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Original question:

[FAQ] Proper RC Snubber Design for Motor Drivers

DRV8343-Q1: Questions about the parameters explained in 'Proper RC Snubber Design for Motor Drivers.'

Keiju Nakano
Prodigy 30 points
Other Parts Discussed in Thread: CSD18540Q5B

Tool/software:

I am designing a BLDC driver. The phase outputs oscillate and I plan to add RC snubber circuits to the upper and lower FETs of the half bridge. I would like to follow the design guidelines for a 7-stage RC snubber circuit as described in this topic.

https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/991693/faq-proper-rc-snubber-design-for-motor-drivers

I have a few questions:

The rectifier has a capacitor value of 22pF, where is this listed on the CSD18540Q5B MOSFET datasheet? I could not find this value in the published datasheet.

CSD18540Q5B

To find the parasitic inductance, should the extra capacitor C1 be added to the top FET or the bottom FET? Should I add it to both FETs or just one?

Sorry for the basic question and the bad English.

  • +1
    TI__Genius 12231 points

    Hi Keiju, 

    Thank you for your question and posting to our forum! 

    Keiju Nakano said:
    The rectifier has a capacitor value of 22pF, where is this listed on the CSD18540Q5B MOSFET datasheet? I could not find this value in the published datasheet.

    I believe this data is coming from the maximum reverse capacitance (listed as 20pF in the datasheet image below), but the value is just increased up to the standard 22pF capacitance that is used.

    Keiju Nakano said:
    To find the parasitic inductance, should the extra capacitor C1 be added to the top FET or the bottom FET? Should I add it to both FETs or just one?

    For RC snubbers you can have a capacitor for each high-side and low-side FET to dampen, as seen in this diagram here: 

    I hope this information has been helpful! Please reach out again if you have further questions.

    Best Regards,

    -Joshua 

  • 0 in reply to Joshua Maize
    Prodigy 30 points

    Thank you for your response.
    -I understand about the capacitance of the rectifier.

    -About C1, I understand that the capacitors in the snubber circuit will eventually be added to each of the high-side and low-side FETs.
    However,
    e2e.ti.com/.../pastedimage1617744455172v3. png.
    In step 2 above, when adding C1 to measure the shift of oscillation, I would like to know if it is measured with a capacitor of the value of C1 added to each of the high side and low side, or only to one of them.

  • +1 in reply to Keiju Nakano
    TI__Genius 12231 points

    Hi Keiju, 

    Thank you for the clarification. 

    It appears that you add this C1 to both highside and lowside, as you are characterizing each MOSFET. It seems likely this guide is only covering the snubbing of a singular highside FET where the important motor phase output is the source, but in actuality you may want to dampen the lowside transient response too and will most likely need to characterize that circuit element.

    That is my current understanding of the process, and hope the information is helpful.

    Best Regards,

    -Joshua

  • 0 in reply to Joshua Maize
    Prodigy 30 points

    I understood the steps to add C1 to the circuit. I was able to actually add it to the circuit and take measurements. Thank you so much for your help.