Step Up Down Choppers - Engineering Notes

A chopper is a static circuit that converts fixed dc input voltage to a variable dc output voltage directly. As choppers involve one stage conversion, these are more efficient.

Step Up / Step down Chopper

Step Up Down chopper Circuit diagram

• When chopper is ON, current flows from source Vs to CH, L and back to V.

 

Step Up down waveform

• Energy stored in inductor during Ton, Win = Vs. Ton (I1 + I2 ) / 2.
• When chopper is OFF, inductor current tends to decrease by KVL VL - Vo = 0.

 

 

VL = Vo

Woff = Vo . Toff (I1 + I2) / 2

• Inductor stored energy now discharges through the path, load, diode D and L during the time chopper is off.
• Assuming the system to be lossless, the energy balance equation 

Win = Woff

Vs .Ton = Vo . Toff

Vo = Vs . ( duty cycle / 1 - duty cycle)

• for step down chopper, Vo = duty cycle * Vs
• for step up chopper,  Vo = ( 1/ 1 - duty cycle) Vs
• for step up / down chopper, Vo = Vs . ( duty cycle / 1 - duty cycle)
• For 0 < duty cycle < 0.5, circuit works as step down chopper.
• For 0.5 < duty cycle < 1, it operates as a step - up chopper.