Low ground capacitance filter and design method

Abstract

The invention belongs to the technical field of electromagnetic compatibility control design, and particularly relates to a low-to-ground capacitance filter and a design method. The filter comprises a low-frequency filter circuit, an intermediate-frequency filter circuit and a high-frequency filter circuit which are cascaded in sequence, wherein the low-frequency filter circuit is composed of a common-mode inductor Lc1 and a plurality of differential-mode capacitors Cx connected to the two ends of the common-mode inductor Lc1 in parallel, and the two ends of each differential-mode capacitor Cx are connected with a protection resistor R in parallel; the intermediate frequency filter circuit is composed of a differential mode inductor Lc2, a common mode capacitor Cy connected between a live wire and the ground in parallel and a common mode capacitor Cy connected between a zero wire and the ground in parallel. The high-frequency filter circuit is composed of a common-mode inductor Lc3 and a differential-mode capacitor Cx1 connected to the two ends of the common-mode inductor Lc3 in parallel, and the two ends of the differential-mode capacitor Cx1 are connected with a protection resistor R in parallel.

Description

Technical Field

[0001] The invention belongs to the technical field of electromagnetic compatibility control design, and in particular relates to a low-ground capacitance filter and a design method thereof. Background Art

[0002] High-frequency power supply filters can effectively filter out high-frequency interference signals in circuits and are key equipment for electromagnetic compatibility design and control of electrical systems. However, traditional high-frequency power supply filters must add a large capacitance to ground capacitor Cy, which forms a low-impedance path to filter high-frequency signals in the circuit to the ground. However, for complex platforms, there are a large number of power supply filters, and a large number of high-frequency interference signals are diverted to the ground, resulting in a large amount of high-frequency interference in the public ground. These high-frequency interference signals on the ground will return to the circuit through the ground capacitor Cy, causing new interference. How to effectively filter out high-frequency interference signals in the circuit without affecting the normal transmission of DC and power frequency currents, while preventing interference signals in the ground from flowing back into the circuit through the ground capacitor Cy and causing interference, is a key problem that power supply filters must solve in complex electrical platforms. Summary of the invention

[0003] In view of this, the present invention provides a low ground capacitance filter and a design method, wherein the filter can remove the ground capacitance Cy without affecting the normal transmission of DC and power frequency currents.

[0004] The technical solution for implementing the present invention is as follows:

[0005] A low-to-ground capacitance filter comprises: a low-frequency filter circuit, a medium-frequency filter circuit and a high-frequency filter circuit which are cascaded in sequence; wherein:

[0006] The low-frequency filter circuit is composed of a common-mode inductor Lc1 and a plurality of differential-mode capacitors Cx connected in parallel to both ends of the common-mode inductor Lc1, and a protective resistor R is connected in parallel to both ends of the differential-mode capacitor Cx;

[0007] The intermediate frequency filter circuit is composed of a differential mode inductor Lc2, a common mode capacitor Cy connected in parallel between the live wire and the ground, and a common mode capacitor Cy connected in parallel between the neutral wire and the ground;

[0008] The high-frequency filter circuit is composed of a common-mode inductor Lc3 and a differential-mode capacitor Cx1 connected in parallel to both ends of the common-mode inductor Lc3 , and a protective resistor R is connected in parallel to both ends of the differential-mode capacitor Cx1 .

[0009] Furthermore, the inductance of the common-mode inductor Lc1 of the present invention is 30mH; the inductance of the differential-mode inductor Lc2 is 3mH; the inductance of the common-mode inductor Lc3 is 50uH; the capacitance value of the common-mode capacitor Cy is 0.23nF; the capacitance value of the differential-mode capacitor Cx is 10uF; and the capacitance value of the differential-mode capacitor Cx1 is 1.1uF.

[0010] Furthermore, the common-mode inductor Lc1 magnetic core of the present invention selects an iron-based nanocrystalline magnetic core, which is a magnetic core wrapped with a non-metallic sheath or a non-magnetic metal sheath; the differential-mode inductor Lc2 is obtained by winding an iron powder core magnetic core; the common-mode inductor Lc3 magnetic core is wound with a MnNi and / or NiZn ferrite core, and is in a circular shape with dimensions of: inner diameter φ18mm×outer diameter φ30mm×height 15mm.

[0011] Furthermore, the capacitor described in the present invention is a safety capacitor.

[0012] Furthermore, the common-mode inductor Lc1 of the present invention is composed of two inductors packaged in separate metal shells, the differential-mode inductor Lc2 is composed of two inductors packaged in a combined metal shell, and the high-frequency common-mode inductor Lc3 is packaged in a separate metal shell.

[0013] Furthermore, the common mode inductor Lc1 and the differential mode inductor Lc2 in the filter circuit of the present invention are both arranged in parallel with respect to the bottom surface of the metal shell.

[0014] Furthermore, the common-mode inductor of the present invention is wound in a bilaterally symmetrical manner, and the conductors used to wind the inductor are multi-strand soft wires.

[0015] Furthermore, the filter housing of the present invention is made of stainless steel, the shielding body of the internal device is made of aluminum and subjected to surface conductive oxidation treatment, and the filter circuit and the housing are fixed by fasteners.

[0016] A method for designing a low ground capacitance filter, the specific process is:

[0017] Step 1, based on the filter capacitance to ground and filtering indicators, complete the filter circuit design, the circuit includes a low-frequency filter circuit, an intermediate frequency filter circuit and a high-frequency filter circuit cascaded in sequence; wherein the low-frequency filter circuit is composed of a common-mode inductor Lc1 and a plurality of differential-mode capacitors Cx connected in parallel at both ends of the common-mode inductor Lc1, and the two ends of the differential-mode capacitor Cx are connected in parallel with a protective resistor R; the intermediate-frequency filter circuit is composed of a differential-mode inductor Lc2, a common-mode capacitor Cy connected in parallel between the live wire and the ground, and a common-mode capacitor Cy connected in parallel between the neutral wire and the ground; the high-frequency filter circuit is composed of a common-mode inductor Lc3 and a differential-mode capacitor Cx1 connected in parallel at both ends of the common-mode inductor Lc3, and the two ends of the differential capacitor Cx1 are connected in parallel with a protective resistor R;

[0018] Step 2: According to the filter circuit design requirements and the characteristics of the inductor material, complete the filter common mode and differential mode inductor design;

[0019] Step 3: According to the filter circuit design results, complete the filter differential mode capacitor design and selection;

[0020] Step 4: Based on the filter capacitance requirements and combined with the basic principles and element analysis of the filter distributed capacitance, the filter distributed capacitance process design and control are carried out, and the distributed capacitance design results are verified and tested;

[0021] Step 5: Based on the design requirements of low-ground capacitance filter, combined with the selection scheme of inductance and capacitance, complete the design of filter housing, shape and appearance;

[0022] Step 6: Complete the final processing of low ground capacitance and conduct testing.

[0023] Beneficial Effects

[0024] First, compared with the traditional power supply filter, the present invention removes the ground capacitor Cy, does not affect the normal transmission of DC and industrial frequency current, effectively filters out high-frequency interference signals in the circuit, and ensures that the interference signals in the earth cannot enter the circuit loop through the ground capacitor. The design of this new power supply filter is of great significance for the application of filters in large and complex electrical platforms.

[0025] Second, the low ground capacitance filter that is finally processed and formed has extremely low ground capacitance, and other filtering, size, weight and other performances are basically the same as normal filters. The filters used in existing equipment can be replaced in situ.

[0026] Third, the low-ground capacitance filter has extremely low ground capacitance, and the actual measured ground capacitance of the filter is less than 1nf, which can effectively suppress the impact of ground current on filter performance, and at the same time greatly reduce the risk of electromagnetic interference of actual boat hull ground current on sensitive equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 This is the filter circuit schematic diagram;

[0029] Figure 2 is the schematic diagram of the filter circuit;

[0030] Figure 3 Comparison of MnZn ferrite and iron-based nanocrystalline core, (a) insertion loss comparison, (b) magnetic permeability comparison, (c) magnetic flux density B S contrast;

[0031] Figure 4The internal structures of different magnetic core materials: (a) MnZn ferrite crystal structure, (b) amorphous structure, and (c) nanocrystal structure.

[0032] Figure 5 The iron-based nanocrystalline magnetic core, (a) a magnetic core with an anti-stress sheath, (b) a similar magnetic core real picture;

[0033] Figure 6 Comparison of material properties of MnZn material and NiZn material, (a) frequency characteristics of MnZn material, (b) impedance characteristics of NiZn material, (c) magnetic permeability / frequency characteristics of MnZn / NiZn ferromagnetic material;

[0034] Figure 7 This is a physical picture of the high-frequency magnetic core;

[0035] Figure 8 is the distributed capacitance of the filter circuit to the metal shell;

[0036] Fig. 9 is the position where the toroid of the inductor element is parallel to the ground;

[0037] Fig.10 is the position of the inductor element loop perpendicular to the ground;

[0038] Fig.11 Simulation model for layout 1;

[0039] Fig.12 Simulation model for layout 2;

[0040] Fig.13 Simulation model for layout 3;

[0041] Fig.14 Simulation model for layout 4;

[0042] Fig.15 This is a test diagram of capacitance to ground;

[0043] Fig.16 The figure is a schematic diagram of the device packaging inside the filter circuit 2;

[0044] Fig.17 It is the mark of the components inside the filter circuit 2;

[0045] Fig.18 It is the electric field simulation distribution diagram;

[0046] Fig.19 L5 is rotated 90° around the Y axis;

[0047] Fig. 20 L5 is rotated 90° around the Z axis;

[0048] Fig.21Rotate L5 and its package 90° around the Z axis;

[0049] Fig. 22 Rotate the L5 package 90° around the Z axis;

[0050] Fig.23 The simulation results of L1, its package and each shell surface;

[0051] Fig.24 Rotate L3, L4 and their packages 90° around the X axis;

[0052] Fig.25 Rotate L3, L4 and their packages 90° around the Y axis;

[0053] Fig.26 The simulation results of L4 and its packaging and each shell surface;

[0054] Fig. 27 is a schematic diagram of the filter structure;

[0055] Fig.28 It is a schematic diagram of the filtering part of the filter;

[0056] Fig.29 Schematic diagram of the direct-pass part of the filter;

[0057] Fig.30 This is a schematic diagram of inductor winding;

[0058] Fig.31 Inductor fixing method; (a) Schematic diagram of inductor L1 module; (b) Schematic diagram of inductor L2 module; (c) Schematic diagram of inductor L3 module;

[0059] Fig.32 The dimensions and installation dimensions of the filter;

[0060] Fig.33 FIG. 1 is a diagram showing the appearance of the filter. FIG. DETAILED DESCRIPTION

[0061] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0062] It should be noted that the following embodiments and features in the embodiments may be combined with each other in the absence of conflict; and, based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without making any creative work are within the scope of protection of the present disclosure.

[0063] It should be noted that various aspects of the embodiments within the scope of the appended claims are described below. It should be apparent that the aspects described herein may be embodied in a wide variety of forms, and any specific structure and / or function described herein is merely illustrative. Based on the present disclosure, it should be understood by those skilled in the art that an aspect described herein may be implemented independently of any other aspect, and two or more of these aspects may be combined in various ways. For example, any number of aspects described herein may be used to implement the device and / or practice the method. In addition, other structures and / or functionalities other than one or more of the aspects described herein may be used to implement this device and / or practice this method.

[0064] The embodiment of the present application provides a low-to-ground capacitance filter, comprising: a low-frequency filter circuit, a medium-frequency filter circuit and a high-frequency filter circuit connected in cascade in sequence; wherein:

[0065] The low-frequency filter circuit is composed of a common-mode inductor Lc1 and a plurality of differential-mode capacitors Cx connected in parallel to both ends of the common-mode inductor Lc1, and a protective resistor R is connected in parallel to both ends of the differential-mode capacitor Cx;

[0066] The intermediate frequency filter circuit is composed of a differential mode inductor Lc2, a common mode capacitor Cy connected in parallel between the live wire and the ground, and a common mode capacitor Cy connected in parallel between the neutral wire and the ground;

[0067] The high-frequency filter circuit is composed of a common-mode inductor Lc3 and a differential-mode capacitor Cx1 connected in parallel to both ends of the common-mode inductor Lc3 , and a protective resistor R is connected in parallel to both ends of the differential-mode capacitor Cx1 .

[0068] This embodiment takes a low ground capacitance filter with an insertion loss of not less than 20 dB and a ground capacitance of not more than 1 nF in a frequency band of 10 kHz to 100 MHz as an example to illustrate a filter design method.

[0069] Step 1: Complete the filter circuit design based on the filter's capacitance to ground, filtering and other indicators;

[0070] According to the above requirements, the design results of the low ground capacitance filter are as follows:

[0071] It is composed of "two-level common mode (low frequency)" + one-level common mode (high frequency). The circuit principle is shown in Figure 1 .

[0072] By simulating the filter circuit, the parameters of the common-mode inductance of the low-frequency circuit, the common-mode inductance of the high-frequency circuit and the differential-mode capacitance are determined respectively. The final filter circuit is as follows: Figure 2 As shown, it includes: a low-frequency filter circuit, a medium-frequency filter circuit and a high-frequency filter circuit which are cascaded in sequence; wherein,

[0073] The low-frequency filter circuit is composed of a common-mode inductor Lc1 and a plurality of differential-mode capacitors Cx connected in parallel at both ends of the common-mode inductor Lc1, and a protective resistor R is connected in parallel at both ends of the differential-mode capacitor Cx; the intermediate-frequency filter circuit is composed of a differential-mode inductor Lc2, a common-mode capacitor Cy connected in parallel between the live wire and the ground, and a common-mode capacitor Cy connected in parallel between the neutral wire and the ground; the high-frequency filter circuit is composed of a common-mode inductor Lc3 and a differential-mode capacitor Cx1 connected in parallel at both ends of the common-mode inductor Lc3, and a protective resistor R is connected in parallel at both ends of the differential capacitor Cx1.

[0074] The circuit design of the entire filter is based on electromagnetic simulation. After the performance of the sample, it can meet the insertion loss design index with sufficient margin. The parameters of the filter are as follows:

[0075] (1) Low-frequency common-mode inductor Lc1 inductance: 30mH;

[0076] (2) Low frequency differential mode inductor Lc2 inductance: 3mH;

[0077] (3) High-frequency common-mode inductor Lc3 inductance: 50uH;

[0078] (4) Common mode capacitor Cy capacitance value: 0.23nF;

[0079] (5) Differential mode capacitor Cx capacitance value: 10uF;

[0080] (6) Differential mode capacitor Cx1 capacitance value: 1.1uF;

[0081] Step 2: According to the filter circuit design requirements and the characteristics of the inductor materials, complete the filter common mode and differential mode inductor design, selection and processing;

[0082] (I) Design, selection and processing of low-frequency filter common mode inductor Lc1

[0083] Through circuit simulation and combined with engineering experience, the inductance (30mH) of the common mode inductor in the low-frequency circuit was determined. The magnetic cores commonly used for winding common mode inductors are mainly MnZn ferrite cores and iron-based nanocrystalline cores. By analyzing the characteristics of the two cores, it is determined which core can better meet the design requirements of this project.

[0084] (1) The low-frequency common mode inductor Lc1 core should be an iron-based nanocrystalline core.

[0085] Figure 3 The performance comparison between MnZn ferrite and iron-based nanocrystalline cores is given. Under the same external dimensions, the comprehensive performance of iron-based nanocrystalline cores is much better than that of MnZn ferrite in terms of frequency band, magnetic permeability, and insertion loss. Especially in the low frequency band of 10kHz, the insertion loss is more than ten dB higher.

[0086] Obviously, in the low frequency band, since the magnetic permeability of the iron-based nanocrystalline core is much higher than that of the MnZn ferrite, a smaller volume and lower number of turns can be used to achieve the purpose of small volume and high inductance, which is beneficial to reduce the distributed capacitance of the filter circuit to the ground.

[0087] Figure 4 The internal structure of different core materials is given. Due to the special structure of the nanocrystalline core, it has: ultra-high and flat magnetic permeability in a wide frequency range (ensuring high inductance in a wide frequency range), high magnetic flux density B S (strong anti-saturation ability), extremely low coercive force (small remanence), high Curie temperature (suitable for high temperature), small magnetostriction (small electromagnetic noise) and other outstanding advantages.

[0088] Based on the above analysis, this project initially selected iron-based nanocrystalline cores with high saturation magnetic induction intensity (1.25T), high magnetic permeability (100000), low (magnetic) loss, and good temperature characteristics (Curie temperature above 500°C) for winding common-mode inductors.

[0089] As for which specific iron-based nanocrystalline magnetic core (electrical properties, volume, size, shape) to choose in the project, the final selection can only be made based on material trial production and performance testing.

[0090] (2) Stress control technology of low-frequency common mode inductor Lc1 core

[0091] According to the simulation results, low-frequency filtering requires an inductor with a large inductance (30mH (full load current 70A)). Since the iron-based nanocrystalline core material is very sensitive to stress, the material is prone to breakage under stress, destroying the lattice and causing a sharp drop in magnetic permeability. Therefore, stress control of the low-frequency inductor core is particularly important.

[0092] Controlling stress involves the protection of the magnetic core itself and the structure and process of inductor winding and installation. For the protection of the magnetic core itself, a special sheath needs to be designed. Depending on the process of winding the inductor, a non-metallic sheath (low stress winding process) or a non-magnetic metal sheath (high stress winding process) can be used, such as Figure 5 shown.

[0093] (II) Design, selection and processing of low-frequency differential mode inductor Lc2

[0094] Since the inductance of the differential mode inductor is small, a conventional iron powder core is used to wind the inductor.

[0095] (III) Design, selection and processing of high-frequency common mode inductor Lc3

[0096] (1) For low frequency (≤1MHz) high frequency inductor core, choose NiZn ferrite. For 1MHz to 300MHz high frequency inductor core, choose NiZn common mode inductor core.

[0097] Figure 6 The performance comparison of the magnetic cores used in common high-frequency inductors is given in . The magnetic cores used in high-frequency inductors are also called ferrite cores, which are divided into two types: MnZn ferrite and NiZn ferrite.

[0098] MnZn ferrite has high magnetic permeability and good conductivity, and is suitable for low-frequency (≤1MHz) applications.

[0099] NiZn ferrite has low magnetic permeability and high resistance, which is suitable for high frequency occasions. NiZn ferrite can be used in the frequency range of 1MHz to 300MHz. Its main features are low magnetic permeability, high resistivity, and low high-frequency eddy current loss. It is the inductor core material with the best magnetic properties in the high-frequency band above MHz. Therefore, NiZn ferrite is selected according to the needs of the project.

[0100] In order to avoid magnetic saturation, high-frequency inductors still use common-mode inductors. Considering the actual power requirements, NiZn common-mode inductors use circular ring cores, and their dimensions are initially determined as: inner diameter φ18mm×outer diameter φ30mm×height 15mm. Figure 7 shown.

[0101] Step 3: According to the filter circuit design results, complete the filter differential mode capacitor (line-to-line, non-ground) design and selection;

[0102] Through the simulation of the above filter circuit, it is concluded that the differential mode capacitor in the filter circuit is 10uF. The correct selection of differential mode capacitors has a great influence on the low-frequency performance of EMI electromagnetic interference filters. The low-frequency insertion loss is achieved through the leakage inductance of differential mode capacitors, differential mode inductors or common mode inductors. Since the common mode inductor of large current has a large volume, it will increase the distributed capacitance from the filter circuit to the ground. For this reason, differential mode capacitors with appropriate parameters are used, and large differential mode capacitance is achieved through parallel connection, thereby reducing the volume of the common mode inductor. For this reason, this project selected special safety capacitors with excellent low-frequency characteristics. In addition to ensuring good filtering characteristics, it also ensures the safety of the filter.

[0103] Step 4: According to the filter capacitance requirements to ground, combined with the basic principles and element analysis of the filter distributed capacitance, carry out the filter distributed capacitance process design and control, and verify the distributed capacitance design results.

[0104] (I) Control of distributed capacitance of filter circuit to ground

[0105] When designing the filter circuit mentioned above, the limitation of the filter's functional capacitance to ground has been taken into consideration. The capacitance of the filter to ground consists of two parts: one is the filter's functional capacitance to ground: that is, the common mode capacitance of the filter to ground; the other is the distributed capacitance of the filter circuit to ground: that is, the distributed capacitance of the filter circuit itself to ground.

[0106] In order to improve the high-frequency insertion loss of the filter, it is necessary to minimize the distributed capacitance of the filter circuit to the ground and increase the functional capacitance to the ground during design. Therefore, the control of the capacitance of the filter circuit to the ground focuses on the control of the distributed capacitance of the filter circuit to the ground.

[0107] (II) The basic principle of distributed capacitance:

[0108] According to the definition of capacitance, two conductor plates close to each other, with dielectric material ε filled in between, constitute a capacitor.

[0109] The calculation formula of the board capacitance is shown in formula (1):

[0110]

[0111] In the formula: ε—dielectric constant (F / m); S—conductor plate area; d—the distance between the two conductor plates.

[0112] From the formula we can see that:

[0113] The larger the dielectric constant ε of the dielectric, the more obvious the degree of polarization and the larger the capacitance;

[0114] The larger the area of ​​the conductor plate, the greater the capacitance;

[0115] The smaller the distance between the conductor plates, the greater the electric field strength at the same voltage, the more obvious the polarization of the dielectric is, and the greater the capacitance.

[0116] Obviously, for the entire filter circuit, the identification of the distributed capacitance factor to the ground must be carried out in accordance with the above three principles so that targeted control measures can be taken.

[0117] (III) Identification of the elements of the distributed capacitance of the filter circuit to ground

[0118] The composition of the capacitance to ground: Usually the filter housing is made of metal materials to ensure the structural and installation strength. Therefore, for the filter, the elements that constitute the capacitance to ground are obviously the filter circuit (one electrode) and the filter housing (the other electrode). In the filter circuit, only the inductor is a conductor. Therefore, the determining factor of the filter circuit electrode is the inductor.

[0119] (1) Factors constituting the distributed capacitance to ground

[0120] First, S and d in formula (1):

[0121] The filter circuit and the six surfaces of the metal shell form distributed capacitance (set as Cy1, Cy2, Cy3, Cy4, Cy5, Cy6), and the L and N lines of the filter circuit and the metal shell form distributed capacitance (set as Cy7, Cy8). Since all the above capacitors are in parallel, according to the parallel formula of capacitors, the total distributed capacitance of the filter circuit and the metal shell is the sum of the values ​​of the above eight capacitors, as shown in Figure 8 shown.

[0122] The position of the filter element relative to the ground

[0123] by Fig. 9 , Fig.10 Take the two filter circuits shown as an example, where the low-frequency common-mode inductor Lc1 is composed of two inductors that are separately packaged, the low-frequency differential-mode inductor Lc2 is composed of two inductors that are packaged together, and the high-frequency common-mode inductor Lc3 is packaged independently. Relative to Cy2 on the bottom surface of the filter housing, it is different because the area of ​​the inductor element projected on the bottom surface is different. Obviously, the inductor element ring in the filter circuit is placed perpendicular to the ground, the S formed with the ground is smaller, and the distributed capacitance formed is also smaller.

[0124] Due to the distributed capacitance and the circuit area S i (different shell surfaces) is proportional to the distance d i (different shell surfaces), so the larger the volume of the filter circuit, the closer the distance to the shell, and the larger the area parallel to the shell, the larger the distributed capacitance. For this reason, the volume of the filter circuit (using new magnetic conductive materials) and the area parallel to the shell (the installation position of the filter inductor relative to the shell) must be minimized;

[0125] To this end, the volume of the filter circuit must be reduced as much as possible (using new magnetic conductive materials), a new structure to reduce distributed capacitance must be adopted, and simulation software must be used to simulate the capacitance of the filter circuit relative to the housing to determine the optimal layout.

[0126] Second, ε in formula (1)

[0127] Traditional filters usually use potting materials to fill the gap between the filter circuit and the metal shell to fix the filter circuit and increase the insulation strength. However, as can be seen from formula (1), since the ε of the potting material must be greater than that of air, potting will inevitably increase the distributed capacitance. For this reason, the potting material cannot be used to pot the filter circuit, and other fixing methods must be used to fix the filter circuit.

[0128] Simulation analysis of distributed capacitance of filter circuit: CST software is used to simulate through the electrostatic field solver in the low-frequency simulation studio.

[0129] (IV) Simulation of different placement of 2 components in the filter circuit

[0130] Simulation of “Filter circuit - 2 levels of common mode inductors (low frequency)” + 1 level of differential mode inductors (high frequency).

[0131] (1) Basic parameter settings

[0132] Metal filter housing dimensions: 534×180×113mm;

[0133] Common mode inductor: outer diameter 96mm, inner diameter 40mm, thickness 61mm;

[0134] Differential mode inductor: outer diameter 94mm; inner diameter 40mm; thickness 42mm;

[0135] Insulated terminal: diameter 12mm; thickness 6.5mm;

[0136] ·Red and blue line diameter: 5mm;

[0137] The center of the inductor is located on the middle surface of the metal filter housing.

[0138] (2) Four layouts of filter circuits

[0139] like Fig.11 As shown, layout 1: the differential mode inductor is vertical with a center spacing of 81 mm; the common mode inductor is horizontal with a spacing of 150 mm, and the centers are both on the middle horizontal plane of the metal shell.

[0140] like Fig.12 As shown, layout 2: The differential mode inductor is the same as the first one, the common mode inductor on the left is rotated 90° with the center of this inductor as the rotation center and the z-axis as the rotation axis to become vertical, and the common mode inductor on the right remains unchanged.

[0141] like Fig.13 As shown, layout 3: Compared with the second model, the differential mode inductor and the common mode inductor on the left remain unchanged, and the common mode inductor on the right is rotated 90° with the center of this inductor as the rotation center and the z-axis as the rotation axis to become vertical.

[0142] like Fig.14 As shown, layout 4: the four inductors are placed horizontally, the center distance between the two differential-mode inductors is 113mm, the center distance between the two common-mode inductors is 120mm, and the center distance between the differential-mode inductor on the right and the common-mode inductor on the left is 118.5mm.

[0143] (3) Simulation results

[0144] The simulation results are shown in Table 2. It can be seen from the simulation results that the more vertically placed inductors there are, the smaller the distance between the surface of the inductor ring and the inner surface of the box, and the more induced charge on the surfaces of the two. Therefore, when the inductor is placed horizontally, the coupling capacitance is the smallest.

[0145] Table 2 Distributed capacitance of different layouts of filter circuit

[0146]

[0147]

[0148] (4) Verification test

[0149] Test, verify and analyze the simulation results to provide a design basis for later engineering design.

[0150] Test methods

[0151] Use LCR tester to test the capacitance between the core terminal and the shell of the filter. Fig.15 The test results are shown in Table 3. The test results show that the simulation results of the distributed capacitance are basically consistent with the actual test results. At the same time, since the distributed capacitance of layout 4 (the device is parallel to the bottom surface and the center of the device is at the center of the housing) is small, the layout of the filter device can be selected accordingly.

[0152] Table 3 Comparison of test and simulation results of distributed capacitance of filter circuit 2 with different layouts

[0153] Layout Number Distributed capacitance (pF) test Distributed capacitance (pF) simulation 1 70 61.17 2 69 68.59 3 77 75.76 4 51 46.98

[0154] (V) Simulation of distributed capacitance of filter circuit for different placement positions of inductor packaging

[0155] In order to reduce the weight of distributed capacitance and potting, and to facilitate the subsequent processing and production, the inductors are packaged in metal shells and then assembled. The distributed capacitance is simulated in this state (the packaging of the internal device is shown in the figure). Figure 16-17 ).

[0156] (1) Basic parameter settings

[0157] Metal filter housing dimensions: 534×180×113mm;

[0158] Common mode inductor L1: outer diameter 76mm, inner diameter 30mm, thickness 61mm, encapsulated in aluminum shell;

[0159] Common mode inductor L2: outer diameter 96mm, inner diameter 40mm, thickness 61mm, encapsulated in aluminum shell;

[0160] Differential mode inductors L3 and L4: outer diameter 94mm, inner diameter 40mm, thickness 42mm, encapsulated in aluminum shell;

[0161] Common mode inductor L5: outer diameter 30mm, inner diameter 18mm, thickness 61mm, encapsulated in aluminum shell.

[0162] Insulated terminal: diameter 12mm; thickness 6.5mm;

[0163] ·Red and blue line diameter: 5mm.

[0164] Material setting: Set the four lead terminals and three partitions to insulating materials, and set the shell, inductor package, and inductor to PEC ideal conductor materials. Fig.18 shown.

[0165] (2) Filter circuit layout

[0166] During simulation, the influence of the layout on the distributed capacitance is determined by changing the relative position relationship of L5, L2, and L3 / L4 relative to the filter housing one by one.

[0167] In the first case, only L5 is changed, and other inductors remain unchanged. L5 can be changed in four layouts, as follows: Figure 19-22 As shown, the simulation results of L1 and its package and each shell surface are as follows Fig.23 shown.

[0168] In the second case, L3 is changed (L4 is the same), and L3 can make two layout changes, as shown in Figures 24 and 25. The simulation results of L4 and its package and each shell surface are as follows Fig.26 shown.

[0169] Due to the distributed capacitance from the inductor to the metal shell of the filter, it is reflected as the series connection of the distributed capacitance between "the inductor and the metal packaging shell" and "the metal packaging shell and the filter shell". From the calculation formula of the series capacitance, it can be known that:

[0170] The series connection of multiple capacitors will reduce the total distributed capacitance;

[0171] If multiple capacitors with different capacitance values ​​are connected in series, the total series capacitance depends on the capacitor with the smaller capacitance value.

[0172] The withstand voltage of the series capacitor is improved.

[0173] From the simulation of the distributed capacitance layout of L5, L3 / L4, it can be seen that the total distributed capacitance will inevitably become smaller. From this, it can be concluded that the distributed capacitance is smaller and the withstand voltage value is higher when the inductor is packaged in a metal shell and then installed in a metal filter shell.

[0174] (3) Verification test

[0175] Test verification and analysis are carried out on the simulation results to provide a design basis for the later engineering design. The test results are shown in Table 4. According to the test results, the distributed capacitance of the inductor is reduced after packaging, so the simulation conclusion is valid and the corresponding design can be carried out according to the simulation conclusion.

[0176] Table 4 Distributed capacitance of different layouts of filter circuit 2

[0177]

[0178]

[0179] Step 5: Based on the design requirements of low-ground capacitance filter, combined with the selection scheme of inductance and capacitance, complete the design of filter housing, shape and appearance;

[0180] (I) Overall design plan

[0181] (1) The low-frequency common-mode filter circuit uses a common-mode inductor, which uses an iron-based nanocrystalline magnetic core and uses an anti-stress protective sleeve for stress protection. The common-mode inductor adopts a low-stress winding process;

[0182] (2) The high-frequency filter circuit uses a low-pass filter circuit composed of common-mode inductors and differential-mode capacitors, where the high-frequency inductors are wound with MnNi and NiZn ferrite cores with excellent high-frequency suppression capabilities;

[0183] (3) Since the dielectric constant of air is the smallest among all substances (relative dielectric constant ε r 1), therefore, the filter circuit is not potted with insulating potting material, and the filter circuit and the metal housing are fixed with fasteners;

[0184] (4) Reduce the distributed capacitance of the circuit (inductance) to ground

[0185] The inductors Lc1 and Lc2 are arranged parallel to the bottom of the metal shell;

[0186] Metal encapsulate the common mode inductor to reduce the distributed capacitance between the circuit and the metal surface.

[0187] The final design result is as follows Figure 27-29 shown.

[0188] (II) Feasibility analysis

[0189] The above design scheme fully considers the requirements of the design indicators proposed in the tender, and conducts sufficient demonstration, simulation and corresponding verification tests in combination with the application conditions of the filter, which can ensure that the design scheme is feasible and the technical difficulty is controllable.

[0190] Inductor Design

[0191] (1) Winding method: Due to the relatively large current, the common mode inductor is wound symmetrically on both sides. The wire used to wind the inductor is a multi-strand soft wire to reduce the stress of the wire on the magnetic core during the winding process of the inductor and ensure the stability of the magnetic permeability of the magnetic core. Fig.30 shown.

[0192] Inductor fixed: The inductor is large and heavy. Due to the overall weight limitation and the requirement of low ground capacitance, the inductor is packaged and fixed in a metal shell. Fig.31 .

[0193] Shell design

[0194] The technical requirements require the filter to have two sets of terminals, filtering and direct-through. Therefore, the filter must include a filtering circuit and a non-filtering circuit, and isolation between filtering and direct-through must be performed during shell design.

[0195] Since the filter operating environment has high requirements for salt spray, 316L stainless steel is used to make the filter housing. In order to reduce the overall weight, the shielding body of the internal components is made of 2A12 hard aluminum and the surface is treated with conductive oxidation. In order to ensure the reliability of the filter function, the filter housing is constructed by continuous welding in the overall design. The structural design takes into account the convenience of operation and maintenance. At the same time, in order to protect the internal components of the filter, the operation area and the filter are designed in a cavity, and the filter components are modularized.

[0196] The cover plate and five filter modules are connected to the housing by screw connection to facilitate operation and maintenance. Spring washers are added to prevent the screws from loosening due to vibration.

[0197] To facilitate the maintenance and replacement of each module, the five filter modules are connected to the insulating block by screw connection. To meet the requirements of the use environment, the filter surface coating is uniformly medium green-gray except for the grounding surface (see HJB37A-2000).

[0198] Filter shape and appearance

[0199] (1) Filter dimensions

[0200] The filter dimensions are shown in Fig.32 , meeting the maximum shape requirements in the technical requirements.

[0201] (2) Schematic diagram of the filter appearance

[0202] An illustration of what the filter looks like Fig.33 , meeting technical requirements.

[0203] Weight Assessment

[0204] The weight assessment of the low ground capacitance filter is shown in Table 5, which meets the technical requirement of ≤15kg.

[0205] Table 5 Weight details of low ground capacitance filter

[0206] Serial number name Weight(g) Remark 1 case 7000 — 2 Devices and Equipment 6000 — 3 Cables 200 — 4 Device packaging materials 1000 — 5 total 14200 —

[0207] Step 6: Complete the final processing of low ground capacitance and test it

[0208] The final test results of the low ground capacitance filter are shown in Table 6 below:

[0209] Table 6 Measured results of insertion loss of filter circuit

[0210]

[0211]

[0212] The capacitance value of each line-to-ground capacitor in the AC filter circuit is only 0.86nF, the size meets the requirements, and the weight test is 14.7kg.

[0213] In summary, the low ground capacitance filter meets the design requirements.

[0214] The embodiments of the present application have the following effects:

[0215] 1) During the processing of the filter, while not directly using the ground capacitance, various process designs are used to minimize the distributed ground capacitance of the filter itself;

[0216] 2) In the selection and processing of inductors, select inductor materials with special properties, control the size of the inductor, and fully consider the dual requirements of filter filtering and ground capacitance suppression;

[0217] 3) The low ground capacitance filter finally processed and formed has extremely low ground capacitance, and other performances such as filtering, size, and weight are basically the same as those of normal filters.

[0218] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A low ground capacitance filter, characterized in that: include: A low-frequency filter circuit, a medium-frequency filter circuit and a high-frequency filter circuit are cascaded in sequence; wherein, The low-frequency filter circuit is composed of a common-mode inductor Lc1 and a plurality of differential-mode capacitors Cx connected in parallel to both ends of the common-mode inductor Lc1, and a protective resistor R is connected in parallel to both ends of the differential-mode capacitor Cx; The intermediate frequency filter circuit is composed of a differential mode inductor Lc2, a common mode capacitor Cy connected in parallel between the live wire and the ground, and a common mode capacitor Cy connected in parallel between the neutral wire and the ground; The high-frequency filter circuit is composed of a common-mode inductor Lc3 and a differential-mode capacitor Cx1 connected in parallel to both ends of the common-mode inductor Lc3 , and a protective resistor R is connected in parallel to both ends of the differential-mode capacitor Cx1 .

2. The low ground capacitance filter according to claim 1, characterized in that: The inductance of the common mode inductor Lc1 is 30mH; the inductance of the differential mode inductor Lc2 is 3mH; the inductance of the common mode inductor Lc3 is 50uH; the capacitance value of the common mode capacitor Cy is 0.23nF; the capacitance value of the differential mode capacitor Cx is 10uF; and the capacitance value of the differential mode capacitor Cx1 is 1.1uF.

3. The low ground capacitance filter according to claim 2, characterized in that: The common-mode inductor Lc1 core is an iron-based nanocrystalline core, which is a core wrapped with a non-metallic sheath or a non-magnetic metal sheath; the differential-mode inductor Lc2 is obtained by winding an iron powder core; the common-mode inductor Lc3 core is wound with a MnNi and / or NiZn ferrite core, and is in a circular shape with dimensions of: inner diameter φ18mm×outer diameter φ30mm×height 15mm.

4. The low ground capacitance filter according to claim 1, characterized in that: The capacitor is a safety capacitor.

5. The low ground capacitance filter according to claim 1, characterized in that: The common mode inductor Lc1 is composed of two inductors packaged in separate metal shells, the differential mode inductor Lc2 is composed of two inductors packaged in a combined metal shell, and the high frequency common mode inductor Lc3 is packaged in a separate metal shell.

6. The low ground capacitance filter according to claim 2, characterized in that: The common mode inductor Lc1 and the differential mode inductor Lc2 in the filter circuit are both arranged in parallel with respect to the bottom surface of the metal shell.

7. The low ground capacitance filter according to claim 2, characterized in that: The common mode inductor is wound in a bilaterally symmetrical manner, and the conductor of the wound inductor is a multi-strand soft wire.

8. The low ground capacitance filter according to claim 1, characterized in that: The filter housing is made of stainless steel, and the shielding body of the internal device is made of aluminum with surface conductive oxidation treatment.

9. The low ground capacitance filter according to claim 8, characterized in that: The filter circuit and the housing are fixed by means of fasteners.

10. A method for designing a low ground capacitance filter, characterized in that: The specific process is: Step 1, based on the filter capacitance to ground and filtering indicators, complete the filter circuit design, the circuit includes a low-frequency filter circuit, an intermediate frequency filter circuit and a high-frequency filter circuit cascaded in sequence; wherein the low-frequency filter circuit is composed of a common-mode inductor Lc1 and a plurality of differential-mode capacitors Cx connected in parallel at both ends of the common-mode inductor Lc1, and the two ends of the differential-mode capacitor Cx are connected in parallel with a protective resistor R; the intermediate-frequency filter circuit is composed of a differential-mode inductor Lc2, a common-mode capacitor Cy connected in parallel between the live wire and the ground, and a common-mode capacitor Cy connected in parallel between the neutral wire and the ground; the high-frequency filter circuit is composed of a common-mode inductor Lc3 and a differential-mode capacitor Cx1 connected in parallel at both ends of the common-mode inductor Lc3, and the two ends of the differential capacitor Cx1 are connected in parallel with a protective resistor R; Step 2: According to the filter circuit design requirements and the characteristics of the inductor material, complete the filter common mode and differential mode inductor design; Step 3: According to the filter circuit design results, complete the filter differential mode capacitor design and selection; Step 4: Based on the filter capacitance requirements and combined with the basic principles and element analysis of the filter distributed capacitance, the filter distributed capacitance process design and control are carried out, and the distributed capacitance design results are verified and tested; Step 5: Based on the design requirements of low-ground capacitance filter, combined with the selection scheme of inductance and capacitance, complete the design of filter housing, shape and appearance; Step 6: Complete the final processing of low ground capacitance and conduct testing.

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