Research on Resonant Converters for High Power Conversion Efficiency, Output Voltage Regulation, and EM Noise Reduction

Abstract

Resonant converters are widely used in applications such as induction heating (IH), PCs, data centers, and LED drivers, known for their soft-switching capability and buck-boost operations through pulse frequency modulation (PFM). However, challenges like switching losses, circulating current, and electromagnetic (EM) noise arise at high switching frequencies. To address these issues, this thesis proposes a maximum voltage gain tracking algorithm and two output voltage regulation methods under spread spectrum modulation (SSM) for EM noise reduction.

The maximum voltage gain tracking algorithm is implemented in a two-stage structure consisting of a totem-pole boost power factor correction (PFC) converter and a series resonant converter (SRC) for IH application. The algorithm is proposed with the insight that the power conversion efficiency of both the totem-pole boost PFC and the SRC improves as the DC link voltage decreases. By estimating the impedance of the resonant network, the SRC reaches its maximum voltage gain point, thereby improving the efficiency and total harmonic distortion (THD) of a two-stage IH system.

Two regulation methods compensate for output voltage fluctuations caused by SSM using a partial power processing (PPP) concept and the second harmonic ripple from the totem-pole boost PFC, respectively. Both approaches allow the LLC resonant converter to operate as a DC transformer (DCT) and perform SSM near the resonant frequency, achieving high power conversion efficiency and reduced EM noise. The output voltage is tightly regulated by the PPP converter and totem-pole boost PFC, respectively. Compared with the conventional methods, the PPP method improves cost-effectiveness by using a PPP converter with small-rated components. In contrast, the method utilizing the second harmonic ripple provides greater cost-effectiveness by using the existing structure. Additionally, the electrolytic capacitor used in a DC link capacitor can be replaced with a film capacitor, which enhances the reliability of the power conversion system. The effectiveness of these three methods will be validated through experimental results.