Step up Chopper

The step up chopper is one in which output DC voltage is greater than the input DC voltage.
The basic diagram for the step up chopper is shown in the figure A.

Working of Step up Chopper & Duty cycle

When the chopper is switched ON during T_ON time, the energy stored in the inductor via path V_dc – L – CH – V_dc.
The direction of current passing through inductor is shown in the figure B when the chopper CH is switched on.
When chopper is switched OFF during T_OFF time, the current passing through inductor is zero and voltage across inductor is L ( di/dt ).
The stored energy of inductor is transferred to the load.
The circuit diagram of step up chopper during chopper OFF time is shown in the figure C.
The load / output voltage is equal to

V_O = V_dc + V_L

= V_dc + L (di/dt)

W_i = V_dc I T_ON.......................(1)

When chopper is switched OFF, the energy stored in the inductor is transferred to the load.

W_o = ( V_o – V_dc)I T_OFF.........(2)

If there are no losses in the system, the input energy is equal to output energy

V_dc I T_ON= ( V_o – V_dc)I T_OFF

V_dc T_ON= V_o T_OFF– V_dc T_OFF

V_dc ( T_ON + T_OFF ) = V_o T_OFF

V_o = [ ( T_ON + T_OFF ) / T_OFF]V_dc

V_o = [ T / T_OFF]V_dc

V_o = [ T / ( T –T_ON)]V_dc

V_o = [ 1 / ( 1 – T_ON / T )]V_dc

V_o = [ 1 / ( 1 – K )]V_dc

When K = 0 ( chopper is in OFF condition )

V_o = V_dc

When K = 1 ( chopper is in OFF condition )

V_o = ∞

When the duty cycle lies is in the range of 0 < K < 1, the output voltage lies is in the range of V_dc < V_o < ∞.

The application of step up chopper is in the regenerative braking of DC Motor.
The output voltage is greater than the input voltage therefore the DC Motor works as DC generator and load current flows from load to supply side.