Common mode Choke, also known as common mode choke, is commonly used in computer switching power supplies to filter common mode electromagnetic interference signals. In the design of the board, the common mode inductor also acts as an EMI filter to suppress the electromagnetic radiation generated by the high-speed signal line from radiating outward.
1. As an important part of magnetic components, inductors are widely used in power electronic circuits. Especially in the power circuit is an indispensable part. Such as electromagnetic relays in industrial control equipment, electrical power meters (electric meters) of power systems. The filter of the input and output of the switching power supply device, the tuner for the TV receiving and transmitting terminals, and the like are inseparable from the inductor. The main functions of inductors in electronic circuits are: energy storage, filtering, turbulence, resonance, and so on. In the power supply circuit, since the circuit processing is the energy transfer of a large current or a high voltage, the inductor is mostly a "power type" inductor.
It is precisely because the power inductor is different from the small signal processing inductor. In the design, the topology of the switching power supply is different, and the design method has its own requirements, which makes the design difficult. The inductors in current power circuits are mainly used for filtering, energy storage, energy transfer, and power factor correction. The inductor design covers many aspects of electromagnetic theory, magnetic materials and safety regulations. Designers need to have a clear understanding of the working conditions and related parameter requirements (such as: current, voltage, frequency, temperature rise, material properties, etc.). The most reasonable design.
2, the classification of inductors:
Inductors can be divided into different types according to their application environment, product structure, shape, use, etc. Usually, the inductor design starts with the application and application environment as the starting point. In the switching power supply, the inductor can be divided into:
Common Mode Choke
Normal Mode Choke (Normal Mode Choke)
Power Factor Correction - PFC Choke
Coupling Choke
Energy storage wave inductor (Smooth Choke)
Magnetic amplifier coil (MAG AMP Coil)
The common mode filter inductor requires the same inductance value and the same impedance as the two coils. Therefore, the inductors are designed with symmetry, and the shapes are mostly TOROID, UU, ET and the like.
3. Working principle of common mode inductor:
Common mode filter inductors are also known as common mode choke coils (hereinafter referred to as common mode inductors or CM.M.Choke) or Line Filter.
In the switching power supply, due to the sharp change of the current or voltage in the rectifier diode and the filter capacitor and the inductor, an electromagnetic interference source (noise) is generated, and at the same time, high-order harmonic noise other than the power frequency is also present in the input power source, and these interferences are not Tanning will cause damage to the load device or the switching power supply itself, so the safety agencies in several countries have made electromagnetic interference (EMI) emissions.
The corresponding regulations. At present, the switching frequency of switching power supplies is becoming more and more high-frequency EMI. Therefore, EMI filters must be installed in the switching power supply. The EMI filter needs to suppress the normal mode and common mode noise to achieve a certain regulation. standard. The normal mode filter is responsible for filtering out the differential mode interference signal between the two lines at the input or output. The common mode filter is responsible for filtering out the common mode interference signals of the two incoming lines. The actual common mode inductance can be divided into AC CM.M.CHOKE, DC CM.M.CHOKE and SIGNAL CM.M.CHOKE due to different working environments. It should be distinguished when designing or selecting. But its working principle is exactly the same, the working principle is shown in Figure (1):
As shown in the figure, two sets of coils with opposite directions are wound on the same magnetic ring. According to the rules of the right-handed spiral tube, when the opposite ends of the input terminals A and B are added, the differential mode voltages of the same amplitude are applied. When there is a current i2 indicated by a solid line, a magnetic flux Φ2 indicated by a solid line is generated in the magnetic core, and as long as the two windings are completely symmetrical, magnetic fluxes in two different directions in the magnetic core cancel each other. The total flux is zero, the coil inductance is almost zero, and there is no impedance to the normal mode signal. If a common mode signal of the same polarity and equal amplitude is applied to both ends of the input terminals A and B, the current i1 indicated by a broken line is generated, and the magnetic flux Φ1 indicated by a broken line is generated in the magnetic core, and the magnetic core is magnetic. The same direction is used to strengthen each other, so that the inductance value of each coil is twice that of the single existence, and XL = ωL. Therefore, the coil of this winding method has a strong suppression effect on common mode interference.
The actual EMI filter is composed of L and C. The differential mode and the common mode suppression circuit are often combined in the design (Figure 2). Therefore, the design should be based on the size of the filter capacitor and the required safety regulations. The standard makes the decision on the inductance value.
In the figure, L1, L2, and C1 constitute a normal mode filter, and L3, C2, and C3 constitute a common mode filter.
4, the design of common mode inductor
Before designing a common-mode inductor, the first thing to consider is that the coil must meet the following principles:
1 > Under normal working conditions, the core will not be saturated due to the power supply current.
2 > The high-frequency interference signal should have sufficient impedance and a certain bandwidth, and have the minimum impedance to the signal current of the working frequency.
3 > The temperature coefficient of the inductor should be small, and the distributed capacitance should be small.
4 > DC resistance should be as small as possible.
5 > Inductive induction is as large as possible, and the inductance value needs to be stable.
6 > The insulation between the windings must meet the safety requirements.
Common mode inductor design steps:
Step 0 SPEC acquisition: EMI allowed level, application location.
Step 1 The inductance value is determined.
Step 2 core material and specifications are determined.
Step 3 Winding turns and wire diameter are determined.
Step 4 proofing
Step 5 test
5, design examples
Step 0 : EMI filter circuit as shown in Figure 3.
CX = 1.0 Uf Cy = 3300PF EMI rating: Fcc Class B
Type : Ac Common Mode Choke
Step 1: The inductance (L) is determined:
It can be seen from the circuit diagram that the common mode signal is suppressed by the common mode filter composed of L3 and C2 and C3. The actual L3 and C2 and C3 form a two-way LC series circuit, respectively absorbing the noise of the L and N lines. As long as the cutoff frequency of the filter circuit is determined, and the capacitance capacity C is also known, the inductance L can be obtained by the following equation.
Fo= 1/(2π√LC)L → 1/(2πfo)2C
Usually the EMI test bandwidth is as follows:
Conducted interference: 150KHZ → 30MHZ (Note: VDE standard 10KHZ - 30M)
Radiation interference: 30MHZ 1GHZ
The actual filter can't reach the steep impedance curve like the ideal filter, and the cutoff frequency can usually be set at around 50KHZ. Here, assuming fo = 50KHZ, then
L =1/(2Ï€fo)2C = 1/ [( 2*3.14*50000)2 *3300*10-12] = 3.07mH
L1, L2, C1 form (low pass) normal mode filter, the line capacitance is 1.0uF, then the normal mode inductance is:
L = 1/ [( 2*3.14*50000)2 *1*10-6] = 10.14uH
In this way, the theoretically required inductance value can be obtained. If a lower cutoff frequency fo is to be obtained, the inductance value can be further increased, and the cutoff frequency is generally not lower than 10 kHz. In theory, the higher the inductance, the better the EMI suppression effect, but the too high inductance will make the cutoff frequency lower, and the actual filter can only achieve a certain bandwidth, which will make the suppression effect of high frequency noise worse. The noise component of the switching power supply is between 5 and 10 MHz, but it is also more than 10 MHz. In addition, the higher the inductance, the more the winding turns, or the higher the CORE ui, which will cause the low frequency impedance to increase (the DCR becomes larger). The increase in the number of turns increases the distributed capacitance (see Figure 4), allowing the high-frequency current to flow through the capacitor. Excessive ui makes CORE extremely saturated, and it is extremely difficult to manufacture and costly.
Step 2 CORE material and SIZE OK
It can be seen from the above design requirements that the common mode inductor needs to be less saturated, so it is necessary to select a material with a low B-H angle ratio. Because a higher inductance value is required, the ui value of the core is also high, and must also have Low core loss and high Bs value, CORE material meeting the above requirements, currently the most suitable Mn-Zn ferrite material CORE.
The COEE SIZE is not specified at the time of design. In principle, as long as it meets the required inductance and is within the allowable low frequency loss range, the designed product volume can be minimized.
Therefore, CORE material and SIZE extraction should be investigated in terms of cost, allowable loss, and installation space. Common mode inductors commonly use CORE ui between 2000 ~ 10000. Iron Powder Core also has low iron loss, high Bs and low B-H angle ratio, but its ui is low, so it is generally not applied to common mode inductors, but this type of core is a normal mode inductor. Preferred material.
Step 3 determines the number of turns N and the diameter dw
First determine the specification of CORE. If T18*10*7, A10 and AL=8230±30% are used in this example, then:
N = √L / AL = √(3.07*106 ) / (8230*70%) = 23 TS
The wire diameter is based on the current density of 3 ~ 5A / mm2. If space is allowed, the current density can be selected as low as possible. Assume that the input current I i = 1.2A in this example, take J = 4 A / mm2
Then Aw = 1.2 / 4 = 0.3 mm2 Φ0.70 mm
The actual common mode inductor must also be tested by real-world samples to confirm the reliability of the design, because the difference in manufacturing process will also lead to differences in inductance parameters and affect the filtering effect. For example, the increase of distributed capacitance will make high-frequency noise. It is easier to transfer, and the asymmetry of the two windings makes the difference between the two groups become larger, forming a certain impedance to the normal mode signal.
6, summary
1 > The function of the common mode inductor is to filter out the common mode noise in the line. The design requires that the two windings have a completely symmetrical structure with the same electrical parameters.
2 > The distributed capacitance of the common mode inductor has a negative effect on suppressing high frequency noise and should be minimized.
3 > The inductance of the common mode inductor is related to the noise band to be filtered and the capacitance of the capacitor. The sense value is usually between 2mH and 50mH.
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