Chopper (DC to DC Converter)

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Chopper

                   Chopper is a converter which is convert fixed DC to variable DC voltage .
  • When output voltage is less then the input voltage its called Step-down chopper(Vo < Vs).
  • When output voltage is greater  then the input voltage its called Step-up chopper(Vo > Vs).

Two types of Chopper.

  1. Step-up Chopper (Boost Converter).
  2. Step-Down Chopper (Buck Converter).

 Step-up Chopper (Boost Converter)

  • Step-up chopper convert the low DC voltage to high DC voltage.
  • Its also known as DC booster.
  • When output voltage is less then the input voltage its called step-down chopper(Vo < Vs).
  • We can use SCR , MOSFET , IGBT instead of CH(switch).
  • Output voltage appear across R.
  • Large capacity inductor is connected in series with the supply voltage. Booster Configuration is shown in fig:
Stepup chopper, booster
Fig: Stepup converter (booster)

Working:-

Stepup chopper
Fig 1 :
Stepup chopper
Fig 2 :
  • At the first instant chopper switch is open.
  • Now close the switch and current flowing through path Vs – L – CH – Vs. shown in fig 1:
  • During this period inductor store the charge. that stored charge is given by the L(di / dt).
  • After that at next instant open that switch again.
  • Hence the current flowing through path during this mode is Vdc – L – D – R – Vdc. shown in fig 2:
  • During this time inductor will dissipate the charge which it has store during first instant.
  • Therefore, the output voltage is given by the summation of  supply voltage and the charge dissipated by the inductor both.
  • Output voltage is given by  Vo = Vs + L(di / dt).

 

Total time (T) = Ton + Toff (chopping period)

During the period Ton ,

Won = voltage across L *average current through L * Ton

= Vs * ((I1 + I2)/2)*  Ton…………………………….equ(1)

During the period Toff,

Woff = voltage across L *average current through L * Toff

= (Vo-Vs) * ((I1 + I2)/2)*  Toff………………………..equ(2)

compare equ (1) & (2);

Vs * ((I1 + I2)/2)*  Ton =  (Vo-Vs) * ((I1 + I2)/2)*  Toff

Vs *Ton =  (Vo-Vs) * Toff

Vs *Ton =  Vo*Toff  -Vs *Toff

Vo*Toff = Vs *Ton + Vs *Toff

Vo*Toff = Vs(Ton+Toff)

Vo*Toff = Vs(T)

Vo = Vs*(T/Toff)

Vo = Vs *T/(T-Ton)

Vo = Vs(1/(1-α))………………………………….equ (3)

where,  (T/Ton)= α

  • α is a ratio of Ton and T. its called Duty cycle.
  • when value of α = 0 , output voltage = supply voltage.(Vs = Vo).
  • by changing the value of α we can change value of output voltage.
Waveform:
waveform of Stepup chopper
fig : waveform of Stepup converter

if any query about this DC converter

comment below…………………. ?

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