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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.

Motor 1 Motor 1 zoom low to high Motor 1 zoom high to low

All scope measurements are made with a differential probe across the motor at DO19+ and DO19-

Motor control circuit

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:

Motor 2

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?

Motor 1 data

UPDATE!

After removing the ferrite beads FB3 + FB4 it looks a lot better. I think Marko is on to something regarding the LC resonance.

After removing FB3 and FB4

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  • \$\begingroup\$ How about a fast diode straight across the motor terminals? EDIT: sorry, you are changing direction too with your H-bridge. How’s your free-wheeling situation? Can you sacrifice some losses and add a snubber? Do you see any difference if you put fast Schottky diodes in parallel with your MOSFETs? \$\endgroup\$
    – winny
    Feb 17, 2020 at 11:27
  • \$\begingroup\$ different motor "so less resistance" ... if you mean less torque load, that implies lower drive current. Have you measured the currents? If you increase the torque load on the second motor, do the spikes come back? \$\endgroup\$
    – user16324
    Feb 17, 2020 at 11:50

2 Answers 2

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One possibility is that you placed MOSFETs too far from IC and you have parasitic inductivity. As the problem persist when adding gate resistance, it would mean that ringing is not an artifact of gate driver circuitry, rather a load problem.

Further you say, that when changing the motor with a different one, this problem vanishes. In my opinion, you have kind of capacitor mounted in between brushes, like EMI filter. With your series inductor this makes a LC circuit with its own resonance as you see on the scope.

EDIT:

Further, it seems some weird form of oscillations, like peaking generator. Could be that you series chokes are saturated?

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  • \$\begingroup\$ It would seem you were correct regarding forming a LC circuit :) See update in post. \$\endgroup\$
    – Martin Augustinus
    Feb 17, 2020 at 14:01
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It is worth considering the need for RC snubbers on your FETs (across the drain/source terminals). Does the ring frequency stay the same with the two motors? if so I would investigate snubbers. Nexperia datasheet AN11160 has good coverage of this. https://assets.nexperia.com/documents/application-note/AN11160.pdf

One thing that that datasheet leaves out is that the resistor in the RC snubber will dissipate heat from the ringing that it absorbs. This is approximately P=CFE^2, where C is the snubber capacitor, E is the voltage being switched by your circuit, and F is the PWM frequency.

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