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DRV8305-Q1: PWM Frequency vs EMC

Mark Spanberg1
Prodigy 50 points
Part Number: DRV8305-Q1
Other Parts Discussed in Thread: DRV8305

I am looking at an automotive design of a motor driver with DRV8305 that is suffering some EMC issues. Mainly in the region 90-150 MHz.

The design has the PWM frequency of 20 kHz and some tests have been made before to slow the flanks up by adding extra RC filters on the gate drivers (lowest current setting already) and also adding miller type capacitance between gate and source of the FETs.

An idea that I would like to try is to increase the PWM frequency to 40 or 50 kHz in order to make the current delta smaller. Higher switching frequency would of course give higher losses in the FET's but here they are already adding a lot of parasitics that might not be necessary with lower ripple.

I am looking at the excellent application note "Automotive 12 V 200 W (20 A) BLDC Motor Drive Reference Design" but couldn't find this there.

Is my idea sane or am I missing something?

Best regards!

  • 0
    TI__Guru 65320 points

    Mark,

    I have sent your post to the applications expert for this device, he will get back to you this week.

    Regards,

    -Adam

  • 0
    TI__Intellectual 2595 points

    Hi Mark,

    I think your concept of reducing the current delta with the higher PWM frequency makes sense, but I don't have any past experiments or data to back that up with.

    One thing I'd like to double check, though: You mentioned that you were already using the slowest IDRIVE setting, and added some RC filters to the gate as well. What is the MOSFET model number, or at least the Qg?

    Normally I expect to see noise from the phase slewing down in the 20-30MHz range, but if you're using a very small gate slewing very fast, I could see that noise reaching up into the 100MHz region.

    Thanks,
    Garrett

  • 0 in reply to Garrett Walker
    Prodigy 50 points

    Hi Garrett,

    Thank you for your answer.

    The Qg is 31nC (41 max).

    BR /Mark

  • 0 in reply to Mark Spanberg1
    TI__Intellectual 2595 points

    Hi Mark,

    With a Qg that size, I'd estimate the slewing time as somewhere in the 300-400ns range, if you don't have any gate resistor. If you do, can you let me know what size resistor you're using?. Also, what TDRIVE setting are you using?

    The issue I want to check for is to make sure you're fully charging/discharging the gate before the end of the TDRIVE period, as the DRV8305 will switch to the IHOLD/ISTRONG pullup/down current after the end of the TDRIVE period, which are higher than your programmed pullup/down currents.

    Thanks,
    Garrett

  • 0 in reply to Garrett Walker
    TI__Guru 65320 points

    Mark,

    I have seen no update on this thread, did your issue get resolved?

    I will mark this thread as closed for now but please post again if you need further assistance.

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Prodigy 50 points

    Hi Adam,

    I am waiting for the answer from the team but please close the thread for now. I will post the results if we do measurements in a higher frequency.

    Thank you for your help.

    BR /Mark