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I have a simple RL circuit with a MOSFET, as shown, with the FET switching at 200 kHZ and 50% duty cycle. I expect, for a given input voltage V2, to have an output about half the voltage (thanks to the 50% duty cycle), so in this case an average amperage of about 7.5A. Instead the actual circuit and my spice simulation show a current of only ~60 mA, and bizarre current behavior at the source of the FET. The FET is an IXFB210N30P3, and the signal generator is in reality a PWM controller and MOSFET driver, separated by an optocoupler.

enter image description here

The current through the resistor and inductor, according to a spice model, look like this.

enter image description here

but incomprehensibly, the current at the source looks like this

enter image description here

with enormous current spikes as the MOSFET switches, but no current when the MOSFET should be on (where green is the gate voltage and red is the source current, see clearer image in link).

I am trying to understand, first of all, why there is so little current in the circuit (a hundredth of what I'd expect), and also why the ground current doesn't seem consistent with the current in the rest of the circuit. This is what the drain voltage looks like, with large voltage spikes due to the inductance of the circuit. I'd discount this as a simulation error if it weren't consistent with what I'm seeing in my scope. I'm happy to attach scope readings or the model itself if they would be helpful (not sure what the SE policy on file uploads is).

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  • \$\begingroup\$ First of all: what do you see when you add a diode across the inductor? \$\endgroup\$
    – user133493
    Jun 19, 2017 at 1:11
  • \$\begingroup\$ @replete I just looked at this setup, and at first I see the same thing, but after about 20-30 ms, I start to see ~5A square waves, like this, and the whole circuit starts oscillating with the gate signal generator. I get about 5A through the diode. \$\endgroup\$
    – JAustin
    Jun 19, 2017 at 1:26
  • \$\begingroup\$ Oddly enough, the magnitude of the current spikes in the no-diode model are proportional to the gate current. The threshold voltage is only 5V, so my only idea is that it has something to do with the gate-source or gate-drain capacitance. Either way, I don't know why I'm getting so little current overall. \$\endgroup\$
    – JAustin
    Jun 19, 2017 at 1:27
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    \$\begingroup\$ For a simplistic first-pass, consider that 3mH @ 200kHz gives you an XL of 3.77kOhms ... \$\endgroup\$
    – brhans
    Jun 19, 2017 at 1:37
  • \$\begingroup\$ I had considered that, but everything here is transient, and the current is at least 60 times V/Z. The magnitude of the current is also independent of the inductance, so I don't think impedance calculations are very meaningful. I was expecting the current to just be halved by the MOSFET, with the inductor smoothing the resulting current. \$\endgroup\$
    – JAustin
    Jun 19, 2017 at 1:49

1 Answer 1

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The current spikes in the Source are the result of very rapid charging and discharging of the Gate-Source capacitance. A realistic circuit would have some resistance in the Gate driver, then the Gate-Source current spikes would be smaller (and the FET would take longer to turn on and off).

Source current doesn't actually drop to zero when the FET is on, it just looks that way because 60mA is so much smaller than 10A.

Drain current is low because L1's inductance slows down the current rise. Inductor current = voltage x time / inductance. At 200KHz and 50% PWM the FET is on for 2.5us, so 15V x 2.5us / 3mH = 12.5mA.

If the FET had zero capacitance then current would ramp up to 12.5mA while it was turned on and fall to zero when it turned off. But the IXFB210N30P3 has large output capacitance which supplies current to the inductor when the FET is off, causing it to continue rising. Eventually it 'walks' up to about 60mA, ramping up and down by ~12.5mA as the FET turns on and off.

Reduce the PWM frequency to about 20kHz and you should see the current variation increase as the inductor is given more time to allow it. The current waveform will then become a sawtooth, and the inductor will resonate with the FET's output capacitance to produce a very high voltage spike when the FET turns off.

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  • \$\begingroup\$ Thanks a lot. This makes sense! If you have a chance...do you have any ideas for how to make the circuit function as intended (~constant current through inductor proportional to the FET duty cycle)? My first thought was to add a capacitor in parallel to the FET, but you end up with essentially infinite current through any such capacitor. I also added tried adding a diode across the inductor/resistor, and it gets me a constant current through the coil, but the current supplied by the source oscillates like this, which my voltage source has a hard time handling. \$\endgroup\$
    – JAustin
    Jun 19, 2017 at 23:06
  • \$\begingroup\$ I end up with big voltage oscillations at the supply as it repeatedly over and undercompensates for the huge current oscillations. \$\endgroup\$
    – JAustin
    Jun 19, 2017 at 23:10
  • \$\begingroup\$ "it gets me a constant current through the coil, but the current supplied by the source oscillates" - that's normal. Current is only drawn from the supply when PWM is on, when off the current recirculates through the diode and inductor. To smooth supply current you must add a reservoir capacitor. Next question is what do you want to do with the inductor current. Without a load to draw off energy the current will be low. \$\endgroup\$
    – Bruce Abbott
    Jun 19, 2017 at 23:48
  • \$\begingroup\$ With the diode in place, I get a smooth, constant ~50A current through the coil off a 100V DC voltage (100V V2)*. The circuit I have drawn is actually the entire circuit - the inductor is a large coil being used to confine plasmas, so my only objective is to get controllable, steady DC current through the coil. *in the imaginary spice world of ideal power supplies. \$\endgroup\$
    – JAustin
    Jun 19, 2017 at 23:54
  • \$\begingroup\$ As for a reservoir capacitor, the sheer size of the currents involved makes things tricky. Spice is showing ~80A 200kHz oscillations plus large spikes as the FET capacitor discharges. \$\endgroup\$
    – JAustin
    Jun 20, 2017 at 0:01

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