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This article is to share with you about the basic concept of Current Loop in EMC. The editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article.

In EMC design, current loop is a very basic, but very important concept. An in-depth understanding of it is very helpful to solve some practical problems in EMC design. Unfortunately, most of the materials on electromagnetic compatibility design have little mention or insufficient emphasis on this concept. We can say that more than half of the common EMC problems are related to incorrect current loop processing.

When it comes to the current loop, let's first recall a law discovered by German physicists nearly two hundred years ago-Kirchhoff's law of current: at any node in the circuit, at any time, the sum of the current flowing into the node is equal to the sum of the current flowing out of the node. From this we can understand that in a system, the current flowing from a node must flow back to this node along a certain path. The path in which the current flows out and into this node forms a current loop.

As shown in figure 1, the current flows from the OUT pin of the output of the LM7805, and after 1k resistance, the LED LED returns to the ground GND pin of the LM7805, which is a complete current loop. In the case of high voltage and high current, the signal in the loop is not easy to be disturbed.

Now the trend of integrated circuits is that the working voltage is getting lower and lower, the current is getting smaller and smaller, and the frequency is getting higher and higher. In this case, the current loop with large area is easy to be interfered by external electromagnetic field, and it is also easy to produce electromagnetic radiation that interferes with others. As shown in figure 2 below, if only the signal line is considered when wiring, and the return path of the signal line is not taken into account, then it is easy to cause electromagnetic compatibility problems.

The other is the interconnection between boards, as shown in figure 3 and figure 4. Figure 3 shows that when there is only one ground wire connecting the two boards, there will be a large current loop. Figure 4 shows that when the ground wire is inserted at a fixed interval, the current loop is greatly reduced.

One of the important tasks of EMC design is to identify the current loop and try its best to reduce the loop area. The following is the more commonly used double-sided PCB version of the situation is a little more complicated than before. Consider the situation where the top floor is wired and all the floors are laid on the ground floor. At this time, the reflux path of the current is related to the working frequency of the signal. If the frequency is low (several KHz, dozens of KHz), the resistance plays the main role, and the reflux path takes the shortest distance along the bottom line (figure 5); if the frequency is higher (more than a few MHz), the reactance plays the main role, and the reflux path goes directly below the signal line at the bottom (figure 6).

Care should be taken not to make slots, holes, or overdense holes in the reflux path, which can lead to changes in the reflux path. Of course, do not place interference-prone devices such as ADC and operational amplifiers on the reflow path.

These are the basic concepts of Current Loop in EMC. The editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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