Step Down Chopper - Engineering Notes

Chopper is a static circuit that converts fixed dc input voltage to  a variable dc output voltage directly.

 

Step Down Chopper:

• During the period Ton, chopper is on and load voltage is equal to source voltage Vs. During the interval Toff, chopper is off, load current flows through the free wheeling diode FD. As a result, load terminals are short circuited by FD and load voltage is therefore zero during Toff.

Step Down Chopper

• During turn on Ton, load current rises where as during turn off Toff  load current decays. 

 

Step Down Chopper waveform

• Average load voltage Vo is given by  

 Vo = Ton . Vs / (Ton  + Toff)

                               = Ton . Vs / T = duty cycle * Vs

 

Vo = duty cycle * Vs

where Ton = on - time; Toff = off - time

T = Ton + Toff = chopping period and 

Duty cycle = Ton / T 

•  Thus load voltage can be controlled by varying duty cycle.

Vo   = f . Ton . Vs

where f = 1 / T = chopping frequency

• Variation of  Ton means adjustment of pulse width, as such this scheme is also called pulse width modulation scheme.
• Average output current, 

Io = Vo / R = duty cycle * Vs  / R