Step Up Chopper - Basics, Operation, Application

Step Up Chopper - Basics, Operation, Application

In our previous posts, we have discussed about the Basics of Chopper and Classification. Different types of chopper like Class-AClass-BClass-CClass-D and Class-E are covered already.

In this post let us discuss about the step up chopper. Before proceeding further it is recommended to read about the chopper circuit basics...

Step Up Chopper:

A step up chopper provides a minimum output voltage equal to the DC supply voltage and the output voltage higher than DC input can be obtained. Stepup_Chopper_CircuitAs shown in the circuit diagram, in this chopper, a large inductor(L) in series with the source voltage VS is essential to get the voltage higher than the supply voltage

Step Up Chopper Operation:

Mode-1: Chopper is ON

  • When switch SW is ON, the closed circuit path is shown below. VS(+)  – L – SW – VS(-)stepUp_Chopper_operation
  • During this period (TON) the inductor stores energy in it.
  • Now the load current will increase from Imin to Imax and supply voltage is applied to the load VO = VS.

Mode-1: Chopper is OFF

  • When the switch is turned off, the inductor current is forced to flow through the diode and load.
  • Now the closed circuit path will be Vs+ - L – D – Load - Vs-.
  • During this period (TOFF) the polarity of the emf induced in L is reversed as shown.
    (Remember that it is inductor property to oppose sudden change in voltage).
  • Thus the voltage across load is

    Vo = Vs + L.\frac{di}{dt}

    Which is more than supply voltage.
  • With switch SW is OFF, the current would fall from Imax to Imin.
  • Applying KCL during this condition,
    { { V }_{ S }-V }_{ L }+{ V }_{ O }=0
    { V }_{ L }={ V }_{ O }-{ V }_{ S }

In summary during TON, energy stored in inductor & during TOFF stored energy is released to load.
During TON, energy input to the inductor is
Win = (Voltage across L) (average current through L) TON

Won = Vs. \left ( \frac{Imax + Imin}{2} \right ).Ton

During TOFF, energy released by the inductor is
Woff = (Voltage across L) (average current through L) TOFF

Woff = (Vo - Vs). \left ( \frac{Imax + Imin}{2} \right ).Toff

considering the lossless system, these two energies will be equal.

Vs. \left ( \frac{Imax + Imin}{2} \right ).Ton = (Vo - Vs). \left ( \frac{Imax + Imin}{2} \right ).Toff

Vo = Vs.\left (\frac{T}{T-Ton} \right )

Vo = Vs.\left (\frac{1}{1-\alpha} \right )

Form the above equation, the average voltage across the load can be stepped up by varying duty cycle.
If switch SW is always OFF, α =0 and VO=VS.
If switch SW is always ON, α =1 and VO = ∞.
When the SCR is kept ON and OFF for equal amount of time(α = 0.5), the load voltage is 200% the input DC voltage.

In practical condition the switch is turned ON and OFF (ie, α is varying) and the required step up average output voltage is obtained.

The step up chopper principle is utilized in the regenerative breaking of a DC motor.

Read More:

Buck Converter Tutorial 
Switch Mode Power Supply [SMPS]: Introduction and Classification 
Power Electronics Interview Questions and Answers: Set-8 
Rectifier Types Comparison Tutorial 

Thanks for reading about Step Up Chopper operation.... please leave your comments below...

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