Transcript Slides, chapter 20

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 hardswitched 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
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Lecture 37
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
Add synchronous rectifier
• Can obtain µ < 0.5
• See Maksimovic PhD
thesis, 1989
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The multiresonant switch
Basic single-transistor
version
Synchronous rectifier version
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Multiresonant switch characteristics
Single transistor version
Analysis via state plane in supplementary course notes
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Multiresonant switch characteristics
Two-transistor version with constant frequency
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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 deadtime zero-voltage transitions
• Beware of patent issues
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Forward converter implementation
• Zero-voltage switching of both transistors
• Analysis in an
upcoming
lecture
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
• Resonant reset of transformer reduces
transistor peak voltage, relative to traditional
forward converter with auxiliary reset winding
• Small increase of rms transistor current
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Lecture 37
Zero-voltage transition converters
The phase-shifted full bridge converter
Buck-derived full-bridge converter
A popular converter for server frontend power systems
Zero-voltage switching of each halfbridge section
Each half-bridge produces a square
wave voltage. Phase-shifted control of
converter output
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Efficiencies of 90% to 95% regularly
attained
Controller chips available
Lecture 37
Phase-shifted control
Approximate waveforms
and results
(as predicted by
analysis of the
parent hardswitched converter)
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Actual
waveforms,
including
resonant
transitions
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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.
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Effect of ZVT: reduction of effective duty cycle
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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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ZVT Analysis
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Interval 1
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Normalized state plane
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Solution of state plane
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Subintervals 2 and 3
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Subinterval 4
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Subinterval 5
ZVS: output current charges Cleg without requiring J > 1
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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
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Subintervals 7 to 11
Subintervals 7 to 11 and 0 are symmetrical to subintervals 1 to 6
Complete state plane trajectory:
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