Dynamic Analysis and Control Design of Second-Order Harmonic Active Filters based on Series-Resonant Converters

Two-stage AC/DC converters are typically realized with an Active Front End (AFE) unit followed by an isolated DC/DC unit. In single-phase systems, the AFE unit introduces a second-order harmonic ripple to the system, which can undesirably propagate to the output voltage. If the DC/DC is realized with a Series-Resonant Converter (SRC), the propagation of the second-order harmonic ripple can be neutralized by controlling its switching frequency. The operating principle and steady-state performances of this active filtering mechanism have been recently analyzed in the technical literature. However, multiple dynamic interactions exist in the system, which can impact the stability and robustness of the SRC frequency controller. These effects are investigated in this work. It is here shown that the intrinsic inductive behavior of the SRC, by interacting with the DC-bus capacitors, can cause resonances at low-frequency and interfere with the closed-loop control of the system, potentially causing instability. A detailed mathematical model has been developed to analyze this phenomenon, and design and tuning guidelines have been provided to ensure robust stability of the controlled system in the desired operating range. The analysis has been validated through numerical simulations and experimental testing.