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  

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