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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Lecture 37

Quasi-square wave buck with two switches Original one-switch version Add synchronous rectifier

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

2 • 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

Lecture 37

The multiresonant switch Basic single-transistor version

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

3 Synchronous rectifier version

Lecture 37

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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Lecture 37

Multiresonant switch characteristics Two-transistor version with constant frequency

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Lecture 37

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

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Lecture 37

Forward converter implementation • Analysis in an upcoming lecture

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

• 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 8

Lecture 37

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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

9 A popular converter for server front end power systems Efficiencies of 90% to 95% regularly attained Controller chips available

Lecture 37

Phase-shifted control

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

10 Approximate waveforms and results (as predicted by analysis of the parent hard switched converter)

Lecture 37

Actual waveforms, including resonant transitions

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Lecture 37

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

FP ZVT

(

J

), where

P ZVT

(

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.

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Effect of ZVT: reduction of effective duty cycle

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Lecture 37

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

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Lecture 37