SDG #304 Flyback Converter Design with Power Integrations LNK562

This is the third approach to the same problem: powering a LoRa node with relays from 230V mains, needing both 5V and 3.3V rails at very low total power. The earlier capacitive dropper and non-isolated buck designs both worked, but neither offered galvanic isolation, which is a real safety consideration on a board with accessible output terminals. A flyback fixes that, and it also allows a second secondary winding so the 3.3V rail comes straight off a centre tap rather than burning power in a linear regulator.

The selection process is the useful part here. Working down the recommended device families, the Inno Switch 3 and Top Switch parts are clearly overspecified for around 2W, while the LinkSwitch and TinySwitch parts land in the right area at roughly 50p in an 8 pin package. The LNK562 wins on simplicity: bias winding feedback instead of an optocoupler, better light load efficiency, and a clampless design that removes the primary snubber entirely.

The design spreadsheet outputs an EE16 core with 204 primary turns, a 51 turn bias winding and a centre tapped secondary, complete with layer by layer winding instructions. What it does not tell you is how to gap the core, which matters because a flyback stores its energy in the gap rather than transferring it continuously. That is deferred to the follow up video.

The resulting KiCad board is deliberately oversized as a development platform, with placeholders for a primary clamp, snubbers across the output diodes and a CLC filter on the 3.3V rail, plus test points and isolation slots so each option can be evaluated or linked out.