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basic power mosfet question

Discussion in 'Electronic Repair' started by yhan, Jul 31, 2006.

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  1. yhan

    yhan Guest


    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.

  2. Ray L. Volts

    Ray L. Volts Guest

  3. GPG

    GPG Guest
    When the control transistor is turned on it biases the gate of the FET
    on. The zener limits the gate voltage
  4. ian field

    ian field Guest

    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.
  5. yhan

    yhan Guest

    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.

  6. GPG

    GPG Guest

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