SDG #311 First tests of a LNK562 Flyback Converter

This build log follows a compact offline flyback supply based on the Power Integrations LNK562, a member of the LinkSwitch family aimed at very low power auxiliary supplies where standby consumption and part count matter more than raw output. The design pairs the controller with a hand wound transformer on a small bobbin, with the winder and enamelled wire doing the tedious work that usually deters people from making their own magnetics.

Assembly is done by hand on a prototype board, and the first mains power-up is treated with the caution any isolated SMPS deserves. That caution is rewarded: a layout error surfaces before it can cause damage, a useful reminder that with switch mode designs a schematic passing review means very little if the copper does not match it. Once corrected, both the 5V and 3.3V rails come up, which confirms that the transformer turns ratio, the feedback arrangement and the secondary side regulation are all behaving as intended.

The more interesting part is the load testing. Small flybacks like this often look fine at no load and then sag, ring or enter unpredictable burst mode as current is drawn, so observing rail behaviour and switching waveforms under increasing load is where the real design verification happens. High resolution oscilloscope captures make it easier to see ripple and switching noise that a cheap handheld meter would miss entirely.

For anyone considering an LNK562 or similar low power offline converter, the conclusion is that the silicon is the easy part. Transformer construction, creepage and clearance, and careful PCB layout determine whether the finished supply is reliable or a nuisance. Further refinement, EMI work and efficiency measurement are identified as the next stage.