Understanding Main MOSFET logic

[thanibee]

Could somebody help me to understand how the First MOSFET (PQ42) in the AMD laptop is switched on through the transistor package (PQ17)?

[Sephir0th]

Re: Understanding Main MOSFET logic

No, because even the partial schematic is to incomplete to tell. Can't even read from where the PQ17 Gates are controlled. So please add board number or link to schematics.

[mon2]

Re: Understanding Main MOSFET logic

The first DCin mosfet is a P-channel mosfet. You can confirm this by the direction of the arrow at the gate of the mosfet schematic symbol or by reviewing the datasheet.

Being P-channel, to enable this mosfet, the gate voltage must be lower than the voltage being passed between the source and drain pins.

The dual transistor below this mosfet in the schematic is a 2N2907A type. This is a PNP transistor.

You can confirm this type by the direction of the arrow in the graphic symbol of the schematic. Being used as a switch, this PNP switch will turn ON if the BASE pin is a logic low=0 volts level. When ON, a positive voltage will pass onto the gate pin of the P-channel mosfet.

So this combination of parts is acting like a logic inverter gate.

A logic low on the 2N2907 base pin will turn OFF the first DCin mosfet and vice versa.

Measure the voltage to ground of the mosfet gate pin. If the voltage is sufficiently lower than the source / drain pins, it should be on.

You can confirm if the mosfet is defective or not by checking the resistance between the mosfet pins.

Always remove the power to the logic board when measuring the resistance or if testing in diode mode.

Check the resistance of:

Source (1-2-3) & drain (5-6-7-8)
Source (1-2-3) & gate (4)
Gate (4) & Drain (5-6-7-8)

Post each measurement.

Reference:
https://www.electronics-tutorials.ws...or/tran_4.html