**Principle of Step Down Chopper**

Q. Describe step up and step down chopper with neat diagram.

Q. Describe step-up and step-down chopper.

**Circuit Diagram Step Down Chopper**

- The circuit diagram and the output voltage and current waveforms of a step down chopper are as shown in Figure (a) and (b) respectively. The load is purely resistive.
- When switch S
_{1}is closed for a time T_{ON}the input voltage V appears across the load. If the switch remains off for a period T_{OFF}, the voltage across the load is zero.Figure (a) - Practically any power device can be used as switch These devices have a finite voltage drop ranging from 0.5 to 2 Volts but for the sake of simplicity we shall power of these neglect the voltage drops semiconductor devices, and assume the switch S to be an ideal switch.
Figure (b)

- We can vary average output voltage by using anyone of the control strategies discussed earlier.

**Average Output Voltage of Step Down ****Chopper**

Q. Describe the principle of DC chopper operation. Derive an expression for its average output voltage.

- The average output voltage is given by,

= FT_{ON} V = DV volts

- And the average load current

_{Ldc}= V

_{Ldc}/ R = DV/R

Q. The dc chopper in Figure (a) has a resistive load of R = 10 Ω and the input dc voltage V = 200 Volts. When the chopper switch remains on, its voltage drop is 2 Volts and the chopping frequency is 1 kHz. If the duty cycle is 30% determine :

(a) The average load voltage

(b) Rms load voltage

(c) Form Factor (FF)

(d) Ripple Factor (RF).

Soln. :

Data : f = 1 kHz ∴ T = 1/f = 1 msec, D = 30% or 0.3, V = 200 Volts and V_{ch} = 2 Volts

Average load voltage :

= (V - V_{ch}) x D

Substituting the values

V_{Ldc} = (200 – 2) x 0.3 = 59.4 Volts