Design of EMI for LCD TV

1 Introduction

With the rapid development of LCD TV technology and the upgrading of products, TVs have become more integrated and intelligent, gradually becoming a family entertainment platform and closely integrated into people's daily lives. Especially with the widespread application of AI (artificial intelligence) technology, TVs have gradually become the entrance and central control platform for the interconnection of household electrical appliances. People spend more and more time with TVs, which causes TVs to be powered on for a long time, which makes the EMI design requirements of TVs more stringent. The EMI (Electro Magnetic Interference) measures discussed in this article can effectively meet the relevant standards.

2. Mechanism of radiation generation

EMI refers to the interference caused by the operation of electronic products to other surrounding electronic products. In electronic circuit design, high-frequency signal lines, integrated circuit pins, and various connectors may become radiation interference sources with antenna characteristics. This interference source couples its signal to another circuit network through space, thereby affecting the normal operation of other electronic products in the same circuit network.

Radiation transmission is the propagation of electromagnetic waves through the medium, and the interference energy is emitted to the surrounding space according to the laws of the electromagnetic field. There are three common radiation coupling methods:

(1) The electromagnetic waves emitted by antenna A are accidentally received by antenna B, which is called antenna-to-antenna coupling;

(2) The electromagnetic field in space is coupled through induction of the wire, which is called field-to-wire coupling;

(3) The induction of high-frequency signals between two parallel conductors is called line-to-line inductive coupling.

3 Problem Description

This article discusses the case of radiation interference sources in the TV motherboard radiating outward through the screen line. The author is currently developing a new TV product. When doing an internal EMI test, it was found that it exceeded the national standard at 125.545 MHz (megahertz). The national standard cannot exceed 40 dBuV (decibel microvolts). The current test value is 43.069 dBuV, which is seriously exceeded. The radiation data test is shown in Figure 1.

4 Cause Analysis

After analyzing the relevant components, it was found that 125.545 MHz is exactly 1.5 times the operating frequency of the screen. It was initially suspected that the interference source was radiating interference through the screen line. Therefore, the anti-interference ability of the screen line was analyzed, and measures that can effectively shield the operating frequency (76.3 MHz) and the frequency multiplier need to be found.

Figure 2 Screen working parameters

To this end, the author designed three screen cables with different shielding schemes for comparative testing. See Table 1 for details.

Table 1 Comparison of three shielded cable tests

Line 1: The plug connected to the TV motherboard has no grounding measures. The line body uses a silver absorbing layer. The actual structure is shown in Figure 3, and the radiation test results are shown in Figure 4.

Figure 3 Schematic diagram of screen line 1

Line 2: The plug connected to the TV mainboard has no grounding measures. The line body uses a black absorbing layer, as shown in Figure 5. The radiation test results are shown in Figure 6.

Figure 5 Schematic diagram of screen line 2

Line 3: The plug connected to the TV mainboard has grounding tin foil, and the line body uses a black absorbing layer. The actual situation is shown in Figure 7, and the radiation test results are shown in Figure 8.

Figure 7 No. 3 screen line

5 Test Results

It can be clearly seen from the above experiments that in the comparison of radiation effects, Line 1 is poor, Line 2 is second, and Line 3 has the best effect. Detailed data is shown in Table 2.

Table 2 Radiation test results of three screen lines

5.1 Eye diagram indicator test

The eye diagram is a waveform displayed on the screen by the baseband signal output by the demodulated low-pass filter under the action of the synchronization signal. When there is transmission distortion caused by interference or distortion, it can be clearly reflected in the eye diagram. In order to further verify the effectiveness of Scheme 3, when the EMI test indicators are met, whether the signal transmission requirements are met at the same time, the eye diagram of Line 3 is specially tested, and the test eye diagram data is shown in Figure 9.

Figure 9 Eye diagram waveform of screen line 3

From the waveform test results, the eye diagram waveform of screen line 3 fully meets the requirements, indicating that the screen line has no compression or distortion in signal transmission.

In order to optimize the shielding effect, the socket that matches this screen line also needs to be improved. Using a grounded socket has a better shielding effect. The specific measures are shown in Figure 10. The PCB can be designed to be compatible. See the package design shown in Figure 11.

Actual picture of screen holder without grounding effect

Physical picture of screen holder with grounding effect

Old screen socket packaging

New screen socket packaging

Figure 10 Illustration of the new and old screen seats

Figure 11 PCB compatible design

6 Conclusion

Through the above experimental analysis, the EMI exceeding the standard due to the screen line as a radiation carrier can be solved by the following measures:

① The screen cable needs to use a cable body with a specific absorbing layer shielding, and shielding and grounding measures should be taken at the connector of the mainboard;

②The screen cable connector of the mainboard needs to use a connector with a grounding pin, which can be effectively grounded to the absorbing layer of the screen cable and enhance the shielding effect.

The EMI rectification measures discussed in this article are common EMI problems encountered by LCD TVs. This article provides an effective, convenient, and easy-to-implement solution. When designing new LCD TVs, you can consider introducing this solution to avoid EMI problems caused by screen cables, which can serve as a reference for peers.