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Temporary solved problem. All 4 heat sinks are 2" square but they are all different. Soon as I test the circuit I learn how hot mosfets get. Maybe mosfets need to be larger. I also have liquid cooling for a pump and HV transformer oil if needed. I have boxes of heat sinks the trick is finding what works. I need some of those insulator pads under the mosfets for liquid cooling. I also have several aluminum bars I can make any, size, shape, design, heat sink I want but its more work than I want.
Your heatsinks should be enough, depending on how much current the MOSFETs handle, and as long as the gate voltage is 10V or more (for full "on" state). The RDSon of around 0.3 ohms requires at least 10V on the gate relative to Source. At 10A current, heat generated will be about 30W, that's a lot.
We don't know those numbers, so hard to guess the actual amount of heatsinking required. Also, it may be worthwhile to use small fans blowing on the heatsinks to help dissipate the heat.
At a 500V rating, is that actually required? Lower voltage MOSFETs will usually have a lower RDSon and thus less heat generated.
Your heatsinks should be enough, depending on how much current the MOSFETs handle, and as long as the gate voltage is 10V or more (for full "on" state). The RDSon of around 0.3 ohms requires at least 10V on the gate relative to Source. At 10A current, heat generated will be about 30W, that's a lot.
We don't know those numbers, so hard to guess the actual amount of heatsinking required. Also, it may be worthwhile to use small fans blowing on the heatsinks to help dissipate the heat.
At a 500V rating, is that actually required? Lower voltage MOSFETs will usually have a lower RDSon and thus less heat generated.
My target is 1500 watts. I was getting 900 watts from the other P55NF06L mosfets. 15a 100v will give me 1500w with IRF460 mosfets but I don't like having the voltage that high 100v seems dangerous. 40v 50a would be good = 2000w. This H circuit is different it might work better and not need to be 1500w to get what I want. This is a cool learning project. The H circuit should generate less heat and 900w could be better than the other circuit with 2 mosfets at 900w. Nice thing about P55NF06L mosfets data sheet says .015 ohms 60v 50a. they could still be the best choice of mosfets.
I don't think that will solve your heating problem. First of all, they are rated at 55A (60A depending on which vendor datasheet you look at). Passing 50A is equal to 50*50*0.018ohms = 45W heat dissipation with worst case RDSon. Even with 0,014 ohms, it is still 35W. You still need a big heatsink. Also, that 55A rating is at 25C. Once it warms up, the current rating is 39A at 100C. Thus, passing 50A when they are already hot may destroy those MOSFETs as well. Also, watch out for the gate voltage on those P55NF60L, the maximum is 16V, not the typical 20V most MOSFETs have.
go to the thrift store and get a "beater" AV receiver like an old sony, pioneer, yamaha or something (as long as it has transistors for output devices).... the one large heat sink in one of these should be more than enough for your needs...
and if the mosfets have a metal pad in their back, make sure you use thermal pads and WHITE heat sink grease (the silver stuff is electrically conductive, and you don't want that). the metal pad is connected to the drain of the mosfet.
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