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basic power mosfet question

hello,

I was wondering how much voltage should we measure in the gate of a P-channel power MOSFET. I ask this because I am trying to repair a Philips monitor. I found out that the B+ going to flyback primary winding and to the collector of HOT is connected in the drain terminal of a power MOSFET (IRF 9620). The low voltage power supply works fine. I can measure dc voltage on the source terminal of the MOSFET (I forgot the exact voltage. I think that was

120 v or higher) but there is no voltage present on the drain. So there is no HOT collector voltage. Now measure the gate dc voltage and the dc gate voltage I think is too high (same voltage as with the source). Gate is connected through the anode of crystal diode and a 22k resistor in parallel with diode. Are these passive components do not provide any voltage drop to the gate? Can anyone give me some explanation about this and how P-channel mosfet works in general. Also what is gate drive?

thanks in advance for explanation and patience.

ian

Reply to
yhan
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n vs. p channel operation:

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Reply to
Ray L. Volts

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When the control transistor is turned on it biases the gate of the FET on. The zener limits the gate voltage

Reply to
GPG

With the B+ voltage you mention this sounds like a buck-converter (step down), there should be a squarewave (PWM) drive of at least 6V to gate wrt source. With a Philips monitor it could come from a dedicated sync jungle chip, but on other makes I've seen these pulses derived from everything from the front panel micro to a 555 pulse generator.

Reply to
ian field

thanks for the schematic. Now I know that the zener limits the gate voltage. But I still can't find out the reason why is there no drain voltage present. Is there a possibility of a defective MOSFET ? Low voltage power supply seems ok. The zener and resistor are ok too when tested by ohmmeter.

ian

Reply to
yhan

It is possible, but trace the control transistor and see if it is turned on.

Reply to
GPG

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