taiwanese capacitor company. There's also a blue 681J (i.e. .68nF) polyester film capacitor of unknown manufacturer; looking at the circuit board this capacitor (C7) was originally a surface-mounted device, but was replaced with a larger capacitor.

The diodes are manufactured by MIC (Master Instrument Corporation, Shanghai). Most chargers use a diode bridge to convert the AC to DC, but this charger uses four independent diodes, which are 1N4007 700V diodes. The secondary rectification uses two Schottky diodes (SR360 3 amp 60V) from MIC. The circuit board uses the unusual mounting of two diodes on top of each other soldered into the same holes. The charger also uses FR107 700V fast recovery diodes.

Like most power supplies, the charger uses a TL431A for the voltage feedback.[1] This TL431A is produced by Wing Shing Computer Components The optocoupler is an ORPC 817B optocoupler from Shenzen Orient Technology Co., Ltd. (I don't want to speculate on the cultural significance of their raising the flag over Iwo Jima company logo.)

Conclusion

The KMS charger occupies an interesting middle ground between dangerous $2 counterfeit chargers and expensive name-brand chargers. Tearing down this 4-port USB charger of unknown origin reveals details of the circuitry. It also illustrates a network of Chinese suppliers and manufacturers, most of which are hardly known in the US. On Amazon, customer ratings for this charger are split between people who love it and people who hate it, which seems reasonable given what I saw in the teardown. Thanks to Gary F. for providing the charger.

Notes and references

[1] To summarize the feedback circuit: R17 and R18 form a resistor divider on the output voltage. If the output voltage is above 5.125 volts, the TL431 control input will be above 2.5 and the TL431 conducts. This energizes the optocoupler, providing current pulling the FB pin lower. Low FB increases the duty cycle, increasing the maximum transformer current, and increasing the output voltage. If the output voltage is considerably too high, or overtemperature is sensed, the switching frequency is decreased, reducing the power transferred to the output. (This is over-simplified; the frequency response of the feedback control loop is controlled via R13, R16, C8, and C9.) An alternative is to sense voltage from the primary side, so the feedback circuit can be eliminated. This reduces the total charger cost by about 20 cents according to a report.

[2] The use of a copper "belly band" in flyback transformers is discussed in Flyback Transformer Design for the UCC28600 (page 2). It provides an electromagnetic radiation shield. The article mentions that the belly band may cause difficulties with creepage requirements and that seems to be the case with the KMS, since there is only 3mm creepage between the primary-grounded belly band and the secondary wiring.

[3] A lot of interesting information about flyback transformer design and construction is in Cookbook for do-it-yourself transformer design

[4] A discussion of how to achieve 5-6mm creepage distance by using 2.5 or 3mm margin tape is in Flyback Transformer Design for the IRIS40xx Series. Note that the margin tape must be on both sides of the winding to achieve this distance, while the KMS transformer only uses the tape on one side.

[5] Safety Considerations in Power Supply Design provides a detailed explanation of safety requirements for power supplies. It explains creepage and clearance

[6] See Understanding power factor and input current harmonics in switched mode power supplies for details on power factor, power supplies have poor power factors, and why poor power factors are a bad thing. Briefly, the power factor is due to the non-linear current through the diodes at peaks, not due to a phase shift. Real power can be measured with an oscilloscope as the average value of the instantaneous power, see Power - Real And Apparent: A Tutorial On Basic Line Power Measurements or Measuring power using the DL750.

[7] For the input power measurements it is very important to use an isolation transformer to avoid destroying your oscilloscope or shocking yourself. For my measurements, a resistor voltage divider reduced the input line voltage - the actual voltage is 11.06 times the displayed probe 1 voltage (C1, yellow). The current was measured through a 5.2 ohm shunt resistor, so the current is 1/5.2 times the displayed probe 2 voltage (C2, cyan). Combining these, the power in watts is 2.13 times the measured C1*C2 value (M1, orange).

A dozen USB chargers in the lab: Apple is very good, but not quite the best

When you buy a USB charger, how do you know if you're getting a safe, high-quality charger for your money? You can't tell from the outside if a charger provides silky-smooth power or if it is a dangerous charger that emits noisy power that cause touchscreen malfunctions[1] and could self-destruct. In this article, I carefully measure the performance of a dozen different chargers, rate their performance in multiple categories, and determine the winners and losers.

The above picture shows the twelve chargers I analyzed.[2] The charger in the upper-left is the cube-shaped Apple iPhone charger. Next is an oblong Samsung adapter and a cube Samsung adapter. The Apple iPad power adapter is substantially larger[3] than the iPhone charger but provides twice the power. The HP TouchPad power charger has an unusual cylindrical shape. Next is a counterfeit iPhone charger, which appears identical to the real thing but only costs a couple dollars. In the upper right, the Monoprice iPhone charger has a 30-pin dock connector, not USB. The colorful orange charger is a counterfeit of the Apple UK iPhone charger. Next is a counterfeit iPad charger that looks just like the real one. The Belkin power adapter is oval shaped. The KMS power supply provides four USB ports. The final charger is a Motorola Charger.

Summary of ratings

The chargers are rated from 1 to 5 energy bolts, with 5 bolts the best. The overall rating below is the average of the ratings in nine different categories, based on my measurements of efficiency, power stability, power quality, and power output. The quick summary is that phone manufacturers provide pretty good chargers, the aftermarket chargers are worse, and $2 counterfeit chargers are pretty much junk. Much to my surprise, the HP TouchPad charger (which isn't sold any more) turned out to have the best overall score. The counterfeit iPhone charger set a new low for bad quality, strikingly worse than the other two counterfeits.

	Model	Overall rating
Apple iPhone	Apple A1265
Samsung oblong	Samsung travel adapter ETA0U60JBE
Samsung cube	Samsung travel adapter ETA0U80JBE
Apple iPad	Apple 10W USB Power Adapter A1357
HP TouchPad	Hewlett Packard LPS AC/DC Adaptor P/N 157-10157-00
Counterfeit iPhone	Fake Apple A1265 "Designed by California"
Monoprice	Monoprice Switching Mode Power Supply MIPTC1A
Counterfeit UK	Fake Apple A1299
Counterfeit iPad	Fake Apple 10W USB Power Adapter A1357
Belkin	Belkin UTC001
KMS	KMS-AC09
Motorola	Motorola AC Power Supply DC4050US0301

Inside a charger

These chargers cram a lot of complex circuitry into a small package, as you can see from the iPhone charger below. (See my iPhone charger teardown for more details.) The small size makes it challenging to make an efficient, high-quality charger, while the commoditization of chargers and the demand for low prices pressure manufacturers to make the circuit as simple as possible and exclude expensive components, even if the power quality is worse. The result is a wide variation in the quality of the chargers, most of which is invisible to the user, who may believe "a charger is a charger".

Inside the iPhone charger

Internally a charger is an amazingly compact switching power supply that efficiently converts line AC into 5 volt DC output. The input AC is first converted to high-voltage DC. The DC is chopped up tens of thousands of times a second and fed into a tiny flyback transformer. The output of the transformer is converted to low-voltage DC, filtered, and provided as the 5 volt output through the USB port. A feedback mechanism regulates the chopping frequency to keep the output voltage stable. Name-brand chargers use a specialized control IC to run the charger, while cheap chargers cut corners by replacing the IC with a cheap, low-quality feedback circuit.[4]

A poor design can suffer several problems. If the output voltage is not filtered well, there will be noise and spikes due to the high-frequency switching. At extreme levels this could damage your phone, but the most common symptom is the touchscreen doesn't work while the charger is plugged in.[1] A second problem is the output voltage can be affected by the AC input, causing 120 Hz "ripple".[5] Third, the charger is supposed to provide a constant voltage. A poor design can cause the voltage to sag as the load increases. Your phone will take longer to charge if the charger doesn't provide enough power. Finally, USB chargers are not all interchangeable; the wrong type of charger may not work with your device.[6]

Counterfeits

Counterfeit chargers pose a safety hazard as well as a hazard to your phone. You can buy a charger that looks just like an Apple charger for about $2, but the charger is nothing like an Apple charger internally. The power is extremely bad quality (as I will show below). But more importantly, these chargers ignore safety standards. Since chargers have hundreds of volts internally, there's a big risk if a charger doesn't have proper insulation. You're putting your phone, and more importantly yourself, at risk if you use one of these chargers. I did a teardown of a counterfeit charger, which shows the differences in detail.

I've taken apart several counterfeit chargers and readers have sent me photos of others. Surprisingly, the counterfeit chargers I've examined all use different circuitry internally. If you get a counterfeit, it could be worse or better than what I've seen.

How do you tell if a charger is counterfeit? The fakes are very similar; it's hard for me to tell, even after studying many chargers. There's a video on how to distinguish real and fake chargers through subtle differences. You can also weigh the charger (if you have an accurate scale), and compare with the weights I give above. The easiest way to get a genuine Apple charger is fork over $29 to an Apple store. If you buy a $2 "Original Genuine Apple" charger on eBay shipped from China, I can guarantee it's counterfeit. On the other hand, I've succeeded in buying genuine used chargers from US resellers for a moderate price on eBay, but you're taking a chance.

The following picture shows a counterfeit charger that burned up. The safety issues with counterfeits are not just theoretical; when hundreds of volts short out, the results can be spectacular.

Photo by Anool Mahidharia. Used with permission

Indicated charger type

A device being charged can detect what type of charger is being used through specific voltages on the USB data pins.[6] Because of this, some devices only work with their own special chargers. For instance, an "incorrect" charger may be rejected by an iPhone 3GS or later with the message "Charging is not supported with this accessory".[7]

There are many different charger types, but only a few are used in the chargers I examined. A USB charger that follows the standard is known as a "dedicated USB charger". However, some manufacturers (such as Apple, Sony, and HP) don't follow the USB standard but implement their own proprietary charger types. Apple has separate charger types for 1 amp (iPhone) and 2 amp (iPad) chargers. HP has a special type for the HP TouchPad.

The point is that USB chargers are not interchangeable, and devices may not work if the charger type doesn't match what the device expects. The table below shows the type of charger, the current that the label claims the charger provides, the current it actually provides, and the charger type it indicates to the device.

The types of the counterfeit chargers are a mess, as they advertise one power level, actually supply a different power level, and have the charger type for a third level. For example, the counterfeit iPhone charger is advertised as supplying 1 amp, but has the 2A charger type, so an iPad will expect 2 amps but not obtain enough power. On the other hand, the counterfeit iPad charger claims to supply 2 amps, but really only supplies 1 amp and has a 1A type.

	Charger type	Label	Measured current	Weight
Apple iPhone	Apple 1A charger	5V 1A	1.79A	23.0g
Samsung oblong	dedicated USB charger	5V 0.7A	.80A	33.1g
Samsung cube	dedicated USB charger	5V 1A	1.17A	23.2g
Apple iPad	Apple 2A charger	5.1V 2.1A	2.3A	67.5g
HP TouchPad	HP TouchPad charger	5.3V 2.0A	2.4A	54.8g
Counterfeit iPhone	Apple 2A charger	5V 1A	.94A	18.8g
Monoprice	Apple dock	5V 1A	1.22A	67.8g
Counterfeit UK	dedicated USB charger	5V 1A	.57A	29.4g
Counterfeit iPad	Apple 1A charger	5.1V 2.1A	1.2A	43.4g
Belkin	Apple 1A charger	5V 1A	1.27A	43.0g
KMS	Apple 2A charger	5V 2.1A	3.4A	99.5g
Motorola	dedicated USB charger	5.1V .85A	.82A	38.6g

Efficiency

People often wonder how much power their charger is wasting while it's idle, and if they should unplug their charger when not in use. I measured this "vampire" power usage and found the chargers varied by more than a factor of 20 in their idle power usage. The Samsung oblong charger came in best, using just 19 mW; this was so low compared to the other chargers that I measured it again a different way to make sure I hadn't made an error. On the other extreme, the fake iPhone charger used 375 mW. The Apple iPhone charger performed surprisingly badly at 195 mW. If plugged in for a year, this would cost you about 21 cents in electricity, so it's probably not worth worrying about.[8] In the following table, I use the official charger Star Rating System (yes, there actually is such a thing).[9][10]

I also measured efficiency of the chargers under load.[11] One of the benefits of switching power supplies over simpler linear supplies is they are much more efficient at converting the input power to output. The chargers I measured all did pretty well, with 63% to 80% efficiency. The HP charger was the winner here.

	Vampire	milliwatts	Efficiency	Percent
Apple iPhone		195		74
Samsung oblong		19		76
Samsung cube		86		77
Apple iPad		62		78
HP TouchPad		91		80
Counterfeit iPhone		375		63
Monoprice		78		72
Counterfeit UK		103		63
Counterfeit iPad		95		66
Belkin		234		66
KMS		179		69
Motorola		59		75

The chargers up close

Apple iPhone and counterfeit

The above photo shows a real iPhone charger (left) and a counterfeit (right); the two chargers are almost identical, down to the green dot. If you look closely, the genuine one says "Designed by Apple in California", while the counterfeit has the puzzling text "Designed by California". The counterfeit also removed the "Apple Japan" text below the plug. I've seen another counterfeit that says "Designed by Abble" (not Apple). I assume the word "Apple" is removed for legal or trademark reasons, since the word "Apple" is often (but not always) missing from counterfeits.

Samsung oblong

I call this charger the Samsung oblong charger, to distinguish it from the Samsung cube charger.

Samsung cube

The Samsung cube charger is shaped very similarly to the Apple iPhone charger. Internally, however, it turns out to be entirely different.

Apple iPad and counterfeit

The photo above shows a real iPad charger (left) and a counterfeit (right). The counterfeit has almost identical text, but without "Designed by Apple in California. Assembled in China", "Listed" under UL, and the manufacturer "Foxlink". Inexplicably this sanitization left "TM and © 2010 Apple Inc".

The above photo shows a real iPad charger on the left and a fake iPad charger on the right, with the plug removed. The most visible difference is the real charger has a round metal grounding post, while the fake has plastic. (The US plug isn't grounded, but in other countries the lack of ground in the counterfeit could pose a safety hazard.)

HP TouchPad

The HP TouchPad charger has a very unusual cylindrical shape, which is striking if perhaps not practical. The charger twists apart, allowing the plug to be replaced for different countries. (It took me weeks to discover this feature.)

Monoprice

The Monoprice charger isn't a USB charger, but instead has a 30-pin iPhone dock connector attached. It is a relatively large charger.

Counterfeit UK

This charger is a counterfeit of the Apple UK iPhone charger. They've removed Apple from the text, but left Emerson Network Power, which I'm sure is not the actual manufacturer. The genuine Apple UK charger can be distinguished by a serial number inside the USB connector.

Belkin

The Belkin charger eschews the minimal design styling of most chargers, with a roughly oval cross section, curves and ribs, and a cover over the USB port.

KMS

The KMS charger is unusual in providing 4 USB ports. It also gives off a blue glow while in use. The plug can be removed and replaced for use in different countries, similar to the iPad and HP TouchPad chargers. I couldn't find any UL safety approval on this charger, but I did find a report of one catching fire.

Motorola

The Motorola charger has the lowest listed power output, 850mA. The back of it has a holographic sticker (like a credit card), which may ward off counterfeiters, even though it's unlikely for anyone to counterfeit this charger. I wonder though why Apple doesn't use holograms or other anti-counterfeiting techniques, given the large number of counterfeit Apple chargers being sold.

Delivery of advertised power

Each charger has an advertised power output, but some chargers produce considerably more and some produce much less. Your device will take longer to charge, if the charger can't put out enough power. This table shows each charger's ability to deliver the rated power, based on my measurements of maximum power. While most chargers meet or exceed the power rating, there are some exceptions.

The counterfeit chargers perform extremely poorly, putting out a fraction of the expected power. Charging your device with one of these chargers will be a slow, frustrating experience. In particular, the counterfeit UK charger only produces a third of the expected power. Although the label claims the charger works on 100-240 volts, it's clearly not designed to work on US power.

The iPad is a surprise, putting out less power than expected. Despite being nominally a 10 watt charger, the label says it provides 5.1V and 2.1A, which works out to 10.7 watts. However, the maximum power I measured is 10.1 watts (4.4 volts at 2.3 amps, as shown in the Power section below). Since the measured power is slightly less than advertised, it only gets four bolts.

	Rating	Label	Watts from label	Measured watts
Apple iPhone		5V 1A	5.0	6.0
Samsung oblong		5V 0.7A	3.5	4.0
Samsung cube		5V 1A	5.0	5.5
Apple iPad		5.1V 2.1A	10.7	10.1
HP TouchPad		5.3V 2.0A	10.6	11.4
Counterfeit iPhone		5V 1A	5.0	2.7
Monoprice		5V 1A	5.0	5.7
Counterfeit UK		5V 1A	5.0	1.7
Counterfeit iPad		5.1V 2.1A	10.7	5.9
Belkin		5V 1A	5.0	5.6
KMS		5V 2.1A	10.5	10.9
Motorola		5.1V .85A	4.3	4.3

Power quality

In this section, I measure the quality of the power produced by the different chargers. I analyze it for voltage spikes, high frequency noise, and line-frequency ripple. The following table summarizes the results in three categories. Spikes indicates extremely brief large voltage spikes in the output, while Noise indicates high-frequency noise in the output, and Ripple indicates low-frequency (120 Hz) fluctuations in the output.[12]

	Spikes	Noise	Ripple
Apple iPhone
Samsung oblong
Samsung cube
Apple iPad
HP TouchPad
Counterfeit iPhone
Monoprice
Counterfeit UK
Counterfeit iPad
Belkin
KMS
Motorola

The following oscilloscope traces show the output signal (yellow) and frequency spectrum (orange). The left images provide high-frequency information on the output voltage. The right images show the low-frequency information on the output voltage.[13]

The desired voltage graph is a flat, thin yellow line indicating totally smooth power. However, some factors mess this up. First, any ripple from the power line will show up as 5 sinusoidal peaks in the first (high-frequency) yellow line. High-frequency noise will widen the yellow line. Voltage spikes will appear as vertical spikes in the yellow line.

The plots also show the frequency spectrum in orange, from 0 at the left to 230 kHz at the right. The desired graph would have the orange spectrum near the bottom of the screen. Thus, the power quality exponentially gets worse as the orange line gets higher. The left (high frequency) spectrum generally shows noise at the switching frequency of the charger (and harmonics). The right (low frequency) spectrum typically shows spikes at multiples of 120 Hz, caused by ripple from the 60 Hz power.[5]

Apple iPhone

The ripple is clearly visible as the waves in the yellow trace on the left and as the spikes (at 120 Hz and 240 Hz) in the orange trace on the right.

The iPhone charger performs extremely well at filtering out spikes and noise, the best of the chargers I measured. Apart from the 120 Hz spikes, the noise spectrum (orange) is flat and very low. The power quality is so good, I checked the results several times to make sure I wasn't missing something.

Samsung oblong

The Samsung charger's output has a lot more noise than the iPhone charger. This is visible in the thickness and jaggedness of the yellow output curves. The orange frequency spectrum on the left shows large peaks at harmonics of the switching frequency. The 120 Hz spike on the right is a bit lower than the iPhone charger, so the ripple filtering is a bit better.

Samsung cube

The Samsung cube charger shows some noise in the output (yellow). The frequency spectrum shows wide peaks at multiples of the the switching frequency, about 90kHz. There's some ripple.

Apple iPad

The iPad charger almost eliminates the ripple; only a small blip is visible in the orange spectrum on the right. The noise level is low, although appreciably worse than the iPhone.

HP TouchPad

There's no ripple visible in the HP charger spectrum on the right. The overall noise level is good.

Counterfeit iPhone

The output from this counterfeit charger is a wall of noise. In order to fit the waveform in the display, I had to double the scale on the left and increase it by a factor of 5 on the right, so the yellow curve is actually much worse than it appears. On the left, note the huge ripple with massive high-frequency noise on top. This output is not something you want to feed into your phone.

Monoprice

The output from this charger is very noisy, as you can see from the thickness of the yellow line. Note that the frequency spectrum (left) has very tall but narrow spikes at harmonics of the 28kHz switching frequency, showing a lot of high-frequency noise. On the positive side, there is hardly any ripple.

Counterfeit UK

This charger has very bad output. The large degree of ripple is visible in the waveform (yellow, left) and the very large spikes in the spectrum (orange, right). The thickness of the yellow waveform shows the large amount of high-frequency noise, which is also visible in the very high peaks in the spectrum (orange, left).

Counterfeit iPad

This counterfeit charger has so much noise in the output that I had to double the scale on the left to get it to fit. Note the very large spikes in the output (yellow). The spectrum (orange, left) is much higher everywhere, indicating noise at all frequencies. Surprisingly, it has only a moderate amount of ripple; the manufacturer seems to have done at least one thing right.

Belkin

The Belkin charger does well at eliminating ripple, but has a lot of noise otherwise. The spectrum (orange, left) shows large peaks. The yellow output is wide, showing a lot of noise, combined with many large voltage spikes of about 1/3 volt.

KMS

The KMS charger has fairly good output, with a small peak in the spectrum (orange, left) at the switching frequency. It has no detectable ripple. However, it has many large voltage spikes in the output, over half a volt, as can be seen on the right.

Motorola

The Motorola charger has a lot of spikes in the output (yellow) . The spectrum (orange, left) shows high frequency noise at the switching frequencies. There's a moderate amount of ripple (yellow, left and orange, right).

Summary

The quality of the output power is radically different between chargers. The counterfeit chargers are uniformly bad, with hardly any effort at filtering the output. The other chargers vary in quality with the iPhone charger setting the standard for noise-free power, but surprisingly poor filtering of ripple. The power quality is a key factor that affects the performance of chargers; spikes and noise are known to interfere with touchscreens.[1]

Power curve

In this section I look at the voltage and current output by the charger as the load increases. The first rating is Voltage Sag, which is the undesired drop in output voltage as the load increases. The second rating is Current Sag, which shows how the current fluctuates as load increases. Finally, Regulation shows the overall stability of the output from the charger.

	Voltage sag	Current sag	Regulation
Apple iPhone
Samsung oblong
Samsung cube
Apple iPad
HP TouchPad
Counterfeit iPhone
Monoprice
Counterfeit UK
Counterfeit iPad
Belkin
KMS
Motorola

The graphs in this section need a bit of explanation, which is provided in the diagram below. The voltage/current load curve shows the performance of the charger under different loads. Each point on the curve shows the current (X axis) and voltage (Y axis) produced by the charger under a particular load condition. Follow the yellow curve clockwise from the upper left to the lower left to see the effect of increasing load. The upper left point of the curve shows the voltage produced by the charger when there is no load on the charger. As the load increases, the charger is supposed to keep a constant voltage and increase the current (i.e. horizontal line), until it reaches the maximum power (upper right). If the load continues increasing, the charger switches to a constant current mode, dropping the voltage while continuing to provide the maximum current (i.e. vertical line).[14] At the lower right, the charger has reached its shutdown point due to excessive load, and rapidly drops to no output in the lower left corner to avoid damage.

[16]

Apple iPhone

The output from the Apple iPhone charger is surprisingly non-constant under load. The charger starts off with 5.2 volts with no load, dropping to 4.6 volts as the load increases, resulting in the downwards slope of the top yellow line. As the load increases, the current keeps increasing, resulting in the slope of the right yellow line. Note however that the yellow line is relatively thin, so the regulation is pretty good at each point.

Note that because this charger has a high current output, this chart has a different current (horizontal) scale than most of the charts to fit the whole trace in the image. Stretch it horizontally to compare with other graphs.

Samsung oblong

For this charger, the voltage is approximately flat, except for a bump under no load (upper left) which is probably a measurement artifact. The vertical yellow line shows the current stays nearly constant as the load increases. The charger shows good voltage and current stability under changing load. The yellow line is a bit wider than the iPhone charger, showing a bit less regulation for a fixed load.

Samsung cube

The voltage curve sags slightly under load. The right hand curve shows the current stays stable, but the line is moderately wide, showing a bit of weakness in regulation.

Apple iPad

Similar to the iPhone charger, the iPad charger shows a lot of voltage sag. The voltage is about 5.1 V unloaded, dropping to 4.4 volts and 2.3 A (10.1 W) at the corner. Unlike the iPhone charger, the iPad charger has pretty good current stability. The regulation is solid, as shown by the narrowness of the yellow trace. Note the scale change due to the high current output.

I'm puzzled by the steep voltage sag on both the iPhone and iPad charger. Since the designers of the Apple charger went to a great deal of effort to build a high quality charger, I conclude they must not consider voltage sag worth worrying about. Or, more interestingly, maybe they built this sag as a feature for some reason. In any case, the chargers lose points on this.

HP TouchPad

The charger has some voltage sag, but the current (vertical) is nice and constant. The yellow line is relatively thin, showing good regulation. Note the scale change due to the high current output.

Counterfeit iPhone

This counterfeit charger shows extremely poor regulation, as shown by the very wide yellow line. It's hard to fit a voltage-current curve to this picture. The amount of power supplied by this charger seems almost random.

Monoprice

The Monoprice charger shows reasonably straight voltage and current lines showing good constant voltage and current outputs. The vertical line shows some width and noise, suggesting the regulation isn't totally stable.

Counterfeit UK

For this charger, the upper line doesn't get very far, showing that this charger doesn't output much current. My suspicion is that it was only tested with 240 volts so it performs poorly with 120 volts, even though the label says it takes 100 to 240 volts. The width of the yellow line shows very poor regulation.

Counterfeit iPad

The output of this counterfeit charger is so poorly regulated that it's hard to tell exactly what's happening with the voltage and current. It looks like the voltage is roughly constant underneath all the noise.

Belkin

The Belkin charger shows voltage sag as the current increases. In addition, the output is fairly noisy.

KMS

The KMS charger shows a lot of voltage sag as the load increases. In addition, the output is all over the place, showing very poor regulation, more like what I'd expect from a counterfeit charger. Note the scale change due to the high current output.

Motorola

The Motorola charger shows a bit of voltage sag, but good current stability. The regulation is good but not perfect, as shown by the width of the yellow line. (The gaps in the vertical line are just measurement artifacts.) Note that the maximum current output of this charger is fairly low (as advertised).

Conclusions

So what charger should you spend your hard-earned money on? First, make sure the charger will work with your phone - for instance, newer iPhones only work with certain chargers. Second, don't buy a counterfeit charger; the price is great, but it's not worth risking your expensive device or your safety. Beyond that, it's your decision on how much quality is worth versus price, and I hope the data here helps you make a decision.

P.S. How about some teardowns?

My previous iPhone charger and fake charger teardowns were surprisingly popular, but if you were hoping for teardowns on the full set of chargers, you'll need to wait for a future blog post. I haven't torn the chargers apart yet; if I need to take more measurements, I don't want to have just a pile of parts. But I do have some preview pictures to hold you over until my teardown article.

The above picture shows the internals of a counterfeit Apple iPhone cube charger. The two boards stack to form the compact cube shape. This charger blatantly tries to pass as a genuine Apple charger; unlike the "Designed by California" charger, this one exactly copies the "Designed by Apple in California" text from the real charger. Note the very simple circuitry[4] - there are no components on the other side of the board, no controller IC, and very little filtering. Also look at the terrible mounting of the transistor on the front right; clearly the build quality of this charger is poor. Finally, note the overall lack of insulation; this charger wouldn't meet UL safety standards and could easily short out. But on the plus side, this charger only cost a couple dollars.

The above $2 charger is notable for its low-profile design; it's about as thin as you can make a charger and still fit the power prongs and the USB port. The transformer is very short to fit into this charger. Like the previous charger, it uses a very simple circuit,[4] has little filtering, and almost no safety insulation.

Finally, the above pictures show the internals of the Samsung cube charger, which has circuit boards packed with tiny components and is much more advanced than the counterfeits (although slightly less complex than the Apple charger). Despite being very similar to the Apple charger on the outside, the Samsung charger uses an entirely different design and circuitry internally. One interesting design feature is the filter capacitors fit through the cut-out holes in the secondary circuit board, allowing the large filter capacitors to fit in the charger.

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Notes and references

[1] For an explanation of how the noisy output from cheap chargers messes up touchscreens, see Noise Wars: Projected Capacitance Strikes Back.

[2] The charger selection may seem slightly eccentric; it is based on chargers I had previously acquired, chargers I could obtain at a reasonable price, chargers supplied by Gary F. and Anthony H. (thanks, guys!), and some counterfeit chargers for comparison.

[3] TI has an interesting new design for a 10 watt inch-cube charger. With this design a tablet charger could be as small as the iPhone charger.

Photo of PMP8286 10W cube charger used with permission from Texas Instruments.

[4] The cheap chargers all use a "ringing choke converter" circuit, which coincidentally is the same power supply topology used by the Apple II. These chargers use an extremely simple feedback mechanism in place of the control IC in higher-quality chargers. See a comic-book explanation or a technical explanation for details.

[5] Since the input AC has a frequency of 60 Hertz, you might wonder why the ripple in the output is 120 Hertz. The diode bridge converts the 60 Hz AC input to 120 Hz pulsed DC, as shown in the diagram below. The pulses are smoothed out with filter capacitors before being fed into the switching circuit, but if the filtering isn't sufficient the output may show some 120 Hz ripple.

Image by WdWd, used under CC BY 3.0

[6] The chargers use specific voltages on the data pins to indicate the charger type to the device being charged. Because of this, an "incorrect" charger may be rejected by an iPhone with the message "Charging is not supported with this accessory".[7] Under the USB standard, a charger should short the two data pins together to indicate that it's a "dedicated" charger and not a real USB device. However, companies such as Apple, HP, and Sony have their own proprietary nonstandard techniques. The following table summarizes the voltages that appear on the D+ and D- lines for different chargers, and how the D+ and D- lines are configured internally.

Charger type	D+ voltage	D- voltage	D+/D- shorted	D+ pullup (kΩ)	D+ pulldown (kΩ)	D- pullup (kΩ)	D- pulldown (kΩ)
dedicated USB	float	float	yes	none	none	none	none
Apple .5A	2	2	no	75	49.9	75	49.9
Apple 1A	2	2.7	no	75	49.9	43.2	49.9
Apple 2A	2.7	2	no	43.2	49.9	75	49.9
HP TouchPad 2A	2.8	2.7	yes	250	300	n/a	n/a
Sony	3.3	3.3	no	5.1	10	5.1	10

Most of this data is based on Maxim USB Battery Charger Detectors, Adafruit's The mysteries of Apple device charging, TouchPad's USB Cable, XDA forum (Samsung), and TPS2511 USB Dedicated Charging Port Controller and Current Limiting Power Switch datasheet. The Apple 2A (i.e. iPad) information is a new result from my measurements. For details on USB charging protocols, see my references in my earlier posting.

Amusingly, semiconductor manufacturers have recently introduced chips that allow chargers to sequentially pretend to be different proprietary chargers until they trick the device into accepting the charger. It seems crazy that companies (such as Apple) design incompatible chargers, and then chip companies invent schemes to work around these incompatibilities in order to build universally compatible chargers. Two example chips are the TI TPS 2511 chip, and SMSC's USC1001 controller, which pretends to be nine different charger types.

[7] If you've wondered why some chargers cause the iPhone to give a "Charging not supported with this accessory" error, Silicon based annoyance reduction made easy describes how devices use proprietary protocols to limit the chargers they will work with.

[8] For the efficiency analysis I use 12 cents / kilowatt-hour as a typical residential energy price, which I got from US Energy Information Administration table 5.3.

[9] The official no-load charger star ratings are discussed at Meeting 30 mW standby in mobile phone chargers.

[10] There are many standards for energy consumption; see 5 W Cellular Phone CCCV (Constant Current Constant Voltage) AC-DC Adapter. For Energy Star ratings, a 5W charger must have under .5W no-load consumption, and 63% efficiency under load. A 10W charger must have under .75W no-load consumption, and 70% efficiency.

[11] Because switching power supplies use power in irregular waveforms, I used a complex setup to measure power consumption. I measured the AC input voltage and current with an oscilloscope. The oscilloscope's math functions multiplied the voltage and current at each instant to compute the instantaneous power, and then computed the average power over time. For safety and to avoid vaporizing the oscilloscope I used an isolation transformer. My measurements are fairly close to Apple's[15], which is reassuring.

You might wonder why I didn't just use a Kill A Watt power monitor, which performs the same instantaneous voltage * current process internally. Unfortunately it doesn't have the resolution for the small power consumptions I'm measuring: it reports 0.3W for the Apple iPhone charger, and 0.0W for many of the others. Ironically, after computing these detailed power measurements, I simply measured the input current with a multimeter, multiplied by 115 volts, and got almost exactly the same results for vampire power.

[12] The spike, noise, and ripple measurements come from the oscilloscope traces. The Spikes measurement is based on the maximum peak-to-peak voltage on the high frequency trace (the low frequency trace yields almost identical results). The Noise measurement is based on the RMS voltage on the high-frequency trace, and Ripple is based on the maximum dB measured in the low-frequency spectrum. These measurements appear on the right in the traces.

[13] In the power quality section, the high-frequency (left) images show 40 milliseconds of the waveform in yellow, and the frequency spectrum up to 234 kHz in orange. The low-frequency (right) images show 1 second of the output voltage in yellow and the frequency spectrum up to 600 Hz in orange. Because the frequency spectrum is measured in dBm, it is logarithmic; every division higher indicates 20 dB which is 10 times the voltage and 100 times the power.

[14] The chargers use a design called constant-voltage, constant-current (CVCC), since they provide a constant voltage (and increasing current) up to the maximum load and then a constant current (and decreasing voltage) if the load continues to increase. [15] The Apple 3GS Environmental Report gives some efficiency measurements for the Apple USB Power Adapter. It lists 0.23W no-load power and 75% efficiency. These values are reasonably close to my measurements of 0.195W no-load consumption and 73.6% efficiency.

[16] Measuring these curves was a bit tricky. I used a NTE2382 power MOSFET transistor as a variable load, manually varying the gate bias to generate the load curve. The transistor needed a large heat sink to dissipate 10 watts. A more complex dynamic load circuit is described here, but the simple circuit was sufficient for me.

The graphs were generated using the X-Y mode on the oscilloscope, with the load voltage as Y and the current as X. I used a .12Ω current sense resistor to measure the load current. This works out to 1/6 amp load current per division for the 20mV/div traces (most of them), and 5/12 amp load current per division for the 50mV/div traces (the high-current devices).

Note that increasing load corresponds to a decreasing resistance across the output: the upper left has infinite resistance (no load), the lower left has zero resistance (short circuit), and the resistance decreases in between. Since the power (in watts) is voltage * current, the maximum power is in the upper right corner, approximately 4W in this case. The load resistance can be computed by Ohm's law, e.g. middle of the upper curve: 5 V / .4 A = 12.5Ω, upper right corner 5 V / .8 A = 6.25 ohms. Middle of the right hand curve: 2.5 V / .8 A = 3Ω, overload point = .5 V / .8 A = .6Ω.

[17] Most of these chargers aren't made by the companies that sell them, and there are some interesting facts about the manufacturers. The manufacturers of the chargers can be looked up from the UL certification number. The oblong Samsung is made in China by Korean RFTech, a manufacturer of mobile phone products. The Samsung cube is made in China by Korean power supply manufacturer Dong Yang E&P. The HP charger is made by Foxlink, who also makes the iPad charger for Apple. The counterfeit chargers are made by anonymous Chinese manufacturers, despite what they claim on the labels. The Monoprice is made by Golden Profit Electronics (formerly ShaYao Electric Factory Three - no word on what happened to factories One and Two). The Belkin charger is manufactured by the obscure company Mobiletec of Taiwan. The KMS charger doesn't give any clues as to the manufacturer, and I can't identify KMS as a company. The Motorola charger is built by Astec (now part of Emerson Network Power). Interestingly, Astec's big break was manufacturing power supplies for the Apple II, as I discuss in my article on the Apple II power supply.

Apple uses a dizzying variety of manufacturers for their chargers. The iPhone charger (A1265) is made by Flextronics, the UK charger (A1299) is made by Emerson Network Power (except the one I have is counterfeit), the iPad charger (A1357) is made by Foxlink Technologies, and the Magsafe (ADP-85) charger (not discussed in this article) is made by Delta Electronics. The A1385 iPhone charger often comes with the iPhone 5 and looks identical to the A1265 I measured, but is manufactured by Emerson Network Power instead of Flextronics. I am told that by using multiple manufacturers, Apple has more negotiating leverage, since they can easily switch manufacturers at any time if they're not happy with the price or quality.

Confusingly, Foxlink (Taiwan), Foxconn (Taiwan), and Flextronics (Singapore) are all manufacturers for Apple with similar names. Foxlink (the name for Cheng Uei Precision Industry) and Foxconn (the name for Hon Hai Precision Industry) are entirely independent companies aside from the fact that the chairmen of both companies are brothers and the companies do a lot of business with each other (statement, Foxlink annual report). Foxconn is the company with continuing controversy over employee treatment. Foxconn and Flextronics are the world's #1 and #2 largest electronics manufacturing companies according to the Circuits Assembly Top 50.