SimPowerSystems

Ideal Switch

Implement an ideal switch device

Library

Power Electronics

Description

The Ideal Switch block does not correspond to a particular physical device. When used with appropriate switching logic, it can be used to model simplified semiconductor devices such as a GTO or a MOSFET, or even a power circuit breaker with current chopping. The switch is simulated as a resistor Ron in series with a switch controlled by a logical g.

The Ideal Switch block is fully controlled by the gate signal (g > 0 or g = 0). It has the following characteristics:

• Blocks any forward or reverse applied voltage with 0 current flow when g = 0
• Conducts any bidirectional current with quasi 0 voltage drop when g > 0
• Switches instantaneously between on and off states when triggered

The Ideal Switch block turns on when a positive signal is present at the gate input (g > 0). It turns off when the gate signal equals 0 (g = 0).

The Ideal Switch block also contains a series Rs-Cs snubber circuit that can be connected in parallel with the ideal switch (between nodes 1 and 2).

Dialog Box and Parameters

Internal Resistance Ron

The internal resistance of the switch device, in ohms (). The Internal resistance Ron parameter cannot be set to 0.

Initial state

The initial state of the Ideal Switch block. The initial status of the Ideal Switch block is taken into account in the steady state calculation of Power System Blockset.

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 farads (F). 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 (g) controls the opening and closing of the switch. The output port (m) is a measurement output vector [Iak Vak] returning the Ideal Switch block current and voltage.

Assumptions and Limitations

The Ideal Switch 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. See the Advanced Topics chapter for more details on this topic.

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

Example

The psbswitch.mdl demo uses the Ideal Switch block to switch an RLC circuit on an AC source (60 Hz). The switch, which is initially closed, is first open at t = 50 ms (3 cycles) and then reclosed at t = 138 ms (8.25 cycles). The Ideal Switch block has 0.01 ohms resistance and no snubber is used.

Run the simulation and observe the inductor current, the switch current, and the capacitor voltage. Notice the high-frequency overvoltage produced by inductive current chopping. Note also the high switch current spike when the switch is reclosed on the capacitor at maximum source voltage.

See Also

Breaker

References

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

Hydraulic Turbine and Governor

IGBT