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Choppers - Fixed DC to Variable DC Converters

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Anoop Mathew HOD, ECE Dept. Vedavyasa Institute of Technology Vedavyasa Hills, Malappuaram Dist. Kerala 673632

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Classification of Chopper § Based on input/output voltage levels Ø Ø

n

Step-down Chopper (Buck converter) Step-up Chopper (Boost Converter)

Direction of output voltage and current Ø Ø Ø Ø

Class A Class B Class C Class D

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Classification of Chopper n

Based on Circuit operation Ø Ø Ø

n

First (Single) Quadrant Chopper Two-quadrant Chopper Four Quadrant Chopper

Based on Commutation Method Ø Ø Ø Ø

Voltage Commuted Chopper Current Commuted Chopper Load commuted Chopper Impulse Commuted Chopper

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Classes of chopper (A,B,C,D &E

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Buck Converter (Step-down chopper)

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Discontinuous load current

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Continuous load current

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Principle of operation Ø

Duty ratio (k) = (Ton/ Ton+Toff)

Ø

At Ton, Switch is closed.

Ø

At Ton,load current freewheels through DF – effectively, load terminals are short circuited and load voltage is zero. © www.anoopmathew.110mb.com


Analysis Ø Ø Ø Ø

Ø Ø Ø

Average load voltage, Vo = Vs.k Output voltage varies linearly with “k”. Ton+Toff = chopping period Load voltage can be controlled by changing the duty ratio. As k reduces, current becomes discontinuous. Average load current, Io = Vs.k/R Vo(RMS) = V 2 k = V k s

s

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Boost Converter (Step-up chopper)

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Waveforms

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Principle of operation Ø Ø

Ø Ø

When S is ON à L charges. When ‘S’ is OFF à inductor current flows through diode and load during Toff Vo = Vs+L(di/dt) High value inductor is preferred for lower ripples.

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Analysis Energy stored in the inductor W 1, during TON :

W1 = Vs. Is. TON During TOFF, Energy released by the inductor to the load is:

W0 = (Vo-Vs). Is. TOFF At steady state and consider the system is lossless; Vs. Is. TON = (Vo-Vs). Is. TOFF ie, Vo = [(TON +TOFF)/ TOFF] x Vs

Vo = Vs/(1-k)

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References [1] A. Maamoun, “DEVELOPMENT OF CYCLOCONVERTERS”, IEEE Trans. 2003. [2] Ned Mohan et al, “Power Electronics”, Weily publications 2005 [3] M.D. Singh & K.B Khanjandani, “Power Electronics”, TMH 2006 [4] Cyril W. Lander “ Power Electronics”, McGraw Hill International Editions 2007

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Thank you Questions ?????? anoopmathew@ieee.org

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Chopper