Electronic Design - How to Quickly Solve EMI Problems in printed circuit boards design

PCB layout, routing, and handling of power planes have a very important impact on the EMI problem of the entire printed circuit boards. This article will discuss how to use EMIStream to solve board-level EMI problems through case analysis. As electronic systems become more complex, EMI problems also increase. In order to make their products meet relevant international standards, designers have to go back and forth between the office and the EMC laboratory to repeatedly test, modify the design, and test again. This not only wastes manpower and material resources, but also delays the time to market of the product, which brings immeasurable losses to the enterprise. Therefore, how to detect EMI problems in time at the product design stage becomes important.

Electromagnetic interference (EMI) is divided into two types: conducted interference and radiated interference. Conducted interference is mainly interference signals generated by electronic equipment, which interfere with each other through conductive media or public power lines. Radiated interference refers to the interference signal generated by electronic equipment, which is transmitted to another circuit network or electronic equipment through spatial coupling. In the printed circuit boards, there are two forms of electromagnetic energy, namely differential mode EMI and common mode EMI. When the device Differential mode EMI occurs when the output current flows into a load. The main method to solve EMI is to reduce the radiated energy generated by various reasons on the circuit board, and the key to controlling EMI is to reduce the path impedance of power supply ground plane resonance and circuit return, and to correctly place bypass and decoupling capacitors.

1. EMIStream analysis process
EMIStream is embedded in the whole process of PCB design. Solving EMI problems in the design stage is beneficial to reduce the number of repeated designs.

2. EMI inspection of layout
2.1 When the layout is completed, import the Allegro data directly into the EMIStream tool. EMIStream and mainstream PCB design tools from Mentor, Zuken, Altium and other companies also have interfaces to ensure complete import of data.
2.2 Set the stacking information, and fill in the EMI according to the stacking information of the PCB board.
2.3 According to the design data of the circuit, correctly fill in the settings of the relevant NET frequency, crosstalk group, differential pair, and power ground signal in the circuit.
2.4 Set the parameters of the rule, we choose to use the default parameters, and select the length check and radiation value check items to check the board. The inspection result is displayed in the form of a dialog box. The user clicks the error prompt to view the NET in question, and then adopts the following two methods to eliminate the EMI problem: (1) Adjust the layout position of the parts to reduce the total length of the NET; (2) Adjust the topology of the network to reduce the intensity of common mode radiation.

3. EMI inspection during layout and wiring and after completion:
3.1 When the layout and wiring is completed, the entire board network inspection is carried out, and all key signals to be detected are selected through NET Parameter, such as clock, data, address line, differential pair, etc. At the same time, 13 rules can be arbitrarily selected as the benchmark for EMI inspection.
3.2 The 13 rules include 2 rules for conducted radiation analysis, 3 rules for current loop analysis, 2 rules for power supply and formation analysis, 4 rules for signal integrity analysis, and 2 rules for component layout analysis.
3.3 The inspection results are displayed in a dialog box, listed one by one from top to bottom according to the seve

rity of the EMI problem of the network. Open each faulty network, all EMI error information will be listed, and some error information will also display modification prompts, listing the radiation value of the network and the differential mode radiation common mode radiation value; at the same time, the network will be high on the PCB layout. Brightness is displayed, and all errors are marked on the network with a red circle. The inspection results are displayed in a dialog box, the faulty nets will be highlighted on the PCB layout, and all errors are marked on the nets with red circles. For example, if an error indicates that the network does not have a complete current loop, click on the error, and the screen will be enlarged and displayed with a red circle to mark the wrong location. At the same time, a dialog box will pop up, showing the reason for the error and giving several suggestions for modification. These suggestions include: (1) Modify the path of the wires to avoid copper foils across different NETs, resulting in incomplete reference planes and impedance mismatches. (2) Modify the shape of the copper foil so that the wire has a complete reference plane. The second error is the radiated dB value of the network, divided into differential and common mode radiated values.
3.4 Then, display the copper foil errors found, such as missing vias on the edge of the GND copper foil, excessive via spacing, etc.
3.5 Crosstalk Check Helps to check whether there is crosstalk in the parts running in parallel on the same layer or the wiring that crosses adjacent layers. It is recommended to modify the traces that are too long in parallel.

4. Power supply ground plane resonance analysis
After completing the inspection of the network and making the appropriate modifications, the next step is to perform a resonance analysis on the power ground plane. EMIStream is modeled by simulating the shape of the board and the capacitance formed between the power and ground planes, and analyzed using SPICE circuit simulation. Large voltage fluctuations are indicated in red and small voltage fluctuations are indicated in blue. First, analyze the power plane of 3V3, click the mouse to select the 3V3 power plane, and fill in the distance and dielectric constant information of the GND plane near the 3V3 power plane. Modify the calculation grid size in the Option option to 3 mm, set the scanning frequency from 30MHz to 2GHz, and the step size is 10MHz. Click RUN to start the analysis. For power/ground plane resonance problems, the resonance can be reduced by adding decoupling capacitors where the voltage fluctuations are relatively large. The EMIStream system comes with RLC models of common capacitors. If you need special RLC capacitor models, users can add them freely. We add capacitors for C104 in a few red places. It should be noted that the effect of using a capacitor in series with a resistor may be better. Repeat the analysis using the same settings. At this time, the analysis results have been significantly improved. The red area just now has become blue-green, and the resonance value below 2G has dropped to below -5dB, which meets the design requirements of the system.

Summary of this article
The EMI problem of PCB design is a very complex problem, which needs to be comprehensively dealt with by various methods. Through this analysis, it can be found that: (1) the combined use of the EMIStream tool and the PCB design tool can greatly improve the design efficiency; (2) it can be Find and solve EMI problems in the PCB design stage, reduce the number of repeated revisions, and save costs; (3) Compared with the usual SI analysis tools, no IBIS model is required, and it is not necessary to analyze one network but all networks. The results can be obtained; (4) It can help PCB engineers immediately, help improve the layout and routing strategy, and reduce the emission of EMI interference on the printed circuit boards; (5) Effectively improve the design quality, shorten the design cycle, and speed up the time to market.