Principle and use of common mode inductors

Common-mode inductors are one of the most powerful components we use because of the common-mode interference that EMC faces to solve. Here is to give you a brief introduction of the principle of common-mode inductance and use of the situation.

Common mode inductance is a ferrite core of the common-mode interference suppression device, which is the same size, the same number of turns of the same coil is symmetrically around the same ferrite ring core, forming a four-terminal device, to the common-mode signal to show a large inductance has inhibitory effect, and for the differential mode signal presents a very small leakage inductance almost no effect. The principle is that through the common-mode current when the magnetic ring in the magnetic flux superposition, which has a considerable inductance, the common-mode current to suppress, and when the two coils flow through the differential mode current, magnetic ring in each other offset, almost no inductance, so the differential mode current can be passed without attenuation. Therefore, the common mode inductance can effectively suppress the common mode interference signal in the balanced circuit, but it has no effect on the differential mode signal of normal transmission of the line.

Common-mode inductors should meet the following requirements when making:

1) The wire on the core of the coil should be insulated from each other to ensure that there is no breakdown short circuit between turns of the coil in the instantaneous over-voltage action.

2) when the coil flows through the instantaneous large current, the core does not appear saturated.

3) The core of the coil should be insulated from the coil to prevent breakdown between the two under the action of transient overvoltage.

4) The coil should be as close as possible to the single layer, so as to reduce the parasitic capacitance of the coil, enhance the coil to the instantaneous overvoltage and the ability to grant.

Usually, at the same time to pay attention to select the desired filter frequency bands, the larger the common-mode impedance is better, so we select the common mode inductance need to see the device data, mainly based on impedance frequency curve selection. In addition, the choice of attention to consider the differential mode impedance of the impact of the signal, the main focus on differential mode impedance, pay special attention to high-speed ports.

With the emergence and widespread popularization of electronic equipment, computer and household appliances, the noise disturbance of power network becomes more and more serious and forms a public nuisance. In particular, the transient noise interference, its rising speed, short duration, high voltage amplitude (hundreds of kv to thousands of volts), the random strong, the computer and digital circuits prone to serious interference, often make people difficult to guard against, which has caused high attention at home and abroad electronics industry.

Electromagnetic Interference filter (EMI filter) is a new kind of combination device which has been applied in recent years. It can effectively suppress the power grid noise, improve the anti-jamming ability of electronic equipment and the reliability of the system, can be widely used in electronic measuring instruments, computer room equipment, switching power supply, measurement and control systems and other fields.

1 construction principle and application of EMI filter

1.11 Construction Principle

Power supply noise is a kind of electromagnetic interference, its conduction noise spectrum is roughly 10khz~30mhz, up to 150MHz. Depending on the direction of transmission, the power supply noise can be divided into two categories: one is the external interference from the input line of the power supply, the other is generated by the electronic equipment and transmitted through the power line noise. This indicates that the noise is a bidirectional interference signal, and the electronic device is not only the object of noise disturbance, but also a noise source. If the characteristics of the formation, noise interference is divided into series mode interference and the two kinds of interference and mode. The crosstalk is the noise between the two power lines (the line-to-line), and the common-mode interference is the noise of two power lines to the Earth (the line to ground). Therefore, the electromagnetic interference filter should be in line with the electromagnetic compatibility (EMC) requirements, but also must be a bidirectional RF filter, on the one hand to filter out the AC power line introduced by the external electromagnetic interference, on the other hand can also avoid their own equipment to external noise interference, so as not to affect the same electromagnetic environment of other electronic equipment normal work. In addition, the EMI filter should suppress the series mode and common mode interference.

1.2 Basic circuits and typical applications

The electromagnetic interference filter is shown in basic circuit 1.

The five-terminal device has two inputs, two outputs and a ground end, and the housing should be connected to the earth when used. The circuit includes a common mode choke (also known as common mode inductance) L, filter capacitance c1~c4. L to the crosstalk does not work, but when the common mode interference, because the two coils of the same direction of the magnetic flux, after coupling the total inductance increases rapidly, so the common mode signal presents a great inductance, making it difficult to pass, it is called common mode choke. Its two coils are wound on a low-loss, high-permeability ferrite ring, where the magnetic field on the two coils strengthens each other when the current is passed. The inductance of L is related to the rated current I of EMI filter, see table 1.

It should be pointed out that when the rated current is large, the line diameter of the common mode choke should be increased so as to withstand the larger current. In addition, an appropriate increase in inductance can improve the low-frequency attenuation characteristics. C1 and C2 use film capacitors with a capacity range of roughly 0.01μf~0.47μf, which is mainly used to filter out serial-mode interference. C3 and C4 are bridged at the output end, and the neutral point of the capacitor is grounded to effectively suppress common mode interference. C3 and C4 can also be parallel in the input, ceramic capacitors are still selected, the capacity range is 2200pf~0.1μf. To reduce leakage current, the capacitance must not exceed 0.1μf, and the capacitor midpoint should be connected to the Earth. The C1~C4 pressure values are 630VDC or 250VAC. Figure 2 shows an internal circuit of a two-stage composite EMI filter with a two-stage (also known as two-section) filter, which results in better filtering noise. A group pulse filter (also known as a group Impulse counter) has been developed to suppress the disturbance of a fast transient group pulse with a repetition frequency of thousands of Hz at the site of some users.

Application of 2 EMI filter in switching power supply

In order to reduce the volume and reduce the cost, the monolithic switching power supply generally uses the simple single-stage EMI filter, the typical circuit 3 shows

Figure (a) and the capacitor C in figure (b) can filter out the crosstalk, the difference is only the figure (a) will be C on the input side, the figure (b) is received from the output side. The circuit shown in figure (c) and (d) is more complex, and the effect of suppressing interference is better. The L, C1, and C2 in figure (c) are used to filter out common-mode interferences, C3 and C4 to filter out the serial-mode interference. R is the discharge resistance, can be accumulated on the C3 charge discharge, to avoid the accumulation of charge to affect the filtering characteristics; After power off can also make the input terminal of the power supply L, N is not charged, to ensure the safety of use. Figure (d) is the common-mode interference filter Capacitor C3 and C4 connected to the output.

EMI filter can effectively suppress the electromagnetic interference of single-chip switching power supply. In Figure 4, curve A is the waveform of the 0.15mhz~30mhz conduction noise on the switching power supply when the EMI filter is not added (i.e., the peak envelope of electromagnetic interference). Curve B is the waveform after inserting the EMI filter shown in 3 (d), which can attenuate the electromagnetic interference 50dbμv~70dbμv. Obviously, this EMI filter works better.

3 Technical parameters and test methods of EMI Filter

3.1 Main technical parameters

The main technical parameters of EMI filter are: Rated voltage, rated current, leakage current, test voltage, insulation resistance, DC resistance, use temperature range, working temperature rise tr, insertion loss adb, form factor, weight and so on. The most important of the above parameters is the insertion loss (also known as insertion attenuation), which is the main index to evaluate the performance of EMI filter. Insertion loss (AdB) is a function of frequency, expressed in db. The noise power transmitted to the load before and after the insertion of the EMI filter is P1, P2, with the formula:

ADB=10LG P1/P2 (1)

Assuming that the load impedance remains unchanged before and after insertion, the p1=v12/z,p2=v22/z. The V1 is the voltage that the noise source directly adds to the load, and the V2 is the noise voltage on the load after inserting the EMI filter between the noise source and the load, and the V2<

ADB=20LG (2)

Insertion loss is expressed in decibels (db), the greater the decibel value, the stronger the ability to suppress noise interference. Since the theoretical calculation is cumbersome and the error is large, it is usually measured by the manufacturer, the corresponding insertion loss is measured according to the noise spectrum, and then the typical insertion loss curve is given to the user.

Figure 5 shows a typical curve. Visible from the diagram, the product can attenuate the noise voltage of the 1mhz~30mhz by 65dB. The formula for calculating the drain current of the EMI filter is

ILD=2ΠFCVC (3)

ILD is the leakage current and f is the frequency of the power grid. Take Figure 1 as an example, F=50HZ,C=C3+C4=4400PF,VC is the pressure drop on the C3, C4, that is, the output of the ground voltage, it is desirable to vc≈220v/2=110v. By the (3) formula is not difficult to calculate, at this time leakage current ild=0.15ma. C3 and C4 If selected 4700pF, then C=4700pfx2=9400pf,ild=0.32ma. Obviously, the leakage current is proportional to C. The requirement of leakage current is the smaller the better, so the safety is high, generally should be hundreds of micro to a few mah. The requirements for leakage currents are more stringent in electronic medical devices.

It should be pointed out that the rated current is also related to the ambient temperature ta. For example, some foreign manufacturers give the following empirical formula:

I=I1 (4)

I1 is the rated current at 40°c. For example, when Ta=50℃, I=0.88i1, and when Ta=25℃, i=1.1511. This indicates that the rated current value increases with the decrease of temperature, which is due to the improvement of the cooling conditions.

3.2 Method for measuring insertion loss

The measurement of insertion loss is shown in circuit 6.

E is the noise signal generator, Zi is the internal impedance of the signal source, ZL is the load impedance, generally take 50 ω. The noise frequency range is selectable 10khz~30mhz. Firstly, the noise pressure drop V1 and V2 are measured at different frequencies, and then the ADB value of each frequency point is calculated by substituting (2), and then the insertion loss curve is plotted. It should be pointed out that the above test method is cumbersome, each time to disassemble EMI filter. The two test circuits can be quickly switched by electronic switches.

Principle and use of common mode inductors