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What should be the oscilloscope bandwidth for testing BLDC motor drivers? Is 20MHz bandwidth a good choice?

  • Switching frequency 16-20kHz
  • Test condition: loaded 20A, 24V
  • Test signals: High side and low side gate signal, phase signal, bus voltage, Hall effect output.
  • Used Oscilloscope Bandwidth: up to 200MHz

Is there any rule of a thumb to follow up for different applications such as automotive, aerospace, medical, etc.?

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  • \$\begingroup\$ The rule of thumb has nothing to do with applications, and everything to do with the frequencies you are interested in measuring. In your case, having a switcher operating at relatively low frequency, the result of inadequate bandwidth would probably be a rounding of what should be square edges. \$\endgroup\$
    – Kyle B
    Nov 2, 2022 at 4:41
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    \$\begingroup\$ The test signals you are referring to tend to have bandwidths you can comfortably measure with a 20MHz scope. 20kHz square wave has a 20kHz fundamental, with an infinite number of odd harmonics. So let's say up to the 13th harmonic so you view something that has some semblence of a nice clean square wave on your scope. That's 20kHz x 13 = 260kHz for the highest harmonic. en.wikipedia.org/wiki/Square_wave#/media/… For motor drivers, I would rather have a 20MHz scope and differential probes & current probes than a 200MHz scope without those probes \$\endgroup\$
    – DKNguyen
    Nov 2, 2022 at 5:01
  • \$\begingroup\$ The bandwidth beyond 260kHz goes into an even cleaner square wave and detecting ringing and other high frequency artifacts that might occur in that square wave. You can see there's a lot of space between 260kHz and 20Mhz to do so. \$\endgroup\$
    – DKNguyen
    Nov 2, 2022 at 5:02
  • \$\begingroup\$ Thank you for your comments and helpful informations, really appreciated. \$\endgroup\$
    – yardi
    Nov 4, 2022 at 8:56

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The fundamental switching frequency of 20 kHz isn't relevant, really.

Ultimately what it boils down to are the edge rates of your power switches (MOSFETs/IGBTs) and your gate driver, since their frequency content is higher than the fundamental switching frequency.

From my experience 200 MHz should be plenty for those type of measurements, for example to evaluate the miller plateau in the gate-source-voltage.

However, affordable high-voltage differential probes (which are very handy for those types of applications) have usually bandwidths in the 20 MHz range. But even with 20 MHz you can get a lot of stuff done, for example evaluating the voltage of an in-line shunt resistor for phase current measurement or any other voltage that is not referenced to ground.

It's hard to give a definite answer what upper bandwidth is useful to you. Relevant frequencies can get very high very quickly, if noise measurements and/or EMC become a thing.

The only thing I can tell you is that I've developed a couple of BLDC drives in my career. And in general, the most important measurements for functional verification and/or debugging can be performed with a bandwidth of 20 MHz easily.

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