Calculating the common-mode inductance of a switching power supply is actually not difficult!

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Calculating the common-mode inductance of a switching power supply is actually not difficult! [Copy link]

As an important component of magnetic components, inductors are widely used in power electronic circuits. In particular, they are an indispensable part in power supply circuits. Such as electromagnetic relays in industrial control equipment, electric power meters (watt-hour meters) in power systems. Filters at the input and output ends of switching power supply equipment, tuners at the receiving and transmitting ends of televisions, etc. are all inseparable from inductors. The main functions of inductors in electronic circuits are: energy storage, filtering, choking, resonance, etc. In power supply circuits, since the circuits process energy transfer with large currents or high voltages, most inductors are "power-type" inductors. It is precisely because power inductors are different from small signal processing inductors that the design methods have different requirements due to the different topological methods of switching power supplies, which makes the design difficult. At present, inductors in power supply circuits are mainly used for filtering, energy storage, energy transfer, and power factor correction. Inductor design covers many aspects of knowledge such as 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.) to make the most reasonable design.

Classification of Inductors

Inductors can be divided into different types based on their application environment, product structure, shape, purpose, etc. Usually, the design of inductors starts with the purpose and application environment. In switching power supplies, inductors can be divided into the following types based on their different uses:

Common Mode Choke

Normal Mode Choke

Power Factor Correction (PFC Choke)

Coupler Choke

Energy storage smoothing inductor (Smooth Choke)

MAG AMP Coil

Common-mode filter inductors require that the two coils have the same inductance value, the same impedance, etc., so this type of inductor adopts a symmetrical design, and its shapes are mostly TOROID, UU, ET, etc.

Working principle of common mode inductor

Common mode filter inductor is also called common mode choke coil (hereinafter referred to as common mode inductor or CM.M.Choke) or Line Filter.

In a switching power supply, due to the rapid changes in current or voltage in the rectifier diode, filter capacitor, and inductor, electromagnetic interference sources (noise) are generated. At the same time, there are also high-order harmonic noises other than the power frequency in the input power supply. If these interferences are not suppressed, they will cause damage to the load equipment or the switching power supply itself. Therefore, the safety agencies of several countries have made corresponding control regulations on the emission of electromagnetic interference (EMI). At present, the switching frequency of switching power supplies is becoming higher and higher, and EMI is becoming more and more serious. Therefore, EMI filters must be installed in switching power supplies. EMI filters must suppress both normal mode and common mode noise to meet certain specified standards. Normal mode filters are responsible for filtering out differential mode interference signals between the two lines at the input or output end, and common mode filters are responsible for filtering out common mode interference signals between the two input lines. The actual common mode inductor can be divided into three types: AC CM.M.CHOKE, DC CM.M.CHOKE, and SIGNAL CM.M.CHOKE due to their different working environments. They should be distinguished when designing or selecting. However, their working principles are exactly the same. The working principle is shown in Figure (1):

Figure 1 The role of common mode inductance

As shown in the figure, two sets of coils with opposite directions are wound on the same magnetic ring. According to the right-hand spiral tube rule, when a differential mode voltage with opposite polarity and the same signal amplitude is added to the input terminals A and B, there is a current i2 shown by the solid line, and a magnetic flux Φ2 shown by the solid line is generated in the magnetic core. As long as the two windings are completely symmetrical, the magnetic fluxes in two different directions in the magnetic core cancel each other out. The total magnetic flux is zero, the coil inductance is almost zero, and there is no impedance effect on the normal mode signal. If a common mode signal with the same polarity and equal amplitude is added to the input terminals A and B, there is a current i1 shown by the dotted line, and a magnetic flux Φ1 shown by the dotted line is generated in the magnetic core. The magnetic fluxes in the magnetic core have the same direction and reinforce each other, making the inductance value of each coil twice that of when it exists alone, and XL =ωL. Therefore, the coil wound in this way has a strong inhibitory effect on common mode interference.

The actual EMI filter is composed of L and C. The differential mode and common mode suppression circuits are often combined together during design (as shown in Figure 2). Therefore, the inductance value must be determined based on the size of the filter capacitor and the safety standards that need to be met during design.

In the figure, L1, L2, and C1 constitute a normal mode filter, and L3, C2, and C3 constitute a common mode filter.

Figure 2 EMI filter circuit

Before designing a common-mode inductor, we must first examine whether the coil meets the following principles:

1 > Under normal working conditions, the core will not be saturated due to the power supply current.

2 > It must have a sufficiently large impedance to high-frequency interference signals and a certain bandwidth, and have a minimum impedance to the signal current of the operating frequency.

3 >The temperature coefficient of inductance should be small, and the distributed capacitance should be small.

4 > DC resistance should be as small as possible.

5 > The inductance should be as large as possible and the inductance value should be stable.

6 > The insulation between windings must meet safety regulations.

Common mode inductor design steps:

Step 0 SPEC acquisition: EMI allowable level, application location.

Step 1: Determine the inductance value.

Step 2: Determine the core material and specifications.

Step 3: Determine the number of winding turns and wire diameter.

Step 4: Proofing

Step 5 Testing

Design Examples

Figure 3 Actual EMI filter

Step 0: EMI filter circuit as shown in Figure 3

CX = 1.0 Uf Cy = 3300PF EMI level: Fcc Class

Type : Ac Common Mode Choke

Step 1: Determine the inductance (L):

From the circuit diagram, we can see that the common-mode signal is suppressed by the common-mode filter composed of L3, C2 and C3. In fact, L3, C2 and C3 form two LC series circuits to absorb the noise on the L and N lines respectively. As long as the cut-off frequency of the filter circuit is determined and the capacitance C is known, the inductance L can be calculated by the following formula.

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)

Radiated interference: 30MHZ 1GHZ

The actual filter cannot achieve the steep impedance curve of the ideal filter, and the cut-off frequency can usually be set at around 50KHZ. Here, assuming fo = 50KHZ, then

L =1/(2πfo)2C = 1/ [( 2*3.14*50000)2 *3300*10-12] = 3.07mH

L1, L2, C1 form a (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 you want to obtain a lower cut-off frequency fo, you can further increase the inductance value. The cut-off frequency is generally not less than 10KHZ. In theory, the higher the inductance, the better the EMI suppression effect, but too high an inductance will make the cut-off frequency lower, and the actual filter can only achieve a certain bandwidth, which makes the suppression effect of high-frequency noise worse (the noise component of a general switching power supply is about 5 ~10MHZ, but there are also cases where it exceeds 10MHZ). In addition, the higher the inductance, the more turns of the winding, or the higher the ui of the CORE, which will cause the low-frequency impedance to increase (DCR becomes larger). The increase in the number of turns also increases the distributed capacitance (as shown in Figure 4), so that all high-frequency currents flow through this capacitor. Too high ui makes the CORE very easy to saturate, and it is also extremely difficult to make and the cost is also high.

Figure 4 Equivalent circuit diagram with distributed capacitance Cd

Step 2 Determine the CORE material and size

From the above design requirements, it can be seen that the common mode inductor should not be easy to saturate, 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 must also be high. At the same time, it must also have a lower core loss and a higher Bs value. The core material that meets the above requirements is currently the most suitable Mn-Zn ferrite material core.

There is no specific regulation for COEE SIZE during design. In principle, as long as the required inductance is met and the low-frequency loss is within the allowable range, the size of the designed product can be minimized.

Therefore, the CORE material and SIZE selection should be considered based on cost, allowable loss, installation space, etc. The ui of the common CORE used in common mode inductors is about 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 used in common mode inductors, while this type of magnetic core is the preferred material for normal mode inductors.

Step 3 Determine the number of turns N and wire diameter dw

First, determine the specifications of the CORE. For example, in this example, T18*10*7, A10, AL = 8230±30%, then:

N = √L / AL = √(3.07*106 ) / (8230*70%) = 23 TS

The wire diameter is selected based on the current density of 3 ~ 5A / mm2. If space permits, the current density can be as low as possible. Assuming 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 through actual samples to confirm the reliability of the design, because differences in manufacturing processes will also lead to differences in inductance parameters and affect the filtering effect. For example, the increase in distributed capacitance will make high-frequency noise easier to transmit, and the asymmetry of the two windings will increase the difference in inductance between the two groups, forming a certain impedance to the normal-mode signal.

summary

1 > The function of the common-mode inductor is to filter out the common-mode noise in the circuit. The design requires that the two windings have a completely symmetrical structure and the same electrical parameters.

2 > The distributed capacitance of the common mode inductor has a negative impact on suppressing high-frequency noise and should be minimized.

3 > The inductance of the common mode inductor is related to the noise frequency band to be filtered and the matching capacitor capacity. The inductance is usually between 2mH ~ 50mH