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Choppers
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Principle & Working of Buck Converter ( Step-Down Chopper )

In the previous articles we have seen basic operation and principle of a chopper.In this tutorial we will see Principle of Step-Down Chopper (Buck-Converter) with a neat sketch of circuit diagram

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Buck-Converter ( Step-Down Chopper ) Principle & Working

In general, d.c. chopper consists of power semiconductor devices (SCR, BJT, power MOSFET, IGBT, GTO, MCT, etc., which works as a switch), input d.c. power supply, elements (R, L, C, etc.) and output load. (Refer below figure). The average output voltage across the load is controlled by varying on-period and off-period (or duty cycle) of the switch.
Buck Converter ( Step-Down Chopper ) Circuit Diagram
Buck Converter ( Step-Down Chopper ) Circuit Diagram

Buck-Converter ( Step-Down Chopper )  Operation

A commutation circuitry is required for SCR based chopper circuit. Therefore, in general, gate commutation devices based choppers have replaced the SCR based choppers. However, for high voltage and high-current applications, SCR based choppers are used. The variations in on and off periods of the switch provides an output voltage with an adjustable average value. The power-diode (DP) operates in freewheeling mode to provide a path to load-current when switch (S) is OFF. The smoothing inductor filters out the ripples in the load current. Switch S is kept conducting for period Ton and is blocked for period Toff. The chopped load voltage waveform is shown in figure.
Buck Converter ( Step-Down Chopper ) Output
Buck Converter ( Step-Down Chopper ) Output
During the period Ton, when the chopper is on, the supply terminals are connected to the load, terminals. During the interval Toff, when the chopper is off, load current flows through the freewheeling diode DF. As a result, load terminals are short circuited by DF and load voltage is therefore, zero during Toff. In this manner, a chopped dc. voltage is produced at the load terminals.

From output of buck converter figure, the average load-voltage E0 is given by
From Eq 1, it is obvious that the output voltage varies linearly with the duty-cycle. It is therefore possible to control the output voltage in the range zero to Edc.
If the switch S is a transistor, the base-current will control the ON and OFF period of the transistor switch. If the switch is GTO thyristor, a positive gate pulse will tum-it ON and a negative gate pulse will turn it OFF. If the switch is an SCR, a commutation circuit is required to turn it OFF.
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What Is Chopper ? Classification Of DC To DC Power Converters/Choppers

What Is Called A DC Chopper ? 

A d.c. chopper is a static device (switch) used to obtain variable d.c. voltage from a source of constant d.c. voltage.
In the right below figure you can see circuit diagram of a chopper. Therefore, chopper may be thought of as d.c equivalent of an ac. transformer since they behave in an identical manner. Besides, the saving in power, the dc.chopper offers greater efficiency, faster response, lower maintenance, small size, smooth control, and, for many applications, lower cost, than motor-generator sets or gas tubes approaches.
Chopper

Solid-state choppers due to various advantages are widely used in trolley cars, battery-operated vehicles, traction-motor control, control of a large number of d.c. motors from a common d.c. bus with a considerable improvement of power factor, control of induction motors, marine hoists, forklift trucks and mine haulers. The objective of this chapter is to discuss the basic principles of chopper operation and more common types of chopper configuration circuits.

Classification Of  Choppers or DC To DC Power Converters

DC choppers can be classified as:

(A) According to the Input/Output Voltage Levels
(i) Step-down chopper: The output voltage is less than the input
(ii) Step-up chopper: The output voltage is greater than the input
voltage.

(B) According to the Directions of Output Voltage and Current
(i) Class A (type A) chopper
(ii) Class B (type B) chopper
(iii) Class C (type C) chopper
(iv) Class D (type D) chopper
(v) Class E (type B) chopper
The voltage and current directions for above classes are shown in below figure.

Chopper

(C) According to Circuit Operation
(i) First-quadrant chopper: The output voltage and both must be positive.(Type A).
(ii) Two-quadrant chopper: The output voltage is positive and current can be positive or negative (class-C) or the output current is positive and the voltage can be positive or negative (class-D).
(iii) Four-quadrant chopper: The output voltage and current both can be positive or negative (class-E).

(D) According to Commutation Method
(i) Voltage-commutated choppers
(ii) Current-cornmutated choppers
(iii) Load-commutated choppers
(iv) Impulse-commutated choppers

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