60W USB-C Charger
December 2022
A compact GaN-based flyback charger designed from scratch: schematic and magnetics built around Power Integrations' InnoSwitch3-PD controller and their design tool, then laid out as a two-layer PCB with a USB-C PD port capable of up to 60W.
Designing the schematic
This is a 60W USB-C PD charger built around Power Integrations' InnoSwitch3-PD, a flyback controller with the secondary-side control and USB PD negotiation built in. The schematic was built up with the manufacturer's own design tool, which picks compatible transformer cores and component values and adapts the rest of the circuit as those choices change. On the primary side, mains is filtered through a common-mode choke and fuse for EMI and safety, rectified and buffered, then fed through a snubber network that dissipates the voltage spikes a flyback topology inevitably produces when the primary switches off. A pair of 1.78MΩ resistors bleeds just enough current from the DC bus to start the controller before the transformer's own auxiliary winding can take over. On the secondary side, a synchronous rectifier FET (rather than a diode) rectifies the output, a second FET can fully disconnect the USB-C connector when no load is attached (as USB-C PD requires), and a Y-capacitor bridges primary and secondary for EMI compliance without risking a hard short if it fails.
Protection and monitoring
A shunt resistor sized from the datasheet (32mV at the full 3A output) lets the controller monitor output current and enforce the 60W/20V/3A limit the transformer and components are rated for. An NTC thermistor under the USB-C connector watches for the contact resistance that builds up after repeated plugging and unplugging, throttling the output if the connector starts overheating. Two zener diodes and two capacitors on the USB data lines protect the PD negotiation circuitry from static discharge when a cable is plugged in.
Laying out the PCB
The board is a compact two-layer design with the rectifier, buffer and snubber along the top edge and the transformer in the middle, splitting the primary and secondary sides. A minimum 5mm creepage distance is kept between primary and secondary traces/components throughout for safety isolation, with slots cut into the board to further increase that isolation where space was tight.
Thermal check
Before ordering boards, I worked through the datasheet numbers for worst-case dissipation in the controller's internal switch: at the rated 1.5A primary current the on-resistance gives about 0.9W of loss, which combined with the package's 50°C/W junction-to-ambient thermal resistance works out to roughly 45°C of self-heating, comfortably inside the chip's 150°C rating without needing a heatsink.
The assembled board
The finished, hand-soldered board, built and tested on the bench.