The design of high frequency transformer includes: coil parameter design, magnetic core material selection, magnetic core structure selection, magnetic core parameter design, assembly structure selection, etc. The calculation and selection of the coil parameters of the high-frequency transformer, the selection of the magnetic core material, the selection of the magnetic core structure, the design of the magnetic core parameters and the selection of the assembly structure are described in detail below.
   (1) Calculation and selection of high-frequency transformer coil parameters
   The coil parameters of the high frequency transformer include:
  Number of turns, wire section (diameter), wire form, winding arrangement and insulation arrangement.
   The number of turns of the original winding is determined according to the applied excitation voltage or the excitation inductance (storage energy) of the original winding, and the number of turns cannot be too many or too few. If the number of turns is too large, leakage inductance and winding hours will be increased; if the number of turns is too small, when the applied excitation voltage is relatively high, the voltage drop between turns and the voltage drop between layers may increase, and insulation must be strengthened [5 ]. The number of turns of the secondary winding is determined by the output voltage. The cross-section (diameter) of the wire is determined by the current density of the winding. Also note that the size of the wire section (diameter) is also related to leakage inductance.
   The winding arrangements of high-frequency transformers are:
   ① If the voltage of the primary winding is high and the voltage of the secondary winding is low, the secondary winding can be close to the magnetic core, and then the feedback winding can be wound. The primary winding is in the outermost winding arrangement, which is beneficial to the insulation arrangement of the primary winding to the magnetic core;
   ②If you want to increase the coupling between the primary and secondary windings, you can use half of the primary winding close to the magnetic core, then winding the feedback winding and secondary winding, and then winding half of the primary winding on the outermost layer, which will help reduce leakage inductance.
   In addition, when the original winding is a high-voltage winding, the number of turns should not be too small, otherwise, the voltage difference between the turns or the layers will be large, which will cause a local short circuit.
   For the insulation arrangement, the first thing to pay attention to is that the magnet wire and the insulation material grade of the insulating parts must match the allowable operating temperature of the magnetic core and winding. The grade is low, which cannot meet the heat resistance requirements, and the grade is too high, which will increase unnecessary material costs. Secondly, for coils wound on a cylindrical magnetic circuit, it is best to use a coil bobbin, which can ensure insulation and simplify the winding process. In addition, the outermost and innermost layers of the coil, and the high-voltage and low-voltage windings must be insulated. If only one layer of insulating film is used for general insulation, 2 to 3 layers of insulating film should be used for reinforced insulation.
   (2) Selection of core material for high frequency transformer
   High-frequency transformer cores generally use soft magnetic materials. Soft magnetic materials have high permeability, low coercivity, and high resistivity. High magnetic permeability, at a certain number of turns of the coil, a high magnetic induction intensity can be achieved through a small excitation current, and the coil can withstand a higher applied voltage. Therefore, under the condition of a certain output power, the magnetic core can be reduced. volume. The magnetic core has low coercivity, and the hysteresis loop area is small, so the iron loss is also small. The higher the resistivity, the lower the eddy current and the lower the iron loss.
  Ferrite material is a composite oxide sintered body. Like other soft magnetic core materials, soft ferrite has the advantages of high resistivity, low AC eddy current loss, low price, and easy processing into various shapes of magnetic cores. Disadvantages It has low working magnetic flux density, low magnetic permeability, large magnetostriction, and is more sensitive to temperature changes. It is suitable for high-frequency use, so high-frequency transformers generally use ferrite materials as magnetic cores.
   (3) Selection of core structure of high frequency transformer
   The basic structure of the magnetic core:
   ① Laminated sheet, usually punched out of silicon steel or nickel steel sheet into E, I, F, O, etc. shapes, and stacked into an iron core.
   ②The ring-shaped iron core is made of O-shaped sheets, or it can be wound from long and narrow silicon steel and alloy steel strips.
   ③C-shaped iron core, this kind of iron core can avoid the shortcomings of ring-shaped iron core winding difficulty, and is formed by butting two C-shaped iron cores.
   ④The pot-shaped iron core is a structure that has the magnetic core outside and the copper coil inside, which eliminates the inconvenience of toroidal coils and can reduce EMI. The disadvantage is that the internal coil has poor heat dissipation and high temperature rise.
   Factors that should be considered when selecting the magnetic core structure when designing high-frequency transformers: reduce magnetic leakage and leakage inductance, increase coil heat dissipation area, facilitate shielding, easy coil winding, and convenient assembly and wiring.
   In the design of the high-frequency transformer core structure, the size of the window area must be determined after comprehensive consideration of various factors. In order to prevent electromagnetic interference from the inside to the outside and from the outside to the inside of the high-frequency power transformer, some magnetic core structures have closed and semi-closed shells outside the window. The enclosed shell has a good effect on shielding electromagnetic interference, but it is inconvenient for heat dissipation and wiring, and wiring holes and air vents must be reserved. Semi-enclosed shell, the enclosed area plays a role of shielding electromagnetic interference, and the unenclosed area is used for wiring and heat dissipation. If the window is completely open, wiring and heat dissipation are convenient, and the effect of shielding electromagnetic interference is poor.
   (4) Selection of the magnetic core parameter ΔB of the high-frequency transformer
   When selecting the magnetic core parameters of high-frequency transformers, it must be noted that the working magnetic flux density is not only limited by the magnetization curve, but also by the loss, and is also related to the working mode of power transmission.
   Regarding the operating mode of unidirectional change of magnetic flux:, ΔB is limited by both the saturation magnetic flux density and the loss.
   For the working mode of magnetic flux bidirectional change:, the area surrounded by the working hysteresis loop is much larger than the local loop, and the loss is much larger. ΔB is mainly limited by the loss, and attention should be paid to the DC bias problem.
   For the inductor power transmission method, the permeability is the equivalent permeability after an air gap, which is generally smaller than the permeability measured by the magnetization curve.
   (5) Selection of high-frequency transformer assembly structure
   The high-frequency transformer assembly structure is divided into two types: horizontal and vertical. If plane magnetic cores, chip magnetic cores and thin-film magnetic cores are selected, they all adopt a horizontal assembly structure, and the upper and lower surfaces are relatively large, which is conducive to heat dissipation; the others are all vertical structures. In addition, the clamps and wiring terminals used in the assembly structure should use standard components as much as possible to facilitate outsourcing processing and reduce costs.
   (6) Determination of working point of high frequency transformer
   For the newly purchased magnetic core, since the magnetic induction value provided by the manufacturer is not accurate, it is generally necessary to roughly test it first. The specific method: connect the voltage regulator to the primary coil, observe the output voltage waveform of the secondary coil with an oscilloscope, and change the primary coil The input voltage increases slowly from small to large, until the waveform displayed by the oscilloscope changes oddly, at which time the magnetic core is saturated. According to the formula: U=4.44fN1Φm, the value of Φm can be inferred.