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This letter presents a current-controlled variable magnetizing inductance of transformer for further improving the holdup time on half-bridge LLC converter. When the input voltage is failed, the magnetizing inductance of transformer can be decreased sharply; therefore, the LLC converter holdup time can be improved without adding additional auxiliary circuit. The mathematic model between magnetizing inductance and step-gap structure of transformer is proposed, and the benefit effects using this step-gap structure of transformer have been verified in this letter. Finally, a 180-W, LLC converter utilizing step-gap structure of transformer has been achieved, and detailed analysis and design of the proposed step-gap structure are described.

This paper proposes a novel multi-element resonant converter with self-driven synchronous rectification (SR). The proposed resonant converter can achieve a zero-voltage-switching (ZVS) operation from light load to full load, meanwhile, the zero-current-switching (ZCS) can achieve rectifiers of a secondary-side. Therefore, the switching losses can be significantly reduced. Compared with an LLC resonant converter, the proposed resonant converter can be effective to decrease the circulating energy through the primary-side of the transformer to output a load and provide a wide voltage gain range for over-current protection as well as decreasing the inrush current under the start-up condition. Moreover, the proposed converter uses a simple current detection scheme to control the synchronous rectification switches. A detailed analysis and design of this novel multi-element resonant converter with self-driven synchronous rectification is described. Finally, a DC input voltage of 380-VDC and an output voltage/current of 12-VDC/54-A for the resonant converter prototype is built to verify the theoretical analysis and performance of the proposed converter.

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