PID Controller Tuning in Voltage Mode Controlled LLC Converters for Fast Transient under PFM

In LLC resonant converter, a single-loop voltage mode control (VMC) technique is the simplest solution under pulse frequency modulation (PFM), where a PID controller needs to be designed to achieve adequate transient performance under step changes in load current, input voltage, and/or reference voltage. However, the challenges remain in identifying a suitable analytical framework for controller design because of the higher-order complex plant transfer function (TF), which is sensitive to parameter variations and wide variations of load current and input voltage. An extended describing function (EDF) method is used to derive small-signal TFs, including the use of reduced-order models. Existing loop shaping design approaches fail to directly shape the closed-loop output impedance over a wide load current range, thereby resulting in reduced bandwidth (BW) and phase margin (PM). This digest presents a robust framework to design a PID controller for direct output impedance shaping in VMC LLC converter under PFM. This enables designers to achieve a much higher BW with sufficient PM, thereby substantially improving the transient response and closed-loop damping for large load transients. Analytical design guidelines are presented. Simulation results are presented to demonstrate the superior transient performance using the proposed PID tuning approach over the existing tuning approach.