ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

1 Lecture 37

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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Actual waveforms, including resonant transitions

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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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 5817 Resonant and Soft-Switching Techniques in Power Electronics

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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

• 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.

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Effect of ZVT: reduction of effective duty cycle

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Phase-shifted control

Approximate waveforms and results

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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Diode switching analysis

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Diode commutation: intervals 3 and 4

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Waveforms: ZCS of D6

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Approaches to snub the diode ringing