What is a HF transformer?
HF is abbreviation for high frequency. High-frequency transformer is a power transformer whose operating frequency exceeds the intermediate frequency (10 kHz).
What are the design principle of high frequency transformer?
The design principle of a high frequency transformer is to provide the best functionality at the lowest cost under the specific conditions of operation.
Checking a transformer design is solely based on its marketability. High-frequency transformers are sometimes designed for performance and efficiency, and sometimes for price and cost, depending on customer needs. There is a growing trend in the manufacture of high-frequency transformers to become light, thin, short and small as part of the design and development process.
What are the design requirements for high frequency transformers?
Taking into consideration the design principles of high-frequency transformers, four main design requirements are proposed for high-frequency transformers, and these are use conditions, full functionality, cost reduction, and improvements to efficiency.
1.The conditions of use
There are two aspects to consider when determining conditions of use: reliability and electromagnetic compatibility. Previously, only reliability was considered, but now that environmental protection has become more of a concern, electromagnetic compatibility must also be considered. Reliability refers to that the high-frequency transformer can work normally until its service life under specific service conditions. Temperature is one of the general operating conditions that has the greatest impact on the high-frequency transformer in terms of its performance. High frequency power transformer that is electromagnetically compatible does not generate electromagnetic interference to the outside world, but can also withstand external electromagnetic interference. There are two types of electromagnetic interference: audible audio noise and inaudible high frequency noise. In high frequency transformers, magnetostriction of the magnetic core is the main cause of electromagnetic interference. Electromagnetic interference is large in soft magnetic materials with large magnetostriction. High frequency transformers can be shielded to prevent electromagnetic interference and to increase electromagnetic compatibility. Nevertheless, in order to prevent the propagation of electromagnetic interference in high-frequency transformers, corresponding measures must also be taken in the design of the magnetic core and winding structures.
2.Full functionality
The high frequency transformer has three complete functions: power transfer, voltage conversion, and insulation isolation.
A.Power transfer
Transmission power is relatively high, and operating frequency is relatively low; Transmission power is relatively low, and operating frequency is relatively high. There are two modes of magnetic flux change in power transmission: unidirectional and bidirectional. By using the unidirectional change working mode, the magnetic flux density changes from the maximum value Bm to the residual value Br, or vice versa. Change in magnetic flux density:ΔB=Bm－Br. There are different power transmission methods which require different magnetic core parameters, but when it comes to designing of high-frequency transformers, the selection of magnetic core materials and parameters is still an important aspect of the process.
B.Voltage conversion
High frequency transformers complete their voltage transformation by varying the turns ratio of their primary and secondary windings. As long as the turns ratio remains the same, the voltage conversion ratio between the primary and secondary sides remains the same regardless of the power transmission method. However, winding turns are related to leakage inductance. Leakage inductance is proportional to the square of the number of turns in the primary winding.
C.Insulation isolation
In high-frequency transformers, the primary and secondary windings are insulated to provide insulation isolation. In order to ensure the insulation between the windings, the distance between the two windings must be increased, which reduces the coupling degree between the windings and increases leakage inductance. Furthermore, the primary winding should not have too few turns, as a voltage difference between turns or layers can cause a local short circuit. Thus, the leakage inductance has a lower limit due to the lower limit on the number of turns.
3.Improvements efficiency
Increasing the efficiency of high frequency transformers is one of the most common requirements for their design. For high-frequency transformers, improvement of efficiency is a major design requirement. In general, the efficiency should be increased to more than 95%, and the loss should be reduced to less than 5%, which are two major design requirements. There are two types of losses associated with high frequency transformers: core loss (iron loss) and winding loss (copper loss). This ratio will change with the operating frequency of the transformer. The lower the operating frequency, the more winding turns, and the greater the copper loss of the transformer, the same as if the applied voltage remains unchanged.
4.Cost reduction
For high-frequency transformers, cost reduction is one of the major, and sometimes decisive, design requirements with regards to their design. High-frequency transformers, like other commodities, are a product that is going up against competition in the market. The content of competition includes both performance and cost, both of which are essential. There is no doubt that you will be eliminated in the competition if you do not pay attention to reducing your costs. Material, manufacturing, and management costs are included in the cost of a high frequency transformer. The main method of reducing the cost of high frequency transformers is to reduce costs during the design process. Engineers must know the performance and price of soft magnetic materials and cores, magnet wires, and insulating materials. In addition, understand the core processing heat treatment process, the coil winding and insulation treatment process, and the transformer assembly process. Also, learn about the testing parameters and instrumentation used for quality control and understand the basics of high frequency power transformer production management and market dynamics. This is a great opportunity for engineers to show their professional level and comprehensive quality.
What are the design steps of high-frequency transformer?
1.Design of coil parameters
High frequency transformer’s coil parameters include the number of coils, wire diameter, wire type, winding arrangement and safety insulation. Primary winding’s number of turns is determined by either the applied excitation voltage or the primary winding’s excitation inductance. There should not be too many or too few turns. When there are too many turns, leakage inductance increases, and winding time increases; When the external excitation voltage is high, the voltage drop between turns and layers may increase if the number of turns is too small. Safety insulation must be strengthened. Secondary winding is determined by the output voltage. The diameter of the wire depends on the current density of the winding. The wire diameter is also related to the strong leakage inductance.
2.Transformer structure determination
There are two types of high frequency transformers: horizontal and vertical. The horizontal assembly structure is adopted for the plane core, chip core, and film core, with the upper and lower surfaces being relatively large, which promotes heat dissipation. Depending on the size of the magnetic core required by the high-frequency transformer, the skeleton can be the same size; Select the PIN number of the skeleton based on the number of high-frequency transformer windings; Based on the size and height of the high-frequency transformer on the PCB, select either a vertical skeleton or a horizontal skeleton.
3.The selection of magnetic materials
The magnetic core of a high frequency transformer is typically made of MnZn. Because of different formulations and production processes, magnetic materials come in many grades and have different characteristic parameters. These include frequency ranges of use, initial permeability, specific loss factors, specific temperature coefficients, saturation flux densities, Curie temperatures, resistivities, and densities, among others.
For the selection of magnetic core model rules, there are also certain principles to consider:
- Use the existing transformer skeleton
- It should conform to the cross-sectional area, saturation magnetic flux density, etc., as specified in the circuit design;
- The power and frequency requirements should be met as well as the structural installation requirements;
- Safety and reliability should also be considered when choosing an insulating material.
4.Core type selection
The most common types of magnetic cores are EI, EE, EFD, ETD, UI, UF, and others. Transformers are selected based on the highest operating frequency they use.
When selecting magnetic core types for high-frequency transformers, the following factors should be considered:
- Improve shielding by reducing magnetic flux leakage and leakage inductance, increasing the coil’s heat dissipation area;
- You can wind coils easily, assemble them, and wire them easily.
The size of the window area of a high-frequency transformer core structure should be determined after considering various factors. 为Some high-frequency power transformers have a closed or semi-closed shell outside the window to prevent electromagnetic interference between the inside and outside. Wiring holes and heat dissipation holes should both have good heat dissipation, otherwise it will be inconvenient. Unenclosed part is used for wiring and heat dissipation, while the enclosed part shields electromagnetic interference. In a fully open window, the wiring easily dissipates heat, and electromagnetic interference shielding is poor.
5.Core ΔB Parameter selection
To choose high frequency transformer core parameters, it is important to understand that the working flux density is not only limited by the magnetization curve, but also by losses, as well as how power is transferred.
In the case of unidirectional flux change: ΔB saturation flux density and losses limit the amount of flux.
For the working mode in which the magnetic flux changes in two directions, the area enclosed by the working hysteresis loop is larger than the area of the local flux linkage, and the loss is greater. The main limitation of ΔB is losses, and DC bias has to be considered. Inductive power transfer uses the equivalent permeability of the air gap, which is generally smaller than the permeability measured by magnetization.
