LCD TV EMC design technology overview
The LCD TV structure mainly includes: a liquid crystal display module, a power supply module, a drive module (mainly including a main drive board and a tuner board), and a key button module. General liquid crystal display modules have been EMC tested by manufacturers before production. Here mainly introduces the electromagnetic interference problems that should be paid attention to when designing the power supply module, drive module, button module, and the design of the whole machine.
Electromagnetic compatibility (EMC) is an inevitable and important issue in the design of LCD TVs. If the EMC design is not good, it will cause problems such as water ripples and stroboscopic during the broadcast of the TV. In severe cases, it will lead to unwatchable. EMC design is actually to optimize the design of the electromagnetic interference generated in the product, so that it conforms to the EMC standards of various countries or regions. It is defined as: the ability of a device or system to function normally in its electromagnetic environment and not to constitute unbearable electromagnetic interference (EMI) in anything in that environment.
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 means that the source of interference couples (interferes) its signal to another electrical network through space.
EMC design of power module
The two main functions of the power supply part are to realize the backlight driving the LCD screen and provide DC power for other modules (including the driving module and the key module).
The design of the power module directly affects the entire system. If the design is not good, it will cause large water ripples on the TV. In severe cases, the TV will not be used. It will also seriously affect the normal use of other nearby equipment.
The power supply part of the LCD TV uses switching power supply. The cause of electromagnetic interference problems caused by switching power supplies is very complicated. When designing a switching power supply, it is necessary to prevent the switching power supply from causing interference to 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 adopted in the design:
Soft switching technology: when the switching device is turned on / off, it will generate inrush current and spike voltage, which is the main reason for the electromagnetic interference and switching loss of the switch tube. Soft switching technology is an important method to reduce switching device losses and improve switching device EMC characteristics. This 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, thus spreading the interference spectrum and distributing the interference energy over discrete frequency bands, thereby reducing the intensity of electromagnetic interference at the switching frequency point.
Component layout and wiring: Place the components associated with the power input signal and output signal near the corresponding ports to avoid interference due to the coupling path. Put interrelated components together to avoid interference caused by long traces.
Also try to avoid parallel routing of signal lines. If unavoidable, try to increase the line spacing. Or add a ground wire in the middle to reduce mutual interference.
EMC design of main drive board
The main driver board of LCD TV mainly includes: analog signal part, high-speed digital circuit part, noise source DC-DC power supply part.
Component layout and wiring: In the layout, the three parts of the analog signal part, the high-speed digital circuit part, and the noise source DC-DC power supply part should be reasonably separated to minimize the signal coupling between each other. In terms of device layout, the principle of keeping devices close to each other as close as possible is obtained, so that a better anti-noise effect can be obtained.
DC-DC power supply part and ground: On the printed circuit board, the power line and ground line are the most important. Let analog circuits and digital circuits have their own power and ground paths, respectively. To overcome electromagnetic interference, the most important means is grounding.
On the driving board of the LCD TV, the ground of the power supply part (DC-DC) is mainly separated from the grounds of other parts such as decoding and main chip processing to reduce the interference of the power supply ground on the image display and TV sound.
If there are analog ground and digital ground when designing the circuit, they should be separated when printing the board. To reduce mutual interference. In the layout using double-layer boards and multi-layer PCBs, 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 separated. For example, the AD9883 analog-to-digital conversion chip of AD company is often used in the main driver board of the LCD TV. During the design of the printed board, we can separately lay the analog part and the digital part of the chip. Finally, connect a single point of the two grounds with a relatively short wire. Or connect the two grounds with a 1nF bypass capacitor.
Crystal oscillator: The clock circuit in the digital circuit is one of the main sources of electromagnetic interference in electronic products, and is the main content of EMC design. Crystal oscillator 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 lead the relatively large Ldi / dt current to the ground plane with little loss. As a result, the metal case becomes a monopole antenna. A radiation field is around the crystal.
Therefore, the crystal oscillator circuit should be as far away as possible from the interface circuit, such as serial port, address line, data line, etc. To avoid the interface circuit will bring the harmonic signal of the crystal oscillator out of the printed board to cause electromagnetic interference. Both legs of the crystal oscillator must be added with RC filter circuit. At the same time, the metal shell of the crystal oscillator must be connected to the ground on the printed board. In addition, the crystal and the chip pins should be as close as possible, isolate the clock area with a ground wire, place a local ground plane and connect to the ground wire through multiple vias.
Capacitive decoupling: Use capacitive decoupling to reduce electromagnetic interference. Capacitive decoupling can be divided into three types: overall, local and inter-board.
The overall decoupling capacitor works at a low frequency, providing a stable voltage and current for the entire circuit board. It should be placed close to the power and ground wires of the printed circuit board. 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 inductance of 5μH, and its parallel resonance frequency is about 7MHz, that is, it has a good decoupling effect for noise below 10MHz, and has little effect on noise above tens of MHz . So for noise above 20MHz, use a 0.01μF capacitor to decouple.
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