Apple A1401 12W USB charger

The Apple model A1401 12W USB charger that comes with the iPad has been available for some time. It replaces the A1357 which was rated at 10W but appears to be identical on the outside.

A single multi-layer printed circuit board supports both primary and secondary side circuitry. The 2 sides are separated by an L-shaped slot in which a plastic insulating flange is positioned. Primary and secondary conductors do not come any closer than 5mm around the L217 opto-coupler which provides a high degree of safety. While the Apple A1385 5W USB charger has a single Y-capacitor between the primary and secondary side grounds, the 
A1401 12W USB charger has 2 Y-capacitors, one between primary and secondary side grounds and the other between one of the AC line inputs and secondary side ground.

From a circuit point of view the A1401 12W charger closely resembles the A1385 5W charger. Both use the Fairchild (now ON Semiconductor) FAN301 switching power supply controller IC. The basic isolated flyback converter topology remains the same. On the primary side, the transformer, switching diode, and switching transistor have higher current ratings to supply the higher power. There is a more significant difference on the secondary side. Rather than simple diode rectification a Texas Instruments UCC24610 synchronous rectification controller and a power MOSFET are employed in parallel with the diode rectifier. When a forward voltage is detected across the rectifier diode the power MOSFET is turned on to reduce the rectification voltage drop from around 0.5v to below 0.1v. With several amps average current at the 5volt ouput, this saves at least 1w. 

Delivering the rated 12w output power at 5volts, the A1401 operates at 83% efficiency. This means that a bit more than 2.5w is consumed internally which does noticeably warm the device. Power output can be increased to over 13w before output voltage droop is observed and the device shuts down soon after. The A1401 achieves significantly higher efficiency than the 70% efficient A1385 5W charger due to secondary-side synchronous rectification and amortization of fixed controller power dissipation over a higher output power.