The board I had issue with has 2 P-channel MOSFETs connected in series. each MOSFET has its gate connected to its Source through a resistor. at the other hand, both gate can be pulled to GND by one signal signal. In that case, the voltage input from a power supply at the Source of the first MOSFET is output at the Drain of the last MOSFET. For some configuration reason, I was not able to control the signal at the gate to turn on both MOSFETs. But I needed to have the power available at both the input and the output to power up other components on the board. So based on similar experience with other board and some extra googling today, I believed it’s safe to apply a second power supply with lower voltage level (2volt lower) at the drain of 2nd MOSFET. But I think I was wrong because the board was not working after that. Since I don’t understand, hope someone here can shed a light for me. Isn’t enhanced P MOSFET can only become conduct through body diode when it’s off? If The voltage at the source was higher than the that at the drain, they should stay off, shouldn’t they?

 

hi FZ,
Welcome to AAC.
Do you have a circuit diagram to post, it would be much easier to suggest a solution.?
E

 

Hi,
Here attched the circuit diagram. The control signal was low when two power sources were applied. Put 12V PS on first and its current was limited to ~40mA. Noticed Voltage needle of the 10V PS swung instantly when switching on 12V PS.

 

Rotated schematic for ease in reading: