MOSFET maintains voltage drop across G and S only for 3 seconds

[digitalDNA]

Hello,

While trying to fix an SMPS, I hear a ticking sound.
I've checked all capacitors, but none seem to be shorted.

I then suspected an issue with the flyback transformer which has 3 leads on the primary (used in a push-pull configuration ?) and one of them, which is always connected to GND has a resistance reading in the order of Mega ohms with any of the other 2 leads on the primary.

But I thought let's check the MOSFETs first::

The circuit also contains a PFC circuit, controlled by a UCC3818D (D stands for the SMD version), with no TI logo on it...

I removed the MOSFETs from the circuit and tested them one by one with a multimeter in diode mode.

After "charging" the gate with the red probe on G and the black probe on S, I moved the red multimeter probe to the Drain, D.
At that point, one of the MOSFETs maintains a voltage drop reading across D and S for only 3 seconds. Other MOSFETs maintain the voltage drop as long as the probes are still connected to the MOSFET's leads.

Is this a problem with the MOSFET ? Or is it completely normal ?
Do MOSFETs also produce a ticking sound when defective ?

Thank you in advance,
Roger

[digitalDNA]

Re: MOSFET maintains voltage drop across G and S only for 3 seconds

The MOSFETs are W25NM60N by ST Microelectronics.

[redwire]

Re: MOSFET maintains voltage drop across G and S only for 3 seconds

Your multimeter likely only puts out 3V when you are "charging" the gate.
That mosfet needs 3-4V to start to turn on, so I say the mosfet is OK. It is just getting not enough charge to stay on very long. When a mosfet fails the gate or S-D typically short.
The clicking sound is from the magnetics (inductor or transformer) getting a current pulse.