Step-Up Chopper

Step-Up Chopper

A step-Up chopper is used to obtain voltage higher than the input voltage V. The value of L and C are chosen depending upon the output requirement of the output voltage and current. The basic circuit arrangement of step-up dc chopper is shown beside. When chopper is closed, current will build up in the inductor’L’(energy stored in the inductor) and at the same time, capacitor ‘C’ will get discharged through the load. At the end of ON time, the switch ‘S’(or chopper) will be opened and current i_d will flow through Diode D₁ thus charging the capacitor. Diode D₁ prevents any current flow from capacitor to the source during on time of switch

Expression for output Voltage

Suppose I be the average value of current through the inductor.

Energy delivered to the inductor during ON-time is given by

Ein = V_in I_d t₁

Energy delivered by inductor during OFF-time is given by

E_o = (V_o - V_in) I_d t₂ where V_o represents output voltage, V_in represents input voltage and t₂ represents off period

Now, if the system has no power loss then

Energy delivered by inductor during ON-time = Energy delivered by inductor during OFF-time

Or, V_in I_d t₁ = (V_o - V_in) I_d t₂

Or, V_o t₂ = V_in ( t₁+ t₂)

Or, V_o = V_in ( t₁ + t₂)/t₂

so, as the ratio between ( t₁ + t₂)/t₂ > 1, the output voltage is always greater or equal to the input voltage.

Now, converting it into duty cycle ratio,

Or, V_o = V_in ( T)/(t -t₁)

Or, V_o = V_in ( 1)/(1 -d) where d represents the duty cycle

Again, the expression of the duty cycle also verifies that output voltage is always greater or equal to the input voltage.

References

1) B Classes. CHOPERS(DC-DC Converters). n.d. 10 05 2016. <http://www.bclasses.in/images/material/eee/powerelectronics/newdcchoppers/39.jpg>.