Spread Spectrum Modulation to Reduce EM Noise for LLC Resonant Converter Using Partial Power Processing Concept

This article proposes a method to obtain tight output voltage regulation under the spread spectrum modulation (SSM) using a partial power processing (PPP) concept. The LLC resonant converter performs the SSM by handling most of the power as a dc transformer without output voltage regulation. In contrast, a buck converter processes only partial power to regulate the output voltage and to compensate for the output voltage fluctuations caused by the SSM. Since the output voltage fluctuation can be compensated by the PPP, high power conversion efficiency, electromagnetic (EM) noise reduction, and tight output voltage regulation can be obtained simultaneously. Furthermore, the proposed method has high cost-effectiveness compared with the conventional methods which process the full power to compensate for the output voltage fluctuation due to the SSM. The effectiveness of the proposed method is experimentally verified with a 500-W prototype. The output voltage is tightly regulated under the steady-state operations and transient-state within 1.24 V-pp (2.5%) and within 1.7 V-pp (3.5%) under the rated power. The EM noises are reduced by 26 dB mu V (more than 45%) compared with the no SSM case. The maximumpower conversion efficiency is 95.49% under the nominal input voltage of 400 V.