EMI Solutions in PCB Design

I. Summary

As electronic systems become more and more complex, EMC issues are also increasing. In order to make their products meet relevant international standards, engineers have to travel back and forth between the office and the EMC laboratory, repeatedly testing, modifying, and retesting. This not only wastes manpower and material resources, but also delays the time to market of products, causing immeasurable losses to companies. It is important to find EMI problems in a timely manner during the product design stage. We know that the layout, wiring, and power layer processing of the PCB have a very important impact on the EMI of the entire board. Below we will use an example analysis to introduce how EMIStream can help you solve board-level EMI problems.

Second, the correct text

Theory: Electromagnetic interference (Electromagnetic Interference), referred to as conducted interference and radiated interference. Conducted interference is mainly caused by interference signals generated by electronic devices interfering with each other through conductive media or common power lines; radiated interference refers to interference signals generated by electronic devices being transmitted to another electrical network or electronic device through spatial coupling. 

In PCB circuit boards, electromagnetic energy usually exists in two forms, differential mode EMI and common mode EMI. Differential mode EMI is generated when the current output by the device in the circuit flows into a load. Common mode radiation is generated when the current flows through multiple conductive planes, such as a group of wires or cables on a PCB. 

Calculation of differential radiation

Where Ip represents the current intensity, f represents the frequency of the common mode current, Ls represents the loop area, and d represents the distance from the measurement antenna to the cable.

Calculation of common and differential mode radiation

Where I represents the current intensity, f represents the frequency of the common mode current, L represents the cable length, and d represents the distance from the measurement antenna to the cable.

The main way to solve EMI is to reduce the radiated energy generated by various reasons on the circuit board. The key to controlling EMI is to reduce the resonance of the power ground plane and the impedance of the circuit return path, and correctly place the bypass and decoupling capacitors.

Example: The author uses the EMIStream tool to analyze the EMI problem of the board. The two main functions of this tool are EMI analysis of all networks on the PCB board and power resonance analysis. EMIStream is an application software developed by Japan's NEC Corporation based on many years of EMI design experience. It can be used in various PCB design processes such as Allegro, Mentor, Zuken and Altium to solve EMI problems before PCB manufacturing. EMIStream software has 13 built-in classic EMI inspection rules, all of which have been verified by the actual design results of Japan's NEC internal products. The setting value of each inspection rule is the best theoretical value verified in practice.

1: EMIStream analysis process

The following figure shows the design process of using EMIStream to analyze the circuit board

EMIStream is embedded in the entire PCB design process, solving EMI problems in the design phase and reducing the number of repeated designs.

2: EMI check of layout:

A: After we complete the layout, we import the ALLEGRO data directly into the EMIStream tool. EMIStream has interfaces with other mainstream PCB design tools in the market, such as Mentor, Zuken, Altium, etc., to ensure complete data import.

B: Set the stacking information and fill in EMI according to the stacking information in the PCB board.

C: According to the design data of the circuit, correctly fill in the designation of the relevant NET frequency, crosstalk group, differential pair, and power ground signal in the circuit.

D: Set the parameters of the rule. We choose to use the default parameters and select the length check and maximum radiation value check items to check the board.

The result is displayed in the form of a dialog box. The user clicks the error prompt to view the problematic NET using the following method:

a: Adjust the layout of parts to reduce the total length of NET/

b: Adjust the network topology to reduce the intensity of common mode radiation/

3: EMI inspection during and after layout and routing:

A: After we complete the layout and routing, we implement the whole board network inspection. We select all the key signals that need to be tested through NET Parameter, such as clock, data, address line, differential pair, etc. At the same time, we can arbitrarily check 13 rules as the basis for EMI inspection. 

B: The 13 rules include the following

1: Conducted radiation analysis 2 rules

2: Current loop analysis 3 rules

3: Power supply, ground stratum 2 rules

4: Signal Integrity 4 Rules

5: Parts layout 2 rules