ZVS investigation of llc converters based on FHA assumptions

While resonant converters have been gaining much attention within the recent years, their exact mathematical description is rather complicated. Thus, simplified calculation methods are very attractive, e.g. the First Harmonic Approximation (FHA). Although the steady state solution of these simplified methods is usually quite acceptable and close to the exact solution, the prediction of the ZVS characteristic is either neglected or treated to simple. The ZVS characteristic is typically directly related to the voltage transfer function resulting in a too optimistic estimation of the operation range obtaining ZVS. Therefore, this paper focuses on a more accurate investigation of the resonant LLC converter's ZVS behavior and proposes two approximation methods, which are also based on simplified FHA assumptions allowing a fast estimation of different converter designs.