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The below given circuit works as a switch. The P channel MOSFET has max Vdss = -30V and Vgss = +/- 20V.MOSFET DATASHEET

The voltage divider protects the gate of the MOSFET from exceeding it's Vgss in case if high voltage is applied.

Now the issue is, when external +12V is applied to Output 2 connector as shown in image, the voltage flows from Drain to Source and flows through all the regulator and generate +5V. [Ground is always there]

Whereas +5V should only be generated when 12V is applied to the 12V connector.

The simplest solution is to use a diode at the output 2 which will stop any reverse voltage and only allow forward voltage through it (while switching).

But why the voltage is flowing from drain to source when circuit has no power, why does Led glow (means 5V generated).

What else can be done to rectify this issue apart from using Diode?

enter image description here

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  • \$\begingroup\$ Why is your VSS pin not grounded? \$\endgroup\$
    – Reinderien
    Commented Sep 18, 2017 at 19:07
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    \$\begingroup\$ Look at the MOSFET symbol ... note that it incorporates a diode symbol. Now what would a diode oriented like that do? \$\endgroup\$
    – user16324
    Commented Sep 18, 2017 at 19:10

2 Answers 2

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In each MOSFET, the body forms diodes with the drain and source junctions. The body in a pMOSFET is n-type, while the drain and source p-type. The opposite in in an nMOSFET.

Discrete MOSFETs have the body and source terminal shorted together. This form a diode between source and drain. To see the direction of the diode, just have a look at the arrow in the symbol (in your schematics, the diode is also explicitly shown). On a pMOSFET, the anode is at the drain, the cathode at the source. The opposite occurs on nMOSFET.

Therefore, in your circuit, current will flow from drain to source, regardless if the pMOSFET is ON or OFF.

To avoid this, you can use a back to back MOSFET connection shown below.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Value of R1 depends on? \$\endgroup\$
    – MICRO
    Commented Sep 19, 2017 at 4:09
  • \$\begingroup\$ The larger the value, the slower the turn-off. But also the smaller the current (actually, in your application who cares...). You can choose it between 10k and 100k I'd say. Remember to remove R9 on your circuit (now it's that R1). And you can make the same calculations you made for R9 and R15. \$\endgroup\$
    – next-hack
    Commented Sep 19, 2017 at 5:42
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The body diode is what allows your P channel mosfet to conduct current from drain to source. By applying the 12V to the output 2 connector, you foward-bias the body diode and therefore current has a path to flow.

If the 12V source you have in the schematic (connected to one of the terminals of the 3.30kOhm) resistor is the same as the input voltage to the regulator, that explains it.

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