Ideal Switch & Practical Switch

The Ideal Switch

It is always desired to have the power switches perform as close as possible to the ideal case. Device characteristically speaking, for a semiconductor device to operate as an ideal switch, it must possess the following features:

1.      No limit on the amount of current (known as forward or reverse current) the device can carry when in the conduction state (on-state);

2.      No limit on the amount of the device-voltage ((known as forward or reverse blocking voltage) when the device is in the non-conduction state (off-state);

3.      Zero on-state voltage drop when in the conduction state;

4.      Infinite off-state resistance, i.e. zero leakage current when in the non-conduction state; and

5.      No limit on the operating speed of the device when changes states, i.e. zero rise and fall times.

The switching waveforms for an ideal switch are shown in Fig. 4.2, where isw and vsw are the current through and the voltage across the switch, respectively. Both during the switching and conduction periods, the power loss is zero, resulting in a 100% efficiency, and with no switching delays, an infinite operating frequency can be achieved. In short, an ideal switch has infinite speed, unlimited power handling capabilities, and 100% efficiency.

The Practical Switch

The practical switch has the following switching and conduction

characteristics:

1.      Limited power handling capabilities, i.e. limited conduction current when the switch is in the on-state, and limited blocking voltage when the switch is in the off-state.

2.      Limited switching speed that is caused by the finite turn-on and turn-off times. This limits the maximum operating frequency of the device.

3.      Finite on-state and off-state resistance’s i.e. there exists forward voltage drop when in the on-state, and reverse current flow (leakage) when in the off-state.

4.      Because of characteristics 2 and 3 above, the practical switch experiences power losses in the on and the off states (known as conduction loss), and during switching transitions (known as switching loss).

Typical switching waveforms of a practical switch are shown in Fig. 4.3a. The average switching power and conduction power losses can be evaluated from these waveforms. We should point out the exact practical switching waveforms vary from one device to another device, but Fig. 4.3a is a reasonably good representation. Moreover, other issues such as temperature dependence, power gain, surge capacity, and over voltage capacity must be considered when addressing specific devices for specific applications. A useful plot that illustrates how switching takes place from on to off and vice versa is what is called switching trajectory, which is simply a plot of isw vs vsw . Figure 4.3b shows several switching trajectories for the ideal and practical cases under resistive loads.