When doing EMC testing, how to reduce the difference in EMI testing?

Preface

Conduct EMC electromagnetic compatibility test and verification. Sometimes, the goods meet the standards in laboratory a, but there are differences between laboratory B, Chinese standards and overseas laboratories. During registration, testing and certification, the company must transfer the certificate, conduct market spot check, and face the verification of specific laboratories of different customers, as well as the facts it must face. So, how can we improve the accuracy of the test, or improve the consistency with other laboratories in the same industry? Or compare the EMC laboratory of China's EMC foreign capital laboratory. It is an objective difficulty for us to ensure the consistency of laboratory information in the top field of EMC. Usually, many enterprises will encounter this situation.

EMI sends test data consistency experience sharing

First, all laboratories have uncertainties. In other words, the information of the same test product is tested in laboratory a and then in laboratory B. There are usually data differences. This is normal and we need to understand. Therefore, the general laboratory will require customers to test more test products, instead of leaving work when reaching the standard, or they will fail in the target laboratory. We do not pursue positive consistency, but we must pursue consistency and effective consistency within the scope of estimation.

Then, for different products, EMC project engineers must understand the product EMC, such as filter, magnetic ring, pressure sensitive, X capacitor, Y For capacitors, we should also understand the operating mode of the product, as well as the different operating states and characteristics under different modes. For example, for products with lithium batteries, we must evaluate whether the power of the product is full? If it is full of detection, transmission and radiation, the power will be very small or the boost and buck charging circuits of the internal DCtoDC will not work. In this case, the data test is wrong or not strict. For example, monitors and other commodities involve differences in resolution, source, chroma and saturation. In this case, we still have to evaluate the consistency problems caused by such differences. Because different parameters will indeed lead to internal circuit board, chip working times, current size, especially backlight, which will be very direct to emc different tests.

For power supply products, after all, in different laboratories, there will be different test engineers, different places, and corresponding auxiliary tests nearby, such as power lines and resistance types, such as different sliding rheostats and cement resistors. Similarly, even in the middle of the laboratory, the loads used will be different. For example, LED drivers include sliding rheostats, laboratory cement resistors, semiconductor diode simulations, and even laboratory LEDs. In this case, due to different resistance, different rational characteristics, and different negative load appearances, different equivalent transmission antennas are formed, which will lead to data consistency problems. In particular, are detachable cables tested in different laboratories?

In the process of developing multimedia product EMI for MME, do different laboratories require goods to be heated, and is the heating time consistent? In particular, are the internal software settings of some embedded CPU, DSP and FPGA products consistent? Is the firmware version consistent? Since the software iteration will always be carried out at different stages of the design phase, it is necessary to know whether the drive has been modified. Logic control involves changes in hardware drive current, working time pulse and frequency cycle?

When EMI detects consistency problems, we must carefully communicate with customers. First, is there any EMI in the test product that must be stored by itself? Do you take yourself to different laboratories for testing? After the first EMI test, do you have to go home to the factory for welding because a certain welding is not beautiful? Even when we return to the factory, we need to reassemble and weld a piece. The inside of the qualified sample tested by EMI laboratory is exactly the same PCBA board? Because I met this problem, the customer said that I went back and changed a board, but I changed the same board, I didn't change it, just weld a little more beautiful, and wash a little more. In this way, when you do these actions, there will often be differences in EMI data testing. We must strictly communicate with customers. Even if we change a MOS and a CAP capacitor, even if it is exactly the same, we should also tell customers that this will lead to EMI gap, because different MOSs have different junction capacitance, different reverse recovery time, different capacitance, and different thermal temperature coefficient. At each temperature, the capacitance value will change with temperature, and this change will vary with different materials.

Subsequently, during the implementation of EMI, it must be placed according to the site layout that must be in accordance with the specifications and standards. The length of the cable and the length of the cable can be extended back and forth in the shape of 8, as well as different specifications. For example, the power cord of diagnosis and treatment products does not have to fall to the ground, while the requirement of multimedia consumer products is that the power cord naturally falls to the ground (the new requirement is that the insulating mat must be laid on the road). Ensure that the direction of the radiation test EUT close to the antenna is the standard 3m, 10m wait at the center point of the receiving antenna in the close test of the rotating table. At the same time, the standard of materials with the lowest electrical dielectric index shall be met.

In any case, in order to ensure the consistency of traceability, it is necessary to meet the different test conditions and standards of EMI in different laboratories. When you are uncertain, you must absolutely ensure the consistency. That is right, and there will be no mistakes.

Finally, we should choose a reliable laboratory, which is the most important. In addition to the following recommended laboratories with the most basic hardware quality. However, what is more important is EMC engineers' professional technical level, experience, understanding of specifications, comprehensive testing principle of laboratory testing, working principle of electronic circuits, conversion from electrical excitation to magnetic excitation, concept of high frequency, understanding and understanding of equivalent distribution parameters, and their careful and responsible working attitude. The emergence of EMC engineers can make you more professional, more reliable and assured of EMI's confidence in data testing.