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Basic principle of Step Up Down Chopper
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A chopper can also be used both in step-up and step-down modes by continuously varying its duty cycle. The principle of operation is illustrated in Fig. below. As shown, the output, polarity is opposite to that of input voltage Edc .

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When the chopper is ON, the supply current flows through the path Edc+CHLEdc. Hence, inductor L stores the energy during the T on period. 

When the chopper CH is OFF, the inductor current tends to decrease and as a result, the polarity of the emf induced in L is reversed as shown in above figure.

Thus, the inductance energy discharges in the load through the path,

L+ Load DL

During T on , the energy stored in the inductance is given by

Wi=E dc IsT on (1)

During T off , the energy fed to the load is

Wo=E0IsT off (2)

For a lossless system, in steady-state: Input energy, Wi= output energy, Wo

EdcIsIon=E0IsToff, or E0=EdcTonToff(3)

or E0=EdcTonTTon=Edc1T/TonTon/Ton

Substituting T on T=α, we get, E0=Edc11/α1

or E0=Edcα1α

For 0<α<0.5, the step-down chopper operation is achieved and for 0.5<α<1, step-up chopper operation is obtained.

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