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 characteristics Single transistor version
Analysis via state plane in supplementary course notes

5. Multiresonant switch characteristics Two-transistor version with constant frequency

6. ZVS active clamp circuits The 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

8. 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

9. Zero-voltage transition converters The 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

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

11. Actual waveforms, including resonant transitions

12. Result of analysis Basic 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.

13. Effect of ZVT: reduction of effective duty cycle

14. 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