what is the cause of these oscillations and how I can prevent them

[reinholdPL]

Hello, I made a simple DC motor control circuit. At maximum load, the motor consumes about 4A.
Arduino provides a PWM signal with a frequency of 62.5 kHz.
Everything works fine, nothing gets hot, the system does its job.

However, on the oscilloscope I can see quite large oscillations on the drain.
Interestingly, it changes quite significantly depending on duty of PWM signal fed to the gate.
The ringing disappears completely when duty is over 70%.
Circuit is powered from 12V car battery.
How can I prevent such oscillations?

Yellow signal is the drain voltage, purple is the gate voltage. Thank you for any help.

[mvs]

However, on the oscilloscope I can see quite large oscillations on the drain.
Interestingly, it changes quite significantly depending on duty of PWM signal fed to the gate.
The ringing disappears completely when duty is over 70%.
Circuit is powered from 12V car battery.
How can I prevent such oscillations?

It is back-EMF. You can add a 100n film or ceramic cap across motor terminals to mitigate ringing.

Duty cycle plays a role, because there are two different modes of operation, if you look at motor winding (inductor) current.
In continuous conduction mode (CCM) there are two phases in a cycle:
1 Switch open, diode closed, motor winding current is flowing throught switch.
2 Switch closed, diode open, motor winding current is flowing throght diode.

In discontinuous conduction mode (DCM) there is aditional third phase in a cycle:
3 Switch closed, diode closed, motor winding current is zero.

In phases 1 and 2 the drain node is connected either to gnd or to +12V, so you can't see back-EMF on scope. In phase 3 this node is in high-Z (between rails) and you can see ringing on scope.

[reinholdPL]

thank you for your answer. so if I understand correctly this aditional third phase starts when oscillations shows up on oscilloscope and ends when switch (mosfet) opens and first phase begins again, right?

could this do any harm to my circuit?

[Mechatrommer]

why dont you utilize both IN A and B to get stronger driver? or make traces easily switched between both in case the first driver burnt?

[Benta]

thank you for your answer. so if I understand correctly this aditional third phase starts when oscillations shows up on oscilloscope and ends when switch (mosfet) opens and first phase begins again, right?

could this do any harm to my circuit?

The oscillations start when no current is flowing in either switch or diode. The inductance is then in a high-impedance environment and together with the inter-winding capacitance forms a parallel resonant circuit.

The oscillations are harmless and low power.

[reinholdPL]

why dont you utilize both IN A and B to get stronger driver? or make traces easily switched between both in case the first driver burnt?

Thanks for suggestions. This was just test circuit to see if everything will work like it supposed to do. I have another pcb where I have utilized both channels.