DVD digital video player electromagnetic interference design - Power Circuit - Circuit Diagram

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Abstract: Electromagnetic interference is a problem that cannot be ignored in electronic products. Measures should be taken during design. Good design will reduce design time and product cost. Starting from the requirements of the national standardization organization for electromagnetic interference, the three main reasons for the electromagnetic interference generated by the digital video disc player are analyzed. The targeted discussion from the three aspects of Switching Power Supply, audio/video output and PCB circuit board, design analysis Several methods to suppress electromagnetic interference have passed the EMI test.
Keywords: digital video disc player; electromagnetic interference; switching power supply; audio/video output; PCB board

0 Introduction Electromagnetic Interference (EMI) refers to an electronic device that continuously discharges interference into the environment in which it is located beyond the allowable range. In order to protect the environment, the National Standards Organization has established a number of mandatory standards, requiring electronic equipment manufacturers to reduce the electromagnetic interference of their products to a certain extent. Therefore, from the beginning of design, we must pay attention to the EMI design of electronic equipment, from the selection of components, connectors, layout and wiring of printed boards, grounding points and other aspects to reduce electromagnetic interference. This article takes the DVD-1000 digital video disk machine as an example to introduce the experience of EMI design.

1 EMI design requirements For the EMI requirements of electronic products, China issued the “GB13837-1997 Standards for Limits and Measurement Methods for Radio Interference Characteristics of Sound and Television Broadcast Receivers and Related Equipment” in July 1997, which specifies Five limits for electromagnetic interference.
(1) The disturbance voltage injected into the power supply. In the range of 9 to 30 kHz, if the interference voltage of the electronic equipment injected into the grid exceeds 66 dBμV, the interference signal will cause serious interference to other equipment through the power network, which may cause other equipment to degrade or fail to work properly.
(2) Antenna end disturbance voltage. When the receiving device is connected to a common antenna or the antennas are very close together, the interference signals are easily interfered by the distribution cable or the coupling between the antennas.
(3) RF output useful signal and disturbance signal levels. When the RF output of the device is connected to other devices, if the RF output of the device and its harmonic level are too high, the generated high-frequency harmonic radiation will cause interference to nearby devices.
(4) Radiation disturbance field strength or power. The local oscillator of the broadcast receiver and its harmonics, the radiation field strength of an intermediate frequency unit should not be too high, otherwise it will pollute the electromagnetic environment.
(5) Harassment power. Devices such as audio power amplifiers and audio-visual discs radiate energy at frequencies above 30 MHz mainly through their connected power lines and other connections. When the peak of the disturbance power on the power line and other wires is greater than 55 dBμV, it will affect the normal operation of other devices.

2 EMI propagation mechanism of digital video player
The propagation of EMI is mainly divided into conduction interference and radiation interference. Radiated interference refers to the interference between various systems. It radiates various electromagnetic interference generated inside the electronic product to the external electromagnetic field through the lead wires and components in the form of space electromagnetic waves, thereby causing interference to other electronic devices. For example, the high-frequency signal lines of DVD player and various connectors may become sources of radiation interference with antenna characteristics. Conducted interference is the interference formed by the signal coupling between the internal conduction of the system through the conductive medium. It is usually due to the intersection of the impedance (internal resistance of the power supply, the impedance between the ground lines) and the mutual inductance (guide line). Internal interference caused by the intersection of parasitic capacitance. For example: the radiation of a television set.
There are many specific methods for eliminating or reducing electromagnetic interference, including shielding technology, power supply filtering, grounding technology, and the use of EMI devices. For DVD digital video disc players, because there is no antenna and RF output port, the antenna end disturbance voltage is very small, mostly less than 30 dBμV. The main propagation paths are as follows: the pulse current and voltage generated by the switching power supply change rapidly, forming The switching power supply switching pulse and higher harmonics radiate and conduct outward through the components and the power line to generate electromagnetic interference; the audio/video output signal radiates outward through the audio/video signal connection line; various clocks required for digital circuit operation The signal and its higher harmonics, such as the CPU chip operating clock, MPEG decoder operating clock, video synchronization clock (27 MHz, 16.934 4 MHz, 40.5 MHz), form an interference pulse that radiates outward through the component.

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