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.
- When the chopper is switched ON during TON time, the energy stored in the inductor via path Vdc – L – CH – Vdc.
- 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 TOFF 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
VO = Vdc
+ VL
= Vdc + L (di/dt)
- When chopper is switch ON, the energy stored in the inductor is
Wi = Vdc
I TON.......................(1)
- When chopper is switched OFF, the energy stored in the inductor is transferred to the load.
Wo = ( Vo
– Vdc )I TOFF.........(2)
- If there are no losses in the system, the input energy is equal to output energy
Vdc I TON
= ( Vo – Vdc )I TOFF
Vdc TON
= Vo TOFF –
Vdc TOFF
Vdc ( TON
+ TOFF ) = Vo TOFF
Vo = [ ( TON + TOFF )
/ TOFF ] Vdc
Vo = [ T / TOFF ] Vdc
OR
Vo = [ T / ( T –TON ) ]
Vdc
Vo = [ 1 / ( 1 – TON / T ) ]
Vdc
Vo = [ 1 / ( 1 – K ) ] Vdc
When K = 0 ( chopper
is in OFF condition )
Vo = Vdc
When K = 1 ( chopper
is in OFF condition )
Vo = ∞
- When the duty cycle lies is in the range of 0 < K < 1, the output voltage lies is in the range of Vdc < Vo < ∞.
Application of DC Step up Chopper
- 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.
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