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Zero-voltage transition converters The phase-shifted full bridge converter

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.

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Zero-voltage transition converters The phase-shifted full bridge converter

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  1. Zero-voltage transition convertersThe 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

  2. Actual waveforms, including resonant transitions

  3. Issues with this converter It’s a good converter for many applications requiring isolation. But… • Secondary-side diodes operate with zero-current switching. They require snubbing or other protection to avoid failure associated with avalanche breakdown • 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 • 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

  4. 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 • 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.

  5. Effect of ZVT: reduction of effective duty cycle

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

  7. Diode switching analysis

  8. Diode commutation: intervals 3 and 4

  9. Waveforms: ZCS of D6

  10. Approaches to snub the diode ringing

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