Shielding-passive Cancellation Hybrid Technique For Suppressing The Common-mode Noise In Isolated Power Converters

Isolated power converters have the ability of galavanic isolation and have found wide applications.Since the power devices are operated at high frequency,there exist voltage pulsating nodes in the isolated converters,which result in common-mode(CM)electromagnetic interference(EMI).Effective suppression of the original CM noises is beneficial to reduction of the CM EMI filter,and thus the power density of the isolated converters can be improved.In this thesis,the shielding-passive cancellation hybrid technique for suppressing the original CM noises in isolated power converters is studied.In isolated converters,there are two main propagation paths for CM noises.One is the parasitic capacitance between the primary and secondary windings of the transformer,and the other is the parasitic capacitance between the voltage pulsating nodes to the protective earth.Transformer shielding technique is always adopted to block the electric coupling between the primary and secondary windings and thus reduce the CM noises.In this thesis,the existing shielding techniques are briefly reviewed and the limitations are pointed out.Then,based on the single-layer shielding technique,the method of lifting the electrical potential of the shielding layer is proposed for making the average electrical potentials of the shielding layer and the adjacent secondary winding equal.As a consequence,the displacement current between the shielding layer and the adjacent secondary winding is effectively suppressed.An auxiliary winding is introduced to lift the potential of the shielding layer,and the calculation of the auxiliary winding turns number and the selection of the point in the shielding layer for connecting to the auxiliary winding are also given.Furthermore,the shielding-passive cancellation hybrid technique is proposed in this thesis.The auxiliary winding in the shielding technique is reused as the passive cancellation winding,and it is connected to the protective earth through a compensation capacitor.By properly choosing the capacitance of the compensation capacitor,the compensation current can be equal to the displacement current caused by the voltage pulsating node in the primary-side circuit.Thus,the CM noises in the isolated converter can be further suppressed.The implementation of the shielding-passive cancellation hybrid technique in a variety of isolation power converters is given in this thesis.To verify the effectiveness of the shielding technique with electrical potential lift of the shielding layer and the shielding-passive cancellation hybrid technique proposed in this thesis,a 1-k W halfbridge LLC resonant converter prototype has been fabricated and tested in the lab.The experimental results are provided.It is demonstrated that the shielding technique with electrical potential lift of the shielding layer has better CM noise suppresstion effect compared to the single-layer shielding technique,and the CM noise is further suppressed with the shielding-passive cancellation hybrid technique.