Software Method for EMC-Assisted Design of Switching Power Supply Printed Circuit Board

Abstract: This paper proposes a software design idea for EMC auxiliary design of switching power supply printed circuit board based on electric field analysis, that is,

The interference distribution diagram is used as a guide, and the coupling coefficient is used as a reference to adjust the wiring design in time. The article finally gives an experimental verification.

Keywords: EMC wiring interference coupling

1 Introduction

To reduce the EMI of electronic equipment, the design of printed circuit boards (PCBs) is the key. A good wiring scheme can reduce the interference level without modifying the circuit topology or adding any components. However, the design of PCBs is currently just an experimental design process that relies on experience in most cases. It is called the "trial & error" design method abroad, which is very blind. The main interference coupling modes on PCBs are conducted interference and near-field interference (including electric field interference and magnetic field interference). They can often be represented by stray resistance, capacitance, and inductance. One of the design goals of PCBs is to try to reduce these stray parameters and reduce unnecessary interference coupling between printed circuits.

Many literatures have listed some methods to reduce the stray parameters between printed circuits, but these methods are often too general and still rely heavily on experience in practical applications. At present, there are also auxiliary design software packages that use numerical technology to extract PCB stray parameters and establish simulation models. Although the simulation results can match the measurement results well, this type of method is essentially to transplant the trial & error design method from the hardware platform to the software platform, and cannot guide how to wire to reduce the stray parameters between lines. After all, these methods analyze interference from the perspective of centralized circuits, and EMI is essentially a field problem, so there are still considerable limitations.

2 Basic principles

Electric field coupling is caused by displacement current interference, which is described by Maxwell's equations: The changing electric field will generate displacement current, where the displacement current density (x, y, z, t) and the electric displacement density (x, y, z, t) are functions of space and time. According to experience, the interference generated by most switching power supplies is concentrated below 200MHz, and the amplitude of interference above 200MHz is already very small. The geometric dimensions of most PCBs are much smaller than the wavelength of 200MHz electromagnetic waves, and can be approximated as a quasi-static field. Under this condition, the field quantity can be written as the product of mutually independent spatial quantities and time quantities. Therefore, equation (1) can be written as: where φ(x, y, z) is the spatial component of the potential φ(x, y, z, t) at any point (x, y, z) in space, and φ(t) is the time component of the potential at that point. (x, y, z) is the spatial component of the displacement current density (x, y, z, t) at that point, and is its time component. Under quasi-static field conditions, these spatial quantities and time quantities are independent of each other. To reduce the electric field interference between printed circuits, it can be achieved by reducing the time component and the space component (x, y, z). Extending the on/off time of the switching device can reduce it, but this will increase the switching loss and reduce efficiency. Another way is to reduce (x, y, z), which can be achieved by selecting a suitable wiring scheme and placing the sensitive circuit in a smaller place. For switching power supplies, the interference source is mainly concentrated on several wires connected to the switching device with a relatively large voltage change rate dv/dt〖2〗. To select a suitable wiring scheme, the interference intensity distribution diagram of the interference source must first be calculated. According to the distribution, placing the sensitive circuit in a smaller place can reduce the degree of interference. This is the basic idea of ​​​​our wiring using the "field" method〖3〗.

The interference coupling level between printed conductors is not entirely determined by their relative positions, but is also related to the size and shape of the conductors. In order to comprehensively evaluate the coupling degree between sensitive conductors and interfering conductors, we propose

Figure 1 Relationship between coupling coefficient and capacitance

A new evaluation parameter - coupling index, as shown in formula (4). The basic idea is to subdivide the sensitive wire into N grids, where is the displacement current density of the nth grid, and ΔA(n) is the area of ​​the nth grid. The sum of the products of all grids and ΔA(n) is used as the coupling index to evaluate the degree of coupling between the sensitive wire and the interference wire. Compared with the calculation of capacitance, the calculation of coupling coefficient is very simple and only takes up very little computer resources. The wiring scheme can be adjusted in time according to the real-time calculation results of coupling coefficient to improve the design. Instead of waiting for the entire PCB design to be completed, the software package is used to extract stray parameters to establish a simulation model, input the simulation software package, and then go back to modify the design if the simulation results are not good.

Table 1 lists nine different wiring designs and gives the corresponding coupling coefficient and capacitance value calculation results. Comparing these results, it can be found that the size, shape and relative position of the printed conductors will affect the coupling coefficient and capacitance value between them. In order to more clearly reflect the relationship between the two, the coupling coefficient and capacitance value are plotted in the same figure and linear regression analysis is performed, as shown in Figure 1. The correlation coefficient is 0.98, indicating that the coupling coefficient can well reflect the degree of coupling between the conductors. It is feasible to perform wiring based on the coupling coefficient.

Table 1 Coupling coefficient and capacitance value for different wiring designs

3 Experimental verification

The test device in Figure 2 is used to further confirm this idea. The printed conductor is connected to the signal generator HP8110A via a shielded cable, and a 10V, 200kHz pulse interference signal is fed as an interference source. The sensitive conductor is arranged as shown in No.5 or No.7 in the expression, and is connected to the spectrum analyzer HP8590L via a shielded cable to measure the interference signal. The entire device is placed in a shielded box. Figure 3 shows the design dimensions and measurement results of the No.5 wiring scheme in Table 1, and Figure 4 shows the design dimensions and measurement results of the No.7 wiring scheme in Table 1. Comparing the coupling coefficient of No.5 in Table 1 (776.35) and the coupling coefficient of No.7 (1432.9), it can be seen that the sensitive conductor in No.7 receives more interference than the sensitive conductor in No.5, which is confirmed by the experimental results of Figures 3(b) and 4(b).

4 Software Framework

The original idea of ​​software design was to get rid of the traditional "trial & error" design method of PCB, hoping that the software could be used in the PCB design process.

Figure 2 Test layout

(a) Wiring size

(b) Spectrum of the disturbed signal

Figure 3 Dimensions and interference measurement results of No.5 wiring

(a) Wiring size

(b) Spectrum of the disturbed signal

Figure 4 Dimensions and interference measurement results of No. 7 wiring

The necessary interference distribution information is given in order to suppress the interference to the lowest possible level in the early stage of PCB design.

The design work mainly includes two steps: preliminary auxiliary design and simulation verification design. In the preliminary design stage, the computer first identifies the interference source according to the size of the dv/dt of each node in the circuit, calculates the interference distribution map of the interference source and displays it on the screen for reference. According to the interference distribution map, the sensitive circuit is placed in the area with weak interference, which can reduce the interference degree of the sensitive circuit [3]. At the same time, the size and shape of the sensitive circuit can be adjusted in time according to the real-time coupling coefficient calculation value, and the interference coupling can be reduced as much as possible in the early stage of PCB design. After the entire PCB design is completed, it enters the simulation design stage. Using finite element technology to extract the PCB's stray parameters, establish a distributed parameter equivalent circuit, and put it into the circuit simulation software package Pspice or Saber, the possible interference level can be calculated and compared with the interference tolerance limit specified in the EMC standard. The entire software design block diagram is shown in Figure 5.

Figure 5 PCB assisted EMC design software block diagram

5 Conclusion

The stray parameters of the printed circuit board have a great influence on the EMC of the switching power supply. Appropriate wiring is critical to reducing the interference between printed circuits. By designing the PCB wiring according to the interference intensity distribution diagram, sensitive circuits can be placed in areas with weaker interference. Accurate calculation of stray capacitance requires a long calculation time, while the coupling coefficient can display the degree of coupling between wires in real time, greatly shortening the calculation time and assisting wiring design. Both calculation and experimental results have confirmed this. The new software-aided design concept provides new ideas for the design of printed circuit boards.