Choppers are switches that convert a fixed dc supply into a variable dc supply at either a constant frequency or variable frequency.
Because they can operate at higher frequencies, Choppers are mostly Mosfets and IGBTs. This helps to reduce losses and improve efficiency.
This circuit basically acts as a Buck converter or step down chopper, reducing the fixed dc that we supply to a level determined according to duty ratio.
This is the circuit of Boost converter which raises the voltage we supply since we have connected an inductor which stores the energy when switch is ON and releases that energy when switch is OFF.
Combining both is Buck-Boost Convert.