Heatsink has a voltage?

Posted by: juinbeh on 08/05/2009

Hi PI Engineers,

When I accidently touched the heatsink attaching to the Source tab of TOP261YN, it gave me a tingle. It measures ~0V between heatsink and PGND, but 36Vac between the heatsink and my finger.

Any idea why? What can I do to make it safe?

Thanks.

Comments

That is because the Source pin is connected to B- (negative terminal of bulk capacitor). This has an AC potential as compared to earth ground (which your finger is at).

This is why during bench testing, you must use an isolation transformer to power the PSU - so that you won't get electrocuted when you touch B+ or B-.

Hi Tucker, that makes sense.
However we intend to connect this heatsink to the main chassis. Is there anything we can do to the circuit to make it safe to touch?

If you will connect the heatsink to the chassis then you need Safety rated isolation (insulation) between the TopSwitch and the heatsink. You will need to be careful about the resulting additional thermal resistance.