High-frequency transformer temperature field analysis method based on magnetothermal coupling model

Abstract

The invention relates to the technical field of electronic component performance analysis, and discloses a high-frequency transformer temperature field analysis method based on a magnetothermal coupling model, which analyzes the influence of temperature on high-frequency loss and magnetic characteristics of a magnetic material through a multi-physical field coupling calculation process and a specific heat transfer process of the high-frequency transformer. And then a three-dimensional magnetic-thermal multi-physical coupling model based on a homogenization equivalence thought is established to calculate and analyze a temperature field of the high-frequency transformer under non-sinusoidal excitation. According to the method, the winding is quickly modeled by adopting a homogenization equivalence thought, so that the calculation cost and difficulty are effectively reduced, and quick and accurate prediction of the temperature distribution of the high-frequency transformer under different operation conditions is realized.

Description

Technical Field

[0001] The invention relates to the technical field of electronic component performance analysis, in particular to a high-frequency transformer temperature field analysis method based on a magnetothermal coupling model. Background Art

[0002] Temperature rise prediction can not only indirectly reflect the loss of high-frequency transformer magnetic components, but also serve as an indicator of the long-term stable operation of the transformer. Therefore, accurate analysis of the hot spot distribution of high-frequency transformers is of great significance to extending the service life of the transformer and improving capacity utilization. As an important magnetic component in power electronics, high-frequency transformers play a vital role in electrical isolation, voltage regulation, and power transmission throughout the circuit. Compared with traditional transformers, high-frequency transformers have obvious advantages in working efficiency, manufacturing cost, transportation, and installation. Due to the high operating frequency and relatively small size and weight of high-frequency transformers, they can be widely used in portable electronic devices.

[0003] Dry-type transformers usually use flame-retardant solid materials and air as insulating media, and have many advantages such as environmental protection, fire and explosion prevention, and maintenance-free. According to statistics, my country has become the country with the largest production and sales of dry-type transformers in the world, and the proportion of dry-type transformers in medium and large urban distribution networks is as high as 40%-50%. For dry-type transformers, internal thermal characteristics are the main limiting factor affecting their stable operation. Since the windings are generally cast with epoxy resin, the thermal conductivity is poor, which leads to an increase in the internal temperature of the windings, accelerates insulation aging, and even causes burning accidents. Therefore, accurately predicting the temperature distribution of dry-type transformers is crucial in both the design and operation stages.

[0004] The calculation of hot spot temperature of transformer winding can adopt the empirical formula in GB / T 1094.7-2008 and IEEE Standard C57.91-2011. This method is simple and convenient, but the accuracy is low. It is more suitable for roughly calculating the hot spot temperature of transformer winding, and the calculation result is greatly affected by the operating conditions of the transformer. In order to achieve accurate calculation of temperature rise distribution, scholars have conducted a lot of research. At present, there are two methods for accurate calculation of temperature field of dry-type transformer, one is thermal circuit model method, and the other is numerical calculation method.

[0005] Among them, the thermal circuit model method integrates environmental factors and working conditions; however, the actual structure of the dry-type transformer is simplified and the calculation accuracy is low; therefore, in the actual operation process, it is necessary to consider radial and axial heat transfer, and expand the thermal circuit model into a thermal network model, which is more in line with the actual heat transfer process of the transformer. However, the thermal network model cannot reflect the fluid dynamic characteristics of the air domain of the dry-type transformer, and it is difficult to accurately calculate the convective heat transfer thermal resistance. The model must be modified in combination with the fluid field simulation results, thereby improving the calculation accuracy of the thermal network model. Although the thermal network method is simple and feasible, it requires more parameters to be considered, makes a lot of assumptions, and can only obtain the average temperature of each part of the transformer.

[0006] The numerical calculation method proposes a two-dimensional multi-physics field of the transformer, ignoring the influence of the proximity effect and the skin effect. The internal structure of the transformer is very complex and will be affected by various factors such as the winding bending angle. Therefore, the numerical calculation method uses a three-dimensional magnetothermal coupling model of the high-frequency transformer, but does not take into account the influence of temperature on the magnetic properties. Therefore, after establishing a three-dimensional magneto-thermal-fluid coupling model, it is necessary to consider the influence of temperature on magnetic loss. However, this places high demands on the computer's memory and data processing capabilities, and even if adaptive meshing is used, it cannot meet the needs of actual use.

[0007] Therefore, those skilled in the art need a method that can accurately and quickly predict the winding loss, core loss, and temperature field distribution of a high-frequency transformer under non-sinusoidal excitation. Summary of the invention

[0008] The purpose of the present invention is to solve the above problems and to design a temperature field analysis method for a high-frequency transformer based on a magnetothermal coupling model.

[0009] The technical solution of the present invention to achieve the above object is a method for analyzing the temperature field of a high-frequency transformer based on a magnetothermal coupling model, the method comprising:

[0010] Analyze the multi-physics coupling calculation process and the heat transfer process of high-frequency transformers;

[0011] Build a magnetic properties test platform to measure the magnetization and loss curves of nanocrystalline core materials at different temperatures, and analyze the effect of temperature on the high-frequency loss and magnetic properties of magnetic materials;

[0012] Based on the high-frequency transformer model and the idea of ​​homogenization equivalence, a three-dimensional magnetic-thermal multi-physics coupling model is established. The loss distribution calculated by the magnetic field is introduced into the thermal field as a heat source, and the temperature distribution of the high-frequency transformer under non-sinusoidal excitation is calculated.

[0013] The multi-physics field coupling calculation process is:

[0014] A magnetic-thermal multi-physics field coupling model is established, the calculation results of the magnetic field are input into the temperature field as excitation, and the average loss of one cycle of the leakage magnetic field is used for iterative calculation;

[0015] Among them, the transient electromagnetic field is controlled by Maxwell's equations and can be expressed in vector form as:

[0016]

[0017] Where, E is the electric field intensity vector, B is the magnetic flux density vector, J is the current density vector, ρ is the charge density, ε0 ​​is the vacuum dielectric constant, and μ0 is the vacuum permeability;

[0018] Based on Maxwell equations and Dirichlet boundary conditions, the transient magnetic field equation of the high-frequency transformer can be derived as follows:

[0019]

[0020] I| Γ =C (5)

[0021] Where A is the vector magnetic potential; A x , A y , A z are the components of the vector magnetic potential A on the x, y, and z axes respectively; J is the time-varying non-sinusoidal excitation current density; σ is the electrical conductivity; μ is the magnetic permeability; I| Γ is a type of homogeneous boundary condition; t is time; C is a constant;

[0022] According to the relationship between the areas of sinusoidal and non-sinusoidal magnetic induction waveforms, the correction coefficient is determined and the Steinmetz equation is modified as follows:

[0023]

[0024] Where P c is the core loss, k, α, β are the correlation coefficients of the material, which can be obtained through the BP curve, B m is the peak value of magnetic flux density;

[0025] The Dowell formula is used to calculate the winding loss. High-frequency transformers usually use Litz wire winding structure. The AC resistance factor can be obtained by the area equivalent method, as shown in the following formula:

[0026]

[0027] Where: δ is the skin depth; N s is the number of strands of Litz wire; D L is the diameter of the Litz wire; d s is the strand diameter; d L is the turn spacing; hw is the winding height; h c is the core window height; n is the harmonic number.

[0028] The heat transfer process of the high-frequency transformer includes: heat conduction, heat convection and heat radiation;

[0029] Among them, heat conduction is a heat transfer method in which heat is transferred from the higher temperature part of an object to the lower temperature part. Heat conduction can occur between solids, inside solids, and inside fluids. According to Fourier's law, heat conduction satisfies the following formula:

[0030]

[0031] In the formula, q is the heat flux, λ is the thermal conductivity, and T is the temperature;

[0032] Convection refers to the heat transfer between the fluid and the solid surface when the fluid flows over the solid surface. The calculation of convection heat transfer can be obtained by Newton's law of cooling. The specific equation is:

[0033] q=hΔT(9)

[0034] In the formula, h is the convective heat transfer coefficient, T is the temperature;

[0035] The convective heat transfer coefficient is related to many factors such as the material properties of the medium (density, thermal conductivity, etc.), surface shape and size, and placement method. In engineering, it is generally obtained through analytical methods, experimental measurements, and other methods in combination with specific application scenarios.

[0036] Thermal radiation refers to the transfer of energy by an object through electromagnetic waves. Combined with the Stefan-Boltzmann Law, in actual engineering calculations, the heat transferred by radiation can be obtained by the following modified formula:

[0037] q=εσT 4 (10)

[0038] Where ε is the thermal emissivity of the object, which is related to the material and surface area of ​​the object; σ is the blackbody radiation constant, which is a natural constant with a value of 5.67×10 -8 ; T is the absolute temperature of the radiator;

[0039] During the operation of the transformer, the heat generated by the magnetic components is transferred from the center to the surface through heat conduction. Convection mainly occurs in the fluid medium between solids. Heat is first transferred from the winding and the core surface to the air, resulting in an increase in temperature and a decrease in density, causing thermal convection. In addition, there is heat radiation between the core, primary winding and secondary winding, and between the surface of the object and the external environment.

[0040] The analysis process of the effect of temperature on the high-frequency loss and magnetic properties of magnetic materials is as follows:

[0041] According to the size and characteristics of the sample, the magnetic field intensity H and magnetic flux density B were calculated using Faraday's law of induction and Ampere's law respectively:

[0042]

[0043] Where l is the equivalent magnetic path length of the core; A e is the cross-sectional area of ​​the core; N1 and N2 are the turns of the two windings respectively. The BH curves of the nanocrystalline core at different temperatures were obtained by data processing using Matlab software;

[0044] The area enclosed by the hysteresis loop at different temperatures is the loss of the nanocrystalline core in one magnetization cycle at that temperature, and the calculation formula is as follows:

[0045]

[0046] Each turn of Litz wire and the overlapping Nomex insulation are considered as a whole. The thermal conductivity of the whole after equivalent can be obtained by the following formula [:

[0047]

[0048] Among them, k i is the thermal conductivity of the insulating material, k cond is the thermal conductivity of the Litz wire, τ is the volume fraction (τ = vol cond / col tot ).

[0049] Beneficial Effects

[0050] A high-frequency transformer temperature field analysis method based on a magnetothermal coupling model produced using the technical solution of the present invention has the following advantages:

[0051] 1. In order to balance the calculation accuracy and speed, this application adopts the method of partitioning and gridding, and builds a magnetic property test platform to explore the influence of temperature on the magnetic properties of materials, and obtains the magnetization and loss characteristics of 1K107B nanocrystalline core at 30℃ to 150℃;

[0052] 2. This application considers the influence of temperature on the magnetic properties of materials, establishes a magnetic-thermal multi-physics field coupling model, and builds a high-frequency transformer temperature rise test platform. The model calculation value is compared with the experimental value, and the error is controlled within ±5%, which verifies the rationality and accuracy of the proposed model;

[0053] 3. This application has carried out detailed modeling of the insulation structure of the high-frequency transformer winding. The homogenization equivalent method is used to equate each turn of Litz wire and the overlapping Nomex insulation layer as a whole. The HS correction formula is used to calculate the overall thermal conductivity. The relative error before and after the model is equivalent is within the allowable range, ensuring the effectiveness of the calculation model. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a flow chart of transient magnetic-thermal coupling calculation of high-frequency transformer according to the present invention;

[0055] Figure 2 It is a heat transfer schematic diagram of the shell type transformer of the present invention;

[0056] Figure 3 It is a model diagram of a shell-type transformer prototype according to the present invention;

[0057] Figure 4 It is a physical picture of the ring core magnetization curve measurement system (i.e., magnetic property test platform) of the present invention;

[0058] Figure 5 It is the magnetization curve diagram of the nanocrystalline iron core in the range of 30°C-150°C of the present invention;

[0059] Figure 6 It is the nanocrystalline core loss curve in the range of 30°C-150°C of the present invention;

[0060] Figure 7 It is the core magnetic flux density and loss distribution cloud diagram under no-load condition described in the present invention;

[0061] Figure 8 It is the core temperature distribution cloud diagram under no-load condition described in the present invention;

[0062] Fig. 9 is a distribution diagram of winding current density under the short-circuit condition of the present invention;

[0063] Fig.10 It is the main air leakage magnetic field distribution diagram of the winding of the present invention;

[0064] Fig.11 is the temperature rise distribution diagram of the winding under the short-circuit condition of the present invention;

[0065] Fig.12 is a cross-sectional view of the high-frequency transformer winding of the present invention;

[0066] Fig.13 This is a diagram of a transformer temperature rise measurement platform according to the present invention;

[0067] Fig.14 This is a graph showing the temperature rise test results of the high frequency transformer of the present invention;

[0068] Fig.15 This is a comparison chart between the simulated value and the experimental value of the high-frequency transformer temperature;

[0069] Fig.16 This is a structural parameter table of the single-phase shell-type transformer prototype described in the present invention. DETAILED DESCRIPTION

[0070] The present invention will be described in detail below in conjunction with the accompanying drawings. Figure 1-16 As shown;

[0071] Since the 1940s, people have been committed to using computers to explore various physical phenomena. In the beginning, due to the limitations of hardware conditions, people mainly studied various isolated physical phenomena. But in fact, physical phenomena do not occur alone, and transformer operation is a process in which multiple physical fields are intertwined. At present, the multi-physical field analysis of high-frequency transformers at home and abroad mainly focuses on magnetic fields and temperature fields, while stress fields and acoustic fields are still in their infancy and need further exploration. In order to shorten the simulation time, this application established a 1 / 4 high-frequency transformer three-dimensional model, and used the Maxwell and Thermal modules integrated in the Workbench to analyze and calculate the magnetic field and temperature field distribution.

[0072] The magnetic properties of the nanocrystalline core and the thermal properties of the air will change with temperature. In order to improve the calculation accuracy of the temperature field simulation, this application proposes a magnetic-thermal multi-physics field coupling model. The calculation result of the magnetic field is input into the temperature field as an excitation, and the average loss of one cycle of the leakage magnetic field is iteratively calculated. In addition, the material properties at different temperatures should also be considered, so it is necessary to analyze the transient temperature field of the high-frequency transformer. The calculation process of the magnetic-thermal multi-physics field coupling of the high-frequency transformer is as follows: Figure 1 shown.

[0073] The transient electromagnetic field is governed by Maxwell's equations, which can be expressed in vector form as:

[0074]

[0075] Where E is the electric field intensity vector, B is the magnetic flux density vector, J is the current density vector, ρ is the charge density, ε0 ​​is the vacuum dielectric constant, and μ0 is the vacuum permeability.

[0076] Based on Maxwell equations and Dirichlet boundary conditions, the transient magnetic field equation of the high-frequency transformer can be derived.

[0077]

[0078] I| Γ =C (5)

[0079] Where A is the vector magnetic potential; A x, A y , A z are the components of the vector magnetic potential A on the x, y, and z axes respectively; J is the time-varying non-sinusoidal excitation current density; σ is the electrical conductivity; μ is the magnetic permeability; I| Γ is a type of homogeneous boundary condition; t is time; C is a constant.

[0080] Considering that the waveform of the excitation current is a square wave, the use of the traditional Steinmetz equation will produce a large error. The correction coefficient is determined based on the relationship between the areas of the sinusoidal and non-sinusoidal magnetic induction intensity waveforms. The modified Steinmetz equation is as follows:

[0081]

[0082] Where P c is the core loss, k, α, β are the correlation coefficients of the material, which can be obtained through the BP curve, B m is the peak value of magnetic flux density.

[0083] This application uses the Dowell formula to calculate the winding loss. High-frequency transformers usually use a Litz wire winding structure. The AC resistance factor can be obtained by the area equivalent method, as shown in the following formula:

[0084]

[0085] Where: δ is the skin depth; N s is the number of strands of Litz wire; D L is the diameter of the Litz wire; d s is the strand diameter; d L is the turn spacing; h w is the winding height; h c is the core window height; n is the harmonic number.

[0086] The heat transfer methods of high-frequency transformers mainly include heat conduction, heat convection and heat radiation. The following is a brief introduction to its basic principles and the heat dissipation process of each part.

[0087] a. Heat conduction

[0088] Heat conduction is a way of transferring heat from a higher temperature part of an object to a lower temperature part. Heat conduction can occur between solids, inside solids, and inside fluids. According to Fourier's law, heat conduction satisfies the following formula:

[0089]

[0090] Where q is the heat flux, λ is the thermal conductivity, and T is the temperature.

[0091] b. Thermal convection

[0092] When a fluid flows over a solid surface, the heat transfer between the fluid and the solid surface is called convective heat transfer. The calculation of convective heat transfer can be obtained by Newton's law of cooling, and its specific equation is:

[0093] q=hΔT (9)

[0094] Where h is the convective heat transfer coefficient and T is the temperature.

[0095] The convective heat transfer coefficient is related to many factors such as the material properties of the medium (density, thermal conductivity, etc.), surface shape and size, and placement method. In engineering, it is generally calculated through analytical methods, experimental measurements, and other methods in combination with specific application scenarios.

[0096] c. Thermal radiation

[0097] Thermal radiation refers to the transfer of energy by an object through electromagnetic waves. Combined with the Stefan-Boltzmann Law, in actual engineering calculations, the heat transferred by radiation can be obtained through the following modified formula:

[0098] q=εσT 4 (10)

[0099] Where ε is the thermal emissivity of the object, which is related to the material and surface area of ​​the object; σ is the blackbody radiation constant, which is a natural constant with a value of 5.67×10 -8 ; T is the absolute temperature of the radiator.

[0100] Figure 2 The heat transfer process of a high-frequency transformer is shown in Figure 1. During the operation of the transformer, the heat generated by the magnetic components is transferred from the center to the surface through heat conduction. Convection mainly occurs in the fluid medium between solids. Heat is first transferred from the winding and the core surface to the air, resulting in an increase in temperature and a decrease in density, causing thermal convection. In addition, there is heat radiation between the core, primary winding and secondary winding, and between the surface of the object and the external environment.

[0101] This application takes a single-phase shell-type high-frequency transformer as the research object, which is mainly composed of an iron core, a primary winding, a secondary winding, a non-magnetic steel plate, an epoxy resin, etc. Figure 3 shown.

[0102] Generally speaking, the winding of a high-frequency transformer is usually made of copper wire wrapped with an insulating layer, mainly including Litz wire, round conductor, copper foil winding, etc. Compared with copper foil winding, Litz wire has a simple process and low cost. Compared with solid round conductor, Litz wire has smaller skin effect and proximity effect, and low AC loss. This application adopts a Litz wire winding structure, each turn of Litz wire is wrapped with overlapping Nomex insulation paper, and the coil is cast with epoxy resin. The detailed technical specifications of the high-frequency transformer are as follows: Fig.16 shown.

[0103] Nanocrystalline alloy soft magnetic materials have low loss and high magnetic induction characteristics. The magnetostriction coefficient (2×10 -6 ) is much lower than amorphous alloy (27×10 -6 ), so the high-frequency transformer core is made of nanocrystalline alloy. Due to the large volume and mass of the prototype core, experimental measurement is difficult, so the Antai 1K107B nanocrystalline ring sample of the same manufacturer, model and batch is used instead for magnetic property measurement. The nanocrystalline ring sample is placed in a Binder FED56 constant temperature box, and the hysteresis loop of the nanocrystalline alloy material in the temperature range of 30℃-150℃ is measured by computer sampling voltammetry. The core loss measurement system (i.e., magnetic property test platform) is built as shown in the figure. Figure 4 shown.

[0104] The entire test system is divided into three parts: the first is the heating and temperature control system; the second is the non-sinusoidal excitation power generation system; and the third is the magnetic property measurement system. The advantage of this design is that, except for the nanocrystalline sample, only the windings wrapped around the sample are exposed to the high temperature environment, which can reduce the impact of temperature changes on the experimental results.

[0105] This application uses computer automatic test software and LMG500 three-phase power analyzer to process the data during the measurement process and display the magnetic characteristic curve on the screen. According to the size and characteristics of the sample, the magnetic field intensity H and magnetic flux density B are calculated using Faraday's induction law and Ampere's law respectively:

[0106]

[0107] Where l is the equivalent magnetic path length of the core; A e is the cross-sectional area of ​​the core; N1 and N2 are the turns of the two windings respectively. The BH curves of the nanocrystalline core at different temperatures were obtained by data processing using Matlab software. The measurement results are shown in Figure 5 shown.

[0108] It can be seen from the figure that the saturation point of magnetic flux density decreases with increasing temperature. In the linear region, the magnetization curves of 1K107B at different temperatures are not much different, and the magnetic permeability is almost unaffected by temperature changes, with stable magnetic properties. In the nonlinear region, as the temperature increases, the magnetic permeability and saturation magnetic flux density decrease significantly. The area enclosed by the hysteresis loop at different temperatures is the loss of the nanocrystalline core in one magnetization cycle at that temperature, and the calculation formula is as follows:

[0109]

[0110] According to the above formula, the core loss at 30-150℃ is calculated. The calculation results are summarized as follows: Figure 6 As shown, it can be seen that the loss of 1K107B fluctuates with the increase of temperature, generally showing an upward trend.

[0111] Common coupling methods used in numerical simulations are unidirectional coupling and bidirectional coupling. In the study of thermal stress fields, the unidirectional coupling method is usually used to analyze the deformation of the object by taking the calculated unit node temperature as the "body load" in the stress field. For the magnetic field-thermal field, the influence of temperature on parameters such as the electrical conductivity and magnetic permeability of the conductor is not considered during unidirectional coupling. However, bidirectional coupling can feed back the temperature rise into the magnetic field and recalculate the magnetic field by correcting the material parameters to obtain more accurate losses.

[0112] This application uses nanocrystalline 1K107B as the core material of the transformer. When the core works in a saturated state, problems such as waveform distortion, excessive loss, and excessive current will occur, which can easily lead to accidents. Considering safety factors, this application only studies the magnetic flux density distribution when the core is running in a non-saturated state (0.9T). Figure 5 It can be seen that in the unsaturated state, temperature has almost no effect on the magnetic properties of nanocrystals, while the bidirectional magnetic-thermal coupling mainly considers the effect of temperature on the magnetic properties of the iron core. Although bidirectional coupling has higher calculation accuracy, the amount of calculation increases exponentially, the calculation efficiency is low, and there are many limitations in practical applications. Therefore, this application only discusses unidirectional magnetic-thermal coupling.

[0113] from Figure 7 (a) It can be seen that, considering the anisotropy of the material, the flux density distribution at the corners of the core is not significantly different from that in other parts. However, there is an uneven distribution in the stacking direction of the nanocrystalline laminates. The maximum flux density is 0.34T and the minimum flux density is 0.24T. Along the stacking direction of the laminates, the flux density on the inside is significantly higher than that on the outside, and decreases layer by layer from the inside to the outside. This is because the air gap between the insulating material and the strip prevents the formation of eddy currents in this direction, thereby affecting the distribution of the magnetic flux. By observing (a) and (b), it is found that the loss distribution of the transformer has the same characteristics as the flux density distribution. From Figure 7 (b) It can be seen that the maximum loss of the core is 3.38×10 4 W / m 3 , the minimum loss is 1.70×10 4 W / m 3 .

[0114] The hottest point of the high-frequency transformer is located on the inner core column of the core, with the highest temperature of 130.89℃. This phenomenon is related to the heat dissipation conditions. Since this area is completely surrounded by the winding, it is not conducive to heat dissipation, and the air gap between the two cores is very small, so heat transfer almost depends on heat conduction. The lowest temperature appears in the outer vertical side yoke part, where air circulation, convection and radiation heat dissipation play a greater role.

[0115] The winding current density distribution is as follows: Fig. 9 As shown, the current density of the primary winding (8.882×10 5 A / m 2 ) is greater than the current density of the secondary winding (6.929×10 5 A / m 2 ). Due to the end effect, the magnetic field at the end of the winding is stronger and can produce a larger current density. Therefore, the loss at the end of the winding is slightly greater than the loss in the middle part of the winding.

[0116] The leakage magnetic field distribution of the winding main air channel is as follows: Fig.10 As shown in the figure, it can be seen that the leakage magnetic field of the winding cross section is symmetrically distributed up and down. The main air path leakage magnetic density between the primary winding and the secondary winding is the largest (7.685×10 -3 T), and the axial component is dominant. Under the influence of the end effect, the magnetic flux lines at the end of the winding are bent, generating transverse leakage flux.

[0117] Depend on Fig.11 (b) It can be seen that the temperature first rises and then falls along the winding arrangement direction. This is because the winding is surrounded by conductors, the heat cannot be dissipated, and the heat dissipation effect is poor, so the temperature in the middle of the winding is higher than the temperature on both sides. Since the ratio of the number of turns of the high and low voltage windings is 1:1, and the secondary winding is equivalent to the heat source wrapping the primary winding. In addition, the secondary winding is greatly affected by air convection and has a better heat dissipation effect. Therefore, the temperature of the secondary winding is lower.

[0118] Simulation time is related to multiple factors, including model complexity, physics and analysis types, solver settings, etc. The complexity of the model (including the number of elements, nodes or units) may have a significant impact on the simulation time. The high-frequency transformer model proposed in this application takes into account the complex insulation structure of the winding, and requires detailed mesh division to obtain accurate results, which leads to longer simulation time.

[0119] Therefore, this application adopts the homogenization equivalent method, Fig.12 The homogenization process of the winding is shown. Each turn of Litz wire and the overlapping Nomex insulation are regarded as a whole. The thermal conductivity of the whole after equivalent can be obtained by the following formula:

[0120]

[0121] Among them, k i is the thermal conductivity of the insulating material, k cond is the thermal conductivity of the Litz wire, τ is the volume fraction (τ = vol cond / col tot ).

[0122] The relative error of the model before and after the equivalent is within the allowable range. This method eliminates the complex modeling process of the finite element method and saves the calculation cost while ensuring the calculation accuracy.

[0123] Embodiment 1;

[0124] In order to verify the accuracy of the transformer temperature field calculated by finite element simulation, a temperature rise test platform was built for the 10kHz high-frequency transformer. Fig.13 shown.

[0125] This application uses a DC power supply with a full-bridge inverter as the excitation source, combines transformer-related theoretical knowledge and engineering experience, installs thermocouples at the predicted hotspots, and uses a FLUKE thermal imager and a multi-channel temperature rise recorder to measure the overall temperature rise and local hotspot temperature rise of the transformer, providing data support for verifying the accuracy of the established coupling model. Fig.14 The initial temperature distribution of the high-frequency transformer and the temperature distribution after 8 hours of operation are shown. Fig.14 (b) The temperature rise distribution characteristics can be seen intuitively, and the highest temperature can reach 135.7℃. This is because the primary winding generates losses by inducing magnetic flux in the core, which in turn causes the core temperature to rise. In addition, heat is transferred from the core to the surrounding windings through radiation and convection, causing the winding temperature to rise.

[0126] The temperature curves of various components of the high-frequency transformer are as follows: Fig.15 As shown in the figure, it can be seen that when the transformer starts to operate, the temperature continues to rise and reaches a stable state after a period of time. The overall temperature simulation results of the transformer are basically consistent with the experimental results, and the error is controlled within ±5%, which verifies the effectiveness of the model. This method can obtain the maximum temperature rise point that may occur during operation and can be used for the optimization design of high-frequency transformers.

[0127] Before the transformer reaches a stable temperature rise state, the calculated value is significantly higher than the measured value. The possible reasons for the error are: (1) In order to simplify the calculation when building the model, the influence of metal structural parts was ignored; (2) The structural equivalence of the Litz wire deviates from the actual complex torsional structural characteristics, and there is an insulation layer between the single-strand round conductors, which has a certain effect on the temperature conduction of the winding; (3) In actual operation, the transformer will be affected by various conditions, including changes in ambient temperature, air ventilation conditions, etc., while the simulation considers the temperature distribution under relatively ideal conditions.

[0128] It should be noted that, in this application, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions. The sentence "includes an element defined by ... does not exclude the existence of other identical elements in the process, method, article or device including the element".

[0129] The above technical solutions only reflect the preferred technical solutions of the technical solutions of the present invention. Some changes that may be made to certain parts thereof by technicians in this technical field all reflect the principles of the present invention and fall within the protection scope of the present invention.

Claims

1. A method for analyzing the temperature field of a high-frequency transformer based on a magnetothermal coupling model, characterized in that: The method includes: Analyze the multi-physics coupling calculation process and the heat transfer process of high-frequency transformers; Build a magnetic properties test platform to measure the magnetization and loss curves of nanocrystalline core materials at different temperatures, and analyze the effect of temperature on the high-frequency loss and magnetic properties of magnetic materials; Based on the high-frequency transformer model and the idea of ​​homogenization equivalence, a three-dimensional magnetic-thermal multi-physics coupling model is established. The loss distribution calculated by the magnetic field is introduced into the thermal field as a heat source, and the temperature distribution of the high-frequency transformer under non-sinusoidal excitation is calculated.

2. The method for analyzing the temperature field of a high-frequency transformer based on a magnetothermal coupling model according to claim 1 is characterized in that: The multi-physics field coupling calculation process is: A magnetic-thermal multi-physics field coupling model is established, the calculation results of the magnetic field are input into the temperature field as excitation, and the average loss of one cycle of the leakage magnetic field is used for iterative calculation; Among them, the transient electromagnetic field is controlled by Maxwell's equations and can be expressed in vector form as: ▽·B＝0(2) Where, E is the electric field intensity vector, B is the magnetic flux density vector, J is the current density vector, ρ is the charge density, ε0 ​​is the vacuum dielectric constant, and μ0 is the vacuum permeability; Based on Maxwell equations and Dirichlet boundary conditions, the transient magnetic field equation of the high-frequency transformer can be derived as follows: I∣ Γ =C (5) Where A is the vector magnetic potential; A x , A y , A z are the components of the vector magnetic potential A on the x, y, and z axes respectively; J is the time-varying non-sinusoidal excitation current density; σ is the electrical conductivity; μ is the magnetic permeability; I| Γ is a type of homogeneous boundary condition; t is time; C is a constant; According to the relationship between the areas of sinusoidal and non-sinusoidal magnetic induction waveforms, the correction coefficient is determined and the Steinmetz equation is modified as follows: Where P c is the core loss, k, α, β are the correlation coefficients of the material, which can be obtained through the BP curve, B m is the peak value of magnetic flux density; The Dowel l formula is used to calculate the winding loss. High-frequency transformers usually use Litz wire winding structure. The AC resistance factor can be obtained by the area equivalent method, as shown in the following formula: Where: δ is the skin depth; N s is the number of strands of Litz wire; D L is the diameter of the Litz wire; d s is the strand diameter; d L is the turn spacing; h w is the winding height; h c is the core window height; n is the harmonic number.

3. The method for analyzing temperature field of a high-frequency transformer based on a magnetothermal coupling model according to claim 1 is characterized in that: The heat transfer process of the high-frequency transformer includes: heat conduction, heat convection and heat radiation; According to Fourier's law, heat conduction satisfies the following formula: q=-λ▽T(8) In the formula, q is the heat flux, λ is the thermal conductivity, and T is the temperature; The calculation of heat convection can be obtained by Newton's law of cooling, and its specific equation is: q=hΔT(9) In the formula, h is the convective heat transfer coefficient, T is the temperature; Thermal radiation can be obtained by formula (10): q=εσT 4 (10) Where ε is the thermal emissivity of the object, which is related to the material and surface area of ​​the object; σ is the blackbody radiation constant, which is a natural constant with a value of 5.67×10 -8 ; T is the absolute temperature of the radiator.

4. The method for analyzing temperature field of a high-frequency transformer based on a magnetothermal coupling model according to claim 1 is characterized in that: The analysis process of the effect of temperature on the high-frequency loss and magnetic properties of magnetic materials is as follows: According to the size and characteristics of the sample, the magnetic field intensity H and magnetic flux density B were calculated using Faraday's law of induction and Ampere's law respectively: Where l is the equivalent magnetic path length of the core; A e is the cross-sectional area of ​​the core; N1 and N2 are the number of turns of the two windings respectively. The data was processed by Matlab software to obtain the BH curves of the nanocrystalline core at different temperatures; The area enclosed by the hysteresis loop at different temperatures is the loss of the nanocrystalline core in one magnetization cycle at that temperature, and the calculation formula is as follows:

5. The method for analyzing temperature field of a high-frequency transformer based on a magnetothermal coupling model according to claim 1 is characterized in that: The three-dimensional magnetic-thermal multi-physics coupling model adopts the homogenization equivalent method to regard each turn of Litz wire and the overlapping Nomex insulation as a whole. The thermal conductivity of the whole after equivalent can be obtained by the following formula: Among them, k i is the thermal conductivity of the insulating material, k cond is the thermal conductivity of the Litz wire, τ is the volume fraction (τ = vol cond / col tot ).

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