Mosfet (Power System Blockset)

Implement a Mosfet model

Library

Description

The Metal-Oxide-Semiconductor-Field-Effect-Transistor (Mosfet) block is a semiconductor device controllable by the gate signal (G > 0) if its current Id is positive (Id>0). The Mosfet block is connected in parallel with an internal diode which turns on when the Mosfet block is reverse biased (Vds < 0). The model is simulated as a series combination of a variable resistor (R_t) and inductor (L_on) in series with a switch controlled by a logical signal (G>0 or G=0).

The Mosfet block turns on when the drain-source voltage is positive and a positive signal is applied at the gate input (G >0).

With a positive current flowing through the device, the Mosfet block turns off when the gate input becomes zero. If the current Id is negative (Id flowing in the internal diode) and the gate signal is zero (G = 0), the Mosfet block turns off when the current Id becomes zero (Id = 0).

Note that the on-state resistance Rt depends on the drain current direction:

Rt= Ron if Id > 0, where Ron represents the typical value of the forward conducting resistance of the Mosfet block
Rt= Rd if Id < 0, where Rd represents the internal diode resistance

The Mosfet block also contains a series Rs-Cs snubber circuit, which is usually connected in parallel with the Mosfet block. You can specify a snubber that is purely resistive (Cs = Inf) or purely capacitive (Rs=0). If you specify either Rs=Inf or Cs=0, the snubber is eliminated and it disappears on the Mosfet icon

The initial current Ic flowing in the Mosfet block is usually set to zero, so that the simulation is started with Mosfet blocked. However, you may specify an Ic value corresponding to a particular state of the circuit. In such a case, all states of the linear circuit must be set accordingly. Initializing all states of a power-electronic converter is a complex task. Therefore, this option is useful only with simple circuits.

Parameters and Dialog Box

Inputs and Outputs

The first input and output are the Mosfet connection to drain (d) and source (s). The second input (g) is a logical Simulink signal applied to the gate. The second output is a Simulink measurement vector [Id, Vds] returning the Mosfet current and voltage.

Assumptions and Limitations

The Mosfet block implements a macro-model of the real Mosfet device. It does not take into account either the geometry of the device or the complex physical processes [1].

In the Simulink representation, the Mosfet is modeled as a nonlinear element interfaced with the linear circuit as shown below.

To avoid an algebraic loop, the Mosfet inductance Lon cannot be set to zero. Each Mosfet adds an extra state to the electrical circuit model. Since the Mosfet is modeled as a current source, it cannot be connected in series with an inductor, a current source, or an open circuit, unless a snubber circuit is used.

You must use a stiff integrator algorithm to simulate circuits containing mosfets. ode23tb and ode15s usually gives best simulation speed.

Example

The following example illustrates the use of the Mosfet block in a Zero-Current-Quasi-Resonant Switch converter. In such a converter, the current produced by the Lr-Cr resonant circuit flows through the device, thus causing it to turn on and off at zero current [1]. The switching frequency is 2 MHz and the pulse width is 72 degrees (duty cycle: 20%). This example is available in the psbmosconv.mdl file

Run the simulation and observe the capacitor voltage, the mosfet current, the gate pulse signal, the diode current, and the state-plane trajectory (inductor current versus capacitor voltage).

References

[1] Mohan N., Power Electronic, Converters, Applications and Design, John Wiley & Sons, Inc., New York, 1995.