# Mosfet amp spike in SPICE when connecting power

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.

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

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.