Duet 2 Mosfet switch off ringing

zapta

@peter_o said in Duet 2 Mosfet switch off ringing:

these effects vanished, when I used a cheap external mosfet board with optocouplers.

Are you sure that that board had an open drain configuration with no additional components attached to the mosfet's Drain pin? Having additional components such as the resistor/diode in the diagram below may suppress voltage changes by the fan when the mosfet is off.

Peter_O

@zapta

Quite sure. The schematics of the Duet 2 seem to be basic:
https://github.com/Duet3D/Duet-2-Hardware/blob/master/Duet2/Duet2v1.04/Duet2_1.04c_Schematic.pdf

No gate driver as in your example.

@alankilian

Alan, in addition to my last post I think we can see cases within my screenshots, where the inductance effects of the motor are high enough to trigger the gate again and swith the mosfet on for some time. That would be a little more than just flapping around.

dc42

@peter_o, 2-wire fans were never intended to work with PWM. So it's a minor miracle that some of them do, and it's no surprise that they generate strange transients when you PWM them.

The ringing of this type of circuit can be controlled to some extent by adjusting the turn-off time of the mosfet. On Duet 2 and Duet 3 this turn off time is controlled by the value of the mosfet gate series resistor. The choice is a compromise. A high value gate resistor leads to a slow turn-off and turn-on, which increases the power dissipation in the mosfet. A low value gives fast turn-on and turn-off, but that may excite ringing in the load (if it is inductive) and generate EMI.

Opto isolators are slow unless used carefully; so your mosfet switch probably has a slow turn-off time. If it's a large mosfet driving a small load such as a fan, it will be able to handle the increased power dissipation.

Peter_O

Hi @dc42 ,
thanks for the explanaitions and your time!

So, if I would insist in bringing that EMI away, I should probably use a 4 wire fan.
Otherwise it's just the way 2 wire fans are kind of running by PWM and I should come to terms with it.

BTW: I never had any doubt, that the duet boards are carefully optimized designs. It's just abount gaining some small understanding on my side.

Thx again!