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Balance Technique for CM Noise Reduction in Critical-Mode-Based Three-Phase Soft Switching Bidirectional AC-DC Converters

circuit topology
Fig. 1. Circuit topology of three-phase ac-dc converter with balance inductors.
  The application of wide bandgap (WBG) semiconductor devices for high-frequency power conversion system design brings issues related to electromagnetic interference (EMI) noise. More specifically, the much higher dv/dt brought by the WBG devices compared with their Si counterparts at switching instants causes much higher common-mode (CM) noise. In a typical three- phase PV inverter product, the EMI filter, which is used for attenuating EMI noise to meet the standard, occupies around one-fourth of the system volume. Therefore, reducing the EMI noise of the power converter can help reduce the volume of an EMI filter and improve the total power density of the converter.
  The balance technique has proved to be an effective method for reducing CM noise in single-phase interleaved Totem-Pole PFC converters. By adding inductors on the return path and coupling the additional inductors with the original inductors, the balance conditions and therefore CM noise reduction are achieved. With optimal PCB winding inductor design and implementation, experimental results show a uniform 20 dB CM noise reduction from 150 kHz to 30 MHz.
  The balance technique is extended to three-phase ac-dc systems in this work. A split capacitor branch is added at the dc side, and then the midpoint of the split capacitor branch is connected with the neutral point of the ac filter Y-capacitor by six additional inductors. After that, these six additional inductors are coupled with the six original inductors to achieve balance. Fig. 1 shows the circuit topology with the additional inductors for achieving balance.
  For better effectiveness of CM noise reduction, external capacitors can be added to the system between the bus and the earth, or between the phase leg switch node and the earth, to fine tune the system parasitic capacitance and meet the balance condition. Fig. 2 shows the simulation results of CM noise, which indicates around 30 dB CM noise reduction is achieved with the balance technique in three-phase ac-dc system.
CM noise
Fig. 2. Simulated CM noise with and without balance.
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