For applications with hold-up time requirement, the conventional LLC converter requires a small magnetising inductance L _m to provide enough output voltage gain under the hold-up operation. However, it deteriorates the efficiency in normal operation. Different from the conventional one, the proposed converter has a bidirectional switch paralleled with the secondary winding of the transformer. In normal operation, the secondary bidirectional switch remains off, and the proposed converter works around the resonant frequency f _r as the conventional one. While during the hold-up time, the converter provides sufficient output voltage gain with phase-shift control between primary and secondary switches. However, its output voltage gain is mainly determined by the phase shift angle φ and the normalised load p _on but not the L _m. Then it can have a larger L _m than the conventional LLC converter, decreasing primary conduction and turn-off losses, and improving the efficiency in normal operation. A 200 V/420 W prototype was built, and experimental results show that the efficiency can be improved by 1% compared with the conventional LLC converter in normal operation, which has verified the effectiveness of the proposed solution.

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Modified LLC resonant converter with secondary paralleled bidirectional switch for applications with hold-up time requirement

References

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