Electrical – short circuit response P-MOSFET


i plan on doing a SC test on a power supply that the output is switched on with a p-mosfet i was just wondering if there is defined recovery time for the pmos to cool down before you can do another short circuit test.

Best Answer

If I understand your question correctly, you want to interrupt a voltage line with a series-connected P-channel transistor and turn it on by connecting its gate to GND. The bias in this case is the voltage \$V_1\$ connected to the source (see below sketch). Then you realize that the P-channel heats up seriously when you do a short circuit test. This is a classic in safety tests in which the quality department rejects the design because risks of fire exists when you short \$V_{out}\$ to ground. The reason is because you end-up in an equilibrium situation in which the bias collapses and ends up forcing the P-channel into a linear mode where it heats up big time. This is solution (a). A better solution is to use an N-channel (lower \$r_{DS(on)}\$ and cheaper than P-channel) as shown in solution (b):

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In this application, the source \$V_2\$ is greater than \$V_1\$ by the amount of needed \$V_{GS}\$ to bias the MOSFET (10 V for a classical type). Then, when you short \$V_{out}\$, the MOSFET bias no longer disappears and it sustains the short circuit current with a low \$r_{DS(on)}\$ (low power dissipation). Of course the current is high and you may want to take additional precautions to harness it but the MOSFET is no longer in danger with risks of fire. Finally, if the body diode causes problems, you can connect two back-to-back MOSFETs as commonly done in battery protection circuits for instance (solution c).

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