How to improve the filter’s ability to suppress differential mode noise

How to improve the filter’s ability to suppress differential mode noise

With the development and application of microelectronics technology, electromagnetic compatibility has become an important topic for studying the safe and stable operation of microelectronic devices. The technologies used to suppress electromagnetic interference mainly include filtering technology, layout and wiring technology, shielding technology, grounding technology, sealing technology, etc. The propagation path of the interference source is divided into conducted interference and radiated interference. The frequency range of conducted noise is very wide, from 10kHz to 30MHz, only from the cause of the interference, it is not necessarily a good way to solve the interference problem by controlling the rise and fall time of the pulse.To understand the difference between common mode and differential mode signals, it is important to understand the difference between the pulse magnetic circuit and the working module correctly.

1 Overview

With the development and application of microelectronics technology, electromagnetic compatibility has become an important topic for studying the safe and stable operation of microelectronic devices. The technologies used to suppress electromagnetic interference mainly include filtering technology, layout and wiring technology, shielding technology, grounding technology, sealing technology, etc. The propagation path of the interference source is divided into conducted interference and radiated interference. The frequency range of conducted noise is very wide, from 10kHz to 30MHz, only from the cause of the interference, it is not necessarily a good way to solve the interference problem by controlling the rise and fall time of the pulse. For this reason, understanding the difference between common mode and differential mode signals is essential to correctly understand the relationship between the pulsed magnetic circuit and the working module. Among the various technologies for suppressing electromagnetic interference, the use of filtering technology plays a key role in reducing common mode interference in local area networks (LAN), communication interface circuits, and power circuits. Therefore, mastering the working principle of the filter and the structure of its practical circuit and its correct application is an important link in the system design of microelectronic devices.

2 Differential mode signal and common mode signal

Differential mode signals are also called normal mode, series mode, line-to-line induction and symmetrical signals. In a two-wire cable transmission loop, the voltage of each line to ground is represented by the symbols V1 and V2. The differential mode signal component is VDIFF. The pure differential mode signal is: V1=-V2; its size is equal, the phase difference is 180°; VDIFF=V1-V2, because V1 and V2 are symmetrical to the ground, so there is no current flowing on the ground wire, the circuit of the differential mode signal As shown in Figure 1. All the differential mode current (IDIFF) flows through the load. Differential mode interference invades the two signal lines back and forth, and the direction is consistent with the direction of the signal current. One is generated by the signal source, and the other is generated by electromagnetic induction during transmission. It is stringed together with the signal and has the same phase. Interference is generally difficult to suppress.

 How to improve the filter’s ability to suppress differential mode noise  

The common mode signal is also called ground induction signal or asymmetric signal. The common mode signal component is VCOM, and the pure common mode signal is: VCOM=V1=V2; equal in magnitude, with a phase difference of 0°; V3=0. The circuit of the common mode signal is shown as in Fig. 2. The interference signal invades between the line and the ground, and the interference current flows through one-half of the two lines, and the ground is the common loop; in principle, this interference is relatively easy to eliminate. In the actual circuit, due to the unbalanced line impedance, the common mode signal interference will be converted into crosstalk interference that is not easy to eliminate.

 How to improve the filter’s ability to suppress differential mode noise

3 filter

The filter can suppress the interference signal input on the AC power line and various interference induced on the signal transmission line. Filters can be divided into AC power filters, signal transmission line filters and decoupling filters. AC power filters are widely used in switching power supply systems, which can not only suppress external high-frequency interference, but also inhibit the switching power supply from sending interference. Transient interference from industrial frequency power supply or lightning strikes invades Electronic equipment through power lines. This interference is transmitted in common mode and differential mode and can be filtered out by power filters. In the filter circuit, there are a lot of special filter components (such as ferrite magnetic ring), they can improve the filter characteristics of the circuit, the appropriate design and use of filters is an important means of anti-interference technology. For example, the noise emitted by the switching power supply through conduction and radiation is divided into differential mode and common mode, and differential mode noise is suppressed by a π-type filter, as shown in (a).

In (a), LD is a filter choke. To have the ability to suppress common mode noise, the filter circuit shown in Figure 3(b) should be used. In Figure 3(b), LC is a filter choke. Since the two coils of the LC have the same winding direction, when the power input current flows through the LC, the generated magnetic fields can cancel each other, which is equivalent to no inductance effect. Therefore, it uses a magnetic core with high permeability. For common mode noise, LC is equivalent to a large inductance, which can effectively suppress common mode conduction noise. The capacitors CY connected in parallel to the input terminals of the switching power supply respectively bypass the common mode noise. The capacitor CX connected in parallel at both ends of the common mode choke suppresses common mode noise. R is the discharge resistance of CX, which is recommended by VDE0806 and IEC380 safety technical standards. The parameter range of each component in Fig. 3(b) is: CX=0.1μF~2μF; CY=2.0nF~33nF; LC=a few to several tens of mH, and different parameter values ​​will be taken depending on the working current, such as when the current is 25A LC=1.8mH; when the current is 03A, LC=47mH. In addition, in the selection of filter components, we must ensure that the resonant frequency of the input filter is lower than the operating frequency of the switching power supply.

The filter shown can further improve the ability to suppress differential mode noise. In addition to the power supply voltage applied to the CX, various peak voltages of electromagnetic interference existing between the phase line and the neutral line will also be superimposed. In order to ensure that the capacitor does not endanger personal safety after failure, and taking into account the worst case in the application, CX safety levels are divided into two categories, namely X1 and X2. The X1 level is used for the occasions where the peak voltage of the equipment is greater than 12kV, and the X2 is used. For general occasions where the peak voltage of the equipment is less than 1.2kV. In addition, by limiting the capacity of CY, the leakage current flowing through the capacitor can be controlled under the action of the specified voltage and frequency. If it is a filter installed on a movable device, its AC leakage current should be less than 1mA. If it is a filter installed on a fixed and grounded device, its AC leakage current should be less than 35mA, and then according to the leakage current Ii The requirement to calculate the capacity of CY, the relational formula is:

Ii=2πfCYU

In the formula: f-power frequency;

U-power supply voltage.

LD is a differential mode choke used to further suppress differential mode noise. Because the introduction of LD will reduce the charging current of the capacitor CX, the purpose of suppressing differential mode noise is achieved.

4 Installation and wiring of the filter

The installation and wiring of the AC filter directly affect the performance of the filter. Pay attention to the following points in the installation and wiring:

(1) The filter should be installed at the bottom of the cabinet as close as possible to the power inlet of the equipment, and insulated. Do not allow the power cord that has not passed through the filter to bypass the cabinet. If the AC power enters the cabinet and there is a gap between the power supply filter For longer distances, this line should be shielded.

(2) The shell of the power filter must be connected to the shell at the shortest distance with a wire with a large cross-sectional area, and try to keep the grounding point of the power filter and the shell grounding point as short as possible, and the input and output lines should be close Wiring at the bottom of the case to reduce coupling, and strictly separate the input and output lines. It is never allowed to bundle the input and output lines of the filter together or close together, otherwise, when the interference frequency reaches several megahertz or more, this At this time, the input and output lines will be coupled to each other to reduce its attenuation effect on high-frequency interference signals. The socket type AC power filter realizes the isolation of input and output from the structure, and is an ideal anti-interference component for some electronic equipment that directly uses the casing as a shield. The output wire of the filter should be twisted-pair wire or shielded wire, and the shielding should be reliably grounded.

(3) Other electrical appliances (lights, signal lights, etc.) or electromagnetic switches in the cabinet should be connected to the load from the front end of the filter, or a separate filter should be installed for these interference sources.

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