EMI Reduction of PFC Rectifier and LLC Converter with Automatic Output Ripple Improvement

This paper proposes the method of the Electro-Magnetic Interference (EMI) noise reduction of the AC-DC rectifiers and the DC-DC converters with the insulated transformers. For the Power Factor Correction (PFC) rectifier, the power factor is the most important item, but the EMI noise emitted from the clock pulse is not remarked. For the DC-DC converters such as the forward converter, the efficiency is the most important. We have focused on the EMI noise reduction for the PFC rectifier and insulated DC-DC converters with the frequency modulation of the clock pulses. First, the spectrum level of the clock pulse is introduced to be much reduced by shaking the clock frequency for the PFC rectifier and the forward converter. Next, we have investigated the EMI reduction of the LLC converter whose operating frequency varies to make the output voltage stable. It is difficult to modify the operating frequency to reduce the EMI noise. We have investigated to reduce the EMI noise by shaking the duty ratio of the resonant signals for the LLC converter. In this case, the output voltage ripple is increased by much EMI noise reduction. Finally, the technology to stabilize the increased ripple is introduced.