How to solve the ground loop interference encountered in the conduction emission process of the motor controller

A good grounding design can not only ensure that the circuit does not interfere with each other, but also can reduce the interference emission of the circuit. The grounding technology is a common technology to solve the electromagnetic compatibility problem, and the low cost effect is obvious. However, improper grounding can also introduce disturbances to the circuit, such as ground loop disturbances.

A good grounding design can not only ensure that the circuit does not interfere with each other, but also can reduce the interference emission of the circuit. The grounding technology is a common technology to solve the electromagnetic compatibility problem, and the low cost effect is obvious. However, improper grounding can also introduce disturbances to the circuit, such as ground loop disturbances.

1. The generation of the ground loop interference problem

How to solve the ground loop interference encountered in the conduction emission process of the motor controller

Figure 1. Initial scan results

How to solve the ground loop interference encountered in the conduction emission process of the motor controller

Figure 2. Scanning results after adding the grounding point, 6.1M exceeds the standard

2. Test and analysis of ground loop interference

Locate the noise spot with a spectrum analyzer and near-field probe to determine the disturbance is coming from the motor controller’s DCDC output line. The DCDC module is the largest source of interference in the motor controller. The interference is easily conducted outward through the output cable or radiated directly through the space, or even coupled to other power lines and signal lines. In addition, the DCDC output negative pole is directly If it is connected to the chassis, that is, it has been connected to the ground reference plane. If it is not handled properly, it may cause the ground potential to be unstable.

Since the low-voltage side of the motor controller conducts emission only needs to test the 12V/24V power line, and does not directly test the DCDC output line, it is speculated that the 6.1M interference is that the DCDC module is coupled to the 12V/24V power line through the wiring harness. First, put a ferrite magnetic ring on the relevant signal line of the DCDC module, which does not improve the 6.1M frequency point; wrapping the signal line with copper foil and bonding the inner wall of the case is also ineffective; the CAN communication line of the DCDC module The upper set of ferrite rings has no effect…

I tried many methods and still couldn’t do anything about the 6.1M frequency point. I recalled where the 6.1M frequency point came from, and guessed whether it was caused by the addition of the ground point, so I tried to remove the added ground point, and the 6.1M frequency point changed immediately. OK, as shown in Figure 3.

How to solve the ground loop interference encountered in the conduction emission process of the motor controller

Figure 3. Scan result after removing the ground point

Restoring the added grounding point, 6.1M exceeds the standard again, the phenomenon can be recovered, indicating that the interference at 6.1M is indeed caused by the grounding problem. Observe the position of the added grounding point, which is just between the positive and negative poles of the DCDC output line, as shown in Figure 4. The initial grounding point of the machine is shown in Figure 5. Use a copper tape braided mesh to connect to the grounding reference plane. Grounding at two points will cause Ground loop interference, 6.1M exceeding the standard is likely to be caused by ground loop interference.

How to solve the ground loop interference encountered in the conduction emission process of the motor controller

When the DCDC module is working, the DC+ and DC- output lines will carry a strong interference current, and DC- is directly connected to the chassis. The test model diagram is shown in Figure 6. When there is no ground point B, the interference current mainly forms a loop through the stray capacitance between the DC output line and the ground reference plane; when the ground point B is added between DC+ and DC-, If a direct low-impedance path is provided for the interference current, the ground loop current will appear, resulting in a larger common-mode current, and then the phenomenon of conducted emission exceeding the standard will occur.

How to solve the ground loop interference encountered in the conduction emission process of the motor controller

Figure 6. Model diagram of the test system (ground point C is required by the test standard: the negative electrode of the low-voltage battery is grounded)

3. Solving the problem of ground loop interference

The inherent cause of ground loop interference is the existence of ground loop current, which is caused by the voltage formed by the difference in potential of two ground points. Commonly used methods to solve the problem of ground loop interference include single-point grounding, isolation transformers or optocoupler isolators, and installation of common mode choke coils to increase ground loop impedance. In this rectification test, the ground loop interference can be avoided and weakened only by keeping the added ground point away from the DCDC output line to ensure that the potentials of the two ground points are similar. As shown in Figure 7, by arranging the additional grounding point on the other side of the machine, away from the interference output end, it is possible to avoid the 6.1M exceeding the standard and suppress the 32M, 41M and 65M interference points, as shown in Figure 8, combined with other measures, It can pass the conducted emission test.

4. Summary

Two-point grounding and multi-point grounding can easily cause ground loop interference. When the machine casing has output cables, special attention should be paid to prevent the potential difference of the grounding points from being too large. When the frequency is relatively low, single-point grounding should be selected as far as possible. Grounding technology is the easiest and cheapest technology to solve electromagnetic compatibility problems, and it is also the most exquisite technology. Therefore, considering the grounding method and grounding position in the early stage of design will be of great help to subsequent EMC testing and rectification.

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