Design Scheme of Electromagnetic Interference for DVD Digital Video 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. Starting from the requirements of the National Standardization Organization for electromagnetic interference, the three main reasons for electromagnetic interference generated by the digital video disc player are analyzed. From this, the three aspects of the switching power supply, audio / video output and PCB circuit board are discussed and designed. Several methods of suppressing electromagnetic interference are passed, and the EMI test is passed.
Electromagnetic interference (Electro MagneTIc Interference, EMI) refers to an electronic device that continuously emits interference that exceeds the allowable range into its environment. In order to protect the environment, the National Standardization Organization has formulated a number of mandatory standards that require electronic equipment manufacturers to reduce the electromagnetic interference of their products to a certain degree. Therefore, from the beginning of the design, we must pay attention to the EMI design of electronic equipment, comprehensive consideration from the aspects of the selection of components and connectors, the layout of the printed board, the grounding point, etc., to reduce electromagnetic interference. This article takes the DVD-1000 digital video disc player as an example to introduce some experience of EMI design.
1 EMI design requirements For the EMI requirements of electronic products, China promulgated the standard "GB13837-1997" Limits and Measurement Methods of Radio Interference Characteristics of Sound and Television Broadcast Receivers and Related Equipment "" in July 1997, which specifically specifies Five kinds of electromagnetic interference limits.
(1) The disturbance voltage injected into the power supply. In the range of 9-30 kHz, if the interference voltage injected by the electronic equipment into the power grid 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 preventing it from working properly.
(2) Disturbance voltage at the antenna end. When the receiving device is connected to an external public antenna or the antennas are very close to each other, the interference signals can easily cause mutual interference through the distribution cable or the coupling between the antennas.
(3) The useful signal and disturbance signal level of the RF output device. When the RF output terminal of the device is connected to other devices, if the RF output signal 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, and the radiated field strength of an intermediate frequency unit must not be too high, otherwise it will pollute the electromagnetic environment.
(5) Disturbance power. The radiation of audio power amplifiers, audio-visual disk drives and other devices at frequencies above 30 MHz is mainly radiated outward through the power cord and other connections connected to them. When the peak value of the disturbance power on the power cord and other wiring 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 conduction interference and radiated interference. Radiated interference refers to the interference between various systems. It radiates electromagnetic fields in the form of space electromagnetic waves through wires and components through various frequencies generated inside electronic products, thereby causing interference to other electronic equipment. For example, the high-frequency signal line of DVD player and various connectors may become the radiation interference source with antenna characteristics. Conducted interference is the interference caused by the coupling of signals through a conductive medium within the system. It is generally due to the cross-linking of impedance (internal resistance of the power supply, the impedance between the ground wires), and the cross-linking of mutual inductance (guide line Time) and internal interference caused by the interconnection of parasitic capacitance. For example: line radiation of 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 the DVD digital video disc player, because there is no antenna and RF output port, the disturbance voltage at the antenna end is very small, mostly less than 30 dBμV, and its main propagation routes are as follows: the pulse current and voltage generated by the switching power supply change rapidly and form The switching pulses and high-order harmonics of the switching power supply are radiated and conducted outward through the components and the power line to generate electromagnetic interference; the audio / video output signal is radiated outward through the audio / video signal connection line; various clocks required for digital circuit operation Signals and their higher harmonics, such as interference pulses formed by the CPU chip working clock, MPEG decoder working clock, and video synchronization clock (27 MHz, 16.934 4 MHz, 40.5 MHz), radiate outward through the components.
3 EMI design
3. EMI design of the switching power supply The switching power supply is an important part of the electronic equipment. The electrical signals running in electronic equipment are much smaller in magnitude and energy than switching power supplies, which means that switching power supplies are usually the strongest source of interference for electronic equipment. There are many reasons for the interference caused by the switching power supply. For example, the input current of the switching power supply contains more noise due to the random changes in the load and voltage in the external power grid; the high-speed switching of the internal power switch tube between the cut-off and the turn-on produces rich high Sub-harmonic interference, abrupt peak current interference caused by reverse recovery of rectifier diodes, conducted interference caused by high-rate changing voltage and distributed capacitance to ground, etc. In the DVD digital video disc player, in order to reduce the EMl interference of the switching power supply, the following measures were taken:
3.1.1 EMI design of the switching transformer In order to reduce the interference in the power supply circuit, first of all, the switching transformer adopts the flyback excitation method. The principle diagram of the flyback transformer switching power supply is shown in Figure 1. Ui is the input voltage of the switching power supply, T is the high-frequency transformer, and SW is the control switch. Due to the existence of the rectifier diode, the flyback switching power supply does not provide power output to the load during the control switch is turned on, and the stored energy is converted into back electromotive force to the load output only during the control switch is turned off. Therefore, when the input power changes , The switching power supply can not immediately reflect the output voltage and current, and it will only work when the power is turned off in the next working cycle, that is, the transient response of the output voltage of the flyback switching power supply is slow; at the same time, due to the energy storage filter capacitor The large capacity makes the output voltage stable and reduces high-frequency interference.
In addition, the flyback power transformer should pay attention to the size of the air gap of the transformer. The larger the air gap, the stronger the load capacity of the transformer, but the greater the leakage inductance of the transformer. Under the premise of meeting the load capacity, the leakage inductance should be as small as possible. To reduce the interference caused by 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 of the transformer to cut off the interference propagation path of the primary and secondary.
3.1.2 Design of the output filter of the switching transformer A good filtering technique has a significant effect on the EMl of the switching power supply. Connect the LC common mode-differential mode filter and π-type filter (as shown in Figure 2) at the output of the switching transformer to suppress EMI interference in the switching power supply. Cx is the power supply jumper capacitor to filter out common-mode interference signals. Commonly used ceramic capacitors or polyester film capacitors take 0.22 ~ 0.47μF; Cy is the bypass capacitor to filter differential mode signals below 4 700 pF. Excessive capacitance will affect the insulation performance of the equipment; L1 is a common-mode coil, consisting of two windings wound on the same high-permeability magnetic core, with an inductance of tens of mH. Adjust the inductance and capacitance so that the resonance frequency is close to the interference frequency or close to the center frequency of the interference frequency. For high frequency electromagnetic interference, you can use three-terminal capacitor or feed-through capacitor for filtering.
In addition, the leads of Cx and Cy and the ground lead should be as short as possible to make the ground impedance as small as possible, and the noise can bypass the ground through the capacitor; the filter should be as close as possible to the input port. Avoid coupling the input and output of the filter and lose the filtering effect.
3.l.3 Adopt MOS type switch integrated circuit Use STR6651 MOS type switch integrated circuit, it is a semiconductor device with majority carrier movement, the required driving power is small, the energy of electromagnetic radiation of the components of the driving circuit is small . In addition, the switch integrated circuit contains a pulse spike absorption circuit, which greatly reduces spike interference.
3.2 EMI design of audio / video
3.2.1 Using cables with filter connectors In audio / video design, the weakest EMI design link is the cable connecting the equipment. Ordinary shielded cable is a metal braided mesh, and the holes on it will leak at high frequencies, which cannot meet the shielding of high-frequency interference. To this end, a cable with a filter connector is used, and a low-pass filter is installed on the hole of each connector to eliminate high-frequency interference on the signal line. Thereby reducing the radiation above the cable.
3.2.2 Connect EMI devices in series at the audio / video output port Different EMI devices have different high-frequency attenuation frequencies, and their parameter selection should be determined according to the EMI test. For example, in the DVD-1000 digital video disc player, the audio EMI parameters are: 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 The audio / video output wiring is to reduce the grounding resistance and eliminate the effect of distributed capacitance. The audio / video output RCA block uses a large area of ​​grounding, and its grounding terminal is connected to the metal shell; at the same time, it strengthens the video D / A and audio The design of D / A makes 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 the device, each functional circuit is centered on the core chip, and related components are arranged around the core chip; the signal of each functional circuit is smooth, and the phenomenon of signal reflow is minimized; the interference device is tried as much as possible Keep away from I / O ports and connecting lines such as key board and remote control receiving board; each signal connecting line cannot cross a circuit with great interference; arrange reset circuit and clock circuit reasonably, try to be as close as possible to the device pin; arrange the position of filter capacitor reasonably The filter capacitor should be as close as possible to the pin of the device that needs to be filtered.
In the PCB board wiring, analog and digital circuits must be routed in their functional areas, and priority is given to important devices and important signals (clock / address / data / reset lines, etc.) to ensure that these connections are the shortest; the metal of the crystal oscillator The case should be well grounded, and no signal lines should be run under the crystal device; for the EMI devices of the connection line terminals and I / O ports, they must be placed near the connection line terminals and I / O ports, and the signal traces must pass through the EMI device first To the terminal etc.
3.4 EMI test results of DVD digital video disc player After the above EMI design and measures, the DVD-1000 digital video disc device has effectively reduced high-frequency interference and passed the domestic EMI test. image 3. Figure 4 shows the test report of the injected power disturbance voltage at the L and N terminals.
4 Conclusion DVD digital video players have many potential radiation sources, which can interfere with the environment, so that the design cannot pass the 3C demonstration. As a designer, we should implement EMI awareness as early as possible, eliminate electromagnetic interference, and fundamentally improve product quality and reliability.
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