Pattern on a PCB

Question

I've never worked on power supplies. What is the purpose of the pattern in this image (yellow) ? I don't have this PCB, I only have this photo.

Answer 1

The pattern is a pair of spark gaps, a kind of low-cost voltage limiter. The closely-spaced pointy conductors favor the formation of a spark, and the soldermask is removed (the tan rectangular area) so that they are exposed directly to the air.

The whole idea is that if anything is going to arc over, it will happen preferentially here, protecting other areas of the circuit.

Answer 2

That looks like an area with two pairs of pointy “teeth” with a small air gap between them, and no solder mask over that area. Most likely this is part of the board’s EMC protection, providing a place for extreme overvoltage (like lightning) to arc across those gaps instead of finding a path through the board’s circuitry.

There are other, more commonly used forms of overvoltage protection, like transorbs / zeners which may also be on the board somewhere nearby. Often EMC protection has multiple layers to address multiple vulnerabilities.

Answer 3

According to this NXP document, PCB spark gaps have an approximate breakdown voltage of V = 3000pd + 1350 where p is the pressure in atmospheres and d is the gap in mm. So a 0.5mm spark gap would have a breakdown voltage of about 2,800V at STP.

The CM choke has a relatively high CM reactance (because that's what it does) in order to reduce EMI. Often something like 1mH for common-mode inputs.

However that means a high voltage can appear right across the CM choke and it might cause arcing to some other less desirable place. At least that's my assumption.

Answer 4

Those are spark gaps.

You sometimes see them on things like mains inputs on power supplies.

Usually under the mains input common mode choke.

In this picture they also seem to be under the common mode choke.

So in case there is some large overvoltage surge event, the arcing is engineered to happen there, at the common mode choke.