basic power mosfet question

Jul 31, 2006 5 Replies
Y

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



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R

n vs. p channel operation:

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G

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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

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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.

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Y

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

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

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