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basic power mosfet question

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yhan

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Jul 30, 2006, 9:33:59 PM7/30/06
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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

Ray L. Volts

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Jul 31, 2006, 3:12:00 AM7/31/06
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"yhan" <chei...@yahoo.com> wrote in message
news:1154309639.1...@p79g2000cwp.googlegroups.com...

> Can anyone give me some explanation about this and how P-channel
> mosfet works in general. Also what is gate drive?

n vs. p channel operation:

http://en.wikipedia.org/wiki/MOSFET#MOSFET_structure


GPG

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Jul 31, 2006, 9:12:14 AM7/31/06
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>From your description it may be like this
http://www.geocities.com/gpg212/scirepair2.GIF
When the control transistor is turned on it biases the gate of the FET
on. The zener limits the gate voltage

ian field

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Jul 31, 2006, 2:58:14 PM7/31/06
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"yhan" <chei...@yahoo.com> wrote in message
news:1154309639.1...@p79g2000cwp.googlegroups.com...

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.


yhan

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Jul 31, 2006, 7:56:07 PM7/31/06
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> http://www.geocities.com/gpg212/scirepair2.GIF
> When the control transistor is turned on it biases the gate of the FET
> on. The zener limits the gate voltage

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

GPG

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Jul 31, 2006, 10:55:00 PM7/31/06
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It is possible, but trace the control transistor and see if it is
turned on.

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