It really seems as if Ltspice is indeed flawed, not only did I catch op-amp input terminals taking milliamps of current, but now nmos fets take huge amounts of current into the gate.

Previous post had no mosfets: LTSpice Has bizzare results for high input impedance devices

Try for yourself.



Gate Current:

Gate Current


Another mosfet with low gate charge


enter image description here

Gate Current:

enter image description here

  • \$\begingroup\$ At the right frequency, it can take amps to drive a MOSFET's gate. \$\endgroup\$ – Brian Drummond Mar 5 '18 at 23:01
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    \$\begingroup\$ 400 uA is hardly huge. Gate capacitance in FETs can result in dozens of mA of current when switching, and can e.g. fry microcontroller pins if you're unlucky and don't account for it. \$\endgroup\$ – whatsisname Mar 5 '18 at 23:11
  • \$\begingroup\$ Your previous post linked was a design flaw by user driving differential inputs of Op AMp at 2V \$\endgroup\$ – Sunnyskyguy EE75 Mar 5 '18 at 23:40
  • \$\begingroup\$ "...huge amounts of current into the gate" - 400uA isn't much compared to a Drain current of 6000A! \$\endgroup\$ – Bruce Abbott Mar 5 '18 at 23:48
  • \$\begingroup\$ You should only drive MOSFET's with a square or rectangular wave that is buffered by a push-pull bjt pair. To a MOSFET, how you drive the gate is everything... \$\endgroup\$ – Sparky256 Mar 6 '18 at 0:13

What you're seeing is perfectly fine - your FET has a huge gate charge and charging that at 400Hz does take some current. The waveform is distorted because dQ/dU of the FET is not constant. (That is, depending on the gate voltage, the capacitance of the gate changes.)

Q_gate = 405nC (from datasheet)
f = 800Hz (gate being charged two times per full cycle)

405nC * 800Hz = 324µA

That's exactly what you're seeing in the simulation.

  • \$\begingroup\$ Does the new circuit have normal gate current also? \$\endgroup\$ – user33915 Mar 5 '18 at 23:30
  • \$\begingroup\$ @user33915 That one also behaves absolutely normal. Q_gate=150nC for the IRF2805S which brings us to about 120µA (exactly what you're seeing) if you plug it into the formula. Miller effect seems to be much lower on that FET, too, which is why the waveform is less distorted. \$\endgroup\$ – Jonathan S. Mar 5 '18 at 23:34

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