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TO-220 tab leaking to heatsink?

Posted by jorisdallaire (A) on September 5, 2014 at 08:48:28

Hello,

I know this is a tube forum, but many have experience with three-legged fuses and heatsinking. Besides it is used in a tube amp so, i'm legit ;-) I trust you guys can shed some light on a strange problem i have with a heatsink. I did an isolated mount but it is still floating at B+; the heatsink is un-anodized aluminum, salvaged for an old PC CPU.

At first i re-used a thermal pad, also salvaged from an old PC PSU. Added silicone compound, used screw shoulder as it must be done. The heatsink had B+ on it, so i thought the pad was not good anymore.

I then used a new insulating kit. The pad is orange, and i guess it is not a mica, as it is feels thinner than a mica; maybe kapton? Mounting sequence: apply thin and uniform layer of thermal paste on heatsink and tab; put insulating sheet on heatsink and remove trapped air; put shoulder on "greased" device , and device on insulator; hand screw.

There was a strange phenomenon i observed after mount. I checked the resistance between tab and heatsink; it measured 3-4Meg. I was surprised,as i expected no contact at all, but i thought it might still be OK.

I ran tests on the breadboard. The heatsink still had B+ on it, and the resistance between tab and heatsink lowered as the heatsink became hotter. It settled around 50K.

Have any of you had a similar problem? I suspect the thermal paste is slightly conducting; maybe I operated over its dielectric breakdown limit with 350V B+? Dielectric strength is 300V/mil (datasheet link below), but i have no idea how thick is an average paste application.

Which thermal paste you guys are using for high voltages?

One other possibility is that the edge of the hole on the heatsink is not perfectly flat (it is a handmade tapped hole). I filed the surface carefully, but i might try countersinking the hole. I just want to exclude the possibility of thermal paste failure first.

Thanks for any insight.
Joris

• Link to the datasheet of the paste in question.   (Open in New Window)

Edits: 09/05/14

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