EMC Background Noise Solution for Multi-load Product Under Test_Century Wisdom (Suzhou) Testing Technology Co., Ltd

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EMC Background Noise Solution for Multi-load Product Under Test

Release time:

2023-06-12 15:25

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　　1. Preface

As we all know, the open test site is an important EMC test site, but because of its high cost, difficult to obtain in the urban area, inconvenient use and other reasons, it is generally difficult to obtain the ideal test results, so at this stage, most of the semi-anechoic chamber simulation open test site to carry out EMC test. The semi-anechoic chamber is an essential part of a complete EMC laboratory. Background noise level is an important index to judge the performance of semi-anechoic chamber. The background noise level should be as low as possible, but the background noise should be at least 6 dB below the limit. If the background noise level is too high or even exceeds the limit, it will seriously affect the evaluation of the radiation disturbance level of the tested product.

In the laboratory radiation disturbance test, some output ends of the tested products need to be connected with one or more loads to simulate the normal working state of the products. If the load is placed in a dark room, the load itself will produce electromagnetic interference to the outside world and seriously affect the radiation disturbance evaluation of the tested products. This article provides a solution for the background noise of the tested products with multiple loads.

　　2. Multi-load product introduction

In recent years, the country has vigorously developed environmental protection, and the people's awareness of environmental protection has gradually increased. More and more car owners choose to buy new energy vehicles as a means of transportation. The development of new energy vehicles is just around the corner. The new energy vehicle charging pile is to maintain the supply station of new energy vehicles, and the detection volume of new energy vehicle charging piles in the laboratory is also increasing.

The newly developed charging pile has its own large-capacity storage battery, which can rely on its own storage battery to supply the tram when the power grid is cut off, and the storage battery can also rely on the power grid to replenish energy at any time. In the EMC laboratory test, the charging pile shall be tested under different working modes: ① standby mode ② battery charging mode ③ single gun or double gun discharge mode (simulating single or two electric cars charging). In the laboratory, mode ③ needs to receive a single or double pure resistance load to simulate the charging state of the tram.

New energy charging pile is only a multi-channel load product I have encountered, and some multi-channel load products will not be introduced here.

Figure 1 Double Gun Plug New Energy Charging Pile

　　3. Laboratory status and status before rectification

At present, most laboratory semi-anechoic chamber power supplies power to the equipment under test in the laboratory through a power filter hung on the chamber shell (see Figure 2). Power filter is a kind of frequency selection device, can make the specific frequency components of the signal through, attenuate other frequency components to filter out the interference noise introduced by the power supply, to meet the requirements of semi-anechoic chamber background noise. General darkrooms are equipped with AC/DC power filters of different voltages and currents, but regular darkroom manufacturers will not reserve more spare power filters, and the number of filters for multi-channel load products will definitely be insufficient.

Figure 2 Power Supply Filter

When testing single-channel or multi-channel load products, the load is placed in a dark room, and the load itself will generate electromagnetic interference signals, which will increase the background noise of the dark room. If the load is placed outside the dark room through the hourglass transmission port of the dark room through the power line, the electromagnetic interference signals of the load will also conduct interference to the dark room through the power line, which will affect the background noise of the dark room, but will be smaller than the background noise directly placed in the dark room. In the state before rectification, fig. 3 below shows that the power plug-in strip is used to take the mains electricity from the dark room through the hourglass transmission port to the dark room, and the plug-in strip is connected with a vacuum cleaner to simulate the load of the measured object. Do the radiation disturbance test according to the arrangement in Fig. 3, and the background noise does not meet the limit of 6dB (see Fig. 4).

Fig. 3 The load of the simulated measured object directly passes through the copper hourglass and is placed outside the dark room (before rectification)

Fig. 4 Test data of background noise before rectification

　　4. Rectification plan

Filter is one of the main means to suppress the conduction interference of electrical and electronic equipment. In order to suppress the interference to the background noise in the low frequency band, it is first decided to take filtering measures at the load end of the measured object and configure a mobile filter to specifically suppress the interference of low frequency signals (see Figure 5). The filter can be configured for the over-standard interference signal of different frequency bands, and the mobile filter can also solve the problem of insufficient reserved filter on the dark room shell.

Figure 5 Mobile filter

In order to meet the needs of power supply, ventilation, testing and monitoring signals in the semi-anechoic chamber to be transmitted to the outside of the anechoic chamber, there are also some important transmission interfaces between the anechoic chamber and the civil buildings, shielding control room shielding amplifier room and other rooms. The laboratory reserves an hourglass transmission port with a large opening Frankonia the semi-anechoic chamber, which can pass through multiple cables at the same time to meet the requirements of multiple load tests. The hourglass gap is filled with granular copper block, which can shield the outside interference signal from entering the dark room. (See Figure 6)

Fig. 6 Hourglass Transfer Port

　　5. Results after rectification

After adding the above filter at the load end, simulate the background noise of the single-channel or multi-channel load measured object scanning radiation disturbance RE project again (see Figure 7 for the layout). After rectification, the interference in the low frequency band is obviously improved, and the background noise is at least 6 dB lower than the limit value (see Figure 8 data).

Fig. 7 Test Layout after Modification

Fig. 8 Test Data of Background Noise after Rectification

　　6. Concluding remarks

Multi-load products under test have higher and higher requirements for laboratory hardware facilities for electromagnetic compatibility tests. Only when the laboratory's own background noise is qualified can accurate test results be obtained. The rectification method of this article can be used as a reference for similar laboratories.

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