Safe Heat Sinking

shelly_d

shelly_d

Not An Audiophool
All:

I'm toying with a tube project that uses a MOSFET transistor as part of the voltage regulator for the B+. Specifically an IRF830. The data sheet on the transistor states that the mounting tab is wired to the drain. (link here: http://www.mouser.com/catalog/specsheets/91063irf.pdf).

The design constraints I have on space for this means that the transistor will be mounted vertically with the tab and attached heat sink standing straight up. My issue is that there will be about 312V:yikes: on the drain (and therefore on the tab as well) and I would like to find a way of heat sinking this so that the heat sink itself is not at that voltage.

Anyone have any ideas?

Thank you in advance.

Shelly_D
 
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Last edited:
Most insulators have a very high breakdown voltage. I don't know what that is, but it's never been a problem before. See if you can get a datasheet for an insulator and use that for your guideline.

I would be a little concerned about the Pd (Power Dissipation) of the MOSFET. Multiply the voltage across the MOSFET by the current through it (E*I) to figure your power. With a large voltage across it, it won't take much current to get that one hot.
 
gwar said:
i got thousands of the mica insulators, need a few hundred?
Click to expand...

Thank you sir for the offer. I would also need the shoulder washer and hardware as well.


avionic said:
Piece of cake.Mount it to the heatsink with the proper case style (TO-220) insulator either mica and thermal compound or a sil-pad ..With either insulator you will need a plastic bushing for the mounting screw. NTE use to make transistor mounting kits that included mica insulator,bushing ,screw ,washer and nut.
Click to expand...

Avionic:

Thanks for that link. I thought I remembered that something like that existed but had a great deal of difficulty trying to find it on Mouser. Didn't know what to search for.

Thanks Friends:thmbsp:.
 
Jon_Logan said:
Most insulators have a very high breakdown voltage. I don't know what that is, but it's never been a problem before. See if you can get a datasheet for an insulator and use that for your guideline.

I would be a little concerned about the Pd (Power Dissipation) of the MOSFET. Multiply the voltage across the MOSFET by the current through it (E*I) to figure your power. With a large voltage across it, it won't take much current to get that one hot.
Click to expand...

I too am concerned, hence the heat sinking issue (now solved by the ever friendly AudioKarma group:thmbsp:). This is being used as a voltage regulator for a tube circuit. The output of the rectifier will be about 420V or so. The output of the regulator will bring that down to 300V so I'm dissipating 120V or so across the transistor. The regulator is supplying B+ to one tube. There are 6 tubes total and 6 regulators, one for each tube. I'm powering two 12AX7's (each one has the two sections in parallel) and 4 6DJ8,s each one with the 2 sections in series with each other. I believe I'd be drawing no more then 8 to 15 ma on any one regulator. At 20ma draw I believe I'm dissipating about 2.5 watts or so on each transistor. If my analysis is correct:scratch2: I'm not anticipating too much trouble dissipating the heat - but that's why we prototype these things - too discover where we blew it:D.

By the way, I'm working off a design I found online not a brand new one I made up. I'm not good enough with transistors to come up with this on my own.

Thanks for your help. I appreciate all that I get.
 
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