-
8/19/2019 Zero-Voltage Transition Converters
1/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Zero-voltage transition converters:
the phase-shifted full bridge converter
Buck-derived full-bridge converter A popular converter for
server front-
ECEN 58171
Zero-voltage switching of each half-
bridge section
Each half-bridge produces a square
wave voltage. Phase-shifted control of
converter output
end power systems
Efficiencies of 90% to 95% regularly
attained
Controller chips available
Phase-shifted control
Approximate waveformsand results
(as predicted byanalysis of the
parent hard-switched converter)
ECEN 58172
-
8/19/2019 Zero-Voltage Transition Converters
2/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Actual
waveforms,
including
resonant
transitions
ECEN 58173
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
ECEN 58174
• Reduction in effective duty cycle can be expressed as a
function of theform 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.
-
8/19/2019 Zero-Voltage Transition Converters
3/15ECEN5817, ECEE Department, University of Colorado at
Boulder
ZVT Analysis
ECEN 58175
Interval 1
ECEN 58176
-
8/19/2019 Zero-Voltage Transition Converters
4/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Normalized state plane
ECEN 58177
Solution of state plane
ECEN 58178
-
8/19/2019 Zero-Voltage Transition Converters
5/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Subintervals 2 and 3
ECEN 58179
Subinterval 4
ECEN 581710
-
8/19/2019 Zero-Voltage Transition Converters
6/15
-
8/19/2019 Zero-Voltage Transition Converters
7/15
-
8/19/2019 Zero-Voltage Transition Converters
8/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Phase-shift control
ECEN 581715
Phase shift control
ECEN 581716
-
8/19/2019 Zero-Voltage Transition Converters
9/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Phase shift control: result
ECEN 581717
Effect of ZVT: reduction of effective duty cycle
ECEN 581718
-
8/19/2019 Zero-Voltage Transition Converters
10/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Issues with this converter
It’s a good converter for many applications requiring isolation.
But…
1. Secondary-side diodes operate with zero-current switching.
They require
snubbing or other protection to avoid failure associated with
avalanche
breakdown
2. The resonant transitions reduce the effective duty cycle and
conversion ratio.
To compensate, the transformer turns ratio must be increased,
leading to
increased reflected load current in the primary-side
elements
3. During the D’Ts interval when both output diodes conduct,
inductor Lc stores
energy (needed for ZVS to initiate the next DTs interval) and
its current
circulates around the primary-side elements—causing conduction
loss
ECEN 581719
Diode switching
analysis
ECEN 581720
-
8/19/2019 Zero-Voltage Transition Converters
11/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Diode commutation: intervals 3 and 4
ECEN 581721
ZCS of D6
ECEN 581722
-
8/19/2019 Zero-Voltage Transition Converters
12/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Approaches to snub the diode ringing
ECEN 581723
Approaches to snub the diode ringing
(a) conventional diode snubber
ECEN 581724
-
8/19/2019 Zero-Voltage Transition Converters
13/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Approaches to snub the diode ringing
(b) conventional passive voltage-clamp snubber
ECEN 581725
Approaches to snub the diode ringing
(c) simplify to one passive voltage-clamp snubber
ECEN 581726
-
8/19/2019 Zero-Voltage Transition Converters
14/15ECEN5817, ECEE Department, University of Colorado at
Boulder
Approaches to snub the diode ringing(d) improvement of
efficiency in voltage-clamp snubber
ECEN 581727
Approaches to snub the diode ringing
(e) active clamp lossless snubber
ECEN 581728
-
8/19/2019 Zero-Voltage Transition Converters
15/15
Approaches to snub the diode ringing
(f) primary-side lossless voltage clamp
ECEN 581729
