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How did I fry these MOSFETs ?

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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.


At the very least, you should have calculated what maximum case temperature the FET can take with 55W of dissipation (thermal resistance specs) and then monitor case temperature during the experiment.

Another option is to monitor junction temperature by connecting reverse voltage through a current limiting resistor and, during the test, briefly disconnecting the PSU and measuring forward voltage of the body diode. This measurement would need to be calibrated beforehand, for example with boiling water. And I'm not sure how briefly is briefly enough, maybe it would require some electronic switch and a fast sample-and-hold.

Ok I never heard of that before, I do have thermocouples, and actually a cheap IR sensor, I never thought I'd need them.

I want to try again, but with a lot less power, I'm still hoping I can get 4 fet's on 1 plate. But yeah I probably just cooked them.

I should breadboard an op-amp section too, for some control, with a shutoff. I made a simple NTC shutoff before.

Relay in the PSU is not an issue yeah.
You could measure the Rds on when you are doing future tests to verify its near the rated 18mR, but that would not be entirely conclusive. Tayda should be reasonable reputable.

One issue is the IRFZ44 has no rated DC SOA, so there is no real safe spec for the way you are using it.
Of course, the heat probably didn't help, so getting an idea of the FET case temp as mentioned would be a good idea.


Ok, yeah and I got these mosfets, just because they were cheap. The main PSU's I want to test are some computer PSU's, so LV anyways, but super high currents from the 12V.

I better get a thermocouple or 2 on there today when I try again......and start low...I don't want to waste anymore of these mosfets.

Or I need more expensive ones, more suited to the role. In those threads, it doesn't sound like they can handle much of anything I wanted to use them for. I bought 20 of them. Oh well.


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