Characteristics of Switches
There are many types of power switching devices. Each device, however, has its advantages and disadvantages and is suitable to specific applications. The motivation behind the development of any new device is to achieve the characteristics of a “super device.” The characteristics of any real device can be compared and evaluated with reference to the ideal characteristics of a super device
Ideal Characteristics
The characteristics of an ideal switch are as follows:
- In the on-state when the switch is on, it must have (a) the ability to carry a high forward current IF, tending to infinity; (b) a low on-state forward voltage drop VON, tending to zero; and (c) a low on-state resistance RON, tending to zero. Low RON causes low on-state power loss PON. These symbols are normally referred to under DC steady-state conditions.
- In the off-state when the switch is off, it must have (a) the ability to withstand a high forward or reverse voltage VBR, tending to infinity; (b) a low off-state leakage current IOFF, tending to zero, and (c) a high off-state resistance ROFF, tending to infinity. High ROFF causes low off-state power loss POFF. These symbols are normally referred to under dc steady-state conditions.
- During the turn-on and turn-off process, it must be completely turned on and off instantaneously so that the device can be operated at high frequencies. Thus it must have (a) a low delay time td, tending to zero; (b) a low rise time tr, tending to zero; (c) a low storage time ts, tending to zero; and (d) a low fall time tf, tending to zero.
- For turn-on and turn-off, it must require (a) a low gate-drive power PG, tending to zero; (b) a low gate-drive voltage VG, tending to zero; and (c) a low gate-drive current IG, tending to zero.
- Both turn-on and turn-off must be controllable. Thus, it must turn on with a gate signal (e.g., positive) and must turn off with another gate signal (e.g., zero or negative).
- For turning on and off, it should require a pulse signal only; that is, a small pulse with a very small width tW, tending to zero.
- It must have a high dv/dt, tending to infinity. That is, the switch must be capable of handling rapid changes of the voltage across it.
- It must have a high di/dt, tending to infinity. That is, the switch must be capable of handling a rapid rise of the current through it.
- It requires very low thermal impedance from the internal junction to the ambient RJA, tending to zero so that it can transmit heat to the ambient easily.
- The ability to sustain any fault current for a long time is needed; that is, it must have a high value of i2t, tending to infinity.
- A negative temperature coefficient on the conducted current is required to result in an equal current sharing when the devices are operated in parallel.
- Low price is a very important consideration for reduced cost of the power electronics equipment.
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