Common mode chokes are often used in computer switching power supplies to filter common mode electromagnetic interference signals. This article helps you learn about the Features, Working Principle, Selection Method, Precautions, and Core Material of common mode chokes. Click for more.
Catalog
I Introduction | |
II Common Mode Choke Features | |
III Working Principle | 1. Principle of Common Mode Chokes |
2.Differential and Common Mode NoiseSource | |
3. How do Common Mode Chokes Suppress Common Mode Signals? | |
IV Common Mode Choke Selection | |
V Precautions | |
VI Core Material of Common Mode Choke Coil | 1. Magnetic-ring Iron Core |
2. UF/UU Common Mode Choke |
I Introduction
Various high-frequency circuits, digital circuits, and analog circuits are mixed on the mainboard inside the computer. When they work, a large number of high-frequency electromagnetic waves interfere with each other, which is EMI. EMI will also be emitted through the motherboard wiring or external cables, causing electromagnetic radiation pollution, not only affecting the normal operation of other electronic equipment but also harm the human body.
The chip on the PC board is both an object and a source of electromagnetic interference during the working process. In general, we can divide electromagnetic interference into two categories: differential mode interference and common mode interference (ground interference).
Take the two PCB lines on the motherboard as an example. The serial mode interference refers to the interference between the two wires; while the common-mode interference is caused by the potential difference between the two wires and the ground.
The differential mode interference current acts between the two signal lines, and its conduction direction and the waveform a is consistent with the signal current. The common-mode interference current acts between the signal lines and the ground wire, and half of it flows through the two signal lines separately in the same direction, with the ground wire as the common circuit, as shown in FIG 1.
FIG 1. Common & Differential Mode Interference
If the common mode current generated by the board is not attenuated and filtered, then the electromagnetic radiation is easily generated through the data line. In order to eliminate the interference signal on the signal line, we must reasonably arrange the filter circuit to filter the interference of common and series mode, and the common mode choke is one part of the filter circuit.
The common mode choke is essentially a bilateral filter: on the one hand, it must filter out common mode electromagnetic interference on the signal line, on the other hand, it must suppress itself from emitting electromagnetic interference, to avoid affecting the normal operation of other electronic equipment in the same electromagnetic environment.
II Common Mode Choke Features
1. High initial permeability: 5-20 times that of ferrite. So it has greater insertion loss, and its suppression effect on conduction interference is much greater than ferrite.
2. High saturated magnetic strength: 2-3 times higher than ferrite. It is not easy to be magnetized to saturation in the case of strong current interference.
3.Excellent temperature stability: It has a higher Curie temperature. Under large temperature fluctuations, the alloy's performance change rate is significantly lower than ferrite, which is close to linear.
4. Flexible frequency characteristics: It obtains the required frequency characteristics by flexibly adjusting the process. Different impedance characteristics can be obtained through different manufacturing processes, thus meeting the filtering requirements of different wavebands, and making its impedance value much higher than ferrite.
III Working Principle
1. Principle of Common Mode Chokes
A choke coil is a low-impedance coil used to attenuate high-frequency current in a circuit. In order to improve its inductance, the choke usually has a core of soft magnetic material. A common mode choke has multiple identical coils, and the current flows in these coils in opposite directions, so the magnetic field cancels out in the core.
Common mode chokes are also used to suppress interference radiation. Interference currents are reversed in different coils, improving the EMC of the system. The inductance of common mode chokes for such currents is very high. The circuit diagram of the common mode choke is shown in the figure below.
FIG 2. Common Mode Choke Circuit Diagram
The Common-mode signal and differential-mode signal are only relative quantities. The Common-mode signal is also called common-mode noise or ground noise. It refers to the noise of two wires to ground respectively.
For the input filter of the switching power supply, it is electrical signals of the zero wire and lives wire to the earth. The two wires are connected to the earth respectively through the stray capacitance or stray inductance on the circuit board.
Differential mode signal refers to the direct signal difference between two wires
Suppose there are two signals V1 and V2
The common mode signal is (V1+V2)/2, with the same amplitude and the same phase.
The differential mode signal is:
for V1: (V1-V2)/2;
for V2: -(V1-V2)/2, with equal amplitude and opposite phase.
FIG 3.1 Differential Mode Signal
FIG 3.2 Common Mode Signal
2. Differential and Common Mode Noise Source
For the switching power supply, if the large energy storage filter capacitor behind the rectifier bridge is ideal, that is the equivalent series resistance is zero (ignoring all capacitor stray parameters), all possible differential mode noise sources input to the power supply will be completely bypassed or decoupled. However, the equivalent series resistance of the large-capacity capacitor is not zero, and is the main part of the impedance.
In addition to withstanding the operating current flowing in from the power line, the capacitor must also provide the high-frequency pulse current for the switch tube. However, in any case, the current will inevitably produce a voltage drop, so high-frequency ripples from the differential mode current will appear at both ends.
Theoretically, when the rectifier bridge is turned on, the high-frequency ripple noise should only appear on the input side. In fact, the noise will leak through the stray capacitor.
There are many accidental paths for high-frequency current to flow into the case. When the drain of the main switch tube changes, the current flows through the stray capacitor between the switch tube and the radiator connected to the shell. When the AC grid current turns on the rectifier bridge, the power noise into the case meets almost equal impedance, so the same amount flows into the zero wire and the live wire. This is pure common mode noise.
3. How do Common Mode Chokes Suppress Common Mode Signals?
Common-mode signals are two signals with the same amplitude and phase, generally coming from the power grid. They affect the normal operation of the circuit board and interfere with the surrounding environment in the form of electromagnetic waves.
Since inductance is used to suppress common-mode signals, this must be related to the magnetic field.
First, let's introduce the energized solenoid chokes and the direction of the magnetic field. To judge the direction of the magnetic field, hold the solenoid with the right hand and the four fingers is pointing to the direction of the current, then the thumb is pointing to the direction of the magnetic field.
The total amount of magnetic field linesperpendicularly passing through a section is called the magnetic flux of the section. The magnetic field linesof force are produced by the energized solenoids. They are invisible but actually exist.
The magnetic field linesof force are a closed loop. For energized solenoids, the magnetic field linesmust pass through the inside of the solenoid. The magnetic field linesare proportional to the magnetic strength B. Figure 3 shows a schematic diagram of the magnetic field linesgenerated by the energized solenoid.
FIG 4. magnetic field linesof Solenoid
As shown in FIG 5., the magnetic flux passing through a certain section
Figure 5. Magnetic flux Passing Through the Cross-section
The magnetic flux is F, which is a scalar in Weber, and the code is Wb. The relationship between magnetic flux, magnetic intensity B and cross-sectional area A is:
F=BA
Therefore, the more magnetic field linespassing through the cross-section, the greater the magnetic flux. When a current i is applied to the coil, the inductance L of the coil can be expressed as:
L=NF/i
N is the number of turns of the coil.
When the number of coil turns and current of the coil remain unchanged, the more magnetic field linespass through the magnetic core, the greater the magnetic flux, and the larger the inductance.
The choke is to prevent the change of current, and its essence is to prevent the change of the magnetic flux. This is the basic principle for using common mode chokes to suppress common mode current.
FIG 6. shows the magnetic intensity generated by the common-mode current on the common-mode choke. The magnetic intensity generated by the current I1 is B1, that generated by the current I2 is B2. The two yellow arrows indicate that the magnetic field linesgenerated by the currents I1 and I2 in the ferrite.
The magnetic field linesgenerated by the currents I1 and I2 are additive, and the magnetic fluxes are also additive, so is the inductance. The greater the inductance, the greater the suppression of the current.
FIG 6. Magnetic Flux Distribution of Common Mode Current
In a word, when the common-mode current flows through the common-mode choke, the magnetic flux in the magnetic ring is added together to a considerable inductance and suppresses the common-mode current.
When the differential mode current flows through the two coils, the magnetic field linesare opposite, causing the magnetic flux to cancel each other out. There is almost no inductance, so the differential mode signal can pass through without attenuation.
Therefore, the common mode chokes is not only added to the input filter of the switching power supply but also added to suppress the common mode current on the differential signal line to prevent false triggering.
IV Common Mode Choke Selection
According to the requirements of the rated current, DC resistance, and impedance value of at the rated frequency, the design of common mode choke can be carried out as:
● Calculate the minimum inductance value based on the impedance value;
● Choose core material and core size;
● Determine the number of coil turns;
● Select the wire.
The formula for calculating the minimum inductance value:
Xl is the impedance value at frequency f.
Divide the load (unit: Ohms) by the angular frequency or above when the signal starts to decay. For example, in a 50Ω load, when the frequency reaches 4000 Hz or above, the signal begins to attenuate. So we need to use a 1.99 mH (50/2π×4000)) inductor. The corresponding common mode filter structure is shown in Figure below:
Common Mode Filter Structure
V Precautions
1. The wires wound on the magnetic core should be insulated from each other, to ensure no short circuit occurs between the coil turns under the instantaneous overvoltage;
2.When a large instantaneous current passes the coil, the magnetic core should not be saturated;
3.The magnetic core should be insulated from the coil to prevent breakdown between the two under transient overvoltage;
4.The coil should be wound in a single layer as much as possible. This can reduce the stray capacitance of the coil and enhance its ability to withstand transient overvoltage.
Normally, pay attention to selecting the frequency band to be filtered. The larger the common-mode impedance, the better. In addition, focus on the influence of differential mode impedance on the signal, especially differential mode impedance and high-speed ports.
VI Core Material of Common Mode Choke Coil
When selecting a core for a common mode choke, the shape, size, applicable frequency band, temperature rise, and price must be considered. Commonly used cores are U-shaped, E-shaped, and ring-shaped. The following discusses the U-shape coils and ring-shape coils.
1. Magnetic-ring Iron Core
(1) Advantages
1) Because the initial permeability is 5-20 times that of ferrite, the inhibition of conducted interference is much greater than that of ferrite;
2) The high-saturated magnetic induction of nanocrystalline is better than that of ferrite, so it is not easy to saturate under high current;
3)The temperature rise is lower than that of the UF series, nearly 10 degrees lower at room temperature ( for reference only);
4)The flexibility of the structure makes it adaptable. It can be adapted to different needs by changing the processing technology (see the magnetic ring inductor used in energy-saving lamps, which is quite flexible in use);
5)The distributed capacitance will be smaller because of the wide winding area and small volume.
FIG 7. Magnetic Ring Common Mode Choke
(2) Disadvantages
1) The diameter of the magnetic ring too small to thread by machines, and manual winding is required. This wastes time and energy, with high processing costs and low efficiency.
2)In terms of withstanding voltage, it has little advantage compared with the UF series. Many magnetic ring common mode chokes are separated by cable ties with a certain distance, and the line is fixed with glue. If the inductance is large, the wires will be squeezed together, reducing reliability.
3)Inconvenient Installation and high failure rate.
(3) Application
Because of cost, magnetic rings are mostly used in high-power power supplies.
For the small size, magnetic rings are also a good choice for low-power power supplies with volume requirements.
The overall performance is better than that of the UF series. For projects with low cost pressure, you can consider using magnetic rings. For actual test conduction, the margin of the magnetic ring is lower, and the inductance is smaller than UF.
2. UF/UU Common Mode Choke
FIG 8. UF Common Mode Choke
Material: ferrite
The ferrites are generally MXO-manganese zinc and NXO-nickel zinc.
MXO has low initial permeability(less than 1000u), and can work at a high frequency (>100MHZ), with permeability unchanged. NXO has a higher resistivity than MXO.
(1)Advantages
● Low cost.
● Efficient: can be wound with a machine.
● Frames are usually UU9.8 or UU10.5, making the winding process easier to control
● Easy installation. There are only four pins, there is no problem if the holes are in the right position
(2) Disadvantages
● The packaging location is usually larger than the magnetic ring;
● More heat generation. It can reach 90 degrees when there is a 90V input at room temperature.
(3) Application
Generally used on occasions with strict cost control or low power.
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