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DC Chopper circuits(DC-DC converter)


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A dc chopper converts a fixed-voltage dc source into a variable-voltage dc source. This resource briefs about DC chopper circuits.



A dc chopper converts a fixed-voltage dc source into a variable-voltage dc source
•It can be used to step-down or step-up a dc voltage source.
•Choppers are widely used for traction motor control in electric automobiles, trolley cars, marine hoists, forklift trucks, and mine haulers.
–They provide smooth acceleration control, high efficiency, and fast dynamic response.
–Choppers can be used in regenerative braking of dc motors to return energy back into the supply, and this feature results in energy savings for transportation systems with frequency stops.
•Choppers are used in dc voltage regulators, and also used, in conjunction with an inductor, to generate a current source, especially for the current source, especially for the current source inverter.
Power Flow Control
•Constant-frequency operation.
–The chopping frequency f (or chopping period T) is kept constant and the on-time t1is varied. The width of the pulse is varied and this type if control is known as pulse-width-modulation (PWM) control.
•Variable-frequency operation.
–The chopping frequency f is varied. Either on-time t1or off-time t2is kept constant. This is called frequency modulation. The frequency has to be varied over a wide range to obtain the full output frequencies and the filter design would be difficult.
Classification of Choppers
Class A
•Class B
•Class C
•Class D
•Class E
Class A chopper.
•The load current flows into the load.
•Both the load voltage and the load current are positive.
•This is a single-quadrant chopper and is said to be operated as a rectifier.
Class B chopper
•The load current flows out of the load.
•The load voltage is positive, but the load current is negative.
•This is also a single-quadrant chopper, but operates in the second quadrant and is said to be operated as an inverter.
Class C Chopper
•The load current is either positive or negative
•The load voltage is always positive.
•It is a two-quadrant chopper
•S1 and D2operate as a class A chopper.
•S2and D1operate as a class B chopper.
•Care must be taken to ensure that he two switches are not fired together; otherwise, the supply Vswill be short-circuited.
•A class C chopper can operate either as a rectifier or as an inverter.
Class D Chopper
The load current is always positive.
•The load voltage is either positive or negative,
•A class D chopper can also operate either as a rectifier or as an inverter
Class E chopper
•The load current is either positive or negative
•The load voltage is also either positive or negative.
•It is a four-quadrant chopper.
•Two class C choppers can be combined to form a class E chopper,
•The polarities of the load voltage and load current
•The devices that are operative in different quadrants


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