Modeling And Reduction Of Common-Mode Noise In Isolated Power Converters

Isolated power converters have found wide applications thanks to its ability for achieving voltage matching and galavanic isolation.Due to the high frequency on/off action of the power switches,the displacement current that flows through the parasitic capacitance between the high dv/dt nodes in the circuit and the ground,as well as the transformer inter-winding capacitance,generates the common mode(CM)electromagnetic interference(EMI).EMI filter is usually adopted to suppress the CM noise,however,it is bulky and thus limits the power density and conversion efficiency of the power supplies.Therefore,it is desired to reduce the original CM noise and thus the EMI filter size could be reduced.In this dissertation,the CM noise models of the isolated power converters are established,and the methods for suppressing the original CM noise are proposed.In isolated power converters,the transformer inter-winding capacitance is the critical propagation path for the CM noise.Due to the complexity of inter-winding capacitance model,the inter-winding capacitance model is generally converted to the equivalent lumped capacitance for the convenience of circuit analysis.In this dissertation,by analyzing the characteristics of transformer inter-winding capacitance,the displacement current that flows through a transformer with general winding structure is derived.By adopting the displacement current consistency rule,the expression for lumped capacitance is obtained,and the generalized lumped capacitance model is proposed.Two transformers are fabricated in the lab,one has no shielding layer and the other one has shielding layers.The measured results from the transformers are well in agreememnt with the theoretical ones,verifying the validity of the proposed lumped capacitance model.With the help of the lumped capacitance model,the CM noise equivalent circuit for the basic isolated power converters is constructed in this dissertation.The CM noise equivalent circuit is further simplified,and the equivalent noise source(ENS)concept is proposed.The ENS takes into account the transformer winding structure and the secondary side rectifying and filtering circuit,which is an effective method for analyzing CM noise in isolated power converters.By applying the ENS,the effect of secondary side rectifying and filtering circuit on CM noise is analyzed,and the topologies that have the capability of CM noise cancellation within the circuit are identified.A 40 W flyback converter is built,and the CM noises of the converter with different position of the rectifying diode are measured and compared,verifying the validity of the proposed CM noise model and the ENS.Shielding technique is one of the popular approaches to reduce the CM noise in isolated powerconverters.Although the electric coupling between the primary and secondary windings is blocked by the inserted shielding layer,the displacement between the shielding and its adjacent windings still causes CM noise.The winding cancellation method has the capability of cancelling the displacement current betweent the adjacent primary and secondary winding.Therefore,the shielding technique and winding cancellation method are integrated,and thus the shielding-cancellation(SC)technique is proposed,and the condition for applying SC technique is given.A 60 W flyback converter is fabricated and teste in the lab,and the CM noises of the converter with three kinds of transformers(including the transformer without shielding,the transformer with traditional shielding layer,and the transformer with SC technique)are measured and compared.The experimental results show that the converter adopting the transformer with SC technique has the lowest CM noise.Furthermore,the SC technique is combined with the symmetrical circuit and the passive cancellation circuit,respectively,for the purpose of eliminating the displacement flowing through the parasitic capacitance between high dv/dt node in the primary side circuit and the ground.Phase-shifted full-bridge(PSFB)converter has been widely used in medium-to-high power applications,owing to its merits of low voltage stress and capability of achieving soft-switching for power switches.In this dissertation,the two phase-leg midpoint voltages with respect to the ground,as well as the resonant inductor voltage,are regarded as the voltage sources that generate CM noise in the PSFB converter,and the corresponding CM noise model is established.According to the model,the symmetrical circuit,consisting of symmetrical resonant inductor and symmetrical transformer,is adopted for eliminating the effect of resonant inductor voltage on CM noise.On this basis,the passive cancellation circuit is introduced to eliminate the effect of two phase-leg midpoint voltages on CM noise.Furthermore,the necessity of combining the symmetrical circuit and passive cancellation circuit together for reducing the CM noise is discussed.It is revealed that significant CM noise reduction can be achieved,when symmetrical circuit and passive cancellation circuit are both adopted in PSFB converter.A 1 kW PSFB converter prototype is built to verify the effectiveness of the proposed method.The experimental results demonstrate that significant CM noise reduction can be achieved,when the symmetrical circuit and passive cancellation circuit are both adopted.