Hence the mosfet circuit will be operated at active region for constant current mode,then what about the constant voltage mode for battery charger applications?

 

The first thing I thin is, The goal of a battery charger is to provide a constant voltage across the battery, not the mosfet. Without a circuit diagram, I can't understand what you are asking.

 

Thanks for reply,
I don't have any circuit diagram, just asking if a mosfet or igbt will provide a constant voltage.

 

Not all by their self. A circuit can provide constant voltage, a shunt regulator can provide constant voltage, but a mosfet or an IGBT can not do this all alone.

 

If by active region, you mean the highlighted region shown here for a typical mosfet:

Image link

Note that in that region, the mosfet behaves like an Ohmic device. As current increases, the voltage drop across the Drain-Source increases and visa versa. Therefore, by itself, it will not provide constant voltage anymore than a resistor would.