A Nonlinear Load Current Feedforward Strategy for the Charge-Controlled LLC Converter and Its Digital Implementation to Improve the Dynamic Response

Compared with the traditional single-voltage loop control strategy in
LLC
converters, the charge-controlled
LLC
converter can significantly improve its dynamic response due to its model having the first-order feature. Generally, a load current feedforward method could improve the converter's dynamic response. However, for the
LLC
converter, it is almost impractical due to the unclear relationship between the load current and the control variable. To build this relationship, a mathematical expression between the load current change and the control variable is derived that can precisely offset the load disturbance and achieve faster dynamic response. The derived relationship is nonlinear, related to both input and output voltages and switching frequency, and is suitable for digital implementation. To further analyze the dynamic response, the output impedances with and without the load current feedforward control are established and analyzed, which visually demonstrate the dynamic improvement in the time domain through the calculation of the output impedance and load change spectrum in the frequency domain. Finally, a 120 W
LLC
prototype with nominal 48 V input and 12 V output is built to verify the nonlinear load current feedforward control and its analysis.