MATLAB: Internal loss equation currents in N-MOSFET device is cut off in the negative direction but not in the positive direction

currentdiodeinconsistentn-mosfetpowerSimscapeSimscape Electricalsystems

Why is the current inside the loss equation of a N-MOSFET transistor cut off on the negative side but not the positive side of the curve?

When simulating a network of MOSFET transistors in Simscape Power Systems, why are the currents internal to the loss equation (namely 'I' and 'IHold' in the Simscape Results Explorer) cut off? The N-MOSFET network is supplied by a sinusoidal current, and the positive end of the current plot is not cut off.

Refer to the attached screenshot to see this current attenuation effect.

Best Answer

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:

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.

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.

Related Solutions

MATLAB: Can I use the Mosfet block in the SimPowerSystems library without the internal diode

It is possible to use the Mosfet block of SimPowerSystems Version 4.1 (R14SP3) without the internal diode. You can use a simple Ideal Switch block to model the Mosfet block without the internal diode.

Refer to the attached model as an example.

MATLAB: Impedance Measurement for a complex circuit

1) The 'Impedance Measurement' block does not directly connect with Simscape language based blocks as it is from the Specialized Power Systems library. Elements from this library are implemented as Simulink blocks and not directly compatible with Simscape language blocks.

When modeling with the SPS library, you can using the 'powergui' block to measure the impedance. In 'powergui', browse to the ‘Tools>>Impedance measurement’ option. You then need to connect the 'Impedance measurement’ block across the load you would like to measure. You can then select your desired options/settings to view the ‘Impedance Vs Frequency’ graph and load the data to workspace.

Here is an example from the documentation illustrating this approach:

https://www.mathworks.com/help/physmod/sps/examples/steady-state-analysis-of-a-linear-circuit.html

2) To find the impedance of a complex circuit with Simscape Electrical blocks with RLC, another workaround would be to represent the following mathematical equation in Simulink and then extracting the impedance:

Z = R+j*X = Phasor_V / Phasor_I = |V|/|I| * exp(j*(Vang - Iang))
=  |V|/|I| * ( cos(Vang - Iang) +1j*sin(Vang - Iang) )
Phasor_V = |V|exp(j*Vang)
Phasor_I = |I|exp(j*Iang)

Set the initial conditions of the denominator i.e. the current to a small value so that you do not end up with a 'x/0' condition. Thus the actual output would begin after the initial condition has ended. Attached is a model that represents the above equation in Simulink.

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Related Solutions

MATLAB: Can I use the Mosfet block in the SimPowerSystems library without the internal diode

MATLAB: Impedance Measurement for a complex circuit

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