The main heat-generating components in a switching power supply include semiconductor switching transistors, output power diodes, high-frequency transformers, and filter inductors.
Different devices require different methods to control heat generation. Power transistors are among the devices that generate the most heat in high-frequency switching power supplies. Reducing their heat generation can not only improve the reliability of the power transistors, but also improve the reliability of the switching power supply and increase the mean time between failures (MTBF).
The heat generated by a switching transistor is caused by losses, which consist of two parts: switching process losses and conduction losses. Conduction losses can be reduced by selecting a switching transistor with low conduction resistance. Switching process losses are caused by the size of the gate charge and the switching time. Switching process losses can be reduced by selecting devices with faster switching speeds and shorter recovery times.
However, it is more important to reduce losses through better control methods and buffering techniques. For example, using soft-switching technology can significantly reduce these losses. Reducing the heat generated by power diodes is also crucial. For AC rectifier and buffer diodes, there are generally no better control techniques to reduce losses; however, selecting high-quality diodes can help reduce losses.
For rectification on the secondary side of the transformer, a more efficient synchronous rectification technology can be selected to reduce losses. For losses caused by high-frequency magnetic materials, the skin effect should be avoided as much as possible. The impact of the skin effect can be addressed by winding multiple strands of fine enameled wire in parallel.
High-frequency power transformers are power transformers that operate at frequencies exceeding the intermediate frequency (30kHz). They are mainly used in high-frequency switching power supplies as high-frequency switching power supply transformers, and are also used in high-frequency inverter power supplies and high-frequency inverter welding machines as high-frequency inverter power supply transformers.
Based on their operating frequency, they can be divided into several grades: 10kHz～50kHz, 50kHz～100kHz, 100kHz～500kHz, 500kHz～1MHz, and above 1MHz.
Those with higher transmission power have lower operating frequencies;
It has a relatively low transmission power and a relatively high operating frequency.
Thus, there are differences in both operating frequency and transmitted power, and the design methods for power transformers of different grades differ depending on the operating frequency.
Design principles of high-frequency power transformers
Prioritizing performance and efficiency can sometimes come at the expense of price and cost. Currently, lightweight, thin, short, and small designs are the development trend for high-frequency power supplies, emphasizing cost reduction.
High-frequency power transformers, which present a significant challenge, require considerable effort. Therefore, the "design considerations," performance, and cost of high-frequency power transformers must be carefully considered. If the design principles of high-frequency power transformers are taken seriously, and a better performance-price ratio is pursued, then for high-frequency power transformers in switching power supplies with a capacity of less than 10VA, lighter, thinner, shorter, and smaller designs should be developed.
The market's value laws are ruthless; many high-performance products are often neglected and eliminated because their prices are unacceptable to the market. Often, a new product is ultimately rejected by cost.
We need to save energy and money.
Product costs include not only material costs and production costs, but also research and development costs and design costs.
Therefore, to save time, based on experience, we provide some reference data on the ratio of iron loss to copper loss, the ratio of leakage inductance to magnetizing inductance, the ratio of primary and secondary winding losses, and current density of high-frequency power transformers. We also recommend some solutions for window filling, winding conductors, and structure. Avoid performing step-by-step calculations and simulations. The design principle is to pursue the best performance-price ratio while fulfilling specific functions under specific operating conditions. The sole criterion for evaluating the design is whether the designed product meets market requirements.
Design requirements for high-frequency power transformers
Based on design principles, four design requirements can be proposed for high-frequency power transformers:
Determine usage conditions, complete functions, improve efficiency, and reduce costs.
1. Usage conditions
The conditions for use include two aspects: reliability and electromagnetic compatibility.
Reliability refers to the ability of a high-frequency power transformer to operate normally until its service life under specific operating conditions. Generally, ambient temperature has the greatest impact on high-frequency power transformers. Some soft magnetic materials have a low Curie point and are sensitive to temperature.
For example, manganese-zinc soft magnetic ferrite has a Curie point of only 215℃. Its magnetic flux density, permeability, and losses all change with temperature. Therefore, in addition to the normal temperature of 25℃, reference data for 60℃, 80℃, and 100℃ must also be provided. Thus, the operating temperature of manganese-zinc soft magnetic ferrite cores is limited to below 100℃. This means that at an ambient temperature of 40℃, the temperature rise is only allowed to be below 60℃, equivalent to the temperature of Class A insulation materials.
The electromagnetic wires and insulation components that are matched with the manganese-zinc soft ferrite core generally use Class E and Class B insulation materials. Using triple-insulated electromagnetic wires with Class H insulation and polyamide film increases the cost. Is it because the optimized design of the high-frequency power transformer with Class H insulation can reduce the volume by 1/2 to 1/3?
The already relatively small high-frequency 100kHz/10VA power transformer can be reduced in size by 1/2 to 1/3 if the secondary winding uses triple-insulated wire.
Electromagnetic compatibility (EMC) refers to the ability of a high-frequency power transformer to both avoid generating electromagnetic interference and withstand external electromagnetic interference. Electromagnetic interference includes audible noise and high-frequency noise. The main causes of EMC in high-frequency power transformers include the attractive forces between the magnetic cores and the repulsive forces between the winding conductors. The frequency of these forces changes is consistent with the operating frequency of the high-frequency power transformer. Therefore, a high-frequency power transformer operating at around 100kHz will not generate audible noise below 20kHz without a specific reason. The audible noise frequency of switching power supplies below 10W, approximately 10kHz to 20kHz, must have a cause.
For any questions, please refer to – “Main Sources of Noise – Why are Transformers & Inductors the Focus of Concern?”
The likelihood of electromagnetic interference originating from the high-frequency power transformer itself is low, making it unnecessary to use glass bead adhesive to bond the magnetic core. Shielding is a good way to prevent electromagnetic interference and increase the electromagnetic compatibility of high-frequency power transformers. However, to prevent the propagation of electromagnetic interference from high-frequency power transformers, corresponding measures should also be taken in the design of the magnetic core structure and winding structure. Simply adding an external shielding strip is not necessarily the best solution, as it can only prevent radiated interference, not conducted interference.
2. Complete the function
High-frequency power transformers perform three functions: power transmission, voltage transformation, and insulation isolation.
There are two ways to transfer power.
The first method is the power transmission method of a transformer. The voltage applied to the primary winding generates a change in magnetic flux in the core, inducing a voltage in the secondary winding, thereby transmitting electrical power from the primary side to the secondary side. During the power transmission process, the core operates in two modes: unidirectional change and bidirectional change of magnetic flux.
In unidirectional operation mode, the magnetic flux density changes from its maximum value Bm to the residual magnetic flux density Br, or from Br to Bm. The change in magnetic flux density is ΔB = Bm - Br.
To increase ΔB, it is desirable for Bm to be large and Br to be small. In the bidirectional operating mode, the magnetic flux changes from +Bm to -Bm, or from -Bm to +Bm. The change in magnetic flux density ΔB = 2Bm. To increase ΔB, it is desirable for Bm to be large, but it is not required for Br to be small. Regardless of whether it is a unidirectional or bidirectional operating mode, the power transmission method of the transformer is not directly related to the permeability of the magnetic core.
The second method is the inductor power transfer method. The electrical energy input to the primary winding excites the magnetic core, converting it into magnetic energy for storage. Then, by demagnetizing, the secondary winding induces a voltage, which is then converted into electrical energy and released to the load.
The transmitted power depends on the energy stored in the inductor core, which in turn depends on the inductance of the primary winding. The inductance is related to the permeability of the core; higher permeability results in greater inductance and more stored energy, but it is not directly related to the magnetic flux density.
Although different power transmission methods require different core parameters, the selection of core materials and parameters remains a major aspect of high-frequency power transformer design. Voltage transformation is achieved through the turns ratio of the primary and secondary windings. Regardless of the power transmission method, the voltage transformation ratio between the primary and secondary windings is equal to the turns ratio of the primary and secondary windings; as long as the turns ratio remains unchanged, the voltage transformation is unaffected. However, the number of winding turns is related to the leakage inductance of the high-frequency power transformer. The leakage inductance is proportional to the square of the number of turns in the primary winding. "For a high-frequency transformer that meets insulation and safety standards, its leakage inductance should be 1% to 3% of the primary inductance when the secondary is open-circuited." Many technical specifications state requirements such as leakage inductance = 1% magnetizing inductance or leakage inductance < 2% magnetizing inductance. Power supply designers should set a numerical limit on the acceptable leakage inductance value based on the normal operating requirements of the circuit. During transformer manufacturing, the leakage inductance value should be minimized as much as possible without deteriorating other transformer parameters (such as inter-turn capacitance). Because of its large leakage inductance, the energy stored is also large. When it is suddenly released during power switching, it will generate a voltage spike, which will increase the peak voltage that the switching device is subjected to. This is detrimental to insulation and will also generate additional losses and electromagnetic interference.
Insulation is achieved through the insulation structure of the primary and secondary windings. To ensure insulation between windings, the distance between the two windings must be increased, thereby reducing the coupling between windings and increasing leakage inductance. Furthermore, the primary winding is generally a high-voltage winding, and the number of turns cannot be too small; otherwise, a large difference in voltage between turns or layers can cause local short circuits. Thus, there is a lower limit to the number of turns, which in turn limits the leakage inductance.
In summary, leakage inductance and insulation strength must be considered comprehensively in the design of the insulation structure and overall structure of high-frequency power transformers.
3. Improve efficiency
Improving efficiency is a universal requirement for power supplies and electronic equipment. Increasing the efficiency of high-frequency power transformers can save electricity and also has dual socio-economic benefits in terms of environmental protection. Therefore, improving efficiency is a major design requirement for high-frequency power transformers; generally, efficiency should be increased to over 95%, and losses should be reduced to below 5%. High-frequency power transformer losses include core losses (iron losses) and winding losses (copper losses).
Some people are concerned about the ratio of iron loss to copper loss in transformers. This ratio varies with the transformer's operating frequency. If the applied voltage remains constant, the lower the operating frequency and the more turns in the windings, the greater the copper loss. Therefore, at a 50Hz power frequency, copper loss far exceeds iron loss. For example, at 50Hz, the copper loss of a 100kVA S9 type three-phase oil-immersed silicon steel power transformer is about 5 times that of iron loss. At 50Hz, the copper loss of a 100kVA ASH11 type three-phase oil-immersed amorphous alloy power transformer is about 20 times that of iron loss.
As the operating frequency increases, the number of winding turns decreases. Although winding losses increase due to the skin effect and proximity effect, the general trend is that copper losses decrease with increasing operating frequency. Iron losses, including hysteresis losses and eddy current losses, increase rapidly with increasing operating frequency.
At a certain operating frequency range, copper losses and iron losses may be equal. Beyond this operating frequency range, iron losses will be greater than copper losses.
The reason why iron loss is not equal to copper loss is that, although the choice of wire thickness is affected by the skin effect, it is mainly determined by the transmission power of the high-frequency power transformer and has no direct relationship with the operating frequency. Moreover, using very thin enameled wire as windings will actually increase copper loss and slow down its decreasing trend. It's even possible that, at the designed operating frequency, copper loss will equal iron loss. In small and medium power high-frequency power transformers operating at around 100kHz, iron loss already exceeds copper loss and becomes the main component of high-frequency power transformer losses.
Because iron loss is the main component of losses in high-frequency power transformers, selecting core materials based on iron loss is a crucial aspect of high-frequency power transformer design. Iron loss is also a primary parameter for evaluating soft core materials. Iron loss is related to the operating magnetic flux density and frequency of the core. When describing the iron loss of soft core materials, it is essential to specify the operating magnetic flux density and frequency at which the loss occurs. When using symbols, PB/f must be indicated [where the unit of operating magnetic flux density B is Tesla (T), and the unit of operating frequency f is Hertz (Hz)].
For example, P0.5/400 represents the loss at an operating magnetic flux density of 0.5T and an operating frequency of 400Hz. Another example is P0.1/100k, which represents the loss at an operating magnetic flux density of 0.1T and an operating frequency of 100kHz. Iron loss is also related to operating temperature. When describing the iron loss of soft magnetic core materials, its operating temperature must be specified, especially for soft magnetic ferrite materials, which are quite sensitive to temperature changes. Product specifications must list the iron loss from 25℃ to 100℃. The saturation magnetic flux density of soft magnetic materials does not completely represent the upper limit of the operating magnetic flux density; often, it is the iron loss that limits the upper limit of the operating magnetic flux density. Therefore, in the new classification standard for soft magnetic ferrite materials for power transformers, the product of the allowable operating magnetic flux density and the operating frequency, B×f, is used as the material's performance factor, and the allowable loss value under the performance factor condition is specified. The new classification standard divides soft magnetic ferrite materials into five categories—PW1, PW2, PW3, PW4, and PW5—based on their performance factors. Higher performance factors correspond to higher operating frequencies and higher frequency limits. For example, PW3 soft magnetic ferrite materials operate at 100kHz, have a frequency limit of 300kHz, and a performance factor B×f of 10000mT×kHz. This means that at 100mT (0.1T) and 100kHz, at 100℃, losses are ≤300kW/m³ (300mW/cm³) for category a and ≤150kW/m³ (150mW/cm³) for category b.
At a certain operating frequency, when the winding losses (copper losses) of a high-frequency power transformer are close to the iron losses, for example, in the range of copper loss/iron loss = 100% to 25%, the copper losses cannot be ignored, and measures should be taken to reduce them. Since the primary and secondary windings handle similar power, the copper losses of the primary winding are often taken to be equal to those of the secondary winding in the design to simplify the design calculation process. It is not advisable to solely emphasize designing high-frequency power transformers based on temperature rise, as thermal resistance is not easily determined accurately, making design calculations quite cumbersome. Therefore, to simplify calculations, it is sometimes necessary to recommend some principles and data in advance based on experience. Similarly, to simplify calculations, it is also necessary to recommend different winding current densities for high-frequency power transformers with different operating frequencies and power ratings, but not limited to a single current density value, such as 2A/mm² to 3A/mm². It should be recognized that there is no single method to achieve the design requirements of high-frequency power transformers; a variety of approaches should be explored.
4. Reduce costs
Cost reduction is a primary, and sometimes decisive, design requirement for high-frequency power transformers. As a product, high-frequency power transformers, like other commodities, face market competition. This competition encompasses both performance and cost, neither of which can be neglected. Failure to focus on cost reduction often leads to elimination from the competition.
The cost of a high-frequency power transformer includes material costs, manufacturing costs, and management costs.
Design is a primary means of reducing the cost of high-frequency power transformers. What are the costs and quantities of the materials and components used in high-frequency power transformers? Are they easy to procure? Should a certain amount of inventory be maintained?
Is the processing and assembly of the magnetic core, coil, and overall structure complex or simple? What percentage of the process requires manual labor (mechanizing and automating the production process can reduce manual labor hours and better ensure product consistency and quality)? Are tooling and molds required? What processes and parameters need to be tested in quality control: Which parameters should be tested during processing? Which parameters should be tested during factory testing (factory testing parameters should be selected based on key performance parameters, and the number should not be large, so that product quality can be judged immediately)? Which parameters should be tested during type testing? What testing instruments and equipment should be used, and what are their prices? All these questions are determined by the design. Therefore, designers of high-frequency power transformers, in addition to understanding the theory and design methods of high-frequency power transformers, also need to understand the performance and price of various soft magnetic materials and magnetic cores, the performance and price of various electromagnetic wires, and the performance and price of various insulating materials; they also need to understand the heat treatment process for magnetic core processing, the coil winding and insulation treatment process, and the transformer assembly process; they also need to understand the testing parameters and instruments and equipment for quality control; they also need to understand the basic knowledge of production management and the market dynamics of high-frequency power transformers, etc.
Only designers with comprehensive knowledge can design high-performance, low-cost high-frequency power transformers. Cost reduction is a driving force behind the development of high-frequency power transformer technology. Why are lightweight, thin, short, and small designs the development direction for high-frequency power transformers? One reason is that this reduces both material and manufacturing costs. Increasing the operating frequency can reduce the weight and size of high-frequency power transformers. However, to overcome the negative effects of high frequencies, new soft magnetic and conductive materials must be used, and measures to suppress high-frequency electromagnetic interference must be added. Therefore, the optimal operating frequency for a high-frequency power transformer under specific operating conditions must be determined after comprehensively considering performance and overall cost. Improving efficiency and reducing heat generated by losses can reduce the surface area for heat dissipation in high-frequency power transformers, thereby reducing size and weight. However, reducing losses requires the use of new materials and processes. Therefore, the optimal efficiency for a high-frequency power transformer under specific operating conditions must also be determined after comprehensively considering performance and overall cost.
Design of high-frequency power transformer
The design of a high-frequency power transformer includes core material, core structure, core parameters, coil parameters, assembly structure, and temperature rise verification; these will be discussed in detail below.
Magnetic core material
According to the design requirements of high-frequency power transformers, selecting soft magnetic materials should ideally be the first step in the design process. However, it is now generally accepted that soft magnetic ferrites should be chosen for high-frequency power transformers, which is considered a natural choice. Many papers, monographs, and textbooks on high-frequency power transformers only discuss soft magnetic ferrites, while occasionally mentioning other soft magnetic materials. Furthermore, they don't specify which type of soft magnetic ferrite to choose, as if everyone already knows. Like any soft magnetic core material, soft magnetic ferrites have their own advantages and disadvantages. The advantages of soft magnetic ferrites are high resistivity, low AC eddy current loss, low price, and ease of processing into cores of various shapes. The disadvantages are low operating magnetic flux density, low permeability, large magnetostriction, and sensitivity to temperature changes. Therefore, some high-frequency power transformers are not suitable for soft magnetic ferrites. For example, for high-frequency power transformers with relatively low operating frequencies (below 50kHz) and high power, choosing soft magnetic ferrites would result in a lower yield due to the low operating magnetic flux density, requiring more material, resulting in a large core volume, difficult processing, fragility, and low material consumption, thus negating the advantage of low price. For example, high-frequency power transformers with high operating frequencies (above 500kHz) and relatively low power inherently have small core weight and volume. If soft magnetic ferrite is chosen, PW4 or PW5 type materials must be used, which are not inexpensive. Compared with other soft magnetic materials, the core price is basically the same, and sometimes the larger size puts them at a disadvantage. Even within the operating frequency range suitable for soft magnetic ferrite, careful consideration must be given to which type of soft magnetic ferrite can best meet the design requirements of the high-frequency power transformer in order to achieve a more ideal performance-price ratio.
magnetic core structure
Factors considered when selecting the core structure in the design of high-frequency power transformers include: reducing leakage flux and leakage inductance, increasing the coil heat dissipation area, facilitating shielding, facilitating coil winding, and simplifying assembly and wiring. Leakage flux and leakage inductance are directly related to the core structure. If an air gap is not required, closed toroidal or square core structures should be used whenever possible, especially for high-frequency power transformers, because even a small amount of leakage inductance can easily lead to significant leakage impedance. In a closed core, the magnetic flux is essentially concentrated inside the core, resulting in low leakage flux. Furthermore, regardless of the direction of external interference magnetic fields, they are split into two directions within the core, causing the interference to cancel each other out. However, closed cores are difficult to wind, and heat dissipation in toroidal cores requires passing through the coil, and the inner layer leads also need to pass through the coil for exit, thus requiring enhanced insulation. Open cores are easier to wind, have a larger heat dissipation surface for direct heat dissipation, and are easier to wire. A cylindrical cross-section for the coil's magnetic circuit section reduces the average turn length and lowers losses. The leakage flux and leakage inductance of a short, stout cylindrical magnetic core are greater than those of a tall, thin cylindrical core. One reason is the size of the core; the larger the cylinder, the larger the leakage flux radiation surface. Another reason is the shorter core; the closer distance between the upper and lower yokes facilitates the formation of leakage flux paths. The size and location of the air gap in an unsealed core are closely related to leakage flux and leakage inductance. While ensuring the air gap is sufficient for functional operation, its size should be minimized. Increasing the air gap size not only increases leakage flux and leakage inductance but also reduces the equivalent permeability, increasing excitation power, which is detrimental to the operation of high-frequency power transformers. Furthermore, the air gap is best located in the middle of the coil, which helps reduce leakage flux. The window area is related to the coil's heat loss and heat dissipation area. A larger window area allows for a larger cross-section of the wound electromagnetic wire, resulting in lower resistance, lower losses, and less heat generation. Simultaneously, the coil's overall dimensions are larger, and the heat dissipation area is also larger. Generally, after reserving sufficient window area for the process requirements, it is desirable to wind the window area as full as possible. Failure to fully utilize the window area will unnecessarily increase the core size and transformer's overall dimensions, potentially increasing material costs. Therefore, in the design of high-frequency power transformer core structures, the size of the window area must be determined after comprehensively considering various factors. Since the coil and the core are not a single unit, they must be secured separately with clamps to ensure their respective mechanical stability. Simultaneously, to ensure sufficient insulation distance, air gaps must be maintained between the ends of the coil and between the windings; it is impossible to fill the entire window with the windings. To prevent electromagnetic interference from the inside out and from the outside in of the high-frequency power transformer, some core structures have closed or semi-closed outer shells outside the window. Closed shells provide good shielding against electromagnetic interference, but heat dissipation and wiring are inconvenient, requiring wiring holes and vents. Semi-closed shells provide shielding against electromagnetic interference in the closed areas, while the unclosed areas are used for wiring and heat dissipation. Completely open windows facilitate wiring and heat dissipation, but provide poor shielding against electromagnetic interference.
Core parameters
In the design of high-frequency power transformer core parameters, special attention must be paid to the fact that the operating magnetic flux density is limited not only by the magnetization curve but also by losses and the power transmission mode. For the unidirectional flux variation mode of the transformer power transmission mode, ΔB = Bm - Br, it is limited by both saturation magnetic flux density and, more importantly, losses. However, when a high-frequency power transformer operates with unidirectional flux variation, it changes back and forth along a local hysteresis loop. The core loss is only 30% to 40% of that when it changes back and forth along a larger hysteresis loop in both directions. Material testing is conducted under sinusoidal bidirectional excitation conditions with a ΔB variation of 2Bm. Therefore, Bm can be taken as more than twice the value of B selected when testing material losses. Br is limited by Br on the material hysteresis loop. This can be reduced by opening an air gap, thereby increasing the flux density variation value ΔB. Although opening the air gap increases the excitation current, increasing ΔB reduces the core volume, making it worthwhile. For transformers operating in a bidirectional flux-changing power transmission mode, ΔB = 2Bm. The area enclosed by the operating hysteresis loop is much larger than that of the local loop, resulting in significantly higher losses. Bm is primarily limited by these losses. In this bidirectional operation mode, it's also important to consider the potential for DC bias due to unequal volt-second areas caused by various factors. This can be addressed by adding a small air gap to the core circuit, incorporating a DC blocking capacitor in the circuit design, or employing current-based control. For inductors, the permeability is the equivalent permeability after the air gap is applied, and is generally smaller than the permeability measured from the magnetization curve. After determining the core structure, the coil parameters of the high-frequency power transformer can be directly tested. These parameters include the number of turns, conductor cross-section (diameter), conductor type, winding arrangement, and insulation configuration. The number of turns in the primary winding is determined by the applied excitation voltage or the primary winding excitation inductance (energy storage); the number of turns should not be excessive or insufficient. Excessive turns increase leakage inductance and winding time; insufficient turns, especially with high applied excitation voltage, may increase inter-turn and inter-layer voltage drops, necessitating enhanced insulation. The number of turns in the secondary winding is determined by the output voltage. High-frequency power transformers are primarily used in high-frequency switching power supplies. Switching power supplies allow for output voltage adjustment, with the upper limit limited by the permissible switching duty cycle. When calculating the transformer output voltage from the required load voltage, the switching duty cycle, series diode voltage drop, and transformer internal impedance voltage drop should be considered. The conductor cross-section (diameter) is determined by the winding current density. When winding losses (copper losses) constitute a large proportion of total losses, a current density of 2–4 A/mm² is recommended; when copper losses constitute a small proportion, a current density of 8–12 A/mm² is recommended. However, necessary adjustments should be made after transformer temperature rise verification. It's also important to note that the conductor cross-section (diameter) is related to leakage inductance. With the same number of turns, increasing the conductor diameter reduces the number of turns in the inner layers and increases the number of layers. The leakage magnetic field distribution is larger in the inner layer near the core and smaller in the outer layer, decreasing inversely proportional to the square of the distance from the core. This reduces the number of loops in the inner layer with the larger leakage flux, thus decreasing the leakage inductance.
1) If the primary winding voltage is high (e.g., 220V) and the secondary winding voltage is low, the secondary winding can be placed closest to the magnetic core, followed by the feedback winding, with the primary winding on the outermost layer. This arrangement is beneficial for the insulation of the primary winding to the magnetic core.
2) To increase the coupling between the primary and secondary windings, a winding arrangement can be adopted in which half of the primary winding is close to the magnetic core, followed by the feedback winding and the secondary winding, and then the outermost layer is wound with half of the primary winding. This helps to reduce leakage inductance.
First, when arranging insulation, it's crucial to consider the insulation grade of the magnet wire and insulating components. This grade must match the allowable operating temperature of the magnetic core and windings. A low grade won't meet heat resistance requirements, while an excessively high grade will increase unnecessary material costs. Second, for coils wound on cylindrical magnetic circuits, a coil bobbin is preferable, as it ensures insulation and simplifies the winding process. Furthermore, reinforced insulation is essential between the outermost and innermost layers of the coil, and between high-voltage and low-voltage windings. While general insulation uses only one layer of insulating film, reinforced insulation should use 2-3 layers.
Assembly structure
High-frequency power transformer assembly structures are divided into horizontal and vertical types. If planar, sheet, or thin-film magnetic cores are selected, a horizontal assembly structure is used. This structure has a larger top and bottom surface, which is beneficial for heat dissipation or the addition of heat sinks. Its lower height facilitates mounting on printed circuit boards. Standard parts should be used as much as possible for clamps and terminals in the assembly structure to facilitate outsourcing and reduce costs.
Temperature rise verification
Temperature rise verification can be performed through calculation and sample testing. Generally, temperature rise calculation through sample testing is more common. If the sample test temperature rise does not exceed the allowable temperature rise, it passes. However, if the test temperature rise is more than 15°C lower than the allowable temperature rise, the winding current density and conductor cross-section need to be adjusted, appropriately increasing the current density and decreasing the conductor cross-section. If the sample test temperature rise exceeds the allowable temperature rise, the winding current density and conductor cross-section need to be adjusted, appropriately decreasing the current density and increasing the conductor cross-section. If increasing the conductor cross-section makes it impossible to wind within the window, the core size needs to be increased. If the sample test core temperature rise exceeds the allowable temperature rise, the core's heat dissipation area needs to be increased, and the core size needs to be enlarged.
As the operating frequency of high-frequency power transformers increases, the design is constantly changing, with new soft magnetic materials, new core structures, new wire and insulation materials, new coil and assembly structures, and new design methods constantly emerging.
The design of high-frequency power transformers also has a goal: to achieve the design principles and pursue the best performance-price ratio while fulfilling specific functions under specific usage conditions.
