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:
- 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?
- Why do none of the other linear actuators have capacitors, but are somehow still CE compliant?
- Is there any way I can work around this to PWM-drive the brushed DC motor?