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Xerox Alto restoration, IC reverse engineering, chargers, and whatever Apple iPhone charger teardown: Disassembling Apple's diminutive inch-cube iPhone charger reveals a technologically advanced flyback switching power supply that goes beyond the typical charger.
It simply takes AC input anything between and volts and produce 5 watts of smooth 5 volt power, but the circuit to do this is surprisingly complex and innovative. How it works The iPhone power adapter is a switching power supply, where the input power is switched on and off about 70, times a second in order to get the exact output voltage required.
Because of their design, switching power supplies are generally compact and efficient and generate little waste heat compared to simpler linear power supplies.
In more detail, the AC line power is first converted to high voltage DC  by a diode bridge. The DC is switched on and off by a transistor controlled by a power supply controller IC. The chopped DC is fed into a flyback  transformer which converts it into low voltage AC.
Finally, this AC is converted into DC which is filtered to obtain smooth power free of interference, and this power is output through the USB jack.
A feedback circuit measures the output voltage and sends a signal to the controller IC, which adjusts the switching frequency to obtain the desired voltage.
The side view above shows some of the larger components. The charger consists of two circuit boards, slightly under one inch square each.
The input AC first passes through a fusible resistor stripedwhich will break the circuit if there is a catastrophic overload. The input AC is converted to high-voltage DC, which is smoothed by Iphone price analysis two large electrolytic capacitors black with white text and stripe and the inductor green.
The second transistor clamps voltage spikes, as will be explained below. The chopped DC goes to the flyback transformer yellow, barely visible behind the transistorswhich has low voltage output wires going to the secondary board below.
These wires were cut during disassembly. The secondary board converts the low voltage from the transformer to DC, filters it, and then feeds it out through the USB connector the silver rectangle in the lower left.
The gray ribbon cable just barely visible on the lower right under the capacitor provides feedback from the secondary board to the controller IC to keep the voltage regulated. The picture above shows the flyback transformer yellow more clearly, above the USB jack.
The large blue component is a special "Y" capacitor  to reduce interference. The controller IC is visible above the transformer on the top of the primary board.
The inner side diagram above holds the large components while the outer side diagram below has the controller IC. The large components were removed in the diagrams, and are indicated in italics. Note the thickness of the circuit board traces connecting these capacitors and other high-current components compared to the thin control traces.
The controller IC takes a variety of inputs secondary voltage feedback, input DC voltage, transformer primary current, and transformer demagnetization sensing and adjusts the switching frequency and timing to control the output voltage through complex internal circuitry.
The current sense resistors let the IC know how much current is flowing through the primary, which controls when the transistor should be turned off. The second switching transistor, along with some capacitors and diodes, is part of a resonant clamp circuit that absorbs voltage spikes on the transformer.
This unusual and innovative circuit is patented by Flextronics. Since the controller IC needs to be powered up before the transformer can start generating power, you might wonder how this chicken-and-egg problem gets solved. The solution is the high-voltage DC is dropped to a low level through startup power resistors to provide the initial power to the IC until the transformer starts up.
The auxiliary winding is also used by the IC to sense transformer demagnitization, which indicates when to turn on the switching transistor. The tantalum filter capacitors provide high capacitance in a small package. The USB output also has specific resistances connected to the data pins to indicate to the iPhone how much current the charger can supply, through a proprietary Apple protocol.
The secondary board contains a standard switching power supply feedback circuit that monitors the output voltage with a TL regulator and provides feedback to the controller IC through the optocoupler. A second feedback circuit shuts down the charger for protection if the charger overheats or the output voltage is too high.
Isolation Because the power supply can have up to V DC internally, safety is an important issue. Strict regulations govern the separation between the dangerous line voltage and the safe output voltage, which are isolated by a combination of distance called creepage and clearanceand insulation.
The standards  are somewhat incomprehensible, but roughly 4mm of distance is required between the two circuits. As I discuss in Tiny, cheap, dangerous: Inside a fake iPhone chargercheap chargers totally ignore these safety rules.
You might expect the primary board to have the dangerous voltages and the secondary board to have the safe voltages, but the secondary board consists of two areas: The isolation boundary between these areas is about 6mm in the Apple charger and can be seen in the above diagram.
This isolation boundary ensures that dangerous voltages cannot reach the output. There are three types of components that cross the isolation boundary, and they must be specially designed for safety.The iPhone 8 price in Chinese yuan. For years, many of the earliest buyers of the iPhone in stores in the US (paywall) and elsewhere have been Chinese people looking to resell the phones in China.
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