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BUCK CONVERTER

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Continuous and discontinuous mode of conduction of buck converter

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BUCK CONVERTER

  1. 1. 1
  2. 2. CCoonntteennttss 2
  3. 3. IInnttrroodduuccttiioonn 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. 3
  4. 4. TTyyppeess ooff cchhooppppeerrss Buck(Step-down) choppers: · In step down chopper output voltage is less than input voltage. Boost(Step-up) choppers: · In step up chopper output voltage is more than input voltage. Buck-Boost chopper 4
  5. 5. BBUUCCKK CCOONNVVEERRTTEERR Two modes of operation : Continuous conduction mode(CCM) Discontinuous conduction mode(DCM) 5 Fig 1.Buck Converter
  6. 6. II.. CCCCMM 6
  7. 7. 7 Fig 2. Equivalent Circuit Diagrams
  8. 8. WWaavveeffoorrmmss ffoorr CCCCMM :: 8 Fig 3. Continuous conduction mode waveforms
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  11. 11. Calculation ooff ccaappaacciittoorr ccuurrrreenntt :: 11 Fig 4.
  12. 12. IIII..DDiissccoonnttiinnuuoouuss mmooddee ooff ccoonndduuccttiioonn The discontinuous conduction mode arises when the switching ripple in an inductor current or capacitor voltage is large enough to cause the polarity of the applied switch current or voltage to reverse, such that the current- or voltage-unidirectional assumptions made in realizing the switch with semiconductor devices are violated. The DCM is commonly observed in dc-dc converters and rectifiers, and can also sometimes occur in inverters or in other converters containing two-quadrant switches. . 12
  13. 13. The discontinuous conduction mode typically occurs with large inductor current ripple in a converter operating at light load and containing current-unidirectional switches. Some converters are purposely designed to operate in DCM for all loads. 13
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  15. 15. 15 t DT on = t = D ' T I V V D = - ´ L t I Vo D = ´ l off L t t T DT D T idle on s o l off ' ( ) = - -
  16. 16. 16 i Vs DT VsDD ' T l 2 L D = – Vo ´ = 2L
  17. 17. 17 Fig 5 Discontinuous conduction mode waveforms
  18. 18. DD TVs < ' L DVo R 2 18 (I) L < 2 D' RT (II) crit (III) K < K The dimensionless parameter K is a measure of the tendency of a converter to operate in the discontinuous conduction mode. Large values of K lead to continuous mode operation, while small values lead to the discontinuous mode for some values of duty cycle. The critical value of K at the boundary between modes, Kcrit(D), is a function of duty cycle, and is equal to D’ for the buck converter.
  19. 19. It is natural to express the mode boundary in terms of the load resistance R, rather than the dimensionless parameter K. (iv) Where, 19 R R > < For CCM crit R R crit For DCM R =2 L crit ' D T
  20. 20. The complete buck converter characteristics, including both continuous and discontinuous conduction modes, are therefore : M(Conversion ratio) 20 = For K>Kcrit K 2 2 1 1 4 D D + + = For K<Kcrit
  21. 21. EEffffiicciieennccyy ffaaccttoorrss • Resistance when the transistor or MOSFET switch is conducting. • Diode forward voltage drop (usually 0.7 V or 0.4 V for schottky diode) • Inductor winding resistance • Capacitor equivalent series resistance 21
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  23. 23. Power electronics by P. S. Bhimbhra Modern power electronics by Ned Mohan Buck Converter,Wikipedia DC DC Buck converter applications,International rectifier. 23
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