PWM brushed DC motor noise

Question

I am driving a brushed DC motor (internal to a linear actuator) with a TI DRV8872 motor driver. I am PWMing the motor driver at 20 kHz and varying the duty cycle to control the speed.

The problem is that the motor driver output lines are very noisy on the edge transitions and this noise is getting into the rest of my board; see scope capture of the motor driver input (pink) and motor driver output (blue).

As you can see, the output is not a nice square wave but has weird behavior on the edges.

I tested other brushed DC motors with the exact same setup and did not see this at all. In fact, it was a perfect square wave as expected. So from that, I was able to conclude that my driver setup was correct and the problem was related to the brushed DC motor load (inside of the linear actuator).

I took apart the linear actuator and noticed that the specific one I was using had three 0.1 μF capacitors soldered to it (one inbetween the motor terminals and one from each terminal to ground). As I understand, this is commonly done to prevent the arcing of the brushes from causing noise. The other ones did not have these capacitors. Once I removed the capacitors, everything worked as expected, so it seems the capacitors are causing issues with the PWM drive.

However, since the capacitors are internal to the linear actuator and the company says they are necessary for CE compliance, I can't remove them. My questions are:

1. Why/how are the capacitors causing this weird behavior in the first place? Shouldn't the capacitors actually help by providing a source of energy for the current to quickly pull from?
2. Why do none of the other linear actuators have capacitors, but are somehow still CE compliant?
3. Is there any way I can work around this to PWM-drive the brushed DC motor?

Answer 1

1. It is normal, because \$i_c=C\dfrac{du_c}{dt}\$. You are doing PWM, so \$\dfrac{du}{dt}\$ is very high and the current spikes go to infinity.

2. Not an expert here, but I do think it has nothing to do with CE.

3. You can install additional chokes on both lines to limit \$\dfrac{di}{dt}\$ You can use also a common mode choke that will turn into differential mode if you connect forward and return wires. It has to provide 1% to 3% voltage drop, but you should use a function generator to check for resonance. The PWN frequency have to stay below the resonance frequency.

EDIT:

A rough calculation gives me a two very tiny inductors in range of 2 to 5 micro henries. Without a motor connected, omitting the cable capacitance, then supposing you have 2 x 2.2uH and those three caps of 0.1uF you get:

• Equivalent capacitance 0.1uF + 0.1uF/2= 0.15uF
• Equvalent inductance 2 x 2.2uH = 4.4uH

$$f_r=\dfrac{1}{2\pi\sqrt{LC}} = \dfrac{1}{2\pi\sqrt{0.15\cdot 4.4\cdot 10^{-12}}} $$ $$f_r\approx 196 kHz$$

The resonance frequency does not need to be very far away from your PWM frequency, 30kHz would be good enough.

Doing tests you can protect the motor/cable/caps/driver from overvoltage with fast diodes. This will prevent the oscillations building high voltages.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

just saw this, I'll add my comments. what you are seeing is normal for a brushed motor and is due to the brushes + inductance of the motor armature. If the motor was wire brushes, these make the most 'noise'. Carbon brushes are best and will wear in over time so the spikes will get lower as they wear in.

The caps are a "cheap" way of getting around the CE police. They are only really concerned with emi that occurs at higher frequencies, so the caps do nothing at lower frequency pwm drive.

The normal, tried and true solution is to use a motor snubber circuit which is simply a cap and resistor in series across the motor. A good starting point is .15uf and 2.2 ohms. Make sure you use a film cap rated at least 4X the motor drive voltage. You can tweak these values empirically to lower the spikes, you may never get rid of them. Resistor should be non-inductive, carbon comp is best here..old school.