Ideal Switch (Power System Blockset)

Implement an Ideal Switch model

Library

Description

The Ideal Switch does not correspond to a particular physical device. When used with appropriate switching logic, it can be used to model a simplified semiconductor device such as a GTO or a Mosfet, or even a power circuit breaker with current chopping. The switch is simulated as a resistor (Ron) and inductor (Lon) in series with a switch controlled by a logical signal G.

The Ideal Switch 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 zero current flow when G=0
Conducts any bidirectional current with quasi zero voltage drop when G>0
Switches instantaneously between on and off state when triggered The Ideal Switch turns on when a positive signal is present at the gate input (G>0). It turns off when the gate signal equals zero (G=0).

The Ideal Switch block also contains a series Rs-Cs snubber circuit, which is usually connected in parallel with the Ideal Switch. 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 Ideal Switch icon.

Dialog Box

Because of modeling constraints explained in the Assumptions and Limitations section, the inductance Lon cannot be set to zero. If the switch initial state is set to 1 (closed), the states of the linear circuit are automatically initialized so that the simulation starts in steady-state.

Inputs and Outputs

The first input (1) and output (2) are the Ideal Switch electrical connections. The second input (g) is a Simulink signal applied to the gate. The second output (m) is a Simulink measurement output vector [I12,V12] returning the Ideal Switch current and voltage.

Assumptions and Limitations

It can happen that the switch is connected across a capacitor. Depending on the values of the capacitor, switch resistance and inductance, high frequency poorly damped current and voltage oscillations can be produced when the switch closes if the Ron-Lon parameters are not properly selected. This will result in a slow simulation speed. A good practice is to achieve a damping factor of z>0.5 for the Ron-Lon-C circuit. This condition is obtained when

For example, with Ron =0.01

and C= 1e-6 F, you should select Lon

1e-10 H.

The Ideal Switch is modeled as a nonlinear element interfaced with the linear circuit, as shown below.

To avoid an algebraic loop, the Ideal Switch inductance Lon cannot be set to zero. Each Ideal Switch adds an extra state to the electrical circuit model. The Ideal Switch 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 Ideal Switches. ode23tb and ode15s usually give best simulation speed.

Example

An Ideal Switch is used to switch an RLC circuit on an AC source (60 Hz). The switch, which is initially closed, is first open at t=36 ms and then closed at t=100 ms.

As the switch is used to switch a capacitor, its inductance Lon has been set to a very small value to satisfy the

condition. This example is available in the psbswitch.mdl file.

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.

References

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

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