Modeling Of Common Mode Interference And Design Of The Shielding Layer In Half-bridge LLC Transformer

As a kind of high-performance DC/DC converter,LLC resonant converter is widely used in power and communication fields.It has the characteristics of simple circuit structure,low switching device turn-off loss and high circuit efficiency.Although LLC circuits have good soft switching characteristics,with the development of new energy power electronic devices in the direction of miniaturization and high frequency,the electromagnetic compatibility(EMC)problem caused by the switching action of power switching devices has become a challenge on the efficiency of the circuit.Miniaturization results in the centralized arrangement of circuit devices,which makes the electromagnetic environment of the circuit more complicated.High frequency increases the common mode(CM)interference,which is generated by the power switching device during the switching process.In order to effectively reduce the electromagnetic interference(EMI)in LLC circuit,reduce the volume of the EMI filter in LLC circuit,and improve the efficiency and stability of the circuit,it is necessary to propose an EMC optimization design plan for the LLC circuit.For this reason,the following research is carried out in this paper.First of all,this paper analyzes the main CM interference sources in half-bridge LLC resonant converter through testing,determines the amplitude-frequency characteristics of the high-frequency harmonics in the circuit and the location of main noise points,and lays the foundation for establishing the CM conduction model of the circuit.Then different forms of distributed capacitance between the windings of high-frequency transformer are discussed,and it is clarified that distributed capacitance is an important factor affecting the CM conduction current.A CM port equivalent capacitance model is established for the high-frequency transformer windings.Then,combining the CM interference source model and the CM port equivalent capacitance model of high frequency transformer windings,the CM current conduction model of half-bridge LLC resonant converter is established.Finally,this article introduces three methods of suppressing CM noise in half-bridge LLC circuits,analyzes the shielding method,and obtains a mapping function,which reflects the relationship between the amplitude of CM current and the width of shielding layer.The function is solved to obtain the optimal structure of the high-frequency transformer shielding layer.Based on the above theory,an optimal structure design method for the shielding layer is proposed to reduce the CM noise in the half-bridge LLC resonant converter.In order to verify the effectiveness of the shielding layer design method for half-bridge LLC transformers,this paper takes a low-power half-bridge LLC resonant converter prototype as an example.Firstly,the proposed shielding layer design method is used to calculate and obtain the optimal shielding layer structure and the variation law of CM noise with the change of shielding layer width.Subsequently,the experimental test was carried out to verify the CM noise amplitude-frequency characteristics in the frequency range of 100 k Hz～10MHz.And the calculation results were compared and discussed.It has been verified that the proposed shielding layer structure design method of half-bridge LLC transformer is effective.