Are these MOSFETs the same?

[Steve]

The one on the left is the original, tried replacing it with the one on the right and as you can see it overheated and melted its plastic harness... Did I pick the wrong kind?

PXL_20210326_115234054.jpg (428.57 KiB) Viewed 3073 times

[exxos]

It really depends what it is fitted in, plastic packages are normally insulated from the heatsink for a good reason...

Did you put thermal paste on it and the heatsink presumably ?

[Steve]

@exxos No it has a non-conductive heat pad, so I didn't use any paste. It got extremely hot and melted the plastic holder, to which it then managed to short itself on the chassis. It's on the chassis of a crt terminal.

[Steve]

Hard to photograph but here:

PXL_20210326_121305633.jpg (228.28 KiB) Viewed 3062 times

[exxos]

Hard to photograph but here:

You wouldn't ordinarily use the grey washer with a plastic package, you will just put heat sink compound on the transistor and bolt it to the heatsink..

But you should probably be aware, that plastic packages may have a lesser wattage it can handle because of heat dissipation problems by the plastic package... The datasheet would tell you the differences there..

[Steve]

@exxos Not sure I'm understanding you, what we have here is a transistor on top of non-conductive thermal pad to stop it shorting to ground, but still keep cool on the chassis arm. It is pressed down by that plastic/metal tab, which only touches it with plastic, and it is that plastic/metal tab that is bolted down.

[exxos]

Oh, I just turn the brightness up on my monitor, I thought you was using a plastic package MOSFET...

[exxos]

In any case, if the package is "metal"... Then you need the mica insulation washer to isolate the metal from the heatsink, and you also need to isolate the screw going through the heatsink with the mica insulation washes..

If it is a totally plastic package, you need to not use the washes, and put heat sink compound on the plastic package to the heatsink..

[Steve]

Ok cool, well I've taken the necessary precautions and replaced the transistor and this time it seems OK cheers for the advice