A flyback transformer design method

Abstract

The application discloses a flyback transformer design method, and relates to the technical field of flyback transformers. The method comprises the following steps: determining the size and model of a magnetic core and the model of a skeleton of the magnetic core; determining the winding wire diameter, the number of turns of a primary winding in the winding, the number of turns of a secondary winding in the winding, the number of turns of a Vcc winding in the winding and the penetration depth of the wire; determining the primary inductance of the magnetic core and the air gap value based on the output power and the minimum input voltage of the flyback transformer; determining the theoretical temperature rise, and determining the final winding wire diameter, the number of turns of the primary winding, the number of turns of the secondary winding and the number of turns of the Vcc winding according to the theoretical temperature rise, and then winding the winding on the skeleton of the magnetic core by using a winding process to obtain a skeleton after winding; and finally completing the design of the flyback transformer. The application can solve the problems in the design of the flyback transformer, such as the lack of basis for the selection of the material of the magnetic core, the unreasonable selection of the winding wire type and the wire diameter and the inability to predict the temperature rise of the transformer.

Description

Technical Field

[0001] This application relates to the field of flyback transformer technology, and in particular to a flyback transformer design method. Background Technology

[0002] Flyback transformers, as the core energy conversion device of switching power supplies, are widely used in small to medium power applications such as adapters, chargers, and industrial power supplies. With the rapid development of the switching power supply industry, power equipment is moving towards higher speeds, smaller sizes, and higher efficiency, which will further increase the demand for flyback transformers. Furthermore, as the core component of a switching power supply, the quality of the flyback transformer directly affects the overall performance of the power supply.

[0003] Traditional flyback transformer designs do not consider the influence of core material in the initial design phase; they only consider the applicable power of the core. The selection of core material is based on experience. The selected core may be usable, but it is not the optimal choice. During subsequent testing, issues such as low power efficiency or excessive temperature rise may occur. If repeated testing and adjustments are required, the selected core solution will no longer be optimal, and a significant amount of research and development time will be wasted.

[0004] Therefore, there is an urgent need for a flyback transformer design method to solve the problems in traditional flyback transformer design, such as the lack of basis for core material selection, unreasonable selection of winding line type and wire diameter, and inability to predict transformer temperature rise. Summary of the Invention

[0005] The purpose of this application is to provide a flyback transformer design method that can solve the problems of lack of basis for core material selection, unreasonable selection of winding line type and wire diameter, and inability to predict transformer temperature rise in traditional flyback transformer design.

[0006] To achieve the above objectives, this application provides the following solution: This application provides a flyback transformer design method, including: Obtain the design specifications of the flyback transformer to be designed; the design specifications include: output power, output voltage, operating frequency, maximum duty cycle, flyback transformer efficiency, diode forward voltage drop, and number of output channels; The operating temperature of the flyback transformer is determined based on the output power; the core material is determined based on the operating temperature; The Ap method is used to determine the size and model of the magnetic core based on the core material, the efficiency of the flyback transformer, the operating frequency, and the output power; the model of the core frame is determined based on the number of windings; the number of windings is determined based on the number of output channels. The winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the penetration depth of the wire are determined based on the maximum duty cycle and the output power. The required primary inductance of the core is determined based on the output power and the minimum input voltage of the flyback transformer, and the required air gap value of the core is determined in conjunction with the number of turns of the primary winding; the minimum input voltage is determined based on the output power. The theoretical temperature rise is determined based on the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the model of the magnetic core. Determine whether the theoretical temperature rise is greater than the set threshold standard; If the theoretical temperature rise is greater than a set threshold standard, the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding are optimized using set optimization conditions to obtain the final winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding; if the theoretical temperature rise is less than or equal to the set threshold standard, the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding are used as the final winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding. Using a winding process, based on the final winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the penetration depth of the wire, the winding is wound on the skeleton of the magnetic core to obtain the wound skeleton. The flyback transformer is designed based on the size and model of the magnetic core, the wound frame, the primary inductance, and the air gap value.

[0007] In one embodiment, the process of determining the operating temperature of the flyback transformer based on the output power includes: Obtain the ambient temperature; The core surface area is determined based on the output power. Based on the output power and the magnetic core surface area, the formula is used. Determine the operating temperature rise of the flyback transformer; where, For output power, The specific heat capacity of air. air density, The surface area of ​​the magnetic core. For airflow speed, For the efficiency of the flyback transformer, To increase the temperature; The operating temperature is obtained based on the temperature rise and the ambient temperature.

[0008] In one embodiment, the process of determining the core material based on the operating temperature includes: Obtain power loss models for magnetic cores made of different materials; Using the power loss model, the power loss of magnetic cores made of different materials is determined based on the operating temperature; Arrange the power losses in ascending order; determine the difference between the core saturation magnetic induction and remanence of the material corresponding to the first m power losses; The material with the largest difference is selected as the core material.

[0009] In one embodiment, the Ap method is used to determine the size and model of the magnetic core based on the core material, the efficiency of the flyback transformer, the operating frequency, and the output power, including: Using formula The required Ap threshold for the magnetic core is determined based on the efficiency of the flyback transformer, the operating frequency, and the output power; where, Indicates the Ap threshold. The area of ​​the core window is... For output power, For the efficiency of the flyback transformer, The effective cross-sectional area of ​​the magnetic core, For the flyback transformer topology core, For operating frequency, This represents the maximum operating magnetic flux density of the magnetic core. Determined based on the core material; The size and model of the magnetic core are determined based on the Ap threshold and the magnetic core material.

[0010] In one embodiment, the winding process is a sandwich winding method. Half of the number of turns of the primary winding is wound in the innermost layer of the skeleton, and then a layer of copper wire or copper foil is wound for shielding. Then the secondary winding is wound, and then a layer of copper wire or copper foil is wound for shielding. After the remaining half of the number of turns of the primary winding is wound, the Vcc winding is finally wound. Wherein, when the diameter of the winding wire is greater than the penetration depth of the conductor, multi-strand wire is used to avoid the high-frequency skin effect, and the diameter of the multi-strand wire should be smaller than the penetration depth of the conductor.

[0011] In one embodiment, the process of determining the primary inductance includes: The primary initiation current and primary peak current are determined based on the output power and the minimum input voltage of the flyback transformer. The primary inductance is determined based on the primary starting current, the primary peak current, and the minimum input voltage.

[0012] In one embodiment, determining the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the penetration depth of the conductor based on the maximum duty cycle and the output power includes: The effective value of the current is determined based on the maximum duty cycle and the output power; the winding wire diameter is determined based on the effective value of the current. The number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding are determined based on the maximum duty cycle and the minimum input voltage, respectively. The penetration depth is determined based on the operating frequency.

[0013] In one embodiment, determining the theoretical temperature rise based on the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding includes: Obtain the volume loss model corresponding to the magnetic core material; The lengths of the primary winding, secondary winding, and Vcc winding are determined based on the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding, respectively. Copper loss is determined based on the length of the primary winding, the length of the secondary winding, and the length of the Vcc winding. Using the aforementioned volume loss model, the unit volume loss of the magnetic core is determined based on the operating temperature; Determine the effective volume and actual surface area of ​​the magnetic core; Iron loss is determined based on the unit volume loss and the effective volume; The theoretical temperature rise is determined based on the copper loss, the iron loss, and the actual core surface area.

[0014] In one embodiment, the process of determining the actual magnetic core surface area includes: The magnetic core structure coefficient and the Ap value of the magnetic core are determined based on the model of the magnetic core; The actual surface area of ​​the magnetic core is determined based on the magnetic core structure coefficient and the Ap value of the magnetic core.

[0015] In one embodiment, the process of determining the theoretical temperature rise based on the copper loss, the iron loss, and the actual core surface area includes: The power dissipation density is determined based on the copper loss, the iron loss, and the actual magnetic core surface area. Using formula The theoretical temperature rise is determined based on the power dissipation density; where, Indicates theoretical temperature rise, This represents the power dissipation density.

[0016] According to the specific embodiments provided in this application, this application has the following technical effects: This application provides a flyback transformer design method. It determines the operating temperature based on the flyback transformer's output power, preliminarily determines the core material based on the operating temperature, and uses the Ap method to determine the core size and model based on the core material, flyback transformer efficiency, operating frequency, and output power. Furthermore, it determines the core frame model based on the number of windings, thus solving the problem of lacking a basis for core material selection in traditional flyback transformer designs. By determining the winding wire diameter, primary winding turns, secondary winding turns, Vcc winding turns, and conductor penetration depth based on the maximum duty cycle and output power, it addresses the problem of unreasonable winding wire type and diameter selection in traditional flyback transformer designs. The theoretical temperature rise is determined based on the winding wire diameter, primary winding turns, secondary winding turns, Vcc winding turns, and core model. Optimization conditions are then determined based on the theoretical temperature rise, thereby optimizing the flyback transformer design. This solves the problem of unpredictable temperature rise in traditional flyback transformer design. A winding process is employed, based on the winding wire diameter, primary winding turns, secondary winding turns, Vcc winding turns, and wire penetration depth, to wind the core frame, resulting in the wound frame. Based on the core size and model, the wound frame, the primary inductance, and the air gap value, a final flyback transformer is obtained, providing an effective and reliable design scheme for flyback transformers. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart of a flyback transformer design method according to an embodiment of this application; Figure 2 This is a schematic diagram illustrating the relationship between the output power and core surface area of ​​a flyback transformer according to an embodiment of this application. Figure 3 A schematic diagram of power loss curves for magnetic cores made of different materials provided in an embodiment of this application; Figure 4 A schematic diagram of the loss curve of a CP95 material magnetic core provided in an embodiment of this application; Figure 5 A schematic diagram of a PQ26 / 20 type magnetic core provided in an embodiment of this application; Figure 6 A schematic diagram of the PQ26 skeleton structure provided in an embodiment of this application; Figure 7A schematic diagram of the current waveform of a flyback transformer in DCM mode provided for an embodiment of this application; Figure 8 This is a schematic diagram of the flyback transformer winding process provided in an embodiment of this application; Figure 9 This is a schematic diagram illustrating the temperature rise and loss relationship of a flyback transformer according to an embodiment of this application. Detailed Implementation

[0019] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] The flyback transformer design method provided in this application starts from the core loss curve. By estimating the operating temperature of the flyback transformer, the core material with the lowest corresponding operating temperature is selected to ensure that the iron loss of the flyback transformer is low. At the same time, considering the DC superposition requirements of the flyback transformer, the core material with high anti-saturation capability is further selected according to the saturation flux density Bs and remanence Br of different core materials, and whether the flyback transformer needs high superposition performance.

[0022] The flyback transformer design method of this application is compatible with both CCM (Continuous Conduction Mode) and DCM (Discontinuous Conduction Mode). In selecting the winding wire diameter, calculations are performed using the effective current mode based on the flyback transformer current waveform, while also considering the high-frequency skin effect to ensure optimal selection of wire type and diameter. Furthermore, the temperature rise of the designed flyback transformer is estimated, and reasonable adjustments are made based on the estimation results. This flyback transformer design method is of great significance for improving the performance of switching power supplies, ensuring the quality of electronic products, supporting the application of new technologies, and building a green ecosystem.

[0023] In one exemplary embodiment, such as Figure 1 As shown, a flyback transformer design method is provided, including: S1, Obtain the design specifications for the flyback transformer to be designed. Design specifications include: output power, output voltage. Operating frequency, maximum duty cycle, flyback transformer efficiency, diode forward voltage drop, and number of output channels.

[0024] S2, determine the operating temperature of the flyback transformer based on the output power. Determine the core material based on the operating temperature.

[0025] S3 uses the Ap method to determine the core size and model based on the core material, flyback transformer efficiency, operating frequency, and output power. The core frame model is determined based on the number of windings. The number of windings is determined based on the number of output channels.

[0026] S4 determines the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the penetration depth of the conductor based on the maximum duty cycle and output power.

[0027] S5 determines the required primary inductance of the core based on the output power and the minimum input voltage of the flyback transformer, and determines the required air gap value of the core in combination with the number of turns of the primary winding.

[0028] S6. Determine the theoretical temperature rise based on the winding wire diameter, primary winding turns, secondary winding turns, Vcc winding turns, and core model. Determine if the theoretical temperature rise exceeds a set threshold. If the theoretical temperature rise exceeds the set threshold, optimize the winding wire diameter, primary winding turns, secondary winding turns, and Vcc winding turns using set optimization conditions to obtain the final winding wire diameter, primary winding turns, secondary winding turns, and Vcc winding turns. If the theoretical temperature rise is less than or equal to the set threshold, use the winding wire diameter, primary winding turns, secondary winding turns, and Vcc winding turns as the final winding wire diameter, primary winding turns, secondary winding turns, and Vcc winding turns.

[0029] S7 employs a winding process, where the windings are wound on the core's skeleton based on the final winding wire diameter, primary winding turns, secondary winding turns, Vcc winding turns, and wire penetration depth, resulting in the wound skeleton. A flyback transformer is then derived based on the core's dimensions and model, the wound skeleton, the primary inductance, and the air gap value.

[0030] Among them, when the output power is constant, the current is the largest when the voltage is the smallest. At this time, the magnetic flux density of the core and the current density of the conductor are the largest, and the heat generation and current load of the flyback transformer are the largest. Therefore, designing the flyback transformer when the input voltage is the smallest (i.e. the worst situation) can also meet the design standards of the flyback transformer.

[0031] In one embodiment, the process of determining the operating temperature of the flyback transformer based on the output power in step S2 includes: obtaining the ambient temperature; determining the core surface area based on the output power; determining the rising temperature of the flyback transformer based on the output power and the core surface area using formula (1); and obtaining the operating temperature based on the rising temperature and the ambient temperature.

[0032] (1) In the formula, For the output power of the flyback transformer, The specific heat capacity of air. air density, The surface area of ​​the magnetic core. For airflow speed, For the efficiency of the flyback transformer, The temperature rises.

[0033] For example, the output power of a flyback transformer is determined by the power supply, which is provided by the power supply manufacturer. The output power, output voltage, and current of the flyback transformer are known parameters (as well as the operating frequency, maximum duty cycle, flyback transformer efficiency, diode forward voltage drop, and number of output channels, etc.), and the flyback transformer is designed based on these parameters.

[0034] A mathematical model is used to determine the relationship between the output power of a flyback transformer and its core surface area. The power output of a flyback transformer is directly proportional to the core size; the higher the output power, the larger the core volume. Different core sizes correspond to a minimum applicable power rating. Core surface area... It is mainly affected by the size of the magnetic core, and secondly by the shape of the magnetic core.

[0035] Using the above mathematical model, the surface area of ​​the required magnetic core is obtained based on the output power of the flyback transformer. When a flyback transformer is operating, core losses and winding losses are converted into heat and dissipated into the air. The temperature of the flyback transformer rises due to this heat dissipation, and the magnitude of this temperature rise depends on the magnitude of core and winding losses, the core surface area, and the rate of heat dissipation. Based on the obtained core surface area... The relationship with wind speed (i.e., airflow velocity) determines the operating temperature rise of the flyback transformer. temperature rise Satisfy formula (1). Increase the temperature Plus ambient temperature Obtain the operating temperature of the flyback transformer .

[0036] In one embodiment, the process of determining the core material based on the operating temperature in step S2 includes: obtaining power loss models for cores of different materials; using the power loss models, determining the power loss of cores of different materials based on the operating temperature; sorting the power losses in ascending order; determining the difference between the core saturation magnetic induction and remanence of the materials corresponding to the first m power losses; and selecting the material with the largest difference as the core material.

[0037] For example, a formula model can be obtained to show the relationship between power loss and operating temperature for magnetic cores made of different materials (i.e., a power loss model; this model can be derived by measuring key temperature points of magnetic cores made of different materials to obtain the loss at different temperature points). The operating temperature of the flyback transformer can then be considered. By substituting the power loss models of magnetic cores made of different materials, the power loss of magnetic cores under different materials is obtained. The lower the core power loss, the higher the efficiency of the flyback transformer, and the better the effect of the flyback transformer.

[0038] Further considering the DC-bias capability of the flyback transformer, the core saturation magnetic flux density... The higher the remanence, the better. The lower the value, the less likely the magnetic core is to saturate, and the better the DC superposition effect.

[0039] Therefore, the power losses of magnetic cores made of different materials are sorted in ascending order. Among the materials corresponding to the first m power losses, the material with the largest difference between the saturation magnetic induction intensity and the remanence is further selected as the core material.

[0040] This method utilizes the operating temperature and DC-bias capability of flyback transformers to select core materials with lower losses and stronger anti-saturation capabilities, ensuring the optimal selection of flyback transformer core materials.

[0041] In one embodiment, the process of determining the size and model of the magnetic core using the Ap (Area Product Method) in step S3 based on the core material, flyback transformer efficiency, operating frequency, and output power includes: using formula (2), determining the required Ap threshold of the magnetic core based on the flyback transformer efficiency, operating frequency, and output power; and determining the size and model of the magnetic core based on the Ap threshold and the core material.

[0042] (2) In the formula, Indicates the Ap threshold. The area of ​​the magnetic core window. For output power, For the efficiency of the flyback transformer, The effective cross-sectional area of ​​the magnetic core. For the flyback transformer topology core, For operating frequency, This represents the maximum operating magnetic flux density of the magnetic core. Determined based on the core material.

[0043] For example, the minimum Ap value (i.e., the Ap threshold) of the required magnetic core can be calculated using the Ap method. The Ap value represents the energy storage capacity of the magnetic core. The larger the power of the flyback transformer, the larger the required Ap value. The Ap threshold is determined by the magnetic core window area Aw and the effective cross-sectional area Ae of the magnetic core, as shown in formula (2).

[0044] Maximum operating magnetic flux density of the magnetic core Generally, it is the saturation magnetic flux density of the magnetic core. 0.7 times. The core saturation flux density is determined based on the core material. The Ap value of the selected core must be greater than the Ap threshold. .

[0045] Select a suitable core bobbin model based on the number of windings. The number of windings is known (determined by the number of output channels of the flyback transformer). The windings include the primary winding, secondary winding, and Vcc winding. If the flyback transformer has 'a' output channels (determined before design), then the number of windings is 'a+2'. Each winding requires at least 2 pins, therefore the selected core bobbin must meet this requirement. Simultaneously, determine the core model with the smallest AP value among cores with an AP value greater than the calculated AP value. This core model has the optimal size and energy storage capacity.

[0046] This embodiment enables the selection of the smallest magnetic core that meets performance requirements, avoiding the selection of a core that is too small, leading to core saturation, or a core that is too large, increasing cost and space.

[0047] In one embodiment, step S4 is implemented as follows: The effective value of the current is determined based on the maximum duty cycle and the output power. The winding wire diameter is determined based on the effective value of the current. The number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding are determined based on the maximum duty cycle and the minimum input voltage, respectively. The penetration depth is determined based on the operating frequency.

[0048] The winding process in step S7 uses a sandwich winding method. Half the number of turns of the primary winding is wound in the innermost layer of the bobbin, followed by a layer of copper wire or copper foil for shielding. Then, the secondary winding is wound, followed by another layer of copper wire or copper foil for shielding. After the remaining half of the primary winding is wound, the Vcc winding is finally wound. When the winding wire diameter is greater than the conductor penetration depth, multi-strand wire is used to avoid the high-frequency skin effect. The diameter of the multi-strand wire should be smaller than the conductor penetration depth.

[0049] For example, winding design includes wire diameter, number of turns, type, and winding process.

[0050] 1) Wire diameter.

[0051] The wire diameter is determined as shown in formula (3).

[0052] (3) In the formula, The size of the winding wire diameter, This is the effective value of the current. denoted as current density.

[0053] Flyback transformers generally have two current modes: CCM (Continuous Current Mode) and DCM (Discontinuous Current Mode). These different current modes result in different primary peak currents. and primary starting current The change in primary peak current is determined based on the output power and the minimum input voltage of the flyback transformer. and primary starting current The formula is as follows: (4) (5) In the formula, This is the initial starting current. This is the primary peak current. This is the minimum input voltage for the flyback transformer. For maximum duty cycle, This is the ripple factor.

[0054] By combining formulas (4) and (5), the primary initiation current and primary peak current can be solved.

[0055] RMS value of current Including the primary current RMS value and the effective value of secondary current Primary current RMS value The calculation formula is as follows: (6) Calculate the effective value of the secondary current The secondary peak current must be calculated first. and secondary starting current The calculation formula is as follows: (7) (8) In the formula, This is the secondary starting current. This is the secondary peak current. This is the secondary output voltage (i.e., the output voltage of the flyback transformer).

[0056] By combining equations (7) and (8), the secondary peak current can be solved. and secondary starting current Therefore, the effective value of the secondary current is determined. The size of is determined by the following formula: (9) Through the process described above in this embodiment, the actual electrical waveform is selected based on the actual current operating mode of the flyback transformer. The magnitudes of the primary and secondary currents are then calculated based on the actual current waveform.

[0057] 2) Number of turns.

[0058] Number of turns in primary winding Derived from Faraday's law of electromagnetic induction; number of turns in the secondary winding and the number of turns of the Vcc winding Both are based on the secondary winding voltage. and Vcc winding voltage With minimum input voltage The ratio is obtained by considering the duty cycle. The formula for the number of turns in the winding is as follows: (10) (11) (12) In the formula, This refers to the number of turns in the primary winding. This refers to the number of turns in the secondary winding. For auxiliary winding, For diode forward voltage drop, This is the voltage of the Vcc winding.

[0059] 3) Winding type.

[0060] The primary winding and Vcc winding generally use enameled wire; the secondary winding, to enhance insulation, typically uses triple-insulated wire. Simultaneously, to avoid the effects of the high-frequency skin effect, the penetration depth of the conductor is calculated. In the conductor diameter (i.e., winding diameter) (greater than the penetration depth) In such cases, multi-strand wires are needed to effectively avoid the skin effect at high frequencies. The diameter of the multi-strand wire should be smaller than the penetration depth. The penetration depth The formula is as follows: (12) Wherein, 66.1 is the copper wire penetration depth coefficient. This is the operating frequency of the flyback transformer.

[0061] 4) The winding process uses a sandwich winding method, with half of the primary winding turns wound in the innermost layer of the bobbin. Then, wind another layer of copper wire or copper foil for shielding, and then wind the secondary winding turns. Then, another layer of copper wire or copper foil is wound for shielding, reducing the remaining half of the primary winding turns. After the winding is completed, the Vcc auxiliary winding is wound last. .

[0062] The winding process should follow the principle of low leakage inductance and good coupling effect between the primary and secondary windings. Generally, the primary winding should be wound to fill the width of the bobbin to enhance coupling. If the calculated result is a few turns more than the actual primary winding should fill the width of the bobbin, it can be discarded; if the calculated result is a few turns less than the actual primary winding should fill the width of the bobbin, a few more turns can be added.

[0063] By determining the appropriate wire diameter and number of turns for the flyback transformer using steps 1) to 4) above, the turn count should take into account the diode forward voltage drop. and maximum duty cycle This makes the turns calculation more accurate. Additionally, for wire diameter calculation, the effective value of the current is used. This conforms to the actual current waveform.

[0064] In one embodiment, step S5 specifically includes: S51, determine the primary inductance. Specifically, determine the primary initiation current and primary peak current based on the output power and the minimum input voltage of the flyback transformer. The primary inductance is then determined based on the primary initiation current, primary peak current, and minimum input voltage.

[0065] S52, determine the air gap value. Specifically, the air gap value is determined based on the primary inductance and the number of turns in the primary winding.

[0066] For example, the primary peak current determined by combining formulas (4) and (5) in the above embodiments. and primary starting current The primary inductance is determined using Faraday's law of electromagnetic induction, along with the minimum input voltage, as shown in formula (13).

[0067] (13) In the formula, For primary inductance, This represents the maximum conduction time.

[0068] The core of a flyback transformer generally requires an air gap, primarily to increase core energy storage and improve DC-bias capability, and secondly to maintain stable inductance. The air gap value is shown in formula (14).

[0069] (14) In the formula, is the vacuum permeability.

[0070] By calculating the minimum inductance (i.e., primary inductance) and air gap size required by the flyback transformer through steps S51 and S52, the minimum excitation inductance of the flyback transformer is determined, which facilitates subsequent adjustment and optimization.

[0071] In one embodiment, the process of determining the theoretical temperature rise in step S6 includes: S61, obtain the volume loss model corresponding to the core material.

[0072] S62, the lengths of the primary winding, secondary winding, and Vcc winding are determined based on the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding, respectively.

[0073] S63 determines copper loss based on the length of the primary winding, the length of the secondary winding, and the length of the Vcc winding.

[0074] S64 uses a volume loss model to determine the unit volume loss of the magnetic core based on the operating temperature.

[0075] S65, determine the effective volume and actual surface area of ​​the magnetic core. The process of determining the actual surface area includes: determining the core structure coefficient and the Ap value based on the core's model number; and then determining the actual surface area based on the core structure coefficient and the Ap value.

[0076] S66, iron loss is determined based on unit volume loss and effective volume.

[0077] S67, the theoretical temperature rise is determined based on copper loss, iron loss, and the actual core surface area. Specifically, the power dissipation density is determined based on copper loss, iron loss, and the actual core surface area. Formula (15) is used to determine the theoretical temperature rise based on the power dissipation density.

[0078] (15) In the formula, Indicates theoretical temperature rise, This represents the power dissipation density.

[0079] For example, theoretical temperature rise The calculation includes copper loss. and iron loss Copper loss Copper loss is calculated by multiplying the length of all winding conductors by their corresponding resistivity. The formula is as follows: (16) In the formula, For the primary winding length, The resistivity of the primary winding, For the secondary winding length, The resistivity of the secondary winding, The length of the Vcc winding. Let Vcc be the resistivity of the winding. The lengths of the primary winding, secondary winding, and Vcc winding are determined based on the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding, respectively.

[0080] Iron loss To determine the core loss, a mathematical model (i.e., a volume loss model) is obtained to show the volume variation of cores made of different materials with operating temperature. Based on the operating temperature, the unit volume loss of the core at the corresponding temperature is then calculated. Due to the unit volume loss of the magnetic core Multiply by the effective volume of the magnetic core Obtaining core loss As shown in formula (17).

[0081] (17) Theoretical temperature rise The temperature rise is determined by the heat dissipation from the magnetic core and windings. Heat is dissipated along the surface of the magnetic core. Copper loss and iron loss And the actual core surface area Ato is related. The power dissipation density is determined based on the copper loss, iron loss and the actual core surface area, as shown in formula (18), and then the theoretical temperature rise is determined according to formula (15). .

[0082] (18) In the formula, For power dissipation density, This represents the actual surface area of ​​the magnetic core.

[0083] Among them, the actual magnetic core surface area It can be determined by the structural coefficient of the magnetic core (Determined by the model of the magnetic core) Multiply by the Ap value of the magnetic core to obtain the result, as shown in formula (19): (19) Core structural coefficient It is related to the shape of the magnetic core. Common core shapes are 41.3 for EE type, 50.9 for EI type, 32.5 for PQ type, 39.2 for EER, EC, ETD, and ER type, and 37 for EFD and EPC type.

[0084] This implementation method allows for the prediction of the temperature rise of the designed flyback transformer, facilitating subsequent adjustments and optimizations. Regarding the theoretical temperature rise... If the value exceeds a set threshold, optimization conditions can be set (e.g., increasing the winding wire diameter, which will change the number of turns in the primary, secondary, and Vcc windings) to optimize the winding wire diameter, primary winding turns, secondary winding turns, and Vcc winding turns, resulting in the final winding wire diameter, primary winding turns, secondary winding turns, and Vcc winding turns. Additionally, this can be achieved when the Ap value exceeds the Ap threshold. Other core models can be selected from the available core models (i.e., increasing the core surface area). In an exemplary embodiment, the flyback transformer is designed using the flyback transformer design method described in the above embodiments of this application, based on the specifications of the flyback transformer to be designed.

[0085] The specifications for the flyback transformer are: Input voltage Vin: 85-265Vac; Output power... Output voltage Current operating mode: DCM; Operating frequency Maximum duty cycle The efficiency of the flyback transformer is 0.85; the diode forward voltage drop... It is 0.2V; the number of output channels is one.

[0086] Output power and core surface area Relationship such as Figure 2 As shown. The core surface area is determined based on the output power, and then the rising temperature of the flyback transformer during operation is obtained according to formula (1). .

[0087] Specifically, the specific heat capacity of air, C, is 1000 J / kg, and the air density, ρ, at room temperature is 1.25 kg / m³. 3 The surface area of ​​the magnetic core, At, is 21 cm². 2 The airflow speed is 0.4 m / s.

[0088] Therefore, the operating temperature T = Δt + 25℃ = 92.2℃.

[0089] At an operating temperature of 92.2℃, according to the power loss model (power loss curve as shown in Figure 1), Figure 3 As shown in the figure, the material with the best core material loss is CP97A, followed by CP95 and CP95B. Considering that CP97A has a higher cost and is more suitable for high-end applications, CP95 is preferred in this embodiment. The loss curve of the CP95 core is shown in the figure. Figure 4 As shown.

[0090] Specifically, the flyback transformer designed in this embodiment does not have high requirements for DC-bias capability. The saturation magnetic flux density Bs of CP95 material at 100℃ is 390, the remanence Br is 50, and the maximum working magnetic flux density Bm is 238.

[0091] The Ap threshold is determined according to formula (2) in the above embodiments, as shown in formula (20).

[0092] (20) Then, a suitable magnetic core and frame were selected. Specifically, the PQ26 / 20 magnetic core was chosen, with an Ap value of 0.7188 cm. 4 The PQ26 (6+6) type bobbin was selected, with a width of 9.4mm and a winding bobbin diameter of 14mm. The PQ26 / 20 model magnetic core is as follows... Figure 5 As shown ( Figure 5 Part a is a side view of the magnetic core. Figure 5 Part b is a top view of the magnetic core. Figure 5 The dimensions labeled with Chinese letters are shown in Table 1: A: Bottom length, B: Outer leg length, C: Bottom width, D: Center column diameter, E: Window width, F: Center column thickness. The PQ26 frame structure is as follows... Figure 6 As shown ( Figure 6 Part a is a top view of the skeleton. Figure 6 Part b is an isometric side view of the skeleton. Figure 6 Part C is the right-side view of the skeleton. Figure 6 Part d is the left-side view of the skeleton.

[0093] Table 1. PQ26 / 20 Model Core Dimensions

[0094] The current waveform of the flyback transformer operating in DCM mode is as follows: Figure 7 As shown ( Figure 7 Part a shows the transformer input current waveform. Figure 7 Part b is the waveform of the transformer output current. According to formula (4) and formula (5), the primary peak current ip2 is 2.61A and the primary starting current ip1 is 0.

[0095] According to formula (13), the primary inductance L is 345 μH. In order to meet the working requirements of the flyback transformer and leave a certain margin, the primary inductance of the flyback transformer should be greater than 345 μH, and is taken as 400 μH.

[0096] like Figure 8 As shown, the number of turns Np in the primary winding is obtained from Faraday's law of electromagnetic induction. The number of turns Ns in the secondary winding and the number of turns Nvcc in the Vcc winding are determined based on the secondary winding voltage Vo and the Vcc winding voltage Vcc, and the minimum input voltage. The ratio is obtained by considering the duty cycle. Specifically, the maximum conduction time... The switching cycle of the flyback transformer multiplied by the maximum duty cycle. The value is 7.5μH; the secondary output voltage Vo is 12V, and the Vcc winding voltage is 15V. Calculations using the above formula yield the primary winding turns Np as 32Ts, the secondary winding turns Ns as 4Ts, and the Vcc winding turns Nvcc as 5Ts.

[0097] According to formulas (6) to (9) in the above embodiments, the primary starting current ip2 is 0A, the primary peak current ip1 is 2.61A, the primary current effective value is 0.59A, the secondary peak current is1 is 18A, and the secondary current effective value is 7.78A.

[0098] The current density J = 6 A / mm². Based on the current density calculation formula, the primary wire diameter Dp is 0.45 mm, the secondary wire diameter Ds is 1.29 mm, and the copper wire penetration depth is... The primary wire diameter is 0.54mm, using a single-strand wire. The secondary wire diameter Ds > 0.54mm, using multi-strand wire effectively avoids the skin effect at high frequencies; this translates to a multi-strand wire diameter of 0.1mm. 160P, Vcc winding wire diameter Dvcc is 0.2mm.

[0099] In this implementation case, the number of winding turns is modified to meet the full width of the primary winding. According to the sandwich winding method, the primary winding is divided into two layers, and copper foil shielding is added between the primary and secondary windings, which helps the flyback transformer resist EMI interference.

[0100] The relationship between the core surface area At and the loss P at temperature rises of 25℃ and 50℃ is as follows: Figure 9 As shown, to achieve the corresponding temperature rise standard, under the same loss conditions, the larger the core surface area At (i.e., the larger the core Ap value), the lower the temperature rise; when the core size is the same (i.e., the core surface area At is the same), the lower the winding loss, the lower the temperature rise. By comparing the calculated theoretical temperature rise with the standard temperature rise, when the standard temperature rise is exceeded, the winding loss can be reduced by selecting a core with a larger Ap value (i.e., selecting a core with a larger surface area) or by increasing the winding wire diameter. The estimation of the theoretical temperature rise ΔT is related to the copper loss Pcu and iron loss Pcore of the flyback transformer. The copper loss Pcu is obtained based on the winding wire length and winding resistivity, and the core loss Pcore is obtained by multiplying the unit volume loss Pcv of the core by the effective volume Ve of the core.

[0101] Theoretical temperature rise The temperature rise is determined by the heat dissipation from the magnetic core and windings. Heat is dissipated along the surface of the magnetic core. This is related to the copper loss Pcu, iron loss Pcore, and the actual core surface area Ato. Specifically, the flyback transformer has a copper loss Pcu of 0.445W, a core volume loss Pcv of 324.76kW / m³, an effective core volume Ve of 5490mm³, and a core loss Pcore of 1.7829W. Further calculations yield a theoretical temperature rise of 96.36℃ for this flyback transformer. This theoretical temperature rise is less than the required standard temperature rise of 100℃, which meets the requirement. If it is greater than 100℃, a PQ26 / 25 core can be selected. The Ap value of the PQ26 / 25 core is 0.9971cm. 4 If the Ap value is greater than that of PQ26 / 20, the surface area At of the magnetic core will increase, and the temperature rise of the transformer will decrease.

[0102] This embodiment is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

[0103] In combination with the above embodiments, this application has the following advantages: 1. This application starts with the core material of the flyback transformer and selects the most suitable core in terms of core performance and volume to ensure the optimal solution for the efficiency and volume of the flyback transformer. It provides an effective and reliable design method for traditional flyback transformer design that relies on experience-based selection.

[0104] 2. The calculation of the number of turns and wire diameter of the flyback transformer in this application, and the design method for the number of turns and wire diameter, fully consider the actual operating conditions of the flyback transformer, taking into account the influence of duty cycle changes and diode forward voltage drop. Simultaneously, based on different current operating modes, the effective current value is calculated, and the optimal wire diameter is selected from the effective current value. Furthermore, the influence of the high-frequency skin effect is considered in the selection of wire diameter and wire type to ensure that the flyback transformer has low copper losses, improves the efficiency of the flyback transformer, and reduces the temperature rise of the flyback transformer.

[0105] 3. The flyback transformer design method of this application can effectively predict the temperature rise of the designed flyback transformer. Based on the temperature rise, the winding coil and transformer core can be slightly adjusted to make the flyback transformer design optimal.

[0106] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0107] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. In summary, the content of this specification should not be construed as a limitation of this application.

Claims

1. A flyback transformer design method, characterized in that, include: Obtain the design specifications of the flyback transformer to be designed; The design specifications include: output power, output voltage, operating frequency, maximum duty cycle, flyback transformer efficiency, diode forward voltage drop, and number of output channels; The operating temperature of the flyback transformer is determined based on the output power; the core material is determined based on the operating temperature; The Ap method is used to determine the size and model of the magnetic core based on the core material, the efficiency of the flyback transformer, the operating frequency, and the output power; the model of the core frame is determined based on the number of windings; the number of windings is determined based on the number of output channels. The winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the penetration depth of the wire are determined based on the maximum duty cycle and the output power. The required primary inductance of the core is determined based on the output power and the minimum input voltage of the flyback transformer, and the required air gap value of the core is determined in conjunction with the number of turns of the primary winding; the minimum input voltage is determined based on the output power. The theoretical temperature rise is determined based on the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the model of the magnetic core. Determine whether the theoretical temperature rise is greater than the set threshold standard; If the theoretical temperature rise is greater than a set threshold standard, the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding are optimized using set optimization conditions to obtain the final winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding; if the theoretical temperature rise is less than or equal to the set threshold standard, the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding are used as the final winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding. Using a winding process, based on the final winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the penetration depth of the wire, the winding is wound on the skeleton of the magnetic core to obtain the wound skeleton. The flyback transformer is designed based on the size and model of the magnetic core, the wound frame, the primary inductance, and the air gap value.

2. The flyback transformer design method according to claim 1, characterized in that, The process of determining the operating temperature of the flyback transformer based on the output power includes: Obtain the ambient temperature; The core surface area is determined based on the output power. Based on the output power and the magnetic core surface area, the formula is used. Determine the operating temperature rise of the flyback transformer; where, For output power, The specific heat capacity of air. air density, The surface area of ​​the magnetic core. For airflow speed, For the efficiency of the flyback transformer, To increase the temperature; The operating temperature is obtained based on the temperature rise and the ambient temperature.

3. The flyback transformer design method according to claim 1, characterized in that, The process of determining the core material based on the operating temperature includes: Obtain power loss models for magnetic cores made of different materials; Using the power loss model, the power loss of magnetic cores made of different materials is determined based on the operating temperature; Arrange the power losses in ascending order; determine the difference between the core saturation magnetic induction and remanence of the material corresponding to the first m power losses; The material with the largest difference is selected as the core material.

4. The flyback transformer design method according to claim 1, characterized in that, The Ap method is used to determine the size and model of the magnetic core based on the core material, the efficiency of the flyback transformer, the operating frequency, and the output power, including: Using formula The required Ap threshold for the magnetic core is determined based on the efficiency of the flyback transformer, the operating frequency, and the output power; where, Indicates the Ap threshold. The area of ​​the core window is... For output power, For the efficiency of the flyback transformer, The effective cross-sectional area of ​​the magnetic core, For the flyback transformer topology core, For operating frequency, This represents the maximum operating magnetic flux density of the magnetic core. Determined based on the core material; The size and model of the magnetic core are determined based on the Ap threshold and the magnetic core material.

5. The flyback transformer design method according to claim 1, characterized in that, The winding process is a sandwich winding method. Half of the primary winding turns are wound in the innermost layer of the skeleton, then a layer of copper wire or copper foil is wound for shielding, then the secondary winding is wound, then another layer of copper wire or copper foil is wound for shielding, and after the remaining half of the primary winding turns are wound, the Vcc winding is finally wound. Wherein, when the diameter of the winding wire is greater than the penetration depth of the conductor, multi-strand wire is used to avoid the high-frequency skin effect, and the diameter of the multi-strand wire should be smaller than the penetration depth of the conductor.

6. The flyback transformer design method according to claim 1, characterized in that, The process of determining the primary inductance includes: The primary initiation current and primary peak current are determined based on the output power and the minimum input voltage of the flyback transformer. The primary inductance is determined based on the primary starting current, the primary peak current, and the minimum input voltage.

7. The flyback transformer design method according to claim 1, characterized in that, The winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, the number of turns in the Vcc winding, and the penetration depth of the conductor are determined based on the maximum duty cycle and the output power, including: The effective value of the current is determined based on the maximum duty cycle and the output power; the winding wire diameter is determined based on the effective value of the current. The number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding are determined based on the maximum duty cycle and the minimum input voltage, respectively. The penetration depth is determined based on the operating frequency.

8. The flyback transformer design method according to claim 1, characterized in that, The theoretical temperature rise is determined based on the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding, including: Obtain the volume loss model corresponding to the magnetic core material; The lengths of the primary winding, secondary winding, and Vcc winding are determined based on the winding wire diameter, the number of turns in the primary winding, the number of turns in the secondary winding, and the number of turns in the Vcc winding, respectively. Copper loss is determined based on the length of the primary winding, the length of the secondary winding, and the length of the Vcc winding. Using the aforementioned volume loss model, the unit volume loss of the magnetic core is determined based on the operating temperature; Determine the effective volume and actual surface area of ​​the magnetic core; Iron loss is determined based on the unit volume loss and the effective volume; The theoretical temperature rise is determined based on the copper loss, the iron loss, and the actual core surface area.

9. The flyback transformer design method according to claim 8, characterized in that, The process of determining the actual surface area of ​​the magnetic core includes: The magnetic core structure coefficient and the Ap value of the magnetic core are determined based on the model of the magnetic core; The actual surface area of ​​the magnetic core is determined based on the magnetic core structure coefficient and the Ap value of the magnetic core.

10. The flyback transformer design method according to claim 8, characterized in that, The process of determining the theoretical temperature rise based on the copper loss, the iron loss, and the actual magnetic core surface area includes: The power dissipation density is determined based on the copper loss, the iron loss, and the actual magnetic core surface area. Using formula The theoretical temperature rise is determined based on the power dissipation density; where, Indicates theoretical temperature rise, This represents the power dissipation density.

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