The N-MOSFET devices used contain a diode which will divert current away from the MOSFET once its forward voltage is reached. Since the current is large enough to cause the source-drain voltage (Vsd) to exceed the forward voltage of the integral diode. Current then begins to flow through the diode, which lessens the amount of current flowing through the MOSFET transistor.
Refer to the attached example SLX file, which can better illustrate this phenomenon. The following two scenarios can happen in the attached model:
1)
If the load current is small (in the example, set R = 5 ohm, peak AC current is then ~30A), the negative current only flows through the MOSFET.
No current flows through the diode because the voltage across the diode is less than its forward voltage.
When viewed in the Simscape Results Explorer, the 'I' and 'IHold' signals are consistent with the load current.
2)
If the load current is large (in the example, set R = 1 ohm, peak AC current is then ~150A), the negative current flows through the MOSFET, _initially_.
With the increase of this negative current, the voltage across S and D of the MOSFET (Vsd) increases.
When Vsd is higher than the diode's forward voltage, the diode will conduct current.
When viewed in the Simscape Results Explorer, the 'I' and 'IHold' signals now appear to be inconsistent with the load current, as some current has been diverted through the diode.
Best Answer