An Analytical Simplified Time-Domain Model based Frequency Feedforward-Controlled LLC Converter with Resonant Current Overshoot Suppression

Due to the inherent nonlinearity of the inductor-inductor-capacitor (LLC) resonant converter, traditional linear control strategies are inadequate for handling dynamic processes. Therefore, frequency feedforward control is widely adopted to improve dynamic performance. The feedforward frequency can be obtained by simplifying nonlinear time-domain equations. However, existing models rely on iteration, which limits accuracy and increases computational burden. To improve model accuracy and reduce computation complexity, an analytical simplified time-domain model is proposed. By applying Taylor Series to eliminate sine and cosine functions in time-domain equations, an analytical solution for frequency is derived. Among various simplified models, the proposed model achieves the highest accuracy and the fastest calculation speed. Based on the simplified model, a frequency feedforward control strategy is developed. The feedforward frequency is calculated in the controller, while a linear compensator eliminates the model error. Furthermore, existing feedforward control strategies suffer from abrupt frequency changes, causing severe current overshoot. Therefore, a resonant current overshoot suppression scheme is proposed. When the reference voltage steps up, the switching frequency and output voltage follows a quadratic path, allowing the frequency changing smoothly. Finally, a 500-W LLC converter prototype is built. Compared with existing feedforward control, the proposed method reduces the current overshoot by 84.1% (below resonance region) and 48.4% (above resonance region), while shortening the response time by 11.1% (below resonance region) and 39.0% (above resonance region), respectively.