Design scheme for electromagnetic interference of DVD digital video disk player

Overview: 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. Based on the requirements of national standardization organizations for electromagnetic interference, this paper analyzes the three main reasons for electromagnetic interference generated by digital video disc players. From this, we discuss the three aspects of switching power supply, audio/video output and PCB circuit board, design and analyze several methods to suppress electromagnetic interference, and pass the EMI test.

Electromagnetic interference (EMI) refers to the interference that an electronic device continuously emits to its environment beyond the permitted range. In order to protect the environment, the national standardization organization has formulated several mandatory standards, requiring electronic equipment manufacturers to reduce the electromagnetic interference of their products to a certain extent. Therefore, from the beginning of the design, it is necessary to pay attention to the EMI design of electronic equipment, and comprehensively consider the selection of components, connectors, layout and wiring of printed boards, grounding points, etc. to reduce electromagnetic interference. This article takes the DVD-1000 digital video player as an example to introduce some experience in EMI design.

1 EMI design requirements

Regarding the EMI requirements for electronic products, my country promulgated the "GB13837-1997 "Limits and Measurement Methods of Wireless Interference Characteristics of Sound and Television Broadcast Receivers and Related Equipment" standard in July 1997. This standard specifically stipulates the limits of five types of electromagnetic interference.

(1) Disturbance voltage injected into the power supply. In the range of 9 to 30 kHz, if the interference voltage injected into the power grid by electronic equipment exceeds 66 dBμV, the interference signal will cause serious interference to other equipment through the power grid, thereby reducing the performance of other equipment or making it unable to work normally.

(2) Antenna-end interference voltage: When the receiving device is connected to a common antenna or the antennas are very close to each other, interference signals can easily cause mutual interference through distribution cables or coupling between antennas.

(3) RF output useful signal and interference signal level. When the RF output of the device is connected to other devices, if the RF output useful signal and its harmonic level of the device are too high, the high-frequency harmonic radiation generated will interfere with adjacent devices.

(4) Radiated disturbance field strength or power. The radiation field strength of the local oscillator and its harmonics and the intermediate frequency unit of the broadcast receiver cannot be too high, otherwise it will pollute the electromagnetic environment.

(5) Interference power. The radiation of audio power amplifiers, audio and video disc players and other equipment above 30 MHz mainly radiates energy outward through the power cords and other connecting lines connected to them. When the peak value of the interference power on the power cord and other connecting lines is greater than 55 dBμV, it will affect the normal operation of other equipment.

2 EMI propagation mechanism of digital video disc player

The propagation of EMI is mainly divided into two types: conducted interference and radiated interference. Radiated interference refers to the interference between systems. It radiates electromagnetic fields outward in the form of space electromagnetic waves through leads and components of various frequency interferences generated inside electronic products, thereby interfering with other electronic equipment. For example: the high-frequency signal lines and various connectors of DVD players may become radiation interference sources with antenna characteristics. Conducted interference is the interference formed by coupling signals through conductive media in the system. It is generally caused by the interconnection of impedance (power supply internal resistance, impedance between ground wires), mutual inductance (between conductors) and parasitic capacitance. For example: the line radiation of a TV.

There are many specific methods to eliminate or reduce electromagnetic interference, mainly shielding technology, power supply filtering, grounding technology and the use of EMI devices. For DVD digital video disc players, since there is no antenna and RF output port, the interference voltage at the antenna end is very small, mostly below 30 dBμV, and its main propagation paths are as follows: the pulse current and voltage generated by the switching power supply change very quickly, and the switching power supply switching pulses and high-order harmonics formed are radiated and conducted outward through components and power lines, generating electromagnetic interference; audio/video output signals are radiated outward through audio/video signal connection lines; various clock signals and their high-order harmonics required for the operation of digital circuits, such as the CPU chip working clock, MPEG decoder working clock, and video synchronization clock (27 MHz, 16.9344 MHz, 40.5 MHz) form interference pulses, which are radiated outward through components.

3 EMI Design

3.1 EMI Design of Switching Power Supply

Switching power supply is an important component of electronic equipment. The electrical signals running in electronic equipment are much smaller in amplitude and energy than switching power supply, which means that switching power supply is usually the strongest interference source of electronic equipment. There are many reasons for the interference of switching power supply, such as the random changes of load and voltage in the external power grid, which causes the input current of the switching power supply to contain more noise; the high-speed switching between cut-off and conduction of the internal power switch tube produces abundant high-order harmonic interference, the sudden peak current interference generated by the reverse recovery of the rectifier diode, the high-speed change of voltage and the conduction interference generated by the distributed capacitance to the ground, etc. In DVD digital video disc players, in order to reduce the EML interference of the switching power supply, the following measures are taken:

3.1.1 EMI Design of Switching Transformer

In order to reduce the interference in the power supply circuit, the switch transformer adopts the flyback excitation method first. The schematic diagram of the flyback transformer switch power supply is shown in Figure 1. Ui is the input voltage of the switch power supply, T is the high-frequency transformer, and SW is the control switch. Due to the existence of the rectifier diode, the flyback switch power supply does not provide power output to the load during the control switch is turned on. Only when the control switch is turned off, the stored energy is converted into back electromotive force and output to the load. Therefore, when the input power changes, the switch power supply cannot immediately respond to the output voltage and current. It only works when the power is turned off in the next working cycle, that is, the transient response of the output voltage of the flyback switch power supply is slow; at the same time, due to the large capacity of the energy storage filter capacitor, the output voltage is stable and the high-frequency interference is reduced.

In addition, the flyback power transformer should pay attention to the size of the transformer air gap. The larger the air gap, the stronger the transformer's load capacity, but the greater the transformer's leakage inductance. Under the premise of meeting the load capacity, the leakage inductance should be as small as possible to reduce the interference caused by the leakage inductance. At the same time, in order to reduce the stray capacitance of the primary and secondary sides, a copper shielding layer is added between the primary and secondary sides of the transformer to cut off the interference propagation channel between the primary and secondary sides.

3.1.2 Switching Transformer Output Filter Design

Good filtering technology has a significant effect on the EMI of switching power supplies. Connect LC common-mode and differential-mode filtering and π-type filters (as shown in Figure 2) at the output end of the switching transformer to suppress EMI interference in the switching power supply. Cx is the power supply jumper capacitor, which filters out common-mode interference signals. Ceramic capacitors or polyester film capacitors are commonly used, with a value of 0.22 to 0.47 μF; Cy is a bypass capacitor, which filters out differential-mode signals. It is below 4,700 pF. Too much capacitance will affect the insulation performance of the equipment; L1 is a common-mode coil, which consists of two windings wound on the same high-permeability magnetic core, with an inductance of tens of mH. Adjust the inductance and capacitance to make the resonant frequency close to the interference frequency or close to the center frequency of the interference frequency. For high-frequency electromagnetic interference, three-terminal capacitors or through-hole capacitors can be used for filtering.

In addition, the leads of Cx and Cy and the ground lead should be as short as possible to minimize the ground impedance and allow the noise to be bypassed to the ground through the capacitor; the filter should be as close to the input port as possible to avoid coupling between the filter input and output and thus loss of filtering effect.

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3.1.3 Using MOS switch integrated circuit

The STR6651 MOS switch integrated circuit is used. It is a semiconductor device with majority carrier movement. The required driving power is small, so the electromagnetic radiation energy of the components of the driving circuit is small. In addition, the switch integrated circuit contains a pulse peak absorption circuit, which greatly reduces the peak interference.

3.2 EMI Design for Audio/Video

3.2.1 Use cables with filter connectors

In audio/video design, the weakest EMI design link is the cable connecting the equipment. Ordinary shielded cables are metal braided nets, and the holes on them will cause leakage at high frequencies, which cannot meet the shielding requirements for high-frequency interference. For this reason, cables with filter connectors are used, and a low-pass filter is installed on the hole of each connector to eliminate high-frequency interference on the signal line. This reduces the radiation on the cable.

3.2.2 Connecting EMI devices in series with audio/video output ports

Different EMI devices have different high-frequency attenuation frequencies, and their parameter selection should be determined based on EMI tests. For example, in a DVD-1000 digital video player, the audio EMI parameter is: at 100 MHz, Z = (1 000 ± 250) Ω. In this way, the high-frequency energy of the output signal is greatly reduced, that is, the high-frequency interference is greatly reduced.

3.2.3 Audio/Video Output Wiring

In order to reduce the grounding resistance and eliminate the influence of distributed capacitance, the audio/video output RCA socket adopts a large-area grounding, and its grounding terminal is connected to the metal shell; at the same time, the design of video D/A and audio D/A is strengthened to make the high-frequency harmonic components in the output signal very small. 3.3 EMI design of PCB board

Pay attention to EMI design in the layout of devices. Each functional circuit is centered on the core chip, and related components are arranged around the core chip. The signals of each functional circuit are smooth, and the phenomenon of signal backflow is minimized. Interference devices are kept as far away as possible from I/O ports and connecting lines such as keypads and remote control receiving boards. Each signal connecting line cannot cross circuits with large interference. The reset circuit and clock circuit are arranged reasonably, and they should be as close to the device pins as possible. The position of the filter capacitor is arranged reasonably, and the filter capacitor should be as close as possible to the device pins that need to be filtered.

In PCB board wiring, analog and digital circuits must be routed within their functional areas, with priority given to important devices and important signals (clock/address/data/reset lines, etc.) to ensure that these connections are as short as possible; the metal casing of the crystal oscillator must be well grounded, and signal lines must not be routed under the crystal oscillator device; EMI devices for connecting wire terminals and I/O ports must be placed next to the connecting wire terminals and I/O ports, and signal routing must first pass through the EMI devices and then to the terminals, etc.

3.4 EMI test results of DVD digital video disc player

The DVD-1000 digital video disc player has effectively reduced high-frequency interference through the above EMI design and measures and has passed the domestic EMI test. Figure 3 and Figure 4 are the test reports of the injection power disturbance voltage at the L and N terminals.

4 Conclusion

DVD digital video disc players have many potential radiation sources, which can interfere with the environment and make the design fail to pass the 3C certification. As a designer, you should implement EMI awareness as early as possible, eliminate electromagnetic interference, and fundamentally improve the quality and reliability of the product.