Differential-mode interference characteristics of a switching power supply with a rectifier bridge input stage

1 Introduction

With the increasing application of switching power supplies in modern industrial society, the electromagnetic interference (EMI) problem generated by the PWM switching devices of its power conversion unit has gradually attracted the attention of designers and application personnel [1-3]. With the increasing improvement and strict implementation of international and domestic electromagnetic compatibility (EMC) regulations, it is particularly important to design and solve the EMI problem of switching power supply products. Filters are used to reduce the generation of EMI.

However, the traditional filter design process is achieved through repeated trial and error, lacking systematic mechanism research, which not only wastes money but also delays the timing of product launch on the market.[[4]

Electromagnetic interference is usually divided into differential mode (DM) and common mode (CM) according to the different conduction coupling modes[4]. DM is caused by the switching current with a fast current change rate (dildt) acting on the circuit parasitic current.

The noise caused by the inductor; CM is caused by the rapid voltage change rate

The switching voltage (dv/dt) acts on the parasitic capacitance to ground to form noise interference. When designing the filter, DM and CM need to be considered separately to achieve the optimization of filter parameters and filtering effect.

[5, 6] Most of the literature designs filters for ideal conditions without considering the interaction between DM and CM. However, existing literature shows that when the input stage is equipped with a rectifier bridge,

When the switching power converter operates in discontinuous mode, the imbalance of the common-mode current coupling channel will produce a new type of differential-mode noise component, mixed-mode interference (MM), which is the main source of EMI in most EMI

It is often ignored in research. Although the literature fgl studied the formation mechanism and inhibition method of this newly discovered interference, it did not provide a quantitative analysis, so it has little guiding significance for practical applications.

It is necessary to continue studying the generation mechanism and modeling method of this differential mode interference, which is of great significance for the quantitative design of EMI filters for switching power supplies.

2 Differential Mode Interference Model

Figure 1 shows the test layout of a flyback switching power converter for conducting EMI. The main function of the linear impedance stabilization network (LISN) in Figure 1 is to provide a standard 5052 noise load impedance for the EMI test equipment to ensure the correctness and comparability of the test results [4]. According to general theory, the DM voltage is the difference between the voltages coupled on the two 5052 resistors, and the CM voltage is the average value of the voltages coupled on the two 5052 resistors, which are given by equations (1) and (2) respectively.

3 Analysis of Differential Mode Interference Suppression Methods

3.1 Differential mode X capacitor

Based on the circuit model established in this paper, the common EMI filtering methods are analyzed and studied. First, consider the X capacitor differential

3.2 Differential Mode Reactor

In Mu Wen's analysis, a dual differential mode reactor is used to set up symmetrically

3.3 Common-mode Y capacitor

Conventional EMI analysis believes that the suppression effect of common-mode Y capacitors on differential-mode interference mainly comes from their equivalent series capacitance effect. However, according to the analysis of this article, the inductive component of differential-mode interference, the combined interference

Interference is the noise mode of the common mode action mechanism, so it can be preliminarily determined that the Y capacitor has a suppressive effect on common mode noise and will also greatly reduce the differential mode noise emission on the power grid side. Figure 7a shows the dual Y capacitor

Setting mode, Figure 7b shows the equivalent circuit model for calculating interference.

Then the IDM and MM voltages can be expressed as

4 Conclusion

The differential mode interference generated by the switching power supply with a rectifier bridge input stage on the grid side includes not only the wooden differential mode component, but also the mixed mode component formed by the asymmetric common mode current.

In the design, if the dominance of mixed interference in differential mode interference is not taken into account and only the idealized differential mode is considered, the design of the filter will be affected. The model and analysis method obtained in this paper can be used for EMI filter design.

It can provide a certain reference value for filter design work and help reduce the tedious "trial and error correction" work.