Cost-saving circuit designs of knockoff iPhone chargers

　　A charger that looks like an authentic Apple product, but only costs a few dollars. How is such a low price achieved? Driven by curiosity, a hardware engineer disassembled it and found the circuits, chips, components, and the secret of the low cost of cheap chargers...

　　Many people are curious about what is inside the USB power supplies provided by companies like Apple, Samsung, and RIM. Ken Shirriff, an American hardware engineer, bought an unbranded USB charger on eBay for $2.79 and disassembled it. How is it possible for such a complex charger to cost only a few dollars and look similar to an authentic Apple charger?

　　Unbranded USB charger purchased for $2.79

　　Excluding the European plug, the unbranded charger is only 1 inch thick and is labeled: "For iPhone 4. Input 110-240 volts, 50/60 Hz AC, Output 5.2v, 1000 mA DC. Made in China." Other than that, there are no manufacturer, serial number or safety certification marks on the charger. After disassembling the charger, you can see that most of the space is unused, and the design is actually for the US plug. Since the charger input voltage is 110 to 240 volts, it can be used worldwide.

　　The main components of the knockoff charger

　　The actual power supply circuit is smaller than the charger case, and its main components are shown in the figure. On the left is a standard USB port. The standard USB port takes up a large area, so many devices currently have turned to micro-USB ports. The black and yellow components are transformers, which are used to convert high voltage to low voltage. In front of the transformer is a switching transistor. The component next to the transistor looks like a resistor, but it is actually an inductor for AC input filtering. Under the circuit board are some capacitors for filtering input and output.

　　The actual power supply circuit size is smaller than the charger housing

　　In short, this is a "flyback switching power supply". In this power supply circuit, the input AC power is converted into high-voltage DC power by a diode, and transmitted to the transformer through a switching transistor. After the transformer output, it is converted into low-voltage DC power by a diode. The feedback circuit limits the output voltage to 5 volts.

　　Specific circuit of cottage charger

　　From the details, this is a self-oscillating flyback transformer. Most flyback power supplies use integrated circuit chips to control frequency oscillation, but this charger achieves self-oscillation through transformer feedback. This reduces the number of components required and reduces costs. The main cost of this $2.79 charger is the 75 cents control chip.

　　This picture shows the circuit components. The red boxes and English labels indicate the components on the back of the board. Most of the components are SMD components. The green wires are used for input AC power, which is then filtered by the inductor. The input diode marked 1N4007 and another input capacitor are used to convert AC power to 340 volts DC. The transistor switch marked MJE13003 delivers DC power to the transformer at a variable frequency.

　　At the output, a Schottky diode converts the transformer output to DC, which is filtered by the output capacitor and then sent to the USB port. A simple feedback circuit limits the output voltage.

　　Insulation and safety design of knockoff chargers

　　For safety reasons, the AC power supply must strictly insulate the AC input and DC output. Therefore, the circuit needs to be divided into two parts, the input and output, and there must be no electrical connection between the two parts, otherwise touching the DC output will cause electric shock. The connection between the two parts is achieved through a transformer or an optocoupler. In the charger disassembled this time, the transformer acts as an isolation element for the main power supply, while the optocoupler is used in the feedback circuit.

　　If you look closely at the photo, you can see the insulation boundaries drawn with white lines on the circuit board. Some complex safety regulations specify the distance between the input and output parts, and at least 3 to 4 mm is required. In this power supply circuit, you can see that the distance between the two parts is only about 1 mm, and one of the resistors has a safety distance of even less than 1 mm.

　　In this design, it is difficult for the charger to comply with safety regulations. In fact, there is no safety certification information on the charger shell. Therefore, such a charger will pose a safety risk. In fact, the only measure to protect users from contact with 340 volts is this distance of less than 1 mm.

　　Another hazard of counterfeit and substandard chargers is that the quality of the DC output is very poor, with voltage fluctuations, which may affect the operation of the mobile phone, such as causing the touch screen to malfunction. It can be noted that many cost-saving designs of this charger have brought about voltage instability. For example, there is only one filter diode at the input end, instead of the usual four filter diodes in bridge connection. Compared with normal circuit design, the input and output filter modules of this charger have been minimized. In addition, there is no fuse design for the AC input, which will also bring danger.