Soft-switching converters with constant switching frequency

1. Soft-switching converters with constant switching frequency • With two or more active switches, we can obtain zero-voltage switching in converters operating at constant switching frequency • Often, the converter characteristics are nearly the same as their hard-switched PWM parent converters • The second switch may be one that is already in the PWM parent converter (synchronous rectifier, or part of a half or full bridge). Sometimes, it is not, and is a (hopefully small) auxiliary switch • Examples: • Two-switch quasi-square wave (with synchronous rectifier) • Two-switch multiresonant (with synchronous rectifier) • Phase-shifted bridge with zero voltage transitions • Forward or other converter with active clamp circuit • These converters can exhibit stresses and characteristics that approach those of the parent hard-switched PWM converter (especially the last two), but with zero-voltage switching over a range of operating points

2. Quasi-square wave buck with two switches Original one-switch version • Q2 can be viewed as a synchronous rectifier • Additional degree of control is possible: let Q2 conduct longer than D2 would otherwise conduct • Constant switching frequency control is possible, with behavior similar to conventional PWM • Can obtain µ < 0.5 • See Maksimovic PhD thesis, 1989 Add synchronous rectifier

3. The multiresonant switch Basic single-transistor version Synchronous rectifier version

4. Multiresonant switch characteristicsSingle transistor version Analysis via state plane in supplementary course notes

5. Multiresonant switch characteristicsTwo-transistor version with constant frequency

6. ZVS active clamp circuitsThe auxiliary switch approach Forward converter implementation Flyback converter implementation • Circuit can be added to any single switch in a PWM converter • Main switch plus auxiliary switch behave as half-bridge circuit with dead-time zero-voltage transitions • Beware of patent issues

7. Forward converter implementation • Zero-voltage switching of both transistors • Resonant reset of transformer reduces transistor peak voltage, relative to traditional forward converter with auxiliary reset winding • Small increase of rms transistor current • Analysis in an upcoming lecture

8. Zero-voltage transition convertersThe phase-shifted full bridge converter Buck-derived full-bridge converter Zero-voltage switching of each half-bridge section Each half-bridge produces a square wave voltage. Phase-shifted control of converter output A popular converter for server front-end power systems Efficiencies of 90% to 95% regularly attained Controller chips available

9. Phase-shifted control • Approximate waveforms and results • (as predicted by analysis of the parent hard-switched converter)

10. Actual waveforms, including resonant transitions

11. Result of analysisBasic configuration: full bridge ZVT • Phase shift  assumes the role of duty cycle d in converter equations • Effective duty cycle is reduced by the resonant transition intervals • Reduction in effective duty cycle can be expressed as a function of the form FPZVT(J), where PZVT(J) is a negative number similar in magnitude to 1. F is generally pretty small, so that the resonant transitions do not require a substantial fraction of the switching period • Circuit looks symmetrical, but the control, and hence the operation, isn’t. One side of bridge loses ZVS before the other.

12. Effect of ZVT: reduction of effective duty cycle

13. Summary: recent soft-switched approaches with multiple transistors • Represents an evolution beyond the quasi-square wave approach • Zero-voltage transitions in the half-bridge circuit • Output filter inductor operates in CCM with small ripple • Circuit approaches that minimize the amount of extra current needed to attain zero-voltage switching -- these become feasible when there is more than one active switch • Constant frequency operation • Often, the converter characteristics reduce to a potentially small variation from the characteristics of the parent hard-switched PWM converter • Commercial controllers are sometimes available • Sometimes a conventional voltage-mode or current-mode PWM controller can be used -- just need to add dead times • State-plane analysis of full-bridge ZVT and of active-clamp circuits to come

14. ZVT Analysis

15. Interval 1

16. Normalized state plane

17. Solution of state plane

18. Subintervals 2 and 3

19. Subinterval 4

20. Subinterval 5 ZVS: output current charges Cleg without requiring J > 1

21. Subinterval 6 • Current ic circulates around primary-side elements, causing conduction loss • This current arises from stored energy in Lc • The current is needed to induce ZVS during next subinterval • To maxzimize efficiency, minimize the length of this subinterval by choosing the turns ratio n such that M = V/nVg is only slightly less than 1

22. Subintervals 7 to 11 • Subintervals 7 to 11 and 0 are symmetrical to subintervals 1 to 6 • Complete state plane trajectory: