Chopper

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Choppers- Fixed DC to Variable DC Converters

- Anoop MathewHOD, ECE Dept.Vedavyasa Institute of TechnologyVedavyasa Hills, Malappuaram Dist.Kerala 673632


Classification of Chopper

§ Based on input/output voltage levelsØ Step-down Chopper (Buck converter)Ø Step-up Chopper (Boost Converter)

n Direction of output voltage and currentØ Class AØ Class BØ Class CØ Class D

Contd ...


Classification of Chopper

n Based on Circuit operationØ First (Single) Quadrant ChopperØ Two-quadrant ChopperØ Four Quadrant Chopper

n Based on Commutation Method Ø Voltage Commuted ChopperØ Current Commuted ChopperØ Load commuted ChopperØ Impulse Commuted Chopper


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


Buck Converter (Step-down chopper)


Discontinuous load current


Continuous load current


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.


Analysis

Ø Average load voltage, Vo = Vs.kØ Output voltage varies linearly with “k”.Ø Ton+Toff = chopping periodØ Load voltage can be controlled by changing

the duty ratio.Ø As k reduces, current becomes discontinuous.Ø Average load current, Io = Vs.k/RØ Vo(RMS) = kVkV ss =2


Boost Converter (Step-up chopper)


Waveforms


Principle of operation

Ø When S is ON à L charges.Ø When ‘S’ is OFF à inductor current

flows through diode and load during Toff

Ø Vo = Vs+L(di/dt)Ø High value inductor is preferred

for lower ripples.


Analysis

Energy stored in the inductor W1, 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)


References

[1] A. Maamoun, “DEVELOPMENT OF CYCLOCONVERTERS”, IEEE Trans. 2003.

[2] Ned Mohan et al, “Power Electronics”, Weilypublications 2005

[3] M.D. Singh & K.B Khanjandani, “Power Electronics”,TMH 2006

[4] Cyril W. Lander “ Power Electronics”, McGraw Hill International Editions 2007


Thank you

Questions ??????

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Chopper