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Common Mode Noise Characteristics of Resonant Converters

Fig. 1. CM noise with integrated leakage.
The CM noise characteristics of resonant converters are different from the PWM converters. The soft-switching of the resonant converter may benefit the CM noise of the converter but the resonant inductor degrades such a benefit. It was discovered that utilizing the leakage inductance in the power transformer as the resonant inductor can minimize the effect of the resonant inductor and reduce the high-frequency CM noise of the resonant converters.

In order to integrate the resonant inductor into the transformer, the leakage inductance needs to be increased. It can be achieved by inserting a magnetic layer between the primary and secondary side windings. Such a magnetic layer is not only able to increase the leakage, but also serves as a lossy element in the EMI noise propagation paths so that the high frequency EMI noise can be damped.

CM noise of the resonant converter can be further reduced with the balance technique. Fig. 2 shows that with the balance technique both low and high frequency CM noise are reduced. However, utilizing leakage inductance may limit the low-frequency reduction. It is possible to utilized dv/dt on the resonant capacitor to reduce its impact.

Fig. 2. CM noise with cancellation.
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