Analysis of Stray Capacitance Effects in Dynamics of LLC Converters under PFM for Battery Charging

In digital voltage mode-controlled LLC converter under pulse frequency modulation (PFM), a PID controller needs to be designed to achieve adequate transient performance under step changes in reference voltage, load current, and/or input voltage. However, the unevenly distributed high-frequency ringing and unpredictable voltage regulation issues caused by the stray capacitances cannot be predicted using the traditional fundamental harmonic approximation approach, which may lead to malfunctioning of the circuit components, degrade the transformer's lifespan, and cause devastating effects on the entire system. This paper highlights the typical potential threat that stray capacitances may introduce to the closed-loop dynamics of LLC converter and addresses the feasible degrees of freedom in order to minimize their influence on the converter's performance over a wide operating condition. Analytical predictions are verified using simulation results, and the performance improvement by conditionally adopting the dead time adjustment along with robust controller design is presented.