Hold-Up Time Extension Method for LLC Resonant Converter by Detecting Operation Region

An LLC resonant converter is widely used due to the low-voltage stress and zero-voltage-switching (ZVS) capability. The LLC resonant converter should operate in the inductive region due to the ZVS operation. For the stable ZVS operation, a sufficient margin of the maximum operation gain should be guaranteed because the theoretical peak voltage gain analysis is not well-matched with the experimental peak gain. Therefore, it is hard to use the peak gain of the LLC resonant converter. As a result, to satisfy enough hold-up time, there are many side-effects such as a large link capacitor and an unoptimized design for high efficiency. This letter proposes a new hold-up time extension method for the LLC resonant converter, which can use the peak gain without margin by detecting the accurate peak gain point. By using the proposed method, the hold-up time can be extended, and the link capacitor can be reduced for high power density. The feasibility of the proposed structure is confirmed with a 500 W prototype.