I have some plate steel that I thought might be ok as a heatsink in a big load tester, each 1 is 12"*12"*3/8", and it was easy to drill through, as some on here said.
So I was just trying to see how much heat, 1 plate could handle with 1 mosfet. So right in the centre, I screwed on an IRZ44N, with some thermal paste under it. The plate was sitting up on some wood blocks, the fet and resistors were on a PCB, 90deg to the plate, nothing shorted.
They are rated 55VDSmax, 16VGSmax, 47A, JtoC=1.4C/W. I got them from Tayda-electronics., not their ebay site anyways.
I've tested similar mosfets on a much smaller HS before, with 30-40W, so I figured I'd try ~50-60W, by applying 30V@2A, from my Siglent 3303X-E PSU, hacked to the 1mV-1mA version. Test circuit is below.
So with me doing the gate regulating, starting at 2.5V, and getting it running up to 2A, as it heated up it needed less VGS, and the PSU max current set to 2A, the PSU was voltage limiting to ~25-28V @2A.
Anyways it was doing ok, ran for about 5-10min, I kept turning the gate voltage down a few mV at a time, under 4V, just trying to get the DS voltage to stop limiting lower and lower. If I dropped the VGS way down, it's the current that kept creeping up, so I just let it sit at 2A and Vlim.
The edges of the plate were still cool, but right near the chip was getting too hot to touch, the average wattage from the PSU was ~55W. So I think it's not a very good heatsink that way, and I might have just been cooking it already. But I put a fan over it 1/2 way through.
I turned on/off the gate voltage 1-2 times. Then when it came back up, DS around 26V, the PSU changed trans. taps for the DS rail, switching a relay. I think it went to the tap for under 22V or 24V as it voltage limited, then immediately, it limits to 1V @2A....because the mosfet fried. R-DS is now 0.5ohm, GD and GS are now 66ohm.
So what did I do wrong? Maybe overheat it, the steel is not a great thermal conductor, 55-60W is getting up there. The center of the plate was burning my finger, but a few inches away was just fine, and almost roomtemp on the outer edges.
Or are these counterfeit fets? Or was it the relay, and back voltage, but surely the PSU has snubber dioides, and the fet has a body diode.
Maybe turning on/off the gate, with only 100k to gnd, was bad ? The design I'm going to copy, uses that ~1k, I added the 100k, to allow the gate to discharge better. It's the Array 3700 on the bottom of the site below. Earlier I fried another 1, without having that 100k there, and the gate was not discharging either, when turned off. But with a 1k between the gate and PSU, that should handle anything with the PSU at ~4V.
http://www.kerrywong.com/2018/11/05/teardown-of-an-array-3711a-300w-dc-electronic-load/