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

"yhan" wrote in message
oups.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

yhan wrote:
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?
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

"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.

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

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