20-03-2012, 01:04 PM
DC Choppers
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Introduction
Many industrial applications require power from dc voltage sources. Several of these applications, however, perform better in case these are fed from variable dc voltage sources.
The conversion of fixed DC voltage to an adjustable DC output voltage, through the use of semiconductor devices is called chopping. A chopper is a static device that converts fixed dc input voltage to a variable dc output voltage directly. It is a high speed on/off semiconductor switch (connects source to the load and disconnects the load from the source at a fast speed).
The power semiconductor device used for a chopper circuit can be force commutated thyristor, power BJT, power MOSFET, GTO or IGBT.
Principle of Step Up operation
The step up choppers are used to obtain output voltage greater than the input voltage. The basic circuit for step up operation is shown below
Class A chopper:
It is a step down chopper in which the average value of output voltage and current is always positive. A free wheeling diode (FWD) is connected across the load to prevent the output current being negative. Its region of operation is in the first quadrant only, so it is also called first-quadrant chopper. The circuit and the waveforms are shown below.
Class B chopper:
In this type the average output voltage is positive but the average output current is negative. Therefore the class B chopper operates in the second quadrant. Power flows from the load to the source. It is a step up chopper. It is widely used for regenerative breaking of DC motors. Its circuit and waveforms are shown below:
Class C choppers:
Class C Chopper is a combination of Class A and Class B Choppers. For first quadrant operation, CH1 is ON or D2 conducts. For second quadrant operation, CH2 is ON or D1 conducts. When CH1 is ON, the load current is positive. The output voltage is equal to ‘V’ & the load receives power from the source. When CH1 is turned OFF, energy stored in inductance L forces current to flow through the diode D2 and the output voltage is zero. Current continues to flow in positive direction. When CH2 is triggered, the voltage E forces current to flow in opposite direction through L and CH2 .The output voltage is zero turning OFF CH2, the energy stored in the inductance drives current through diode D1 and the supply Output voltage is V, the input current becomes negative and power flows from load to source.