EMC Design and Improvement of LED Display Screen

2012-2015 is the three years of rapid development of high-density LED display. It is predicted that the development of high-density LED products will go through three stages. The first stage is to enter the professional indoor large screen display market. In the second stage, it will enter the field of business conference and education and gradually replace the projector. The third stage is to enter the high-end home TV market. Limited by the technology of LCD TV, there is no technology in the field of large screen high-end household TV with more than 110 inches, and the projection technology is difficult to meet the requirements of high-end users for viewing effect. Therefore, small spacing LED display technology is expected to achieve brilliant results in this field in the future.

EMC Design and Improvement of LED Display Screen 1

Considering that the high-density LED display will gradually transition from the professional indoor field to the public indoor, and people's increasing demand for their own health and safety, the EMC design and certification related to health and safety is becoming more and more important. The EMC design and improvement of LED display can not be delayed.As we all know, LED is a low-voltage driven semiconductor light source. In the past, the AC-DC conversion and transmission of outdoor display screen were completed outdoors, which is far away from the crowd. When the high-density LED display screen gradually enters the indoor, we have to consider the accumulation of electromagnetic radiation in the limited space in this process.In order to deeply study the EMC of LED display, p2.5 display module of an LED display manufacturer is selected for relevant tests to verify our analysis and find the most effective design and improvement method for LED display EMC.

According to IEC and CISPR standards, the sample test items are divided into EMI and EMS.The test results are as follows:(1) Test standard: en55022:2010 AC: 2011 / cispr22 (2008-09), en61000-6-3:2007 / a1:2011 / AC: 2012, test result: conducted interference.

(2) Test standard: en55022:2010 AC: 2011 / cispr22 (2008-09), en61000-6-3:2007 / a1:2011 / AC: 2012, test result: emission interference.(3) Test standard: en61000-3-2:2006 / a2:2009, test result: harmonic interference.(4) Test standard: en61000-3-3:2013, test results: voltage fluctuation, flicker interference.

(5) Test standard: en 61000-4-2:2009, test result: electrostatic discharge protection.(6) Test standard: en 61000-4-3:2006 a2:2010, test result: RF electromagnetic field protection.(7) Test standard: en 61000-4-4:2004 a1:2010, test result: transient pulse protection.

EMC Design and Improvement of LED Display Screen 2

(8) Test standard: en 61000-4-5:2006, test result: surge voltage protection.(9) Test standard: en 61000-4-6:2009, test result: conducted interference protection.(10) Test standard: en 61000-4-8:2010, test result: power frequency magnetic field protection.

Because the LED display product itself adopts high-end switching power supply, is equipped with transient voltage suppressor and routing specification, it can be obtained from the test results that all tests except emission have passed smoothly. Therefore, emission has become a key problem to be overcome for LED display products to pass EMC certification. According to the previous measurement data, the switching power supply, control card and internal signal transmission of LED display screen will produce varying degrees of electromagnetic radiation. In addition, the working principle of LED display is to achieve high gray and high refresh by adjusting the parameters of current pulse on the LED. Therefore, the higher the refresh rate of the display, the higher the signal frequency on the system card, and the stronger the radiation interference. This is the inevitable result of the high-quality application of the display and one of the main electromagnetic disturbance sources of the display.

The input and output lines of various signal cables and power input and output lines inside the display screen are densely distributed. The electromagnetic radiation generated by the switching power supply and the crystal oscillator on the control card will be coupled with each other among the internal cables to form a radiation channel, and finally transmitted to form conducted interference. This part of radiation is the most easily ignored part, but it is an important part of radiation emission, Therefore, the following measures can be taken for experiments.1 shieldingThrough the test, it is found that the signal connection flat cable between the module and the system adapter board is the key rectification object. At first, the spikes were suppressed by adding magnetic rings (see Fig. 1 and Fig. 2), but the effect was not obvious.

It is really found that the length of the flat cable is significantly improved. When the system adapter board is used to load one module, the module output is cascaded with another module, and the noise value is only increased by 0.5 to 1dB. The system adapter board adds channels, that is, after carrying one more module, the noise value increases by 6 to 8dB. When the channel is increased, the length of the flat cable is 1.3 times that of the cascade, and the noise value is increased by more than 8 times. In addition, through data comparison, it is found that adding a magnetic ring at the power line has obvious noise suppression effect (see Fig. 1 and Fig. 3).

2 filteringDuring the field test, a 3300uf electrolytic capacitor is added at the module power supply VH2. It is found that it has obvious inhibitory effect on radiation, especially on burr and energy spectrum in the range of 30MHz to 1000MHz (see Fig. 4 and Fig. 5). The larger the capacitance, the more obvious the inhibition effect on the burr peak, and the smoother the filtered curve. Of course, the selection of capacitance value needs to be determined according to the actual design requirements.3 formation shielding

When the adapter board is changed from the original 2-layer board to 4-layer board, that is, the grounding layers of the top layer and the bottom layer are added, which can also protect the circuit radiation.When two layers of grounding are added to the LED array driving board, the original 4-layer board is changed into 6-layer board, which can better prevent radiation. When the whole side of copper foil is attached around the adapter plate and fastened with the metal box with screws to form a shell ground, so as to shield the signal (Fig. 6).4 reduce the power supply voltage of the product

Because the value of conduction and radiation is directly proportional to DV / dt or di / DT, the higher the voltage, the greater the value of conduction and radiation. The power supply voltage of the sample is 4.6V. When the voltage is adjusted to 4.2V, it can be seen from the experimental data that the spike burr can also be improved (Fig. 7 and Fig. 8).5. Optimize product structureConsidering that the radiation is related to the maximum aperture size, the shielding effect can be greatly improved when the spacing of circular holes is much larger than the diameter of circular holes. Therefore, the heat dissipation joint of the rear cover was originally a long elliptical hole and then changed to a circular hole with a diameter of 4mm.

6. Improve product processThe back cover of the original sample is an ordinary plastic part, which is improved to add a coating in the back cover. At the same time, it adopts plastic silver copper conductive paint, which is connected with the whole box through screws, so that the diagonal resistance measurement of the spraying surface is not greater than 20 ohms. Through such process improvement, it can also better protect the radiation.7 octave

Because the gclck signal of LED display screen affects the refresh rate of the display screen, we can reduce it by changing the value of gclck. However, from the actual experiment, we find that the frequency doubling value has little effect. This improvement is not desirable in reality.8 conclusion

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