The emced design of LCD TVs became an unavoidable problem

Electromagnetic interference is generally divided into conducted interference and radiated interference. Conducted interference refers to the coupling of signals from one power grid to another through conductive media. Radiated interference refers to the interference source coupling its signal to another power grid through space.

Electromagnetic compatibility is an inevitable and important issue in the design of liquid crystal televisions. If so. Poor EMC design can lead to problems such as water ripples and stroboscopic flashes during TV playback, and in severe cases, it can lead to inability to watch. EMC's design is actually to optimize the electromagnetic interference generated in products to meet the requirements of various countries or regions and EMC standards. Its definition is the ability of a device or system to operate normally in its electromagnetic environment without causing unacceptable electromagnetic interference (EMI) to anything in the environment.

Electromagnetic interference is generally divided into conducted interference and radiated interference. Conducted interference refers to the coupling of signals from one power grid to another through conductive media. Radiated interference refers to the interference source coupling its signal to another power grid through space.

The structure of LCD TV mainly includes: LCD display module, power module, driver module (mainly including main driver board and tuner board), and key modules. Generally, LCD modules are subject to EMC testing by the manufacturer before production. This article mainly introduces the design of power supply module, drive module, and key module, as well as electromagnetic interference issues that should be paid attention to in the design of the entire machine.

Power module EMC design

The two main functions of the power supply section are to drive the backlight of the LCD screen and provide DC power for other modules (including the driver module and the key module).

The design of the power module directly affects the entire system. "If the design is not good, it can lead to large ripples on the TV, which in severe cases cannot be used.". At the same time, it will seriously affect the normal use of other nearby equipment.

The power supply part of the LCD TV is a switching power supply. The reasons for electromagnetic interference caused by switching power supplies are very complex. When designing a switching power supply, it is necessary to prevent the switching power supply from interfering with the power grid and nearby electronic equipment, and strengthen the adaptability of the switching power supply itself to the electromagnetic interference environment.

Main drive board EMC design

The main drive board of LCD TV mainly includes analog signal, high-speed digital circuit, and noise source DC-DC power supply.

Component layout and wiring: In the layout, the analog signal section, high-speed digital circuit section, and noise source DC-DC power supply section are reasonably separated to minimize mutual signal coupling. In terms of equipment layout, the principle of being as close as possible to the relevant equipment should be followed to achieve better anti noise effects.

DC-DC power supply and ground: On printed circuit boards, power and ground cables are the most important. Let analog and digital circuits have their own power supply and ground circuits. The main means to overcome electromagnetic interference is grounding.

On the drive board of a LCD TV, the main part is to separate the power supply (DC-DC) from other parts such as decoding and main chip processing, to reduce the interference of the power supply on image display and TV audio.

If there are analog and digital grounds in the design circuit, they should be separated. To reduce mutual interference. In layout using dual and multilayer PCBs, a layer of copper foil is often used as a special ground plane. The purpose of this is to use ground as a shielding layer.

Integrated chip: In the same integrated chip, analog and digital are also laid separately. For example, the main drive board of a liquid crystal TV often uses AD Company's analog-to-digital conversion chip AD9883. In the design of a printed board, we can separate the ground and digital portions of the analog portion of the chip. Finally, the single points in the two places are connected by a relatively short wire. Or connect a bypass capacitor through a 1nF connection.

The clock circuit in crystal oscillator digital circuits is one of the main sources of electromagnetic interference in electronic products, and is the clock circuit in digital circuits. The main content of EMC design. A crystal oscillator is a strong radiation source. The internal circuitry of the crystal oscillator produces larger circuits. RF Therefore, the ground lead of the crystal cannot fully utilize the large loss Ldi/dt current with a small loss, resulting in a ground plane and a metal shell becoming a monopole antenna. There is a radiation field around the crystal oscillator.

Therefore, the crystal oscillator circuit should be as far away from interface circuits as possible, such as serial ports, address lines, data lines, etc. To avoid the interface circuit bringing out the harmonic signal of the crystal oscillator from the printed board, causing electromagnetic interference. RC filter circuits should be added to both legs of the crystal oscillator. At the same time, the metal housing of the crystal oscillator must be connected to the ground on the printed board. In addition, the crystal oscillator is located as close to the chip pin as possible, using a ground wire to isolate the clock area, placing a local ground plane, and connecting it to the ground wire through multiple holes.

The following main measures should be taken in the EMC design of switching power supplies:

1. Modulation frequency control: Electromagnetic interference varies according to the switching frequency, and interference energy is concentrated at discrete switching frequency points, resulting in high interference intensity. By modulating and distributing the energy of the switching signal over a very wide frequency band, a series of discrete sidebands are generated, thereby expanding the interference spectrum and distributing the interference energy over the discrete frequency band, thereby reducing the electromagnetic interference intensity at the switching frequency points.

Soft switching technology: Switching equipment can generate surge currents and peak voltages when switching, which are the main reasons for electromagnetic interference and switching losses in switching pipelines. Soft switching technology is an important method to reduce the loss of switchgear and improve the EMC characteristics of switchgear. This technology is mainly used to switch the switches in switching power supplies at zero voltage and zero current, thereby effectively suppressing electromagnetic interference.

Component layout and wiring: Place components related to power input signals and output signals near corresponding ports to avoid interference caused by coupling paths. Place interrelated components together to avoid interference caused by excessively long lines.

In addition, try to avoid parallel wiring of signal lines. If unavoidable, increase the line spacing as much as possible. Or add a ground wire in the middle to reduce mutual interference.