Heat sinks and VVRs

[Deric]

When bolting a FET (for VVR) to the chassis of an amp, would adding a heat-sink to the outside of the chassis (directly over the FET) be a good idea?

[fperron_kt88]

I did just that on a 200W major a few years back. I had a cascode arrangement with multiple fets and the heatsink was huge and required forced air, but the thing worked. Ended-up using a simple PPIMV on that one though... So not the best example...

You might want to make sure that all the parameters are working on your side: dark heat sink to maximize passive radiation if no air forced, super good thermal contact between the fet and the chassis (thermal pad instead of mica and grease), then super good contact between the surface of the heatsink and the chassis (bare metal, super thight contact), the whole thing.

I am not a mechanical engineer though, so YMMV. But the simple fact that you go through a layer of aluminium to reach a heatsink that is in aluminium should not be your biggest problem... If the chassis is steel though, you can compute the numbers for the thickness and associated thermal resistance... That might be a deal breaker.

This is the type of thing that I would put through a CFD program just to get some figures and see the real added value of the sink on top of that chassis versus none and the overall performance.

For the record, I have an aluminium chassis wreck clone with a vvr that is bolted through a thermal pad. No heat sink. It is a dual el34 Express design with the VVR board from Dana. Never had an issue, but never had the amp work in more than 23 dec C. ambiant either...

Let me know how it goes and good luck!

[Deric]

Thanks for the input. I do have some NTE "Thermo-Pad"s but I used the spacer (mica?) that came with the FET (NTE2973) and thermal compound. Is the Thermo-Pad a better way to go?

I am using an aluminum chassis. The heatsink I have is a small Rat-Shack TO-220 black finned aluminum one. Something in my brain keeps telling me it will do more harm than good.... but what do I know ?

Edit: Just looked up the datasheet for the Thermo-Pads. Looks like they are the way to go. Still not sure about the heatsink.....

[UR12]

I have never needed a additional heatsink on the Exxpresses I have installed VVRs in. The chassis is usually sufficient by itself as long as it is not the super thin Bud chassis like Ken used on the original. The small RS heatsink for a TO220 device would do very little dissipate any additional heat. I use those on my Lil Devil 2 watt amp without bolting the smaller mosfet to the chassis and they work well in that application but not for an Express. I have a black 2" x 4" x 1" tall heatsink I use on 40-50 watt amps and it works very well.

[jaysg]

I used mouser 532-364424B32G, which is a BGA meant for the back of a microprocessor or large FPGA. I cut out one tyne and drilled a through hole there. It may be overkill. I don't know. I wasn't happy with the idea of seeing how it goes. Amp is a cathode biased 4 x el84 ~30W clean. It gets warm, but not too hot to touch.