Power Electronics Chopper Circuit - An Introduction:
The terms DC–DC converters and choppers are one and same. In the texts usually these terms are interchanged .
The Choppers can be operated in either a continuous or discontinuous current conduction mode.
They can be built with and without electrical isolation.
Chopper - Definition:
A chopper is a static device that converts fixed dc input voltage to a variable dc output voltage directly.
- A chopper is considered as DC equivalent of an AC transformer since it behaves in an identical manner.
- The choppers are more efficient as they involve in one stage conversion.
- The choppers are used in trolley cars, marine hoists, forklift trucks and mine hauler.
- The future electric automobiles are likely to use choppers for their speed control and braking.
- The chopper systems offer smooth control, high-efficiency, fast response and regeneration.
- The chopper is the dc equivalent to an ac transformer having continuously variable turns ratio. Like a transformer, a chopper can be used to step down or step up the fixed dc input voltage.
Chopper - Principle of Operation:
A chopper is a high-speed on/off semiconductor switch. It connects source to load and disconnects the load from source at high-speed.
- In other words, the principle of chopper is application of fixed dc voltage intermittently to the load.
- This is achieved by continuously triggering ON and triggering OFF the power switch(SCR) at rapid speed.
- The duration for which the SCR remains in ON and OFF states are called ON time and OFF time respectively.
- By varying the ON time and OFF time of the SCR, the average voltage across the load can be varied.
Applications of Choppers:
- They are used for DC motor control (battery-supplied vehicles), solar energy conversion and wind energy conversion.
- Choppers are used in electric cars, airplanes and spaceships, where onboard-regulated DC power supplies are required.
- In general, Chopper circuits are used as power supplies in computers, commercial electronics, electronic instruments.
Classification of Choppers:
(a) Depending upon the direction of the output current and voltage, the converters can be classified into five classes namely
Class A [One-quadrant Operation] (1st quadrant only)
Class B [One-quadrant Operation] (2nd quadrant only)
Class C [Two-quadrant Operation] (1,2 quadrants only)
Class D [Two-quadrant Operation] (1,4 quadrants only)
Class E [Four-quadrant Operation] (All four quadrants)
In some text books it will be named as Class A, B,C... whereas in some books it will be be Type A, B, C...
(b) Based turn off process (commutation process)
- Natural Commutated Chopper ( Occurs in AC input circuits)
- Forced Commutated Chopper (Occurs in DC input circuits)
The forced commutation type is further classified as
(c) Based on the output voltage of the output, the choppers are classified as
(i) Step-Down Chopper
In this case the average output voltage is less than the input voltage. It is also known as step down converter
(ii) Step-Up Chopper
Here the average output voltage is more than the input voltage. It is also known as step up converter
(iii) Step-Up/Down Chopper
This type of converter produces an output voltage that is either lower or higher than the input voltage
(c) Depending upon the power loss occurred during turn ON/OFF of the switching device, the choppers are classified into two categories namely
(i) Hard switched Converter
Here the power loss is high during the switching (ON to OFF and OFF to ON) as a result of the non zero voltage and current on the power switches.
(ii) Soft switched or resonant converters
In this type of choppers, the power loss is low at the time of switching as a result of zero voltage and/or zero current on the switches.
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