Electrical isulation?

[larrymoencurly]

Is it bad practice or against any safety rules for a single heatsink to be used for both hot side and cold side components, even if the components are insulated from it with mica or silicone rubber washers? I ask because here's a 1980s UL approved PC PSU from Senstron that uses a single large heatsink for everything, and it doesn't seem to be junk:

[Krankshaft]

Re: Electrical isulation?

Don't know what UL would have to say about it. I have never seen a modern supply with this design even the cheapy Chinese switchers. It's possible UL forbade this practice in the 80s or 90s.

Electronically speaking as long as the FETs or Diodes on the same heatsink are isolated.

Either by full plastic encapsulation of the TO-220 package including around the rear metal tab. Or by using an insulating sheet it's electrically safe.

I wouldn't say it's bad per se as long as the insulative elements integrity isn't compromised you'll have no shorts.

But I wouldn't design it that way. It's best to keep the primary and secondary isolated by more than a coating of plastic or mica.

They may have done it due to space constraints and if they segmented the heat sink down the middle for isolation. It wouldn't provide enough thermal dissipation for one of the components. Even then IMO redesign the PCB to fit another heat sink.

I see that PSU is a real old design with a little feedback transformer near the PWM IC long before the days of opto isolation no doubt.

[larrymoencurly]

Re: Electrical isulation?

I checked the PSU again and now have even less faith in the use of one heatsink for both the hot and cold sides. This particular heatsink can shift around enough on the circuit board and let one screw come within 1/64" of a particular copper trace on the low voltage side, and the two high voltage transistors are held by a clamp that can tilt (only one screw holding it) and short against a metal collector tab. Worse, one of those transistors was installed crooked and would have been shorted, if the clamp had been tilted about 2mm the wrong way.

[redwire]

Re: Electrical isulation?

If the heatsink is grounded- well enough to take a ground fault (say 10 amps) then you don't have to worry about the hot and cold sides getting together.

[larrymoencurly]

Re: Electrical isulation?

I thought consumer products were supposed to be made safe even when operated ungrounded, and isn't that the reason some PSUs with separate heatsinks for the hot side have the heatsink connected to high voltage instead of ground?

I have an ancient Seasonic PSU that looks very much like a Leadman (Powmax) PSU. Both seem cheap and have heatsinks made of bent sheet aluminum, but the Seasonic has separate heatsinks for the hot and cold sides, while the Leadman uses a single heatsink for everything. The Seasonic is UL approved; the Leadman is not.

[370forlife]

Re: Electrical isulation?

Wintech did this on a certain ATX series they made. Wintech is indeed UL approved too.

[redwire]

Re: Electrical isulation?

UL uses a hi-pot test between hot and cold sides, several kV for a minute with no breakdown. It doesn't really deal with "flimsy" construction but 1/64" is not enough, that's for sure.

It would be considered a "two-fault" occurance to get the hot side and cold side transistor insulators to fail (both would have to fail) in order to cause a safety problem.
A "one-fault" failure is like a transformer/opto/Y-cap short.