MOSFET (SimPowerSystems)

Implement a MOSFET model

Library

Power Electronics

Description

The metal-oxide semiconductor field-effect transistor (MOSFET) is a semiconductor device controllable by the gate signal (g > 0) if its current Id is positive (Id > 0). The MOSFET device is connected in parallel with an internal diode that turns on when the MOSFET device 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 device 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 turns off when the gate input becomes 0. If the current Id is negative (Id flowing in the internal diode) and without a gate signal (g = 0), the MOSFET turns off when the current Id becomes 0 (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 device.
Rt = Rd if Id < 0, where Rd represents the internal diode resistance.

The MOSFET block also contains a series Rs-Cs snubber circuit that can be connected in parallel with the MOSFET (between nodes d and s).

Dialog Box and Parameters

Resistance Ron: The internal resistance Ron, in ohms ().
Inductance Lon: The internal inductance Lon, in henries (H). The Inductance Lon parameter cannot be set to 0.
Internal diode resistance Rd: The internal resistance of the internal diode, in ohms ().
Initial current Ic: You can specify an initial current flowing in the MOSFET device. It is usually set to 0 in order to start the simulation with the device blocked.; If the Initial Current IC parameter is set to a value greater than 0, the steady state calculation of Power System Blockset considers the initial status of the MOSFET as closed. Initializing all states of a power electronic converter is a complex task. Therefore, this option is useful only with simple circuits.
Snubber resistance Rs: The snubber resistance, in ohms (). Set the Snubber resistance Rs parameter to inf to eliminate the snubber from the model.
Snubber capacitance Cs: The snubber capacitance, in amperes (A). Set the Snubber capacitance Cs parameter to 0 to eliminate the snubber, or to inf to get a resistive snubber.
Show measurement port: If selected, add a Simulink output to the block returning the diode current and voltage.

Inputs and Outputs

The input port is a logical signal applied to the gate. The output port is a measurement vector [Id Vds] returning the MOSFET device current and voltage.

Assumptions and Limitations

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

The MOSFET block is modeled as a current source. It cannot be connected in series with an inductor, a current source, or an open circuit, unless its snubber circuit is in use. In order to avoid an algebraic loop, you cannot set the MOSFET block inductance Lon to 0. Each MOSFET block adds an extra state to the electrical circuit model. See Advanced Topics, for more details on this topic.

Circuits containing individual MOSFET blocks cannot be discretized. However discretization is permitted for MOSFET/Diode bridges simulated with the Universal Bridge block.

You must use a stiff integrator algorithm to simulate circuits containing MOSFETs. ode23tb or ode15s with default parameters usually gives the best simulation speed.

Example

The psbmosconv.mdl demo illustrates the use of the MOSFET block in a 0-current quasi-resonant switch converter. In such a converter, the current produced by the Lr-Cr resonant circuit flows through the MOSFET and internal diode. The negative current flows through the internal diode that turns off at 0 current [1]. The switching frequency is 2 MHz and the pulse width is 72 degrees (duty cycle: 20%).

Run the simulation and observe the gate pulse signal, the MOSFET current, the capacitor voltage, and the diode current on the four-trace Scope block. Also observe the state-plane trajectory (inductor current versus capacitor voltage).

References

[1] Mohan, N., T.M. Undeland, and W.P. Robbins, Power Electronics: Converters, Applications, and Design, John Wiley & Sons, Inc., New York, 1995.

See Also
Diode, GTO, Ideal Switch, Thyristor

Linear Transformer Multiband Power System Stabilizer