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RthJC (thermal resistance junction-case) for the FET is 0.5 °C/W. RthCS (case-sink) isn't stated but it's not zero and is likely about the same 0.5 °C/W. So, at 125 watts, the junction is probably running 125° C above the 25° of the water pipe or close to 150°
I thought people had already decided it was ok to try to get about 1700W dissipation out of 8 FETs?
No, the initial design was for 1200 watts with 10 FETs. - 120 watts each.
So here we are talking 15 FETs at 60 amps - 1440 watts - 96 watts each.
So basically we are expecting 100 watts from a 300 watt part.
I think during testing Jeremy ran it up to the 70C pipe temperature trip point.
I don't know what the thermal resistance is case to pipe Probably not much since they are soldered to the pipe.
https://www.electro-tech-online.com/custompdfs/2013/02/CD00003180-81350.pdf
()blivion;1107826 [quote said:In theory, this should have been given to us on a silver platter as the parts rated wattage, as that's the exactly the main variable we are talking about in the above. But it could also be expressed as the SOA, because SOA should include the wattage and anything else. AND, it can also be expressed as the package max power, because the thermal transfer area is going to be about the same size for all TO-220 FETs no matter what the exact part is.
What we would really need to know to answer the question once and for all is... What is the thermal impedance from junction to tab? And what is the die area? I think the most important part would be the die thermal impedance. Seems to me that would be the highest number. And it is probably approximately the thermal impedance of silicon X the thickness.
I have been using an infrared thermometer but it is hard to pin point that exact of a location, even though it has a red laser, the laser is not in the right spot.
...This could possibly be why you are reading much hotter temps off the back of the FETs than off the pipe. Black bodies emit more light.