I'm having some issues with a specific 24V brushed DC motor I'm PWM controlling. I'm seeing some rather nasty voltage spikes and ringing on every PWM on and off cycle.
All scope measurements are made with a differential probe across the motor at DO19+ and DO19-
At first I thought it was the internal diodes of the MOSFETs that couldn't handle the inductive spike, so I added another flyback diode directly across the motor - no difference.
Then I tried replacing the 0 Ohm gate resistors with 50 Ohm resistors to slow down the on/off time of the MOSFETs - no difference. (Maybe 50 Ohms was a bit on the low side?)
So I tried a different 24V brushed DC motor (no detailed data on this motor unfortunately) very similar to the first one, except this one had a lower gear ratio - so less resistance:
Everything looks great, so I'm slightly confused and left wondering what to do about the first motor?
Any hardware changes that I can make so the first motor will look as good as the 2nd motor?
After removing the ferrite beads FB3 + FB4 it looks a lot better. I think Marko is on to something regarding the LC resonance.