The Apple model A1385 5W USB charger that comes with the iPhone has been available for several years. There are some significant changes and improvements over the previous A1265 model which are only apparent on the inside.

A molded plastic form now holds the 2 double sided printed circuit boards and other large components such as electrolytic capacitors, transformer, and USB receptacle firmly in position. Connections between the boards are routed through the plastic form to replace the ribbon cable in the older product. The improved mechanical stability should provide better reliability and reduced susceptibility to shocks.

From a circuit point of view the switching power supply controller IC has been replaced with the Fairchild (now ON Semiconductor) FAN301. The basic isolated flyback converter topology remains the same. There is now only a single opto-coupler providing feedback from the secondary board to the primary board. In the old design a second opto-coupler provided temperature feedback from a thermistor located on the secondary board. Now a thermistor is positioned on the primary side within the circuit that provides the constant current mode feedback signal to the controller IC. Having fewer connections between primary and secondary boards means fewer potential points of failure resulting in improved safety and reliability.

Delivering the rated 5w output power at 5volts, the A1385 operates at 70% efficiency. This means that a bit more than 2w is consumed internally which does noticeably warm the device. Power output can be increased to about 6w before output voltage droop is observed and the device shuts down soon after.

While we should not expect a black-turtlenecked CEO to stand up in front of a worldwide audience to introduce the improved features of the A1385 USB charger, it is nevertheless a nicely engineered product.

2017.09.21