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Chapter 20 Quasi-Resonant Converters

Chapter 20 Quasi-Resonant Converters. Introduction 20.1 The zero-current-switching quasi-resonant switch cell 20.1.1 Waveforms of the half-wave ZCS quasi-resonant switch cell 20.1.2 The average terminal waveforms 20.1.3 The full-wave ZCS quasi-resonant switch cell

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Chapter 20 Quasi-Resonant Converters

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  1. Chapter 20Quasi-Resonant Converters • Introduction • 20.1 The zero-current-switching quasi-resonant switch cell 20.1.1 Waveforms of the half-wave ZCS quasi-resonant switch cell 20.1.2 The average terminal waveforms 20.1.3 The full-wave ZCS quasi-resonant switch cell • 20.2 Resonant switch topologies 20.2.1 The zero-voltage-switching quasi-resonant switch 20.2.2 The zero-voltage-switching multiresonant switch 20.2.3 Quasi-square-wave resonant switches • 20.3 Ac modeling of quasi-resonant converters • 20.4 Summary of key points

  2. The resonant switch concept • A quite general idea: • 1. PWM switch network is replaced by a resonant switch network • 2. This leads to a quasi-resonant version of the original PWM converter Example: realization of the switch cell in the buck converter

  3. 20.1 The zero-current-switchingquasi-resonant switch cell • Tank inductor Lr in series with transistor: transistor switches at zero crossings of inductor current waveform • Tank capacitor Cr in parallel with diode D2 : diode switches at zero crossings of capacitor voltage waveform • Two-quadrant switch is required: • Half-wave:Q1 and D1 in series, transistor turns off at first zero crossing of current waveform • Full-wave:Q1 and D1 in parallel, transistor turns off at second zero crossing of current waveform • Performances of half-wave and full-wave cells differ significantly.

  4. The switch conversion ratio µ A generalization of the duty cycle d(t) The switch conversion ratio µ is the ratio of the average terminal voltages of the switch network. It can be applied to non-PWM switch networks. For the CCM PWM case, µ = d. If V/Vg = M(d) for a PWM CCM converter, then V/Vg = M(µ) for the same converter with a switch network having conversion ratio µ. Generalized switch averaging, and µ, are defined and discussed in Section 10.3. In steady state:

  5. Averaged switch modeling of ZCS cells • It is assumed that the converter filter elements are large, such that their switching ripples are small. Hence, we can make the small ripple approximation as usual, for these elements: In steady state, we can further approximate these quantities by their dc values: • Modeling objective: find the average values of the terminal waveforms •  v2(t) Ts and  i1(t) Ts

  6. 20.1.1 Waveforms of the half-wave ZCSquasi-resonant switch cell The half-waveZCS quasi-resonant switch cell, driven by the terminal quantitiesáv1(t)ñTs and ái2(t)ñTs. Waveforms: Each switching period contains four subintervals

  7. Analysis result: switch conversion ratio µ Switch conversion ratio: with This is of the form

  8. Characteristics of the half-wave ZCS resonant switch Switch characteristics: Mode boundary: Js ≤ 1

  9. Buck converter containing half-wave ZCS quasi-resonant switch Conversion ratio of the buck converter is (from inductor volt-second balance): For the buck converter, ZCS occurs when Output voltage varies over the range

  10. Boost converter example For the boost converter, Half-wave ZCS equations:

  11. 20.1.3 The full-wave ZCS quasi-resonant switch cell Half wave Full wave

  12. Analysis: full-wave ZCS Analysis in the full-wave case is nearly the same as in the half-wave case. The second subinterval ends at the second zero crossing of the tank inductor current waveform. The following quantities differ: In either case, µ is given by

  13. Full-wave cell: switch conversion ratio µ Full-wave case: P1 can be approximated as so

  14. 20.2 Resonant switch topologies Basic ZCS switch cell: SPST switch SW: • Voltage-bidirectional two-quadrant switch for half-wave cell • Current-bidirectional two-quadrant switch for full-wave cell • Connection of resonant elements: • Can be connected in other ways that preserve high-frequency components of tank waveforms

  15. Connection of tank capacitor Connection of tank capacitor to two other points at ac ground. This simply changes the dc component of tank capacitor voltage. The ac high-frequency components of the tank waveforms are unchanged.

  16. A test to determine the topologyof a resonant switch network • Replace converter elements by their high-frequency equivalents: • Independent voltage source Vg: short circuit • Filter capacitors: short circuits • Filter inductors: open circuits • The resonant switch network remains. If the converter contains a ZCS quasi-resonant switch, then the result of these operations is

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