"yhan" wrote in message
oups.com...
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


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.