What is a common mode choke?

Common mode inductance, also known as common mode choke coil, is often used in computer switching power supply to filter the common mode electromagnetic interference signal.In the board design, the common mode inductance also plays the role of EMI filter, which is used to suppress the electromagnetic wave radiating from the high-speed signal line.

The theoretical knowledge

The computer's internal motherboard is a mixture of various high-frequency circuits, digital circuits and analog circuits. When they work, they produce a large number of high-frequency electromagnetic waves that interfere with each other. This is EMI.EMI will also emit outward through mainboard wiring or external cables, causing electromagnetic radiation pollution and affecting the normal work of other electronic equipment.

The chip on the PC board card is not only an emi object but also an emi source in the working process.Generally speaking, we can divide these electromagnetic interference into two types: serial mode interference (differential mode interference) and common mode interference (ground mode interference).Take the two PCB wiring on the motherboard (the wires connecting each component of the motherboard) as an example, the so-called serial mode interference refers to the interference between the two wiring;The common mode interference is caused by the potential difference between the two wires and the PCB ground wire.The series mode interference current ACTS on the two signal lines and its conduction direction is consistent with the waveform and the signal current.The common mode interference current ACTS between the signal line and the ground line. The interference current flows through one half of the two signal lines in the same direction, and the ground line is the common loop.

If the common-mode current generated by the card is not attenuated (especially the common-mode current in high-speed interfaces such as USB and IEEE 1394), the common-mode interference current can easily generate electromagnetic radiation through the interface data line -- the common-mode radiation generated by the common-mode current in the cable.The FCC of the United States, CISPR22 of the international radio interference special commission and GB9254 of China all have relevant restrictions on the common mode conduction interference and radiation emission of communication ports of information technology equipment.In order to eliminate the input interference signal and the induced interference on the signal line, we must reasonably arrange the filter circuit to filter the interference of common mode and serial mode. The common mode inductance is a component of the filter circuit.

Common mode inductance is essentially a two-way filter: on the one hand to filter out the common mode electromagnetic interference on the signal line, on the other hand to suppress itself not to send out electromagnetic interference, to avoid affecting the normal work of other electronic equipment in the same electromagnetic environment.

The working principle of

Why common mode inductance can resist EMI?To understand this, we need to start with the structure of the common mode inductance analysis.

Common mode inductance filter circuit, La and Lb are common mode inductance coil.The two coils are wound on the same core with the same number of turns and the same phase (the winding is reversed).In this way, when the normal current in the circuit flows through the common mode inductance, the current in the inductance coil wound with the same phase produces the reverse magnetic field and cancles each other. At this time, the normal signal current is mainly affected by the coil resistance (and a small amount of damping caused by leakage inductance).When a common mode current flows through the coil, due to the codirectivity of the common mode current, a magnetic field in the same direction will be generated in the coil and the inductance of the coil will be increased, so that the coil will show a high impedance and produce a strong damping effect, so as to attenuate the common mode current and achieve the purpose of filtering.

Common mode Choke

In fact, if one end of the filter circuit is connected to the interference source and the other end is connected to the interfered device, then La, C1, Lb and C2 constitute two sets of low-pass filters, which can control the common-mode EMI signal on the line at a very low level.The circuit can not only suppress the incoming EMI signal from the outside, but also attenuate the EMI signal generated by the circuit itself, which can effectively reduce the EMI interference intensity.

Domestic production of a small common mode inductance, the use of high frequency noise suppression strategy, common mode choke coil structure, the signal is not attenuated, small size, easy to use, has the advantages of good balance, easy to use, high quality.It is widely used in double balance tuner, multi-frequency transformer, impedance transformer, balance and unbalanced transformer...And so on.

There is a kind of common mode filter inductor/EMI filter inductor using ferrite core, and bifilar winding countermeasures of noise suppression, high common mode noise suppression and low differential mode noise suppression, suppress interference sources and low differential mode noise signal is difficult to deformation in the high speed signal, small volume, it has good balance, the advantages of convenient use and high quality.It is widely used to suppress EMI noise of electronic equipment, USB circuit of PC and peripheral equipment, DVC, IEEE1394 circuit of STB, LCD panel, low voltage differential signal...And so on.

The leakage inductance of the differential mode

For an ideal inductance model, when the coil is wound, all the flux is concentrated in the center of the coil.But usually the loop is not fully wound or tightly wound, which can cause flux leakage.The common mode inductance has two windings with a considerable gap between them, which results in flux leakage and a differential mode inductance.Therefore, common - mode inductance generally has a certain attenuation capacity of differential - mode interference.

In the design of filter, we can also make use of leakage sense.For example, in ordinary filters, only one common mode inductance is installed, and the leakage inductance of common mode inductance is used to generate an appropriate amount of differential mode inductance, which can suppress the differential mode current.Sometimes, the leakage inductance of the common mode choke coil is artificially increased, and the amount of differential mode inductance is increased to achieve better filtering effect.


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Material quality

Magnetic ring type core advantages:

High initial permeability (which is the basic requirement of common mode inductance), high saturation magnetic induction intensity, stable temperature compared with ferrite (can be understood as a small temperature rise), flexible frequency characteristics, because of high permeability, small can make a large amount of sensitivity, adapt to a wide frequency;

Overall advantages:

Because the initial permeability is 5-20 times that of ferrite, the inhibitory effect on conduction interference is much greater than that of ferrite.

The high saturation magnetic induction intensity of nanocrystalline is better than that of ferrite, so it is not easy to be saturated under large electric current.

The temperature rise was lower than that of the UF series, and someone actually tested it: nearly 10 degrees lower at room temperature (personal test values for reference only);

The flexibility on the structure makes its adaptability good, from the processing process to change, can adapt to different requirements (see energy-saving lamp used on the magnetic ring inductance, use is quite flexible);

The distributed capacitance is smaller because the winding area is wider and the volume is smaller;

Fewer turns, smaller distribution parameters, and better efficiency.

Overall disadvantages:

Magnetic ring aperture is small, the machine is difficult to thread, need manual to wind, time-consuming and laborious, high processing cost, low efficiency.This is all the more important as cost pressures mount.

The advantages of pressure resistance are not big compared with UF: because we can see that many magnetic rings are separated in the middle of common mode by a tie wire band, which is not very reliable. Some magnetic rings are separated in the middle for a certain distance, and the wires are fixed with glue. How about the reliability after a long time?If the amount of inductance requirements are large, the line will be crowded together, the safety of a little doubt.

Installation inconvenience, high failure rate.


Because of the cost factor, magnetic rings are mostly used in high-power power supply, one person said: "the low-power magnetic ring is too high-end," it makes sense.

Of course, because of the small size, the volume of small power supply, magnetic ring is also a very good choice.

The comprehensive performance is better than that of UF system.If the cost pressure is not large, you can consider using the magnetic ring.For an actual test of conduction, the residual of a magnetic ring is much lower.And the sensitivity is lower than UF.

UF/UU series common mode

Material: basically is ferrite, of course this ferrite also has the distinction, generally has MXO- manganese zinc class and NXO- nickel zinc class.The main advantages of nickel-zinc are: the initial permeability is low (less than 1000u), but it can work at a relatively high frequency (more than 100MHZ) and keep the permeability unchanged.Very strong, very great.

NXO has higher resistivity than MXO.The temperature problem of common mode inductance is explained by the damping effect of ferrite on high frequency clutter.

Overall advantages:

The most important point: low cost (0.9 yuan for someone), machine winding, efficient, often UU9.8 or UU10.5;

With the skeleton, the winding process should be better controlled, can do higher inductance;

Good pressure and reliability?For magnetic ring common mode;

Good plugins, easy to install.Four feet, the hole is right, there is no problem;Basically used in a small current of the power supply, because the wire diameter can not be used very thick, so the current can not be too large;

Overall disadvantages:

Space factor: the encapsulation position is large, maybe because it is strong and not as small and exquisite as the magnetic ring;

The heating is quite serious, which is also according to my actual measurement: under the full load of 90V input at room temperature, it can reach 90 degrees faster;


Generally used in the cost control is relatively strict, or low power field


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The design factors

On some motherboards, we can see the common mode inductance, but on most motherboards, we will find that the element is omitted, and some even do not reserve the position.This motherboard, qualified?

There is no denying that common mode inductance has a good inhibiting effect on the common mode interference of the high-speed interface of the mainboard, which can effectively prevent EMI from forming electromagnetic radiation through the cable to affect the normal work of other peripherals and our health.But at the same time, it should be pointed out that anti-emi design of board CARDS is a fairly large and systematic project, and the design of common mode inductance is only a small part of it.High speed interface with common mode inductance design board card, not necessarily the overall EMI design is excellent.Therefore, from the common mode filter circuit we can only see one aspect of the design of the board card, which is easy to be ignored by everyone, making the mistake of not seeing the wood.

Only when we understand the anti-emi design of the board card, we can evaluate the merits of the board card.So, what work does an excellent board card design generally do on EMI performance?

1. Main board Layout design

For excellent mainboard wiring design, most of the clock running line will adopt shielding measures or close to the ground line to reduce EMI.For the multi-layer PCB design, the open-loop principle will be adopted in the adjacent PCB wiring layer, and the wires will go from one layer to another, so that the wires will not form a ring in the design.If the line forms a closed loop, it ACTS as an antenna and enhances the EMI radiation intensity.

The unequal length of signal line will also cause the impedance imbalance of two lines and form common mode interference. Therefore, the signal line will be processed in the form of snake line in the board design to make its impedance as consistent as possible and reduce common mode interference.At the same time, serpentine wiring will minimize the bending swing, in order to reduce the area of the ring area, thereby reducing the radiation intensity.

In high-speed PCB design, the length of the line is generally not an integer multiple of 1/4 of the wavelength of the clock signal, otherwise resonance will be generated and serious EMI radiation will be generated.At the same time, make sure the backflow path is minimal and unobstructed.For the design of decoupling capacitor, its setting should be close to the power pin, and the area surrounded by the power line and ground line of the capacitor should be as small as possible, so as to reduce the ripple and noise of the power supply and reduce EMI radiation.

Of course, the above is only a small part of the principles of EMI - proof PCB design.The Layout design of the main board is a very complex and profound knowledge, and even many DIYer have such a consensus: Layout design is excellent or not, the overall performance of the main board has a very significant impact.

2. Cut off the wiring of the main board

If you want to completely isolate the electromagnetic interference between the motherboard circuit, this is absolutely impossible, because we do not have a way to electromagnetic interference a "package" up, so to use other ways to reduce the degree of interference.The metal wires in the PCB on the motherboard are the main cause of the interference current. They transmit and emit EMI signals like an antenna, so "cutting off" these "antennas" in the right place is a useful way to prevent EMI.When the antenna is broken and surrounded by an insulator, its interference to the outside world will be greatly reduced.If the filter capacitor is used at the disconnection, the electromagnetic radiation leakage can be further reduced.This design can obviously increase the stability of high-frequency work and prevent the generation of EMI radiation, many large motherboard manufacturers have used this method in the design.

3. Design of mainboard interface

I do not know if you notice, the main board will be attached to an open thin iron block, in fact, this is also used to prevent EMI.Although the EMI shielding performance of the chassis is good, the electromagnetic wave will still leak out from the openings on the chassis surface, such as PS/2 interface, USB interface and the openings of parallel and serial ports.The hole size determines the degree of electromagnetic interference leakage.The smaller the aperture of the opening, the greater the reduction of emi radiation.For a square hole, L is its diagonal length.

After using the baffle, the up-turned metal contact of the baffle and the input and output part of the motherboard are well grounded through the chassis, which not only attenuates EMI, but also reduces the size of the square hole and further reduces the L value, so as to shield electromagnetic interference radiation more effectively.

The above three points are just a few main anti-emi design in the main board design in addition to the circuit design, it can be seen that the anti-emi design of the main board is an overall concept, if the overall design is not qualified, will bring greater electromagnetic radiation, and these are not a small common mode inductance can make up for.


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Necessary factors

Loss of common mode inductance = low EMI resistance?Such a statement is obviously rather one-sided.

Admittedly, due to the state's EMI related standards are not sound, some manufacturers in order to save materials on this loophole, in the overall EMI performance are greatly reduced costs of material compression (including common mode inductance omission), the direct result of this is the main board EMI performance is extremely low;But for those excellent overall design, with materials not shrinkage of the motherboard, even without common mode inductance, its overall EMI performance can still meet the relevant requirements, such products are still qualified.Therefore, it is not appropriate to judge the merits of the motherboard simply on whether there is common mode inductance.

Measurement diagnosis

1. An overview of the

The design of power supply filter can be considered from two aspects: common mode and differential mode.The most important part of the common mode filter is the common mode choke coil. Compared with the differential mode choke coil, the common mode choke coil has a significant advantage that its inductance value is very high, and the volume is small.Generally, the way to calculate the leakage inductance is to assume it is 1% of the common mode inductance, in fact, the leakage inductance is 0.5% ~ 4% of the common mode inductance.The influence of this error may not be negligible when designing an optimum performance choke.

2. The importance of leakage

How does leakage form?A tightly wound, fully wound loop in which all flux is concentrated in the coil "core", even if there is no core.However, if the coil is not fully wound or tightly wound, the flux will leak out of the core.This effect is proportional to the relative distance between the wire turns and the permeability of the coil.The common mode choke coil has two windings, which are designed to cause the coil cores along the current they flow to conduct in opposite directions, thereby setting the magnetic field at zero.If, for safety's sake, the coil on the core is not double-wound, there is a considerable gap between the two windings, which naturally causes a flux "leak", that is, the magnetic field is not really zero at the points of interest.The leakage inductance of the common mode choke is the differential mode inductance.In fact, the magnetic flux associated with the differential mode must leave the core at some point. In other words, the magnetic flux forms a closed loop outside the core, not just inside the ring core.

If the core has a differential inductance, then the differential current will cause the magnetic flux in the core body to deviate from zero, if the deviation is too large, the core will occur magnetic saturation phenomenon, so that the common mode inductance is basically the same as the inductance without a magnetic core.

3. Overview of common mode choke coil

In the design of the filter, it is assumed that the common and differential modes are independent of each other.However, the two parts are not truly independent, as the common-mode choke coil can provide considerable differential inductance.This part of differential inductance can be simulated by discrete differential inductance.

In order to make use of differential inductance, the common mode and differential mode should not be carried out simultaneously in the design process of the filter, but should be done in a certain order.First, common mode noise should be measured and filtered out.With the Differential Mode Rejection Network, the Differential Mode components can be eliminated, so the common Mode noise can be directly measured.If a common-mode filter is designed so that the difference mode noise does not exceed the allowable range at the same time, the mixed noise of common mode and difference mode should be measured.Because the known common mode component is below the noise tolerance, only the difference mode component exceeds the standard, and can be attenuated by the difference mode leakage of the common mode filter.For low power supply systems, the differential inductance of the common mode choke coil is sufficient to solve the differential mode radiation problem, because the source impedance of the differential mode radiation is small, so only a very small amount of the inductance is effective.

Although a small amount of differential inductance is very useful, too large a differential inductance can saturate the choke coil magnetically.

4. Method of measuring the saturation characteristics of common mode choke by LISN principle

It is often difficult to measure the saturation characteristics of a common-mode coil core (in whole or in part).Through a simple test, it can be seen to what extent the attenuation of the common mode filter is affected by the inductance reduction caused by the 60Hz programmed current.This test requires an oscilloscope and a differential mode rejection network (DMRN).First, an oscilloscope is used to monitor the line voltage.According to the following method, input signals from the oscilloscope channel A, set the oscilloscope time benchmark to 2ms/div, and then the trigger signal is added on the channel A, when the ac voltage reaches A peak, the wired current will be generated, at this time the filter performance degradation is expected.The input of the DMRN is connected to the LISN, and the output is matched with an impedance of 50 and connected to the B channel of the oscilloscope.When the common-mode choke operates in the linear region, the emission increment detected by channel B during the input current fluctuation does not exceed 6-10db.During the on-line voltage peak, the bridge rectifier is conducting and transmitting the charging current.

If the common-mode choke reaches saturation, the emission will increase as the input surge increases.If the common-mode choke reaches strong saturation, the emission intensity is the same as it would be without the filter, that is, it can easily reach more than 40dB.

The experimental data can be interpreted in other ways.The emission minimum (when the line current is 0) is the effect of the unbiased current of the filter.The ratio of peak emission to minimum emission, namely the degradation factor, is used to measure the effect of line current offset on the actual effect of the filter.The large degradation factor indicates that the magnetic core of the common mode choke coil is not properly used at all, and the "inherent degradation factor" of the better filter is approximately between 2 and 4.It is caused by two phenomena: first, inductance reduction caused by 60Hz charging current (as described above);Second, the bridge rectifier forward and reverse guide.The equivalent circuit of common-mode emission consists of a voltage source with an impedance of approximately 200pF, a diode impedance, and a common-mode impedance of LISN.When the bridge rectifier is positively biased, there is a partial voltage between the source impedance, 25, and the LISN common mode impedance.When the bridge rectifier is negatively biased, the partial voltage phenomenon occurs between the source impedance, the bridge anti-bias capacitance, and the LISN.When the reverse bias capacitance of the diode rectifier bridge is small, it has certain effect on common mode filtering.When the rectifying bridge is positively biased, it has no effect on the common mode filtering.

Due to the partial pressure, the expected value of the intrinsic degradation factor is about 2.The actual value varies considerably depending on the source impedance and the actual size of the diode bridge reverse bias capacitance.In a circuit invented by Flugan, this principle was applied to reduce the conduction emission of ballasts.

5. A method for measuring the saturation characteristics of the common mode choke by the current principle

If the tester is careful, a test device similar to the one in mil-std-461 can be used to detect the saturation characteristics of the common mode choke.The application of this principle is as follows: during the test, two current probes are used, the low frequency probe monitors the line current, and the high frequency probe only measures the common mode emission current.The line current monitor ACTS as the trigger source.However, one of the pitfalls of using the current probe is that the differential current attenuation is a function of the symmetry of the wires in the inner winding of the tube core.If carefully and reasonably arranged winding layout, 30dB or so of differential current attenuation can be obtained.Even if the decay value is reached, the measured difference module component may exceed the expected common module component value.Two techniques can be used to solve this problem: first, connect a high pass filter of 6kHz turning frequency in series with the oscilloscope (note that the terminal impedance of 50 should be matched).Second, connect a wire between each of the 10 MHZ capacitors and the supply bus.In order to measure common-mode radiation, the current probe should be clamped close to these wires carrying minimal current.

6. Differential mode and common mode flux in the common mode choke coil

For a quick and shallow introduction to the role of the common mode choke, the following statement may be considered: "the magnetic fields on both sides of the core of the common mode choke cancel each other, so there is no flux to saturate the core.Although this intuitive account of the action of the common-mode choke is materialized, it is not.

7. Overview of leakage

The common mode choke works because the constant cm is several orders of magnitude larger than the constant dm because the common mode current is usually small and can be obtained by keeping the L/D at a lower value.

In order to obtain the common inductance and minimize the differential inductance, it is better to use the magnetic core with large cross-sectional area to make multi-turn coil.With a larger helical core, which is not required, an effective differential inductance can be incorporated into the common mode choke coil.Because the differential mode flux is away from the core (the ring structure), it can produce very strong radiation.In particular, if the filter is installed on the PCB board, this radiation can be coupled to the power line to enhance the conductive emission.When the magnetic material is brought into the field (for example, the annular core is placed in an iron shell), the differential mode permeability may increase significantly, resulting in the saturation of the core due to the differential mode current.

8. Radiation-free common mode choke coil structure

In order to realize effective filter design, the radiation problem caused by magnetic flux leaving the magnetic core must be solved.Either by limiting the differential mode flux to a magnetic structure (pot core) or by providing a high permeability path for the differential mode flux (E core).

9. Pot shaped core structure

If the common - mode choke is of pot - core construction, then two axes are required.Two sets of coils in a kettle core pane and the magnetic flux paths they generate.The differential mode flux path under the same structure is also shown.

10、E - core structure

There is also a common mode choke coil, which is easier to wind than a ring-cored coil, but radiates more than a kettle-cored coil. The common mode flux of the e-cored coil connects the two sets of coils on the outer lead.In order to obtain high permeability, there should be no air gap on the external lead.On the other hand, the differential mode flux connects the outer lead to the center lead.The permeability in the differential mode path can be obtained by separating the center leads from each other, which are the main areas where radiation is produced.


(1) the wires wound on the coil core should be insulated from each other to ensure that there is no breakdown short circuit between the coils under the action of instantaneous overvoltage;

(2) when the coil flows through the instantaneous large current, the magnetic core should not be saturated;

(3) the magnetic core in the coil shall be insulated from the coil to prevent a breakdown between the two under the action of instantaneous overvoltage;

(4) the coil should be wound in a single layer as far as possible, which can reduce the parasitic capacitance of the coil and enhance the bearing capacity of the coil to the instantaneous overvoltage.

In general, at the same time, pay attention to choose the frequency band needed to filter, the greater the common mode impedance, the better, so in the selection of common mode inductance need to see device data, mainly according to the impedance frequency curve.In addition, the influence of differential impedance on the signal should be taken into account when selecting the differential impedance

The above is a detailed introduction of the common mode choke coil, I hope to have some help for you;We are a Chinese inductance manufacturer, axial inductor, fixed inductor, SMD inductor, etc


Post time: Apr-13-2020