Electromagnetic compatibility (EMC) is an inevitable important issue in LCD TV design. If the EMC design is not good, it will cause problems such as water ripple and strobe during the playback of the TV. In severe cases, it will be impossible to watch. EMC design is actually designed to optimize the electromagnetic interference generated in the product to meet the EMC standards of countries or regions. It is defined as the ability of a device or system to function properly in its electromagnetic environment without posing unacceptable electromagnetic interference (EMI) to anything in the environment.
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Electromagnetic interference is generally divided into two types, conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiated interference refers to an interference source that couples (interferes) its signal to another electrical network through space.
The LCD TV structure mainly includes: a liquid crystal display module, a power module, a driving module (mainly including a main driving board and a tuner board), and a button module. Generally, the liquid crystal display module has been tested by the manufacturer before the production. Here we mainly introduce the design of power modules, drive modules, button modules, and the electromagnetic interference problems that should be paid attention to when designing the whole machine.
Power module EMC design
The two main functions of the power supply part are to realize the backlight for driving the LCD screen and to provide DC power for other modules (including the drive module and the button module).
The design of the power module directly affects the entire system. If the design is not good, it will cause large ripples on the TV. In severe cases, the TV will not be used. At the same time, it will seriously affect the normal use of other devices nearby.
The power supply part of the LCD TV is a switching power supply. The cause of the electromagnetic interference problem caused by the switching power supply is very complicated. When designing the switching power supply, it is necessary to prevent the switching power supply from interfering with the power grid and nearby electronic equipment; it is also necessary to strengthen the adaptability of the switching power supply itself to the electromagnetic interference environment.
For the EMC problem of switching power supply, the following main measures should be taken during design:
Soft switching technology: Inrush current and spike voltage are generated when the switching device is turned on/off, which is the main cause of electromagnetic interference and switching loss of the switching transistor. Soft switching technology is an important method to reduce switching device losses and improve the EMC characteristics of switching devices. The technology is mainly to make the switching tube in the switching power supply switch at zero voltage and zero current to effectively suppress electromagnetic interference.
Modulation frequency control: Electromagnetic interference is changed according to the switching frequency, and the energy of the interference is concentrated on the discrete switching frequency points, resulting in large interference intensity. By distributing the energy modulation of the switching signal over a wide frequency band, a series of discrete sidebands are generated, which spreads the interference spectrum and distributes the interference energy over the discrete frequency bands, thereby reducing the electromagnetic interference strength at the switching frequency point.
Component layout and routing: Components that associate the power input signal with the output signal are placed near the appropriate ports to avoid interference due to the coupling path. Put together the components that are related to each other to avoid interference caused by long traces.
Also try to avoid paralleling the signal lines. If you can't avoid it, try to increase the line spacing. Or add a ground wire in the middle to reduce mutual interference.
Main drive board EMC design
The main driving board of the LCD TV mainly includes: an analog signal part, a high-speed digital circuit part, and a noise source DC-DC power supply part.
Component layout and routing: In the layout, the analog signal part, the high-speed digital circuit part, and the noise source DC-DC power supply part are reasonably separated, so that the signal coupling between them is minimized. In terms of device layout, the principle of keeping the related devices as close as possible is obtained, so that good noise resistance can be obtained.
DC-DC Power Supply Part and Ground: On the printed circuit board, the power and ground lines are the most important. Let the analog and digital circuits have their own power and ground paths. The main means of overcoming electromagnetic interference is grounding.
On the driving board of the liquid crystal television, the ground of the power supply section (DC-DC) is mainly separated from the ground of other parts such as the decoding and the main chip processing to reduce the interference of the power supply to the image display and the television sound.
If there is analog ground and digital ground when designing the circuit, they should be separated when the printed board is laid. To reduce mutual interference. In the layout of the two-layer board and the multi-layer board PCB, one layer of copper foil is generally used as a special ground plane. The purpose of this is that the ground acts as a shield.
Integrated chip: In the same integrated chip, the analog ground and digital ground are also laid separately. For example, the AD9883 analog-to-digital converter chip of AD company, which is often used in the main driver board of LCD TVs, can be used to separate the ground and digital parts of the analog part of the chip during the design of the printed board. Finally, the two points are connected by a relatively short wire. Or connect the two places through a 1nF bypass capacitor.
Crystal oscillator: The clock circuit in digital circuit is one of the main electromagnetic interference sources in electronic products, and it is the main content of EMC design. The crystal is a strong radiation source. The internal circuit of the crystal generates a large RF current, so that the ground lead of the crystal cannot sufficiently bring the relatively large Ldi/dt current to the ground plane with little loss, and as a result, the metal casing becomes a monopole antenna. The periphery of the crystal is a radiation field.
Therefore, the crystal oscillator circuit should be as far as possible from the interface circuit, such as serial port, address line, data line, and so on. To avoid the interface circuit to bring the harmonic signal of the crystal to the printed board to cause electromagnetic interference. The RC filter circuit should be added to both feet of the crystal. At the same time, the metal casing of the crystal must be connected to the ground on the printed board. In addition, the crystal oscillator is placed as close as possible to the chip pins, and the clock region is isolated by a ground line, and a local ground plane is placed and connected to the ground through a plurality of vias.
Capacitor Decoupling: Capacitor decoupling is used to reduce electromagnetic interference. Capacitor decoupling can be divided into three types: global, local, and inter-board.
The overall decoupling capacitor operates in the low frequency state, providing a stable voltage and current for the entire board. It should be placed close to the printed circuit board power and ground. A typical decoupling capacitor value is 0.1μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has a distributed inductor of 5μH, and its parallel resonant frequency is about 7MHz. That is to say, it has better decoupling effect for noise below 10MHz and hardly works for noise above tens of MHz. . Therefore, for noise above 20MHz, a 0.01μF capacitor is used for decoupling.
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