Simple confirmation of dead Mosfets.

[fastline]

I have "another" PS I am working on. This one outputs 6 voltages. The main board is the primary PS and supplies DC voltage to a secondary board for regulation. Not sure what caused the failure but upon inspection, there were no visual signs of damage. I found one iN4004 rectifier diode that was out that seemed to be part of a small bridge for a few things. It was shorted.

I replaced and tested and was not getting power to to the main bridge. Power goes through a series of inductors, caps, etc, then to the bridge. There was a series 25W, 5ohm resistor that was dead and I rarely see that. I replaced the resistor kind of wondering what would have caused enough draw to do that...

Next test, the new resistor failed shortly after take off. Took a bit but I was first just testing for any output voltage and it was there so I then tried to test current to the circuit and it died at that point. I suspected a short in a pair of mosfets from the start.

The mosfets I pulled are IRFPC60. From gate to drain, one is 2 ohms the other is 15 ohms. Per my experience and the data sheet, I should not be conducting there and I just wanted to make sure that my "quick assessment method" was valid unless there is a resister in parallel for some reason? I guess when I check these in circuit, I simple test for shorting from gate to drain since that is a common failure mode. Should these not be all in the Mohms here? I have a bench tester that I made but falling apart and not thinking I need to go any further here. Gate to drain means " its in it to win i"!

Now, I have no idea where that power is going just yet since there is nothing connected to this board so I will have to look further on the matter.

 

[cowana]

While MOSFETs may have a resistor in parallel, I'd expect it to be in the order of a few thousand ohms.

2 or 15 ohms when unpowered sounds like a dead FET to me.

Andrew

 

[Sceadwian]

It's an N channel fet so I would check it by grounding the gate and source leads together and checking the resistance (positive lead to the source) on a multimeter, it should read high impedance. With the gate and source grounded if you reverse your probe leads on the diode test mode you should read a single diode drop (the body diode) the off state impedance and body diode drop should be in the PDF for reference from the meter readings.