Why is there reverse current on Forward Converter demagnetization?

[ckrj21]

Hello,

(no load and no demagnetization winding)

for magnetizing the primary winding, the single MOSFET pulls the terminal to ground. Current starts to increase linearly together with magnetic flux inside the ferrite. Next the MOSFET is switched off and the gate is discharged, and the drain source voltage starts to rise. Once the body diode is in reverse bias the drain-source voltage drops back to ~0 V and a reverse current goes through the primary which discharges to 0 A right until the MOSFET is switched back on again.

So at the transition from on to off of the MOSFET the current completly reverses, but the primary voltage stays mostly the same, since the current change is positive except at the transition.

Now, to my understanding this is not right behaviour. The change of polarity after switching the MOSFET off should mean that the primary inductance has a reverse voltage but the current should still be positive and start discharging towards 0 A.

How could there possibly be negative current?



Since there is no demagnetization winding the core should saturate, could that explain the negative current? Could there be a problem with the measurement at the 3 Ohm shunt resistor at the source pin?

[Faringdon]

Do you mean a single transistor forward?
The attached LTspice shows one that doesnt give current like you show

[ckrj21]

Thank you, LTSpice is very good and pretty much shows the exact same measurements.
I forgot to tell you that I had no load and there is no demagnetization winding present.
Doing that you get the same:



So essentially it is just an inductor now. How can there be reverse current now? Yes there is a high reverse voltage briefly, but it is normally explained that the inductor acts like a source so it should increase the current?

[ckrj21]

yeah ok it makes all sense actually  It was just confusing because I didn't expect the reverse voltage to be able to drive the current into negative values. It just looked like it jumped there. Thanks.

[Faringdon]

OK thanks, here is the one tran forward with the demag winding , in LTspice, as attached.

[T3sl4co1l]

It appears you're in class E.  The flux charged into the inductor, goes into the drain capacitance, which charges back into the inductor, recirculating the energy.  No energy is consumed so the switch returns to negative current, then positive on the next driven cycle.

Tim