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DRV8312: EMI design tips for motor drivers

Part Number: DRV8312

Tool/software:

Hi Team,

In my custom board, I have both the DRV8873HPWPRQ1 (H-bridge motor driver for brushed DC motors) and the DRV8312 (3-phase brushless motor driver). I now want to make it EMI-free and followed the design guidelines in TI's Design 4: EMI Design Tips from TI Precision Labs for motor drivers.

In the EMI design guide, they suggest adding a common-mode choke and a Pi filter for noise filtering. Could you please recommend the cut-off frequencies for the common-mode choke and the inductor value for the Pi filter?

  • Hi Sanath,

    Thank you for your question.  

    Please allow me to look into a suitable suggestion and follow-up by early next week.

    Best Regards,

    -Joshua

  • Hi Joshua,

    Thank you for the response. I will be awaiting the suggestion.

    Thanks and Regards,

    Sanath

  • Yes, please expect a response within the next 2 days.

    Best Regards,

    -Joshua

  • Hi Sanath,

    In the EMI design guide, they suggest adding a common-mode choke and a Pi filter for noise filtering. Could you please recommend the cut-off frequencies for the common-mode choke and the inductor value for the Pi filter?

    For selecting an optimal cut-off frequency for a common-mode choke you will want to base it above your operating PWM frequency (typically >10x). For example, if your PWM is 20kHz, you will want to cut-off withing a range of 200kHz- 500kHz depending on the observed behavior/noise of your system.

    For the PI filter inductor, you will want the cut-off to be lower than your PWM, so again assuming a PWM of 20kHZ, 10kHz cut-off would be acceptable. 

    Using this formula you can select an appropriate inductor value based on your circuit capacitance choice and a cut-off of 10kHz:

       

    These values I chose are based around a PWM frequency of 20kHZ and can be adjusted in the range around 10-20x (common mode choke) or <0.5x (PI filter)  

    Hope this information is helpful in your design implementation. 

    Best Regards,

    -Joshua

  • Hi Joshua,

    Thank you, This helps a lot. 

    Could you please provide an explanation for why the inductor value needs to be less than 0.5X and the CMC should fall within the range of 10 to 20X? This would help in gaining a better understanding.

    Best Regards,

    -Sanath

  • Hi Sanath, 

    Glad to be of help!  

    Yes I can, please let me write up an additional response and follow-up tomorrow. 

    Best Regards,

    -Joshua

  • Hi Sanath,  

    Following up on my previous answer: 

    The reason it's recommended for the common-mode choke cutoff frequency to be around the range of 10x-20x is mainly due to harmonics that can occur much higher than your actual PWM (20kHz). For example,  at a 20kHz signal a 10x choke would attenuate noise for harmonics at the 200+kHz range while still being far enough away from your 20kHz signal that it doesn't get impeded at all.

    For the pi filter (~0.5x) it is largely the same reasoning: the pi filter aims to attenuate signals higher than the cutoff frequency,  so having it be right below (about half) of your main PWM signals allows the filter to cutoff significantly larger noise spikes and harmonics in the lower range (like the first (40kHz), second (60kHz), and more)

    I hope this helps to deepen your understanding! 

    Thank you again and Best Regards, 

    -Joshua