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Original question:

DRV8874: Noise on IPROPI

DRV8231A: Current spikes on IPROPI

Florian Martin
Prodigy 10 points
Part Number: DRV8231A
Other Parts Discussed in Thread: DRV8213

Tool/software:

Hi, I'm trying to measure the current to get a better form of control on my motor using feedback. However, small spikes occur on the IPROPI pin and I would need the cleanest signal as possible.

I've checked it's not a crosstalk issue, is there any possibility that the cause would be the reverse recovery body diode in the H-Bridge mosfets ?

I could see similar waveforms in the lower h-bridge version DRV8213 datasheet, which I'll probably be using in the future.

Do you have any recommandations of snubbers or putting fast switching diodes in parallel of the mosfets to minimize this switching noise as much as possible ? I could experiment but it's hard to say without knowing the characteristics of the internal mosfets.

Thanks

.

  • 0
    TI__Mastermind 44325 points

    Hi Florian,

    You may try adding a 1 to 4.7 nF capacitor to GND on the IPROPI to see if it mitigates the issue. Can we get a capture of the motor current waveform using a current probe (non contact)? What is the drive fPWM and duty cycle used? Is this waveform on IPROPI captured with motor running in one direction? Thank you.

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    Prodigy 10 points

    Hi Murugavel,

    I'm using a 50% to 70% duty cycle PWM with a 80kHz frequency, and the spikes vary along PWM frequency. The motor is alternatively changing direction every 250ms. 

    Probing the current on the motor gives me following waveforms. On the first capture, you can see some big spikes which are inherent of the motor spinning and switching between the internal rotor windings.

    However, there’s a fair big among of noise I can zoom into which seems to be coming from the switching section of the chip since it's at the same frequency of my PWM.

     

    I did try putting capacitors on IPROPI pin which softens the signal's spikes as you can see below (C=33nF for a VRPROPI=3k9), but it's not ideal as I'd like a very accurate representation of motor current since it's used in a feedback loop.

    Any recommandations on snubber or any other type of circuits to get rid of the switching mosfet noise ?

    Regards, Florian

  • 0 in reply to Florian Martin
    TI__Mastermind 44325 points

    Hi Florian,

    Thanks for the additional information.

    Florian Martin said:
    I'm using a 50% to 70% duty cycle PWM with a 80kHz frequency, and the spikes vary along PWM frequency. The motor is alternatively changing direction every 250ms. 

    I assume you are not using on-chip current regulation with this fPWM. Be sure to have this disabled. The regulation is done with fixed tOFF 25 μs. Including 10 μs for tON for example this chopping frequency is not suitable for 80 kHz fPWM - TPWM = 12.5 μs. 

    Florian Martin said:
    However, there’s a fair big among of noise I can zoom into which seems to be coming from the switching section of the chip since it's at the same frequency of my PWM.

    While switching the output drive with PWM corresponding PWM noise on the output is hard to eliminate. However, I'd expect di/dt of the noise to be mitigated by the inductance of the motor. You could try using RC snubbers on the OUT1 and 2 pins to GND and a capacitor 0.1 uF between OUT1 and OUT2 parallel to the motor and see if it mitigates the switching noise.  

    Regarding fast switching diodes, you could try with these as well. For example Schottky diodes in parallel to each MOSFET's internal body diode on OUT1 and 2 and VM and GND correspondingly. Thank you.

    Regards, Murugavel