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DC Choppers

DC Choppers. 1. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT. Introduction. Chopper is a static device. A variable dc voltage is obtained from a constant dc voltage source. Also known as dc-to-dc converter. Widely used for motor control. Also used in regenerative braking.

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DC Choppers

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  1. DC Choppers 1 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  2. Introduction • Chopper is a static device. • A variable dc voltage is obtained from a constant dc voltage source. • Also known as dc-to-dc converter. • Widely used for motor control. • Also used in regenerative braking. • Thyristor converter offers greater efficiency, faster response, lower maintenance, smaller size and smooth control. 2 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  3. Choppers are of Two Types • Step-down choppers. • Step-up choppers. • In step down chopper output voltage is less than input voltage. • In step up chopper output voltage is more than input voltage. 3 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  4. Principle Of Step-down Chopper 4 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  5. A step-down chopper with resistive load. • The thyristor in the circuit acts as a switch. • When thyristor is ON, supply voltage appears across the load • When thyristor is OFF, the voltage across the load will be zero. 5 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  6. 6 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  7. 7 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  8. 8 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  9. 9 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  10. 10 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  11. 11 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  12. 12 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  13. 3.1 Basic DC to DC converters 3.1.1Buck converter SPDT switch changes dc component Switch output voltage waveform Duty cycle D: 0 ≤ D ≤ 1 complement D: D´ = 1 - D

  14. Dc component of switch output voltage

  15. Basic operation principle of buck converter

  16. Thought process in analyzing basic DC/DC converters 1) Basic operation principle (qualitative analysis) –How does current flows during different switching states –How is energy transferred during different switching states 2) Verification of small ripple approximation 3) Derivation of inductor voltage waveform during different switching states 4) Quantitative analysis according to inductor volt-second balance or capacitor charge balance

  17. Actual output voltage waveform of buck converter

  18. Buck converter analysis: inductor current waveform

  19. Inductor voltage and current subinterval 1: switch in position 1

  20. Inductor current waveform during start-up transient

  21. 3.1.2Boost converter • Boost converter example

  22. Boost converter analysis

  23. Subinterval 1: switch in position 1

  24. Subinterval 2: switch in position 2

  25. Inductor voltage and capacitor current waveforms

  26. Conversion ratio M(D) of the boost converter

  27. Continuous- Conduction- Mode (CCM) and Discontinuous Conduction-Mode (DCM) of boost

  28. Methods Of Control • The output dc voltage can be varied by the following methods. • Pulse width modulation control or constant frequency operation. • Variable frequency control. 35 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  29. Pulse Width Modulation • tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant. • Output voltage is varied by varying the ON time tON 36 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  30. 37 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  31. Variable Frequency Control • Chopping frequency ‘f’ is varied keeping either tONor tOFF constant. • To obtain full output voltage range, frequency has to be varied over a wide range. • This method produces harmonics in the output and for large tOFFload current may become discontinuous 38 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  32. 39 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  33. Step-down ChopperWith R-L Load 40 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  34. When chopper is ON, supply is connected across load. • Current flows from supply to load. • When chopper is OFF, load current continues to flow in the same direction through FWD due to energy stored in inductor ‘L’. 41 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  35. Load current can be continuous or discontinuous depending on the values of ‘L’ and duty cycle ‘d’ • For a continuous current operation, load current varies between two limits Imaxand Imin • When current becomes equal to Imaxthe chopper is turned-off and it is turned-on when current reduces to Imin. 42 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  36. 43 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  37. Expressions For Load CurrentiO For Continuous Current Operation When Chopper Is ON (0  t  tON) 44 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  38. 45 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  39. 46 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  40. 47 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  41. When Chopper is OFF 48 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  42. 49 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

  43. 50 Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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