DRV8876: Current Characteristics

Aravind Subramanian

Other Parts Discussed in Thread: DRV8876

Hello,

Using DRV8876 motor driver to drive my Brushed DC motor either in PWM mode at 20 KHz or 40 KHz or in PH/EN mode with slow decay. The coil current falls to zero for every PWM OFF cycle immediately in PWM mode, Current oscillates between negative peak and positive peak in PH/EN mode with slow decay.

This causes huge current ripple and eventually thermal rise due to eddy current loses in my core less brushed DC motor. Is this the expected behaviour of the driver, If not suggest me some changes to get rid of this.

over 3 years ago

0 Pablo Armet over 3 years ago

TI__Mastermind 37683 points

Hi Aravind,

Thank you for posting to the E2E forum.

Can you send me a screenshot of the PWM, output voltage and output current waveforms?

What motor are you using? can you send me the part number?

This device should operate properly as long as the PWM frequency is below 100kHz. If you try to drive a motor at its resonance frequency can cause oscillations on the output current as you are observing.

0 Aravind Subramanian over 3 years ago in reply to Pablo Armet

Prodigy 50 points

Good Day Pablo Armet,

Motor Driver Part Number: DRV8876

Motor Part Number: 22N78 313P

Attached the screenshot of PWM voltage and current waveform in both EN/PH mode with 20 KHz and 40 KHz PWM signals in EN pin for low side slow decay and PWM mode with 20 KHz PWM. Please let me know how to calculate the resonant frequency for a brushed DC motor.

EN/PH mode with 20 KHz (Low side slow decay) - https://postimg.cc/pmRGGrQz

EN/PH mode with 40 KHz (Low side slow decay) - https://postimg.cc/crQkBw3d

PWM mode with 20 KHz - https://postimg.cc/XZrGKKPJ

Thanks in advance.

0 Pablo Armet over 3 years ago in reply to Aravind Subramanian

TI__Mastermind 37683 points

Hi Aravind,

Thank you for the information.

Can you send the images as an attachment. I am not able to open the link as it is restricted by the company's network.

-Pablo

0 Danilo A. over 3 years ago in reply to Pablo Armet

TI__Mastermind 24715 points

Hi Pablo,

For some reason, the customer can't upload the images so I have uploaded them on his behalf.

EN/PH mode 40KHz low side slow decay

EN/PH mode 20KHz low side slow decay

PWM mode 20KHz

Regards,

Danilo

0 Aravind Subramanian over 3 years ago in reply to Pablo Armet

Prodigy 50 points

Hi Pablo,

Sharing image as an attachment din't work for me. Thanks to Danilo from Texas Instrument Technical Support for his timely support.

Please check on the above mentioned concern. Awaiting for your valuable response. Wish you have a great day!!! Thanks.

0 Pablo Armet over 3 years ago in reply to Aravind Subramanian

TI__Mastermind 37683 points

Aravind,

Thank you for sharing the images. There was a recent E2E update which has been causing some problems when uploading images for some people.

Tomorrow is a holiday here in the US so please expect a delay in my response. I will look into your issue in more detail and provide a detailed reply by 4/5.

Thank you in advance for your patience.

0 Grzegorz Pelikan over 3 years ago in reply to Aravind Subramanian

Genius 3580 points

Hi Aravind,

Motor 22N78 313P has R winding = 2.5 Ohm and L winding 0.16mH so L/R constant of that motor

winding is T=L/R = 64us so it would explain shape of current even in slow decay.

The solution probably is to increase PWM frequency and/or add some kind of output filter

L or LC type between driver and motor I guess.

Regards,

Grzegorz

0 Don Dapkus over 3 years ago in reply to Grzegorz Pelikan

TI__Guru**** 189399 points

Our office is closed today for a US holiday as Pablo mentioned.

0 Pablo Armet over 3 years ago in reply to Don Dapkus

TI__Mastermind 37683 points

Hi Aravind,

Grzegorz thank you for your comments. Your explanation makes sense and would explain the current waveform. The LR time constant will determine the current decay rate during the low-side recirculation (slow decay) phase. Increasing the PWM frequency will be the easiest solution. If you require 20kHz or 40kHz for your application, adding the output filter as Grxegorz recommends is one option.

Let me know if this answers your question.

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DRV8876: Current Characteristics