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

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 phi 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.