The paper detail analyze the potential distribution, electric coupling and the principle of induced charge in the primary and secondary windings of the LLC transformer. The analysis shows that copper shielding foil between the primary and secondary windings to block the noise flow from primary to secondary can't eliminate the CM noise completely due to the structural asymmetry. The optimize design methods by adjusting the shielding copper foil is proposed. The fact is revealed that the CM noise by the electric coupling between the primary and the secondary of the transformer can be evaluated by the net induced charge in the secondary windings. Based on the principle of the minimization of the net induced charge in the secondary windings, the optimization design methods of the shielding copper foil in the LLC transformer is analyzed. And the experimental results verify the proposed optimization design methods to be correct and feasible.
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