I'm working on a 3 phase brushless, sensorless driver (Allegro A4960.)

I need to drive different motors in terms of power.

The voltage is 24V but the current per phase varies from 1A to 6A, so I'm looking for an N-MOSFET suitable for this current.

The A4960 evaluation board uses the IRFR2405PBF but I was wondering if the CSD18533Q5A could be better.

I'm not an expert on MOSFETs and motor drivers.

My evaluation is based on IDrain and Rdson.

  • Are these 2 variables sufficient or am I missing something else?
  • Could I not consider the fswitching on the MOSFET to calculate the dissipated power?

In this case I would choose the CSD18533Q5A because of the lower Rdson.

  • \$\begingroup\$ You may want to consider figure of merit when comparing two mosfet's, rather than considering Rds only. electronicdesign.com/power/figure-merit-refresher-course \$\endgroup\$
    – user57037
    Jun 27, 2017 at 7:31
  • \$\begingroup\$ If my memory is working about 20 years ago SGS developed a brushless sensorless motor control for 3 phase motors. It had several application notes that explained it in great detail. I do not know if the information is available anymore. Hopefully this helps as the ap notes had great detail. \$\endgroup\$
    – Gil
    Jan 22, 2023 at 2:24

1 Answer 1


My valutation is based on IDrain and Rdson. Are these 2 variables sufficient or am I missing something else?

Those parameters allow you to consider static power loss situations. You also need to consider peak voltage ratings and the TI device looks comparable but might need more heat-sinking because it looks a smaller device - look for the temperature rise per watt information and make comparisons.

Peak gate drive voltages are both +/-20 volts so that's a no-brainer.

Regards switching losses, you need to compare rise times and fall times given the driver you are using and the output and reverse transfer capacitance of the device. One worry is that the reverse transfer capacitance of the TI part looks remarkably low at 7 pF (typical) yet appears to be around 20 pF in the graphs (a quick look so I could have been mistaken). I would want to clarify this discrepancy before making a decision on which way to go.

Anyway, for your sort of application this is what I'd want to compare.

  • \$\begingroup\$ Hi guys, thank you for your reply. I don't have "space problem" on the PCB, I just want to be sure as much possible that the mosfet will handle the current. And of course the price.. \$\endgroup\$ Jun 28, 2017 at 12:05
  • \$\begingroup\$ @EnricoPallazzo I think it should be OK based on what detail you have in your question. \$\endgroup\$
    – Andy aka
    Jun 28, 2017 at 12:41

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