written 5.8 years ago by |
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 .
When the chopper is ON, the supply current flows through the path Edc+−CH−L−Edc−. 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 −D−L−
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
∴Edc⋅Is⋅Ion=E0IsToff, or E0=Edc⋅TonToff−−−−−(3)
or E0=Edc⋅TonT−Ton=Edc⋅1T/Ton−Ton/Ton
Substituting T on T=α, we get, E0=Edc⋅11/α−1
or E0=Edcα1−α
For 0<α<0.5, the step-down chopper operation is achieved and for 0.5<α<1, step-up chopper operation is obtained.