I intend to drive a DC motor through an H-Bridge with PWM Control. Therefore I'm using only NMOS powerfets to drive the bridge. However I'm facing an issue with the current at the MOS gates. As you can see from the image below, the gate currents suffer periodic spikes.

enter image description here

Additionally, the lower the potentiometer value - the one that regulates the PWM Duty Cycle -, the more ripple I obtain in the current going through my DC Motor model. For instance, the image below shows the current going through the motor when the potentiometer is set to 10%.

enter image description here

How can I verify if the gate current spikes and/or rippled current driving the DC motor are harmful to the motor or not? Since, in theory, the motor is powered by a perfectly DC current.

Here's one side of the H-bridge I'm setting up: enter image description here

The IC I'm using is the IR2104S, since it has built-in dead-time to avoid shoot-through currents. The powerfets are IRF530 NMOS.

Here's a picture showing M2's drain current measured through a low-resistance high-wattage sensor resistor.

enter image description here

  • \$\begingroup\$ For your mosfet graph, double check your blanking period (you do have one right?). This could be from shoot through, which wastes power and heats up the fets. For the motor current graph, if you want to reduce the ripple, then you need to increase the PWM frequency. \$\endgroup\$
    – Aaron
    Commented Dec 8, 2020 at 17:22
  • 1
    \$\begingroup\$ Which real DC motor are you simulating (or is it just a generic model)? Please show us your simulation circuit. \$\endgroup\$ Commented Dec 8, 2020 at 17:42
  • \$\begingroup\$ Anti-shoot-through circuit / method needs to be employed or live with it. \$\endgroup\$
    – Andy aka
    Commented Dec 8, 2020 at 18:40
  • \$\begingroup\$ @Aaron I'm using the IRF530 n-mosfet. I'm afraid it doesn't contain a blanking period plot in the datasheet. \$\endgroup\$
    – MCarsten
    Commented Dec 8, 2020 at 21:07
  • \$\begingroup\$ @BruceAbbott I added a picture showing one side of my H-Bridge (the other side is just a mirrored copy). As for the DC motor, it's just a generic model (a resistance in series with an inductance). \$\endgroup\$
    – MCarsten
    Commented Dec 8, 2020 at 21:07

1 Answer 1


What you are (were, before the edit) calling the "drain" currents measured on the R2 of your provided schematic are actually the "gate" currents. And that's not unexpected - you are charging and discharging the gate capacitance at each switching event. Note how the gate current is zero most of the time, as a gate draws no current in steady state. And the magnitude of your current is quite low to be that of a motor.

At present, you don't have any means in your drawn circuit to measure the drain current at all. Try putting a low value high wattage sense resistor between the drain of M2 and ground, in the usual fashion.

  • \$\begingroup\$ I added the low resistance high wattage sense resistor as required. The measured drain current plot has been added to the original post. \$\endgroup\$
    – MCarsten
    Commented Dec 8, 2020 at 21:21
  • \$\begingroup\$ Does that new actual drain current graph look like your design expectation? To me it looks reasonable at least. \$\endgroup\$ Commented Dec 8, 2020 at 21:25
  • \$\begingroup\$ My motor's nominal current is 2.4A. \$\endgroup\$
    – MCarsten
    Commented Dec 8, 2020 at 21:31

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