gh 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 circuit in detail
The primary circuit board is packed with surface mounted components on both sides. 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.) Input power is connected to the corners of the board, goes through the 10Ω fusible resistor, and is rectified to DC by the four diodes. Two R-C snubber circuits absorb EMI interference created by the bridge. The DC is filtered by the two large electrolytic capacitors and the inductor, producing 125-340V DC. Note the thickness of the circuit board traces connecting these capacitors and other high-current components compared to the thin control traces.
The power supply is controlled by an 8-pin STMicrosystems L6565 quasi-resonant SMPS controller chip. The controller IC drives the MOSFET switching transistor which chops the high voltage DC and feeds it into the primary winding of the flyback transformer. 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.
The controller IC needs DC power to run; this is provided by an auxiliary power circuit consisting of a separate auxiliary winding on the transformer, a diode, and filter capacitors. 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 secondaryOn the secondary board, the low voltage AC from the transformer is rectified by the high-speed Schottky diode, filtered by the inductor and capacitors, and connected to the USB output. 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. An iPhone displays the message "Charging is not supported with this accessory" if the charger has the wrong resistances here.
The secondary board contains a standard switching power supply feedback circuit that monitors the output voltage with a TL431 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. A ribbon cable provides this feedback to the primary board.
IsolationBecause the power supply can have up to 340V 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 clearance), and insulation. The standards are somewhat