A T-type active EMI filter

Abstract

The invention belongs to the technical field of power electronics, and relates to a T-type active EMI filter, comprising: a detection link, an amplification link, an injection link and a multiplexing link; the detection link is composed of a detection resistor and a detection capacitor, and is used to provide an additional flow path for the common mode current; the amplification link is composed of a resistor and an operational amplifier, and is used to reversely amplify the voltage at both ends of the detection resistor; the injection link is composed of an injection resistor, and is used to convert a voltage signal output by the operational amplifier into a compensation current and inject it into the multiplexing link; the multiplexing link is composed of a first capacitor and a second capacitor, and provides a branch for the common mode current and the compensation current while attenuating the system differential mode noise; and the T-type three-capacitor architecture can make an active suppression circuit adopt an operational amplifier and a pure resistor; therefore, the invention reduces the overall volume of the active filter, is more convenient to design and use, has better stability, and can effectively attenuate both the common mode and differential mode noise.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics and relates to a T-type active EMI filter. Background Art

[0002] With the rapid development of the electronics industry, the increase in switching frequency has become a major trend. Although this progress has significantly improved efficiency and made components more miniaturized, it has also brought a series of new challenges, especially in terms of electromagnetic interference (EMI). High-frequency switching operations will produce stronger electromagnetic radiation, and more compact component placement will also bring greater near-field interference. These interferences not only affect the normal operation of the equipment itself, but may also interfere with peripheral equipment and even threaten the stability of the communication system; for grid-connected systems, excessive electromagnetic interference will also affect the safety and reliability of the power grid. Therefore, how to effectively suppress EMI has become an important technical problem in the electronics industry.

[0003] In the process of dealing with EMI problems, the design and optimization of EMI filters are crucial. As the main means of suppressing electromagnetic interference, EMI filters can effectively reduce the propagation of high-frequency signals, reduce interference to sensitive circuits and external devices, and provide key support for ensuring the stability of electronic equipment. Traditional EMI filters are divided into passive and active types. Passive EMI filters are widely used in industry due to their simplicity, reliability, high stability and low cost. However, in order to achieve effective low-frequency interference suppression, passive EMI filters usually require larger inductance and capacitance, which limits the size of the filter in some applications, especially in modern electronic devices that pursue miniaturization. In addition, due to the booming semiconductor chip industry, the cost advantage of passive EMI filters is not significant enough. On the contrary, active EMI filters are gradually being widely used due to their better low-frequency suppression effect, smaller size and adjustable performance.

[0004] Active EMI filter (AEF) is mainly composed of three parts: noise detection, noise amplification and noise compensation. The noise detection part has two methods: current detection and voltage sampling. The noise compensation part and current sampling also have two methods: current compensation and voltage compensation. Therefore, according to the difference between the detection part and the compensation part, the active EMI filter can be divided into current-sensing current-compensating type (CSCC), current-sensing voltage-compensating type (CSVC), voltage-sensing current-compensating type (VSCC), and voltage-sensing voltage-compensating type (VSVC). Since current detection requires the use of current transformers and voltage compensation requires the use of transformers, both of which require a large volume, the use of voltage-sensing current-compensating active EMI filters can achieve a smaller volume. In active EMI filters, EUT (Equipment Under Test) is the "equipment under test", EUT refers to the equipment being tested or its performance evaluated; LISN (Line Impedance Stabilization Network) is the "line impedance stabilization network".

[0005] Active EMI filters are divided into common mode AEF and differential mode AEF according to the type of attenuated noise. Their working principles and design methods are basically similar, but the specific compensation paths are different. Among them, since the attenuation of common mode noise is limited by factors such as ground leakage current, common mode inductor volume and high frequency characteristics, it is more difficult to design passive EMI filters. Therefore, the industry has been seeking to use active EMI filters to suppress common mode noise.

[0006] The traditional VSCC common-mode active EMI filter nowadays obtains the common-mode voltage by superimposing the signals detected on the L and N lines, and then reversely amplifies them through the operational amplifier, and finally injects them back to the L and N lines to compensate for the noise signal. The operational amplifier is referenced to the earth, and the detection and injection points are both on the L and N lines.

[0007] However, existing active EMI filters have the following defects:

[0008] Existing AEF detection and injection require different branches to form a loop, which will result in too many components and the volume cannot be further reduced.

[0009] The existing VSCC common-mode AEF has three obvious disadvantages. The first disadvantage is that the common-mode signal is detected directly from the L line and the N line. The voltage on the L line is relatively high. When the voltage on the L line fluctuates, the input voltage of the operational amplifier fluctuates accordingly, which will affect the stability of the operational amplifier and the AEF system. The second disadvantage is the high cost. The traditional AEF requires four capacitors. A large number of capacitors will not only affect the stability of the system, but also increase the cost. The third is that although there is a certain attenuation of differential mode noise, the degree of attenuation is very low.

[0010] The most obvious disadvantage of the existing CSCC common-mode AEF is that a relatively large current transformer is required to detect the common-mode current, thereby reducing the advantage of the small size of the active AEF.

[0011] Therefore, an active EMI filter with small size, strong stability and the ability to effectively attenuate both common-mode and differential-mode noise without increasing the cost is needed to solve the above technical problems. Summary of the invention

[0012] The technical solution adopted by the present invention to solve the technical problem is: a T-type active EMI filter, including: a detection link, an amplification link, an injection link and a multiplexing link;

[0013] The detection link is composed of a detection resistor and a detection capacitor, and the detection link is used to provide an additional flow path for the common mode current; one end of the detection resistor is connected to the ground, the other end of the detection resistor is connected to one end of the detection capacitor, and the other end of the detection capacitor is connected to the middle node of the series connection of the first capacitor and the second capacitor;

[0014] The amplification link is composed of a first resistor, a second resistor, an operational amplifier and a feedback resistor; the amplification link is used to reversely amplify the voltage across the detection resistor; one end of the first resistor is connected to the ground, and the other end of the first resistor is connected to the non-inverting input end of the operational amplifier; one end of the second resistor is connected to the series middle node of the detection resistor and the detection capacitor, and the other end of the second resistor is connected to the inverting input end of the operational amplifier; the reference ground between the positive and negative poles of the operational amplifier power supply is connected to the ground, and the output end of the operational amplifier is connected to the injection resistor and the feedback resistor; the feedback resistor is connected across the inverting input end and the output end of the operational amplifier;

[0015] The injection link is composed of an injection resistor, and the injection link is used to convert the voltage signal output by the operational amplifier into a compensation current and inject it back into the main multiplexing link; one end of the injection resistor is connected to the output end of the operational amplifier, and the other end of the injection resistor is connected to the middle node of the series connection of the first capacitor and the second capacitor;

[0016] The multiplexing link is composed of a first capacitor and a second capacitor. After the first capacitor and the second capacitor are connected in series, one end is connected to the L line between the LISN and the EUT, and the other end is connected to the N line between the LISN and the EUT;

[0017] In the T-type active EMI filter, part of the common-mode noise current passes through the capacitor of the multiplexing link and enters the branch composed of the detection resistor and the detection capacitor from the neutral point of the capacitor. The compensation current is injected back into the L line and N line of the main power circuit through the capacitor of the multiplexing link; thereby, the common-mode noise current in the main power circuit is reversely offset, forming a feedback loop and reducing the common-mode noise of the EUT.

[0018] Preferably, the first capacitor and the second capacitor are capacitors of μF level; the detection resistor is a resistor of 100Ω level; and the detection capacitor is a capacitor of nF level.

[0019] More preferably, the first capacitor and the second capacitor are 1-2.2 μF.

[0020] Preferably, the compensation current is:

[0021] (1)

[0022] In formula (1), I p represents the current flowing through the LISN, Z LISN Indicates the impedance of the linear impedance stabilization network that needs to be added between the power supply and the target circuit during EMI testing. V out is the output voltage of the operational amplifier, Z out represents the operational amplifier output impedance, R inj Indicates the resistance value of the injection resistor.

[0023] Preferably, the common mode noise current passing through the LISN is:

[0024] (2)

[0025] In formula (2), Z s ,I N represents the equivalent common mode noise source based on the Norton principle, Z LISN It represents the impedance of the linear impedance stabilization network that needs to be added between the power supply and the target circuit during EMI testing. out represents the operational amplifier output impedance, G S represents the total gain of the T-AEF system, R inj Indicates the resistance of the injection resistor, R s Represents the resistance value of the detection resistor, Z Cs To detect the impedance of the capacitor.

[0026] Preferably, the insertion loss of the LISN common mode noise is:

[0027] (3)

[0028] In formula (3), Z s represents the equivalent common mode noise source based on the Norton principle, Z LISN It represents the impedance of the linear impedance stabilization network that needs to be added between the power supply and the target circuit during EMI testing. out represents the output impedance of the operational amplifier, G S represents the total gain of the T-AEF system, R inj Indicates the resistance of the injection resistor, R s Represents the resistance value of the detection resistor, Z Cs To detect the impedance of the capacitor.

[0029] The beneficial effects of the present invention are:

[0030] The present invention uses a multiplexing architecture in which three T-shaped capacitors are used for simultaneous detection and injection. This architecture allows the active suppression circuit to be composed of an operational amplifier and pure resistors. Therefore, the present invention reduces the overall volume of the filter, is easier to design and use, has better stability, and can effectively attenuate both common-mode and differential-mode noise. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a circuit structure diagram of a T-type active EMI filter of the present invention;

[0032] Figure 2 It is a schematic diagram of the flow path of common mode noise and compensation current after adding a T-type active EMI filter of the present invention;

[0033] Figure 3 It is an equivalent common mode EMI noise model diagram of the T-type active EMI filter of the present invention;

[0034] Figure 4 It is a simplified common mode EMI noise model diagram of the T-type active EMI filter of the present invention;

[0035] Figure 5 is a signal flow diagram of a simplified T-type active EMI filter system of the present invention;

[0036] Figure 6 It is a circuit diagram of a T-type active EMI filter of the present invention;

[0037] Figure 7 This is a standard test environment diagram for testing common mode noise of the present invention;

[0038] Figure 8 This is a common mode noise suppression effect diagram of the T-type active EMI filter under the standard test environment of the present invention;

[0039] Fig. 9 It is a common mode noise suppression effect diagram of the T-type active EMI filter on the FSBB of the present invention;

[0040] Fig.10 It is a diagram showing the common mode noise suppression effect of the T-type active EMI filter on PFC of the present invention.

[0041] In the figure, 101 is a LISN; 102 is an EUT; 103 is a first capacitor; 104 is a second capacitor; 105 is a detection resistor; 106 is a detection capacitor; 107 is a first resistor; 108 is a second resistor; 109 is an operational amplifier; 110 is a feedback resistor; and 111 is an injection resistor. DETAILED DESCRIPTION

[0042] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the relevant technologies in the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] refer to Figures 1 to 10 In this embodiment, the biggest feature of the T-type active EMI filter (T-AEF) is the use of a multiplexing architecture that uses three capacitors for simultaneous detection and injection. The T-type architecture borrows the common mode capacitors of traditional passive EMI filters. The T-type architecture can suppress both CM noise and DM noise. And because of the T-type architecture, the active suppression circuit can be composed of an op amp and pure resistors. Its structure is as follows Figure 1 The shown process includes: detection link, amplification link, injection link and multiplexing link.

[0044] The detection link is composed of a detection resistor 105 and a detection capacitor 106, which are used to provide an additional flow path for the common mode current. One end of the detection resistor 105 is connected to the ground, and the other end is connected to the detection capacitor 106. One end of the detection capacitor 106 is connected to the neutral point of the first capacitor 103 and the second capacitor 104, and the other end is connected to the detection resistor 105.

[0045] The amplification link is composed of a first resistor 107, a second resistor 108, an operational amplifier 109 and a feedback resistor 110; it is used to reversely amplify the voltage across the detection resistor 105. One end of the first resistor 107 is connected to the ground, and the other end is connected to the non-inverting input of the operational amplifier 109. One end of the second resistor 108 is connected between the detection resistor 105 and the detection capacitor 106, and the other end is connected to the inverting input of the operational amplifier 109. The non-inverting input of the operational amplifier 109 is connected to the first resistor 107, the inverting input is connected to the second resistor 108 and the feedback resistor 110, the reference ground between the positive and negative poles of the power supply is connected to the ground, and the output is connected to the injection resistor 111 and the feedback resistor 110. The feedback resistor 110 is connected across the inverting input and output of the operational amplifier 109.

[0046] The injection link is composed of an injection resistor 111, which is used to convert the voltage signal output by the operational amplifier 109 into a compensation current and inject it back into the main power circuit. One end of the injection resistor 111 is connected to the output end of the operational amplifier 109, and the other end is connected to the neutral point of the first capacitor 103 and the second capacitor 104.

[0047] The multiplexing link is composed of a first capacitor 103 and a second capacitor 104. The first capacitor 103 and the second capacitor 104 are connected in series, one end of which is connected to the L line between the LISN 101 and the EUT 102, and the other end is connected to the N line between the LISN 101 and the EUT 102. The neutral point of the series connection is connected to the detection capacitor 106 and the injection resistor 111.

[0048] The role of the detection link is to use a resistor and a capacitor in series to form a high-pass filter, and provide a new path for the common mode current to flow into the earth. The size of the common mode current can be determined by detecting the current flowing through this path. Wherein the role of the detection resistor 105 has two, one is to convert the current signal flowing through this branch into a voltage signal, and the second is to prevent the ground leakage current from being too large. The role of the detection capacitor 106 is to adjust the cut-off frequency of the high-pass filter, to prevent the low-frequency signal from flowing into this branch, to reduce the pressure of the operational amplifier, and to prevent the operational amplifier from saturation.

[0049] The function of the amplifier section is to reversely amplify the voltage signal across the detection resistor 105. The operational amplifier should use the ground as a reference, and the amplification factor is determined by the ratio of the resistor R2 and the feedback resistor 110.

[0050] The function of the injection link is to convert the voltage signal output by the operational amplifier into a current signal and then inject it back into the main power circuit.

[0051] The multiplexing link has three functions. First, it provides a path for the common mode current to enter the branch composed of the detection resistor 105 and the detection capacitor 106; second, it provides a path for the compensation current flowing out of the injection link to enter the main power circuit; third, it can act as a differential mode capacitor to attenuate the differential mode noise of the EUT.

[0052] Figure 2 The figure shows the flow path of common-mode noise and compensation current after adding T-AEF. The gray solid line represents the flow path of common-mode noise current. It can be seen that a part of the common-mode noise current passes through the capacitor of the multiplexing link and enters the branch composed of the detection resistor and the detection capacitor from the neutral point of the capacitor. The black dotted line represents the flow path of the compensation current. It can be seen that the compensation current is injected back into the L line and N line of the main power circuit through the capacitor of the multiplexing link. Thereby, the common-mode noise current in the main power circuit is reversely offset, forming a feedback loop and reducing the common-mode noise of the EUT.

[0053] Since the attenuation of differential mode noise by T-AEF is mainly due to the capacitance of the multiplexing link, we will not elaborate on it here. Next, we will deduce the common mode insertion loss of T-AEF from the perspective of small signal analysis. Figure 3 The equivalent common-mode EMI noise model of T-AEF is shown. s ,I N is the equivalent common mode noise source based on the Norton principle. The T-AEF model has been marked in the figure. LISN For EMI testing, it is necessary to add the impedance of the linear impedance stabilization network between the power supply and the target circuit. s Represents the total gain of the T-AEF system, V out and Z out are the output voltage and output impedance of the operational amplifier respectively. The current flowing through the LISN is I p , the compensation current of T-AEF is I offset When selecting the capacitors, C1 and C2 in the reuse link are μF capacitors, while the detection resistor 105 is a hundred Ω resistor and the detection capacitor 106 is a nF capacitor. Therefore, in the conducted EMI frequency band of 150kHz to 30MHz, the equivalent impedance of the capacitors C1 and C2 in parallel is negligible compared to the equivalent impedance of the detection resistor 105 and the detection capacitor 106 in series. Figure 4 As shown in the figure, the equivalent common-mode EMI noise model can be simplified, and the capacitors C1 and C2 in the reuse link can be regarded as wires. The equivalent signal flow diagram of the entire system after simplification is as follows: Figure 5 shown.

[0054] Through this model, the compensation current can be obtained as:

[0055] (1)

[0056] according to Figure 5 The signal flow graph shown can be used to deduce that after adding T-AEF, the common-mode noise current passing through LISN is:

[0057] (2)

[0058] The insertion loss of common mode noise is:

[0059] (3)

[0060] T-AEF has the following differences compared to traditional common mode AEF:

[0061] 1. T-AEF uses a T-type three-capacitor detection and injection multiplexing architecture. The detection link and the injection link use the same capacitor branch, which can get rid of the need to use two capacitor branches for the detection link and the injection link in traditional AEF.

[0062] 2. T-AEF uses only three capacitors, while the traditional VSCC common-mode AEF requires four capacitors. The two capacitors in the multiplexing link can be the common-mode capacitors or differential-mode capacitors of the circuit itself. In this way, T-AEF only needs one detection capacitor, which greatly reduces the use of capacitors.

[0063] 3. T-AEF detection and injection are only performed in the branch consisting of the detection resistor and the detection capacitor, and operate in a low-voltage environment; while the detection and injection of the traditional VSCC type common-mode AEF are performed in the L line and the N line. The stability of T-AEF is more guaranteed.

[0064] 4. Compared with the traditional VSCC type common mode AEF, T-AEF effectively attenuates both common mode and differential mode noise.

[0065] 5. Compared with the traditional CSCC common-mode AEF, T-AEF does not use current transformers, which can greatly reduce the overall size of the filter and is easier to design.

[0066] Design the parameters in T-AEF. First, for the capacitors C1 and C2 in the reuse link, select μF-level capacitors. This is because the μF-level capacitors neither affect the cutoff frequency of the detection link, but also ensure that the compensation current of the injection link can flow from the capacitors C1 and C2 into the main power circuit, and at the same time can effectively attenuate the differential mode noise of the EUT. Because many circuits already have μF-level differential mode capacitors or common mode capacitors, the neutral point of the differential mode capacitors or common mode capacitors in the circuit can be used to construct the ground branch. In this way, the size of the T-AEF itself will be very small. Here we take .

[0067] The detection link needs to detect high-frequency components to prevent the entry of power frequency components. The transfer function of the detection link can be expressed as:

[0068] (4)

[0069] Its cut-off frequency for:

[0070] (5)

[0071] The detection link can be regarded as a high-pass filter, which only detects high-frequency signals. It should be set below the starting frequency of conducted EMI, 150kHz, but not too low, otherwise the attenuation of low-frequency signals will be insufficient and affect the effect. , , cutoff frequency .

[0072] For the signal amplification link, due to the limitation of the gain-bandwidth product of the operational amplifier, the gain should not be too large. Here, the operational amplifier AD829 is selected, which has a bandwidth of 120MHz, a slew rate of 230V / μs, and a supply voltage of ±5V-±15V. .

[0073] For the injection link, just like the detection link, only high-frequency compensation current is allowed to pass through to suppress high-frequency EMI noise current. The appropriate output resistor should be selected according to the device manual of the operational amplifier. In addition, for the value of the injection resistor, the cutoff frequency should take into account the capacitance of the multiplexing link. .

[0074] Figure 6 Shows the Figure 1 The actual object made of the circuit, you can see that the components in the actual object are very small. If the terminals and differential mode capacitors are removed, the volume can be made even smaller.

[0075]

[0076] Since the attenuation of differential mode noise by T-AEF is mainly due to the capacitance of the multiplexing link, the suppression effect of T-AEF on differential mode noise is not experimentally verified. Here, the verification of the suppression effect of T-AEF on common mode noise in three embodiments is given, firstly the test results under standard environment, secondly the verification results in four-switch buck-boost topology, and finally the verification results in PFC topology.

[0077] The test in the standard test environment mainly uses a signal generator and a stable 50Ω impedance to simulate the flow of noise. The test configuration of CM noise is as follows: Figure 7As shown, two channels of the signal generator are used to simultaneously send signals with the same amplitude, frequency, and phase to simulate common mode noise, and then an EMI receiver is used to detect it on the 50Ω resistor side. The test results are shown in Figure 8 It can be seen that after adding T-AEF, the common mode noise in the full frequency band is attenuated by 20-25dB.

[0078] Since T-AEF has good performance in a standard test environment, it is tested in an actual noise environment. In the test of the actual circuit, this example first uses the relatively new four-switch buck-boost circuit (FSBB). The working condition of this DC-DC circuit is 48Vdc to 48Vdc, the output current is 1A, and the power is 48W. The switching frequency is about 200kHz. The supply voltage of T-AEF is ±5V, which is provided by an additional DC power supply. A noise separator is used to convert the noise on the power line into common-mode or differential-mode noise. The test results are as follows Fig. 9 As shown, it can be seen that after adding T-AEF, the common mode noise in the low frequency band below 1MHz is attenuated by 20-30dB.

[0079] In order to prove the universality of T-AEF, an AC-DC circuit was used for testing. This example uses the PFC circuit topology as the test equipment. The working condition of the AC-DC circuit is 220Vac to 24Vdc, the full load output current is 9.4A, and the full load power is 225W. The test results are as follows Fig.10 As shown, it can be seen that after adding T-AEF, the common mode noise in the low frequency band before 3MHz is attenuated by 20-30dB.

[0080] In summary, the present invention uses a multiplexing architecture in which three capacitors are used for simultaneous detection and injection, and the architecture can suppress both CM noise and DM noise; and the architecture can make the active suppression circuit consist of an operational amplifier and pure resistors; therefore, the present invention reduces the overall volume of the filter, is easier to design and use, has better stability, and can effectively attenuate common-mode and differential-mode noise.

[0081] It should be emphasized that the above are only preferred embodiments of the present invention and do not impose any form of limitation on the present invention. Any simple modification of the above embodiments based on the technical essence of the present invention also falls within the protection scope of the present invention. Other equivalent changes and modifications still fall within the scope of the technical solution of the present invention.

Claims

1. A T-type active EMI filter, characterized in that: include: Detection link, amplification link, injection link and reuse link; The detection link is composed of a detection resistor (105) and a detection capacitor (106), and the detection link is used to provide an additional flow path for the common mode current; one end of the detection resistor (105) is connected to the ground, the other end of the detection resistor (105) is connected to one end of the detection capacitor (106), and the other end of the detection capacitor (106) is connected to an intermediate node of the series connection of the first capacitor (103) and the second capacitor (104); The amplification link is composed of a first resistor (107), a second resistor (108), an operational amplifier (109) and a feedback resistor (110); the amplification link is used to reversely amplify the voltage across the detection resistor (105); one end of the first resistor (107) is connected to the ground, and the other end of the first resistor (107) is connected to the non-inverting input end of the operational amplifier (109); one end of the second resistor (108) is connected to the middle node of the series connection of the detection resistor (105) and the detection capacitor (106), and the other end of the second resistor (108) is connected to the inverting input end of the operational amplifier (109); the reference ground between the positive and negative poles of the power supply of the operational amplifier (109) is connected to the ground, and the output end of the operational amplifier (109) is connected to the injection resistor (111) and the feedback resistor (110); the feedback resistor (110) is connected across the inverting input end and the output end of the operational amplifier (109); The injection link is composed of an injection resistor (111), and the injection link is used to convert the voltage signal output by the operational amplifier (109) into a compensation current injection recovery link; one end of the injection resistor (111) is connected to the output end of the operational amplifier (109), and the other end of the injection resistor (111) is connected to the series middle node of the first capacitor (103) and the second capacitor (104); The multiplexing link is composed of a first capacitor (103) and a second capacitor (104); after the first capacitor (103) and the second capacitor (104) are connected in series, one end of the first capacitor (103) and the second capacitor (104) are connected to an L line between the LISN (101) and the EUT (102), and the other end of the first capacitor (103) and the second capacitor (104) are connected to an N line between the LISN (101) and the EUT (102); In the T-type active EMI filter, part of the common-mode noise current passes through the capacitor of the multiplexing link and enters the branch composed of the detection resistor and the detection capacitor from the neutral point of the capacitor, and the compensation current is injected back into the L line and N line of the main power circuit through the capacitor of the multiplexing link; thereby, the common-mode noise current in the main power circuit is reversely offset, forming a feedback loop, and reducing the common-mode noise of the EUT.

2. A T-type active EMI filter according to claim 1, characterized in that: The first capacitor (103) and the second capacitor (104) are capacitors of the μF level; the detection resistor (105) is a resistor of the hundred Ω level; and the detection capacitor (106) is a capacitor of the nF level.

3. A T-type active EMI filter according to claim 2, characterized in that: The first capacitor (103) and the second capacitor (104) are 1 to 2.2 μF.

4. A T-type active EMI filter according to claim 1, characterized in that: The compensation current is: (1) In formula (1), I p represents the current flowing through the LISN, Z LISN It indicates the impedance of LISN (101) that needs to be added between the power supply and EUT (102) during EMI testing, V out represents the output voltage of the operational amplifier (109), Z out represents the output impedance of the operational amplifier (109), R inj represents the resistance value of the injection resistor (111).

5. A T-type active EMI filter according to claim 1, characterized in that: The common mode noise current passing through the LISN (101) is: (2) In formula (2), Z s ,I N represents the equivalent common mode noise source based on the Norton principle, Z LISN It represents the impedance of LISN (101) that needs to be added between the power supply and EUT (102) during EMI testing. out represents the output impedance of the operational amplifier (109), G S Represents the total gain of the system, R inj represents the resistance of the injection resistor (111), R s Represents the resistance value of the detection resistor (105), Z Cs represents the impedance of the detection capacitor (106).

6. A T-type active EMI filter according to claim 1, characterized in that: The insertion loss of the common mode noise of the LISN (101) is: (3) In formula (3), Z s It represents the equivalent common mode noise source impedance to ground based on the Norton principle, Z LISN It represents the impedance of LISN (101) that needs to be added between the power supply and EUT (102) during EMI testing. out represents the output impedance of the operational amplifier (109), G S Represents the total gain of the system, R inj Represents the resistance of the injection resistor (111), R s Represents the resistance value of the detection resistor (105), Z Cs represents the impedance of the detection capacitor (106).

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