High Performance Digital Multi-Loop Control of LLC Resonant Converters for EV Fast Charging with LUT-Based Feedforward and Adaptive Gain

The LLC resonant converter is typically adopted in battery charging applications due to its excellent performance in terms of efficiency, power density and wide input/output voltage regulation. However, this converter is a complex high-order system characterized by a strong non-linear behavior, featuring large variations of the small-signal gain/phase and pole location depending on the operating point. Consequently, these features pose substantial challenges in designing a closed-loop controller and providing constant dynamical performance over a wide operating range. Therefore, this paper proposes a digital multi-loop control strategy for LLC resonant converters ensuring constant closed-loop bandwidth and excellent disturbance rejection performance across the complete converter operating region. The control scheme consists of two cascaded voltage and current loops. To design and tune these controllers, a simplified LLC dual first order small-signal model is proposed. The system non-linear behavior affecting the current control loop is counteracted by a real-time controller gain adaptation process, which ensures constant control bandwidth. In particular, the adaptive gain values are provided by a static switching frequency look-up table (LUT) obtained experimentally. Moreover, the steady state switching frequency value is fed forward at the output of the current loop regulator, providing a further dynamical performance enhancement. The proposed control strategy and controller design procedure are verified both in simulation and experimentally on a 15 kW LLC converter prototype. The results demonstrate the superior reference tracking and disturbance rejection performance of the proposed control strategy with respect to a state-of-the-art solution based on a proportional-integral regulator.