Why only semiconductor devices in Power electronic circuits will operate as switches?
I do not understand your question. Can you restate it more clearly?
suppose i take the case of inverters/choppers/rectifiers, MOSFET,SCRs Diodes are used as switches.we only control the switch on and off state of these devices to get our desired output.it means all these devices are used only as switches in power electronics circuits?
Ah, I see what you mean. Well, it depends on what you mean by power circuit. I assume you mean power DISTRIBUTION systems, yes? There is, for example, a thing called a "power amplifier" in high fidelity electronics and it uses solid state electronics in active mode.
Perhaps the answer to your question is simple....
What is the power dissipated in a switch ? I2 X R ?
R for a switch being very small means a switch doesn't need much of a heatsink.
The same device operated instead in its linear region has substantial R so can get really hot .
Ideal inductors dissipate no heat so we arrange to let them drop the voltage, instead of using a resistive device.
Switching regulators and converters were rare in 1960's. My first encounter with them was in a 1972 computer power supply..
A device is used as a switch where a switch is needed. A device operates in a linear manner where that type of operation is needed. It has nothing to do with the device and more to do with the application. Notice I did not use the word(s) semiconductor. This is because electron tubes can be used in similar manners.
Not entirely a valid statement - there there are power BJTs and MOSFETs operating in linear - particularly in high power, high precision DC supplies. The basic answer to your question is efficiency. If the device is operating in linear region it is dropping a V across the device -- for any given current, this results in losses. In SMPS - the objective is to switch as fast as possible ( for an instant the device is in lenear mode) and the V x I still has losses.
Not all power electronics are (or at least were) semiconductor devices. An example is the mercury arc valve.
The move to semiconductors is because they are cheaper and more versatile.
see also amplidyne, saturable reactor, magnetic amplifier......
Separate names with a comma.