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I'm currently working on a simple driver for a DC motor, only one rotating direction with PWM control. For information, PWM frequency is 1kHz.

I have some problems with overvoltage at the terminals of MOSFET. Those overvoltages are very short and I can not attenuate them. I tried with zener diodes in parallel to power supply but they do not seem fast enough.

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This voltage measurement is taken between the drain and the source of the MOSFET.

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Is there a solution to smooth this voltage spikes? Or is it something acceptable? I used this circuit with a labo power supply.

Thank you in advance for helping me to improve this circuit. Regards.

Edit 1:

I did some tests in order to increase the turn-off time. This is the new diagram, I changed the value of the resistor R2 and I found that a bigger resistance decreased the overvoltage spikes! I need to continue to search but that may be a solution.

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  • \$\begingroup\$ Can you take a picture of how the diode is attached to the motor terminals? \$\endgroup\$ – pgvoorhees Jun 27 '16 at 18:47
  • \$\begingroup\$ Is it something acceptable? The IRF3205 is avalanche rated, to 62 amps and 20mJ. Do the sums. You may have a problem with your use of the word 'simple'. Use of this word may have led you to overlook fundamentals like loop area when placing protection devices like zeners. Zeners are quire quick, connections with excess length, current loops with excess area, can be slow. /edit/ there you go, mr pgvoorhees above is asking you about loop area, or is at least asking to see it! /edit/ \$\endgroup\$ – Neil_UK Jun 27 '16 at 18:47
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    \$\begingroup\$ This seems like it might be measurement artifact to me. D1 is there in the schematic pretty much exactly to provide a path for the reluctant (?) current when the FET shuts off... \$\endgroup\$ – vicatcu Jun 27 '16 at 18:54
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    \$\begingroup\$ You might also want to verify the functionality of D1. \$\endgroup\$ – Tut Jun 27 '16 at 19:35
  • \$\begingroup\$ Thanks you all for your answer! You are right, the current installation is clearly not perfect: the cables between the MOSFET/diode and the motor are ~0.5m long and the all the components are mounted on a protoboard. This is of course an issue but not the only one I think... \$\endgroup\$ – Yann60 Jun 30 '16 at 17:41
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Wow! That is a huge ring wave caused by the mosfet switching OFF, and the motors high inductance dumping its current back into the mosfet drain.

Your clamp diode should be directly across the motor contacts for best clamping effect. M1 and C1 should have the shortest connection path possible, with the motor (+) terminal tied as close to C1 (+) as possible. Try a 330pF 100volt ceramic capacitor across the motor to dampen the ring waves intensity. It only last 5uS, which is not bad. You cannot make the ring wave and initial overshoot go away-only dampen them.

Every SMPS has them to some degree. The idea is to keep the overshoot down by -3db if possible, to protect the mosfet and the clamp diode. This also shortens the duration of the ring wave. Increase the value of Rload if possible to trap the overshoot at the motor and clamp diode. Try to keep the overshoot equal to the supply voltage +10% at most.

EDIT: Are you sure Q2 is wired correctly? If it is a PNP the emitter should be connected to R2. It is designed to dump the gate charge of the mosfet quickly. If this is wired wrong, that could explain much of the massive overshoot and ring wave, as the mosfet is not being switched OFF correctly.

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  • \$\begingroup\$ Whoops you're right, the transistor Q2 was mounted upside down! The sign in the right direction has not changed the operation ... I'll try your recommendations and would keep you informed thank you. \$\endgroup\$ – Yann60 Jun 30 '16 at 17:44
  • \$\begingroup\$ I'm glad your experimenting to solve your own problems. You have discovered that R2 greatly affects ringing (of lack of) at the mosfet drain pin. I use 100 ohms unless it needs to switch faster. Keep R2 as close to the mosfet gate as possible. Move C2 and D1 so they connect directly to the motor. \$\endgroup\$ – Sparky256 Jun 30 '16 at 20:01

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