Page 100 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Principle of Operation:
The principle of operation of chopper can be understood from the circuit diagram below. The circuit consists of a
semiconductor diode, resistor, and a load. For all type of chopper circuit, the output voltage value is controlled by
periodic closing and opening of the switches used in the circuit.
The chopper can be viewed as an ON/OFF switch that can rapidly connect or disconnect the source to load
connection. Continuous DC is given as source to the chopper as Vs and chopped DC is obtained across the load
as V0.
Output Voltage and Current Waveforms
Above are the output voltage and current waveforms of a chopper circuit. From the voltage waveform, it can be
seen that during the period of TON the load voltage V0 is equal to the source voltage Vs. But when the interval
TOFF occurs, the DC voltage step downs to zero, thus making the load short-circuited.

In the current waveform, it can be seen that during the interval TON the load current rises to the maximum value.
During the interval TOFF, the load current decays. In TOFF condition the chopper is off so, the load voltage
becomes zero. But load current flows through the diode FD, making the load short-circuited.
Thus, the chopped DC voltage is produced at the load. The current waveform is continuous which rises during
TON state and decays during TOFF state.
Classification of Chopper:
Based on their operation principle and type of source voltage chopper are of various types. The main classification
of the chopper is DC chopper and AC Link chopper. Based on the commutation process they are classified as a
natural commutated chopper and forced commutated chopper.
Forced commutated chopper is further classified as Jones chopper, Morgan chopper. Based on output voltage
values choppers are classified as a step down the chopper, step up chopper, step-up/down chopper. Based on the
power loss occurred at switching time choppers are classified as Hard switched and soft switched.
1 AC Link Chopper

In this classification of the chopper, the voltage inversion takes place. Here the DC voltage is converted into AC
with the help of an inverter. Now this AC is passed through a step-down or step-up transformers. The output from
the transformers is again converted into DC by a rectifier. AC link choppers are very bulky and occupy a large
amount of space.

85

CITS : E & H - Electronics Mechanic - Lesson 48 - 55