Symmetrical Transformer Design for Suppressing Even-Order Components in CM Noise Applied to Converter With Fixed-Frequency PWM Strategy

To suppress the even-order component in common-mode (CM) noise of the isolated dc-dc converter using the fixed-frequency pulsewidth modulation (PWM) strategy with 180° phase shift, the CM noise conduction paths in converter and noise source characteristics caused by the control strategy are investigated. Since the CM noise sources have equal magnitude and 180° phase difference, the odd-order components can be canceled while the even-order components will be superimposed. Symmetrical transformers with consistent CM path impedances can be used to cancel the odd-order components in CM noise. However, the even-order components will become dominant due to their superposition effect after the odd-order components are eliminated. The symmetrical transformers with lower CM $S_{21}$ curves have a greater attenuation effect on the even-order components. By choosing the symmetrical transformer with low CM $S_{21}$ curves, the even-order components in CM noise can be further reduced. Finally, the CM noise of a full-bridge resonant converter with different symmetrical transformers operating in fixed-frequency PWM strategy is tested. Experimental results indicate that the CM noise for the converter, utilizing the symmetrical transformer with lower CM $S_{21}$ curves, is reduced by more than $12~\text {dB}\cdot \mu $ V compared to that caused by other symmetrical transformers.