When powering this circuit I get a 150 microsecond rush of current at 120 amps. Is this a real issue with mosfets?

Additional info: I've been playing with N and P channel slow start mosfet circuits and on some of the P channel ones I get spikes just like that if my resistors are in certain ranges. I stripped this one down and really expected it to not have a spike in this configuration.

enter image description here

  • 1
    \$\begingroup\$ note M1 has a body diode that will conduct in that orientation \$\endgroup\$
    – sstobbe
    Oct 13, 2017 at 2:26
  • 1
    \$\begingroup\$ Funny enough if you just knew that \$i=C\,\mathrm{d}v/\mathrm{dt}\$ you could just add up \$500\,\mu\mathrm{F}\times 24\,\mathrm{V}/ 100\,\mu\mathrm{s}=120\,\mathrm{A}\$ \$\endgroup\$
    – carloc
    Oct 13, 2017 at 13:34

1 Answer 1


The inrush current is mostly due to the 500uF cap you have. If you downsize it (or add some series resistance to it), you will see the current spike go down. Whether or not this is an issue depends on what the datasheet says—there's a pulsed current maximum spec.

Also, the reason your mosfet is conducting—as shown in the schematic—is because of the body diode. This is an N-channel mosfet so you'd need a voltage at the gate such that \$V_{GS}\$ is high enough to turn it on. This means \$V_G\$ needs to be greater than supply voltage you connected directly at the nmos' source. The mosfet is off in the shown configuration (gate is grounded) but the body diode is forward biased.

If this is a high side switch application so a pmos is an easier choice.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.