Zero-voltage transition converters: the phase-shifted full ...ecee.colorado.edu/~ecen5817/lectures/L40_ECEN5817_notes.pdf · ECEN5817, ECEE Department, Universi ty of Colorado at

ECEN5817, 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-

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g

Zero-voltage switching of each half-bridge section

Each half-bridge produces a square wave voltage. Phase-shifted control of converter output

p pend power systems

Efficiencies of 90% to 95% regularly attained

Controller chips available

Phase-shifted control

Approximate waveforms ppand results

(as predicted by analysis of the parent hard-switched converter)

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ECEN5817, ECEE Department, University of Colorado at Boulder

Actual waveforms, including resonant transitions

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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

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• 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.

ECEN5817, ECEE Department, University of Colorado at Boulder

ZVT Analysis

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Interval 1

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ECEN5817, ECEE Department, University of Colorado at Boulder

Normalized state plane

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Solution of state plane

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ECEN5817, ECEE Department, University of Colorado at Boulder

Subintervals 2 and 3

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Subinterval 4

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ECEN5817, ECEE Department, University of Colorado at Boulder

Subinterval 5

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ZVS: output current charges Cleg without requiring J > 1

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

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• 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

ECEN5817, ECEE Department, University of Colorado at Boulder

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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Averaging

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ECEN5817, ECEE Department, University of Colorado at Boulder

Phase-shift control

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Phase shift control

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ECEN5817, ECEE Department, University of Colorado at Boulder

Phase shift control: result

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Effect of ZVT: reduction of effective duty cycle

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ECEN5817, 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

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p y g

Diode switching analysis

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ECEN5817, ECEE Department, University of Colorado at Boulder

Diode commutation: intervals 3 and 4

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ZCS of D6

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ECEN5817, ECEE Department, University of Colorado at Boulder

Approaches to snub the diode ringing

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Approaches to snub the diode ringing(a) conventional diode snubber

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ECEN5817, ECEE Department, University of Colorado at Boulder

Approaches to snub the diode ringing(b) conventional passive voltage-clamp snubber

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Approaches to snub the diode ringing(c) simplify to one passive voltage-clamp snubber

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ECEN5817, ECEE Department, University of Colorado at Boulder

Approaches to snub the diode ringing(d) improvement of efficiency in voltage-clamp snubber

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Approaches to snub the diode ringing(e) active clamp lossless snubber

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ECEN5817, ECEE Department, University of Colorado at Boulder

Approaches to snub the diode ringing(f) primary-side lossless voltage clamp

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