Time domain analysis of LLC resonant converters in the boost mode for battery charger applications

Abstract

In order to support different types of rechargeable batteries (e.g. Li-Ion, Lead-Acid, NiMh), the design of universal battery chargers must focus on wide output voltage regulation, rather than on constant voltage regulation. The universal battery charger is expected to provide a demanding output voltage range between nominal and 1.5 times nominal, while sustaining the maximum power delivery and maintaining high efficiency. The softly switched LLC resonant converter is one of the best topologies for designing battery chargers due to its ability to produce variable voltage gains in different operating frequencies, while providing soft switching for all semiconductor devices. The objective of this paper is to solve the Time Domain set equations of the LLC resonant converter in boost mode, and extract closed form answer for the voltage gain as a function of converter variables (e.g. input voltage, switching frequency, resonant elements, output load). The closed form answer can precisely predict the behavior of the LLC resonant converter and can be employed in order to design and optimize the LLC resonant converter in the boost mode. In this paper, the Time Domain (TD) analysis of the LLC resonant converter in the operating mode below the resonant frequency will be presented, and a closed form answer for the converter voltage gain will be extracted. The experimental results, extracted from a 1200W platform, shows that using the obtained voltage gain equation for the LLC resonant converter results in a far higher degree of accuracy than does using First Harmonic Approximation method.