Pax Writer said:
Hey folks
Thanks for your answers.
The circuit in which the diode is needed can be found here:
https://www.electro-tech-online.com/threads/simple-pwm-motor-controller.33974/
Look at the illustration uploaded by Audioguru (fifth post from below). This shows that a MOSFET is inserted serially with the motor, which also has a diode in parallel to handle the back EMF (thanks for clarifying this term for me, by the way).
The parallel diode of the MOSFET (the intrinsic diode) does NOTHING for the circuit, it is there because MOSFET's always have an anti-parallel diode (it might not be connected for use mind
and its more a zener then a diode). The diode that allows the current in the machine's coil to carry on flowing is teh one parallel to the machine
Replace the machine with an inductor (effectivly what it is) and think about what happens when the FET opens and current is flowing through that inductor AND there is no anti-parallel diode...
THe fundemental equation for an inductor is: V = Ldi/dt.
You have just opened the switch to try to stop the current, in the ideal case it will open instantly forcing di/dt to tend to infinity, via this equation this produces an infinite voltage across the inductor
In practice the switch has a finite switching time to impose a finite di/dt thus producing a finite V. With no anti-parallel diode this V will just increase until it reaches the breakdown voltage of the FET.
With a diode in anti-parallel, when this induced voltage exceeds the voltage the diode needs to conduct it produces a zero-volt loop (well a 0.6V loop) for the current in the inductor to keep flowing
I personally don't like the term "BackEMF" when talking about open-circuiting a charged inductor (less so in cases like this involving electrical machines).
BackEMF is something that an electrical machine will generate when its rotor is rotating, I personally refer to the voltage in this case as the "induced voltage"
so just to claify the anti-parallel diode of the FET does nothing for this circuit (it would still work if it was removed). It is the anti-parallel diode of the motor that is crucial here