Choppers

Choppers

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  BUCK-BOOST CHOPPERS Choppers Presented by Sundus HayatKhan Roll no: 683-2015
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  Contents  Introduction  ThePrinciples of Basic DC Choppers  Control strategies  Step-Down (Buck) Chopper  Step-Up (Boost) Chopper  Step-Up/Down Chopper
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  Introduction A DC-to-DC converteror chopper, as it is commonly called, is used to obtain variable DC voltage from a constant-voltage DC source. This conversion can be achieved with a combination of an inductor or/and capacitor and a solid state device operated in a high frequency switching mode.
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  Basic Block Diagram
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  Principles of BasicDC Choppers A switch ‘S’ is connected in series can be a power transistor, an SCR, or a GTO thyristor. Ideal switches have the following characteristics:  They have zero resistance (zero voltage drop) when on.  They have infinite resistance (zero leakage current) when off.  They can switch from either state in zero time.
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  Control strategies 1. Pulse-Widthmodulation In pulse-width modulation the switches are turned on at a constant chopping frequency. The total time period of one cycle of output waveform is constant. 2. Frequency modulation In this method, Ton is kept constant while the period (frequency) is varied.
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  Step-Down (Buck) Chopper: Inthis case the average output voltage is less than the input voltage. It is also known as step down converter.
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  On and Offstates  When switch is turned ON, Vs directly appears across the load as shown in figure. So VO = VS.  When switch is turned off, Vs is disconnected from the load. So output voltage VO = 0.
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  Modes  Continuous CurrentMode: Voltage across the load, which is also appearing across the FWD (D). This voltage is equal to the input voltage Vi when the switch is ON and diode D is reverse-biased.  Discontinuous Current Mode: For low values of d, especially with low inductance, the load current decreases and may fall to zero during the part of each cycle when the switch if off. The current again builds up from zero when the switch turns on in the next cycle. The load current is said to be Discontinuous.
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  Step-Up (Boost) Chopper Inthis case the average output voltage is more than the input voltage. It is also known as step up converter.
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  On and Offstates  When switch is ON, it short circuits the load. Hence output voltage during TON is zero. During this period inductor gets charged. So, VS = VL  When switch is OFF, inductor discharges through the load. So, we will get summation of both source voltage VS and inductor Voltage VL as output voltage
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  Step-Up/Down Chopper A buck-boostDC-to-DC chopper circuit combines the concepts of the step-up and step-down choppers. This type of converter produces an output voltage that is either lower or higher than the input voltage.
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  Operating States  Whenthe switch is turned on, the input voltage source supplies current to the inductor, and the capacitor supplies current to the resistor (output load). The diode D is reverse-biased and diode current is zero.  When the switch is turned off, the inductor supplies current to the load via the diode D. The diode is forwarded- biases and provides the path for the load current. The output voltage becomes equal to the inductor voltage