Impedance balance control for reduction of common mode noise in full bridge converter

In this paper, a mechanism of common mode noise generation in a full bridge converter is discussed, and a reduction method is proposed. Common mode noise is generated due to parasitic capacitors around power MOSFETs. In order to explain the mechanism of common mode generation, an equivalent circuit with the parasitic capacitors is proposed. When the circuit satisfies the impedance balance condition, the common mode noise can be eliminated. However, the impedance balance condition is time-varying because a full bridge converter changes the circuit topology during the operation of the circuit. Therefore, large capacitors are inserted for keeping the current paths dominant, and making input impedance of AC side high is applied in order to form the topology satisfying the impedance balance condition. The effectiveness of this method is verified by measurement, and the common mode current in DC side is reduced about 26 dB.