A good grounding design is a required course for every PCB engineer. Grounding not only significantly reduces interference, but it is a common technique for solving electromagnetic compatibility problems. The results are obvious and immediate. However, poor grounding may introduce interference, such as ground loop interference. Here we take the ground loop encountered in the process of conducting and rectifying the motor controller conduction as an example. To illustrate the following
1, the ground loop interference problem
During the EMC test rectification of the motor controller, when testing the positive side of the low voltage side of the conducted emission, 32M, 41M, and 65M exceeded the standard, as shown in Fig. 1; the effect of debugging low-side filter parameters was not obvious. Consider the motor controller in the vehicle. The installation environment and previous rectification experience will increase the grounding point of the motor controller casing to become two-point grounding. The 32M and 65M will be significantly reduced, and the 6.1M will exceed the standard, as shown in Figure 2, and then the 6.1M frequency point will be rectified.
2. Test and analysis of ground loop interference problems
Position the noise points with a spectrum analyzer and a near-field probe to determine the DCDC output line from the motor controller. The DCDC module is the biggest source of interference in the motor controller. The interference is easily conducted through the output cable or directly through the space to radiation, or even coupled to other power lines, signal lines, in addition, the DCDC output negative directly through the sheet metal parts The housing is connected to the ground reference plane. If it is not handled properly, it may lead to unstable ground potential.
Because the low voltage side of the motor controller conducts the test only 12V/24V power line, and does not directly test the DCDC output line, it is speculated that 6.1M interference is the DCDC module coupled to the 12V/24V power line through the harness. First of all, the ferrite core of the DCDC module is covered with a ferrite core, and there is no improvement in the 6.1M frequency point; the use of copper foil to wrap the signal wire and bond the inner wall of the housing has no effect; the CAN communication line in the DCDC module Set of ferrite rings, no effect ...
The test N method still has no way to deal with the 6.1M frequency point. Recalling where the 6.1M frequency point comes from, it is guessed that it will be caused by the increase of the grounding point, so try to remove the increased ground point, and the 6.1M frequency point will change immediately. it is good
Restoring the increased ground point, 6.1M exceeded the standard, and the phenomenon can be restored, indicating that the interference at 6.1M is indeed caused by the grounding problem. Observe the increased ground point position, just between the positive and negative poles of the DCDC output line. Use a copper braid to connect the ground reference plane. Grounding at two points will cause ground loop interference, and 6.1M exceeding the standard may be caused by ground loop interference. of.
3, to solve the problem of ground loop interference
The intrinsic cause of the ground loop interference is the presence of ground loop currents, which are caused by the voltages at the two grounding points being different. The commonly used methods for solving ground loop interference problems include single-point grounding, isolation using an isolation transformer or optocoupler, and installation of a common-mode choke coil to increase the loop impedance. In this rectification test test, only the increased grounding point should be far away from the DCDC output line to ensure that the potentials of the two grounding points are close to each other, and ground loop interference can be avoided and weakened. Place the additional grounding point on the other side of the machine, away from the interference output, to avoid 6.1M violations while suppressing 32M, 41M, and 65M interference points. Combined with other measures, you can pass the conducted emission test.
4, summary
Two-point grounding and multi-point grounding can easily cause ground loop interference problems. Special attention should be paid when the output cable of the machine housing is used to prevent the potential difference of the ground point from being too large. When the frequency is relatively low, one-point grounding should be selected as far as possible. Grounding technology is the simplest and cheapest technology to solve electromagnetic compatibility problems. It is also the most exquisite technology. Therefore, considering the grounding method and grounding position in the early stage of design will greatly help the subsequent EMC testing and rectification.
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