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I'm using a P-channel MOSFET Q57 as a switch for current from VIN_ALW_GATED to +VIN_ALW when VIN_ALW_GATED > 17.259 V.

In case VIN_ALW_GATED was not connected to any reference voltage (floating), and we connect another voltage source to +VIN_ALW, how I can prevent back current to go from the drain of Q57 to the source through the drain-bulk diode? Is this scenario possible?

I'm confused between connecting VIN_ALW_GATE (Gate of Q57) to GND or keep it floating, because, in case VIN_ALW_GATE was connected to GND, and assuming there was leakage current through this bulk-drain diode, it will go to the board GND. I'm trying to compare this with the gate left floating, in which I think there will no be such problem. Is my analysis right?

enter image description here

Simulate this circuit with CircuitLab (Credit to KHFM from my previous question)

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  • \$\begingroup\$ What is your concern? Are you worried about current going through the drain-bulk diode of Q57? \$\endgroup\$
    – sai
    Mar 15 at 7:20
  • \$\begingroup\$ @sai, Yes. That's my concern. In this case, I'm confused between connected VIN_ALW_GATE (Gate of Q57) to GND or not, because if there was leakage current through this substrate diode, it will go to the board GND. I'm trying to compare this with the gate left floating. \$\endgroup\$
    – Firas Abd El Gani
    Mar 15 at 7:31
  • \$\begingroup\$ I see in the PMOS specification that the max body-diode continuous current allowed is -20A. So, I am wondering is there really any concern? given that the current in your case will only be a few mA. \$\endgroup\$
    – sai
    Mar 15 at 7:45
  • \$\begingroup\$ What is the concern if a few mA goes to board GND? \$\endgroup\$
    – sai
    Mar 15 at 7:51
  • \$\begingroup\$ Why in my case it's few mA? @sai \$\endgroup\$
    – Firas Abd El Gani
    Mar 15 at 8:05

1 Answer 1

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If VIN_ALW_GATED is floating and VIN_ALW is connected to a voltage source, there will be a leakage through the drain-bulk diode of Q5 of few mA which I do not expect to cause any other issue. If the few mA (~5mA) leakage current is acceptable, you could live with it.

If you want to avoid this leakage and if a small diode drop is ok, simplest would be to add a diode in series between VIN_ALW and Q57 drain.

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