HV TO-220 kit
This is actually the Aavid 7721-10PPSG shoulder washer, which has a long insulator that goes through the device tab into the pad (Aavid 4170G) and when mounted with a small head 4-40 screw provides pretty good clearance.
This is actually the Aavid 7721-10PPSG shoulder washer, which has a long insulator that goes through the device tab into the pad (Aavid 4170G) and when mounted with a small head 4-40 screw provides pretty good clearance.
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I used the aluminum oxide pads for my "Sic Puppy" Semisouth class A amp, where there were high voltages involved and I wanted to limit signal coupling into the heat sink. I also wanted the extra thermal conductivity of the alumina, as with that design, I push the dissipation limits for the TO-247 package. Ditto the concerns on having a flat mounting surface.
I just picked up an enhanced performance thermal compound put out by MG Chemicals. The usual suspects use Zinc Oxide. The two Arctic formulations I know of use either silver or aluminum oxide. The MG stuff uses boron nitride, so I suspect it's pretty good, and has no conductivity concerns. Basically, it's better than either the ZnO and Al2O3 formulations, and perhaps a bit shy of the performance of the silver-loaded stuff.
I just picked up an enhanced performance thermal compound put out by MG Chemicals. The usual suspects use Zinc Oxide. The two Arctic formulations I know of use either silver or aluminum oxide. The MG stuff uses boron nitride, so I suspect it's pretty good, and has no conductivity concerns. Basically, it's better than either the ZnO and Al2O3 formulations, and perhaps a bit shy of the performance of the silver-loaded stuff.
The alumina pads are pretty good. Assuming a TO-220 contact area of about 1.5 cm x 1.0 cm the total would be about 10m for the .062 thickness giving Rtheta of about 10/15 degreesK/W for the pad. So maybe in the range of 1 degree per watt case-to-sink allowing for the interfaces. Not bad given the 26KV breakdown voltage...
By comparison RthetaJC (junction to case) is about 1.67 degrees/W for a 75 watt device. So the 75 watt device rating assumes a 25 degree C contact surface (150 degree C max junction temp typically).
The heatsink I show is optimistically rated at 2.5 degrees per watt (!) in free air. I think it may be a typo! Anyway taken for granted, if the pad is 1 degree and the sink is 2.5 and RthetaJC is 1.67 the total Rtheta is 5.17 degrees per watt allowing almost 25 watts at 25 ambient, with a sink temp of about 87 degrees C. I have run them at 25 W or so but with a fan. Seems like they're happier in the 12-15W range. My guess is the sink is closer to 4-5 degrees per watt when mounted over a surface like a PC board or chassis plate.
By comparison RthetaJC (junction to case) is about 1.67 degrees/W for a 75 watt device. So the 75 watt device rating assumes a 25 degree C contact surface (150 degree C max junction temp typically).
The heatsink I show is optimistically rated at 2.5 degrees per watt (!) in free air. I think it may be a typo! Anyway taken for granted, if the pad is 1 degree and the sink is 2.5 and RthetaJC is 1.67 the total Rtheta is 5.17 degrees per watt allowing almost 25 watts at 25 ambient, with a sink temp of about 87 degrees C. I have run them at 25 W or so but with a fan. Seems like they're happier in the 12-15W range. My guess is the sink is closer to 4-5 degrees per watt when mounted over a surface like a PC board or chassis plate.
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