DRV8876-Q1: Switching and Motor Current

Labib Labib

Part Number: DRV8876-Q1

We are using DRV8876QPWPRQ1 full bridge driver to drive a brushed DC motor. The brush DC motor has 3 Ohm coild resistance and 13mH inductance. Also, the is also ring varisor across the motor terminal. I have attached the specs of the ring varistor.

We are driving bridge driver in PH/EN Control Mode. I got smother switching across the motor terminal However, the motor current has negative spike. I have atached the ssnapshot here with 55% PWm duty cycle and 20kHz switching. (Green- motor terminal voltage, Blue - Motor Current)

My question is there any issue with the bridge driver configuation? We do braking during off state cycle of the PWM. Let me know if you need any additional question.

Ring varistor 165A E10=17-26v.pdf

8 months ago

0 Jacob Thompson 8 months ago

TI__Mastermind 21980 points

Hey Labib,

I believe this is expected behavior, and the -0.08V spikes is safely within our spec of -0.9V. Looks fine to me!

Best,

Jacob

0 Labib Labib 8 months ago in reply to Jacob Thompson

Prodigy 30 points

Hello Jacob, I am okay with the volatge. It is related to diode's drop. I am not convinced about the negative spikes in current. During braking (off state of the PWM), the motor current is forced to flow in the same direction. Did I miss anything here?

0 Jacob Thompson 8 months ago in reply to Labib Labib

TI__Mastermind 21980 points

Hey Labib,

You can add some capacitors on the output to help reduce the negative voltage spike, see this technical article: How to Achieve Higher System Robustness in DC Drives, Part 1: Negative Voltage

Actually I just found this article that is even better for this application: [FAQ] How to control voltage spikes on motor driver outputs?

I recommend 1nf from OUTx to GND, and a 100nF between OUT1 and OUT2.

Admittedly I am not very familiar with Ring Varistors so I don't know how that might affect this.

Best,

Jacob

0 Labib Labib 8 months ago in reply to Jacob Thompson

Prodigy 30 points

Hello Jacob,

Thank you for your feedback. I am not concerned of the negaive voltage (green trace). It is due to diode drop. My main concern is to having negative spikes in the current waveform (blue trace). I was expecting sawtooth waveform for the motor current.

I tried Y-Capacitor, it doesn't help to reduce negative current spikes,

How to Achieve Higher System Robustness in DC Drives, Part 1: Negative Voltage

The link doesn't work. Please provide me correct link.

0 Jacob Thompson 8 months ago in reply to Labib Labib

TI__Mastermind 21980 points

Hey Labib,

Can you try repeating your test without the Ring Varistor? I did some more research on them and found that the ring varistor across the motor terminals can also contribute to the observed spike. When a voltage change occurs across the motor terminals, the varistor initially acts as a high impedance until the voltage exceeds its breakdown voltage. At this point, the varistor conducts, clamping the voltage across the motor. This sudden change in impedance can cause ringing in the current waveform, leading to negative spikes.

The relatively high inductance (13mH) of the motor can also contribute to the negative spike. When the motor current is suddenly interrupted during braking, the stored energy in the inductor releases as a negative spike in the current.

Labib Labib said:
I was expecting sawtooth waveform for the motor current.

The waveform you showed is a normal current waveform for a brushed DC motor at PWM duty cycle. It shouldn't be sawtooth. If you increased your PWM duty cycle you could get closer to sawtooth, but it likely won't improve your performance and will result in worse thermals

Working link: https://www.ti.com/document-viewer/lit/html/SSZT741

Best,

Jacob

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DRV8876-Q1: Switching and Motor Current