## 14 March 2017

### Working of the Step Up / Step Down Chopper

•  It is a chopper which can work as step up chopper as well as step down chopper.
• When the chopper is switched ON during TON time, the energy stored in the inductor via path Vdc – CH – L – 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 ).
• As the direction of voltage polarity across inductor changes, the stored energy of inductor dissipates via path L – LOAD – D – L.
• When chopper is switch ON, the energy stored in the inductor is

Wi = Vdc I TON.......................(1)
• When chopper is switched OFF, the output energy

Wo = VoI TOFF......................(2)
• The input energy is equal to output energy when there are no losses in the system. Therefore from equation ( 1 ) and ( 2 )

Vdc I TON = VoI TOFF
Vo  = [ TON / TOFF ] Vdc
OR
Vo  = [ TON / ( T  – TON ) ] Vdc
Vo  = [ 1 / ( T / TON – 1 ) ] Vdc
Vo  = [ 1 / ( 1 / K – 1 ) ] Vdc
Where K = Duty cycle = TON / T
Vo  = [ K / ( 1 – K )  ] Vdc
The chopper works as step down chopper if 0 < K < 0.5
Vo < Vdc
The chopper works as step up chopper if 0.5 < K < 1
Vo > Vdc