Basic principles and application cases of power supply noise filters

With the rapid development of modern science and technology, electronics, power electronics, and electrical equipment are increasingly widely used. The high-density, wide-spectrum electromagnetic signals generated by them during operation fill the entire space, forming a complex electromagnetic environment. The complex electromagnetic environment requires electronic equipment and power supplies to have higher electromagnetic compatibility. Therefore, the technology of suppressing electromagnetic interference has received more and more attention. Grounding, shielding, and filtering are the three major measures to suppress electromagnetic interference. The following mainly introduces the EMI filter used in the power supply and its basic principles and correct application methods.

2The role of noise filters in power supply equipment

The power supply of electronic equipment, such as 220V/50Hz AC power grid or 115V/400Hz AC generator, has various EMI noises. Among them, artificial EMI interference sources, such as radio transmission signals of various radar, navigation, communication and other equipment, will induce electromagnetic interference signals on the power line and the connecting cables of electronic equipment. Electric rotating machinery and ignition systems will produce transient processes and radiation noise interference in the inductive load circuit; there are also natural interference sources, such as lightning discharge phenomena and cosmic interference noise. The former has a short duration but high energy, and the latter has a wide frequency range. In addition, electronic circuit components themselves will also generate thermal noise when working.

These electromagnetic interference noises, through radiation and conduction coupling, will affect the normal operation of various electronic devices operating in this environment.

On the other hand, electronic devices also generate various electromagnetic interference noises when working. For example, digital circuits use pulse signals (square waves) to represent logical relationships. Fourier analysis of the pulse waveform shows that its harmonic spectrum range is very wide. In addition, there are pulse trains with multiple repetition frequencies in digital circuits. These pulse trains contain richer harmonics, wider spectra, and more complex electromagnetic interference noise.

Various types of voltage-stabilized power supplies are also a source of electromagnetic interference. In linear voltage-stabilized power supplies, the unidirectional pulsating current formed by rectification can also cause electromagnetic interference; switching power supplies have the advantages of small size and high efficiency, and are increasingly widely used in modern electronic equipment. However, because they are in a switching state during power conversion, they are themselves a strong source of EMI noise. The EMI noise they generate has both a wide frequency range and high intensity. These electromagnetic interference noises also pollute the electromagnetic environment through radiation and conduction, thereby affecting the normal operation of other electronic equipment.

For electronic devices, when EMI noise affects analog circuits, the signal-to-noise ratio of signal transmission will deteriorate. In severe cases, the signal to be transmitted will be submerged by EMI noise and cannot be processed. When EMI noise affects digital circuits, it will cause logical errors and lead to erroneous results.

For power supply equipment, in addition to the power conversion circuit, there are also drive circuits, control circuits, protection circuits, input and output level detection circuits, etc. The circuits are quite complex. These circuits are mainly composed of general or special integrated circuits. When they are malfunctioned due to electromagnetic interference, the power supply will stop working, causing the electronic equipment to not work properly. The use of power grid noise filters can effectively prevent the power supply from malfunctioning due to external electromagnetic noise interference.

In addition, part of the EMI noise that enters from the power input terminal may appear at the power output terminal, which will generate an induced voltage in the load circuit of the power supply, causing the circuit to malfunction or interfere with the signal transmitted in the circuit. These problems can also be prevented by using noise filters.

The functions of using noise filters in power supply equipment are as follows:

(1) Prevent external electromagnetic noise from interfering with the operation of the control circuit of the power supply equipment itself;

(2) Prevent external electromagnetic noise from interfering with the operation of the power supply load;

(3) Suppressing EMI generated by the power supply equipment itself;

(4) Suppress EMI generated by other devices and transmitted through the power supply.

When the switching power supply itself is working and the electronic equipment is in the switching working state, terminal noise will appear at the input end of the power supply equipment, generating radiation and conduction interference, and will also enter the AC power grid to interfere with other electronic equipment, so effective measures must be taken to suppress it. In terms of suppressing the radiation interference of EMI noise, electromagnetic shielding is the best way. In terms of suppressing the conducted interference of EMI noise, the use of EMI filters is a very effective means, of course, it should be combined with good grounding measures.

Internationally, various countries have implemented strict electromagnetic noise restriction rules, such as the FCC in the United States, FTZ and VDE in Germany. If electronic equipment does not meet the noise restriction rules, the product cannot be sold or used.

Due to the above reasons, a grid noise filter that meets the requirements must be designed and used in the power supply equipment.

3 EMI Noise and Filter Types

There are two types of EMI noise on the input leads of power supply equipment: common mode noise and differential mode noise, as shown in Figure 1. The EMI noise between the AC input lead and the ground is called its common mode noise, which can be regarded as interference signals with equal potential and the same phase transmitted on the AC input line, that is, voltages V1 and V2 in Figure 1. The EMI noise between the AC input leads is called differential mode noise, which can be regarded as interference signals with a phase difference of 180° transmitted on the AC input line, that is, voltage V3 in Figure 1. Common mode noise is the interference current flowing from the AC input line to the earth, and differential mode noise is the interference current flowing between the AC input lines. The conducted EMI noise on any power input line can be represented by common mode and differential mode noise, and these two types of EMI noise can be regarded as independent EMI sources to be suppressed separately.

When taking measures to suppress electromagnetic interference noise, the main consideration should be to suppress common-mode noise, because common-mode noise accounts for the main part in the whole frequency domain, especially in the high-frequency domain, while differential-mode noise accounts for a large proportion in the low-frequency domain. Therefore, the appropriate EMI filter should be selected based on this characteristic of EMI noise.

Power supply noise filters can be divided into integrated and discrete types according to their shapes. The integrated type is to encapsulate the inductor coil, capacitor, etc. in a metal or plastic shell; the discrete type is to install the inductor coil, capacitor, etc. on the printed circuit board to form a noise suppression filter. Which form to use depends on the cost, characteristics, installation space, etc. The integrated type has a high cost, good characteristics, and flexible installation; the discrete type has a low cost, but poor shielding and can be freely distributed on the printed circuit board.

4 Basic structure of noise filter

The power supply EMI noise filter is a passive low-pass filter that transmits AC power to the power supply without attenuation, while greatly attenuating the EMI noise transmitted with the AC power; at the same time, it can effectively suppress the EMI noise generated by the power supply equipment and prevent them from entering the AC power grid to interfere with other electronic equipment.

The basic structure of a single-phase AC power grid noise filter is shown in Figure 2. It is a four-terminal passive network composed of concentrated parameter components. The main components used are common-mode inductors L1, L2, differential-mode inductors L3, L4, common-mode capacitors CY1, CY2 and differential-mode capacitors CX. If this filter network is placed at the input end of the power supply, L1 and CY1 and L2 and CY2 respectively constitute low-pass filters between two pairs of independent ports on the AC incoming line, which can attenuate the common-mode interference noise on the AC incoming line and prevent them from entering the power supply equipment. Common-mode inductors are used to attenuate common-mode noise on the AC incoming line. L1 and L2 are generally wound in the same direction with the same number of turns on the ferrite core of the closed magnetic circuit. After being connected to the circuit, the magnetic flux generated by the AC current in the two coils L1 and L2 cancel each other, so that the magnetic core does not cause magnetic flux saturation, and the inductance values ​​of the two coils are larger and remain unchanged in the common-mode state.

The differential mode inductor coils L3 and L4 and the differential mode capacitor CX form a low-pass filter between the independent ports of the AC incoming line, which is used to suppress the differential mode interference noise on the AC incoming line and prevent the power supply equipment from being interfered by it.

The power supply noise filter shown in Figure 2 is a passive network with bidirectional suppression performance. Inserting it between the AC power grid and the power supply is equivalent to adding a blocking barrier between the EMI noise of the two. Such a simple passive filter plays a role in bidirectional noise suppression, and has been widely used in various electronic devices.

5 Main design principles of noise filters

The magnetic cores used in common mode inductors include ring-shaped, E-shaped and U-shaped cores. Ferrite is generally used as the material. Ring-shaped cores are suitable for large currents and small inductances. Their magnetic circuits are longer than those of E-shaped and U-shaped cores, and they have no gaps. They can obtain larger inductances with fewer turns. Due to these characteristics, they have better frequency characteristics. The coil leakage flux of the E-shaped core is small, so when the inductor leakage flux may affect other circuits or other circuits have magnetic coupling with the common mode inductor, and the required noise attenuation effect cannot be obtained, the E-shaped core should be considered to be used to make the common mode inductor.

Differential mode inductor coils generally use metal powder pressed cores. Since the applicable frequency range of powder pressed cores is relatively low, at tens of kHz to several MHz, their DC superposition characteristics are good, and the inductance will not drop significantly when large currents are used, they are most suitable as differential mode inductors.

In Figure 2, the power supply noise filter uses two types of capacitors, CX, CY1 and CY2. They play different roles in the filter and have different safety level requirements. Therefore, their performance parameters are directly related to the safety performance of the filter.

The differential mode capacitor CX is connected to both ends of the AC power line. In addition to the rated AC voltage, it is also superimposed with various EMI peak voltages between the AC power lines. Therefore, the capacitor's withstand voltage and transient peak voltage performance requirements are relatively high. At the same time, it is required that after the capacitor fails, it cannot endanger the subsequent circuits and personal safety. The safety level of CX capacitors is divided into two categories: X1 and X2. X1 is suitable for general occasions, and X2 is suitable for applications where high noise peak voltages will occur.

Common mode capacitors CY are connected between the AC power line and the chassis ground. They are required to have a large enough safety margin in terms of electrical and mechanical performance. In case of breakdown and short circuit, the equipment chassis will be charged with dangerous AC. If the insulation or grounding protection of the equipment fails, the operator may suffer electric shock or even endanger personal safety. Therefore, the capacity of CY capacitors should be limited so that the leakage current is less than the safety specification value at the rated frequency voltage. In addition, it is also required to have sufficient withstand voltage and withstand transient high peak voltage margin, and in case of voltage breakdown, it should be in an open circuit state and will not charge the equipment chassis.

In summary, when designing and selecting power grid noise filters, because they work in high voltage, high current, and harsh electromagnetic interference environments, the safety performance of the inductors and capacitors used must be considered first. For inductor coils, the materials of the magnetic core and winding, the insulation materials and insulation distance, and the temperature rise of the coils should all be taken into consideration. For capacitors, the type of capacitance, withstand voltage, safety level, capacity, leakage current, etc. should all be given priority, and it is especially required to select products that have been safety certified by international safety agencies.

6 Safety performance parameters of filters

6.1 Filter and leakage current

The leakage current of the power grid filter is defined as the current from the filter housing to any end of the AC line at the rated AC voltage. If all ports of the filter are completely insulated from the housing, the value of the leakage current mainly depends on the leakage current of the common mode capacitor CY, that is, it mainly depends on the capacity of CY. Since the size of the filter leakage current involves personal safety, all countries in the world have strict standards for this. For 220V/50Hz AC power grid, the leakage current of the noise filter is generally required to be less than 1mA.

6.2 Filter and test voltage

For AC power grid noise filter, there are two types of test voltages: one is applied to both ends of the AC incoming line, that is, line-to-line test voltage. If the inductor coil and lead wire are well insulated, it mainly depends on the withstand voltage of the capacitor CX; the other is applied between either end of the AC incoming line and the chassis ground, that is, line-to-ground test voltage. It mainly depends on the withstand voltage of CY.

Leakage current and test voltage are both safety performance parameters of noise filters, which are the specific manifestations of the safety performance of inductors, insulation and capacitors CX, CY in the filter, and are closely related to equipment and personal safety. Therefore, in the design, production and use of power grid noise filters, special attention should be paid to the certification and inspection of these technical parameters, and the certification and inspection should be given priority.

7 Technical parameters and correct use of filters

(1) Insertion loss is one of the important technical parameters of noise filters and should be given primary consideration during design and selection. When the filter's safety, conventional electrical performance, environment, and mechanical conditions all meet the requirements, a larger insertion loss value should be selected as much as possible.

The definition of insertion loss is shown in Figure 3. When the filter is not connected, the output voltage of the signal source is V1. When the filter is connected, the voltage of the signal source measured at the output end of the filter is V2. If the output impedance of the signal source is equal to the input impedance of the receiver, both are 50Ω, then the insertion loss of the filter is:

IL=20log(V1/V2) (1)

Because the power supply noise filter can attenuate common-mode and differential-mode noise, it has both common-mode insertion loss and differential-mode insertion loss. However, when selecting the filter, it should be noted that the insertion loss curve given in the product manual is measured under the condition that the input and output impedances are both 50Ω in accordance with the standard. Because the impedance at both ends of the actual filter is not necessarily 50Ω in the full frequency range, its attenuation of EMI signals is not equal to the insertion loss value given in the product manual. Especially when used and installed improperly, it will be much less than the insertion loss given in the standard.

(2) The power supply noise filter is a passive network with reciprocity. In practical applications, it should be reasonably matched to effectively suppress noise. The network structure and parameters of the filter should be selected according to the combination shown in Figure 4 to obtain a better EMI suppression effect.

When the output impedance of the filter is not equal to the load impedance, reflection will occur at this port. The greater the difference between the two impedances, the greater the reflection generated by the port. When the impedance at both ends of the filter is not equal to the external impedance, the EMI signal will be reflected at both its input and output ends. At this time, the attenuation of electromagnetic interference noise by the power supply filter is related to the inherent insertion loss and reflection loss of the filter, which can be used to more effectively suppress electromagnetic interference noise. When actually designing and selecting the use of EMI filters, attention should be paid to the correct connection of the filter impedance to cause the largest possible reflection, so that the filter causes a larger impedance mismatch in a wide frequency range, thereby obtaining better electromagnetic interference suppression performance.

(3) In the actual application of power filter, it is required that there is a good electrical connection between its shell and the system ground, and the grounding wire should be as short as possible, because too long grounding wire will increase the grounding resistance and inductance, which will seriously reduce the common mode suppression capability of the filter, and will also cause the problem of common ground impedance coupling. As shown in Figure 5, if the grounding wire is too long, the common coupling impedance Zg between the filter input and output will also be too large, and the voltage on the load will be:

V0=VZ＋Vg=VZ＋(Ii－IO)Zg(2)

Where: Ii is the noise current of the filter AC input circuit;

IO is the noise current of the filter output circuit.

It can be seen from formula (2) that after the electromagnetic interference signal is attenuated by the filter, the noise current at the output end is much smaller than the noise current at the input end, that is, the voltage drop (Ii-IO)Zg caused by the common ground impedance will be very large, and a very high electromagnetic interference voltage will be generated on Zg, which will be coupled to the output end of the filter through the common ground loop, thereby greatly weakening the noise filter's ability to suppress EMI noise.

The best way to reduce common impedance coupling is to use the electromagnetic shielding of the equipment to isolate the input and output of the noise filter. At the same time, the grounding wire of the filter should be as short as possible. This can minimize the electromagnetic coupling between the input and output of the filter without destroying the shielding structure of the equipment to suppress electromagnetic interference noise.

The ideal installation method of the power supply noise filter is shown in Figure 6.

(4) In summary, the following points should be noted when using power supply noise filters:

①The filter should be installed as close as possible to the AC power inlet of the equipment, and the AC incoming line that does not pass through the filter should be as short as possible inside the equipment;

② The capacitor leads in the filter should be as short as possible to prevent the lead inductive reactance and capacitive reactance from resonating at a lower frequency;

③ A large current flows through the filter grounding wire, which will generate electromagnetic radiation. The filter should be well shielded and grounded;

④ The input and output lines of the filter cannot be bundled together. When wiring, try to increase the distance between them to reduce the coupling between them. Partitions or shielding layers can be added.

8 Conclusion

The design and selection of electromagnetic interference filters are mainly based on the noise interference characteristics and the requirements of system electromagnetic compatibility, and are carried out on the basis of understanding the frequency range of electromagnetic interference and estimating the approximate magnitude of interference. First of all, it is necessary to understand the use environment of the filter (using voltage, load current, ambient temperature and humidity, vibration and shock, installation method and location, etc.), and focus on its safety performance parameters, because it is related to equipment and personal safety. It is also necessary to make the filter have the best suppression effect on EMI noise. The network structure and parameters of the filter should be selected according to the requirements of the access circuit and the principle of producing maximum impedance mismatch. In order to obtain the best electromagnetic noise attenuation characteristics, the filter should be correctly installed on the electronic equipment.