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. As 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(-)
• 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

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

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

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

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.