High frequency transformerThe design includes: the design of coil parameters, the selection of magnetic core materials, the selection of magnetic core structure, the design of magnetic core parameters, the selection of assembly structure and so on. Next, the calculation and selection of coil parameters of high-frequency transformer, the selection of magnetic core materials, the selection of magnetic core structure, the design of magnetic core parameters and the selection of assembly structure are introduced in detail.

 

      (1)  Calculation and selection of coil parameters of high frequency transformer  

 

    Coil parameters of high frequency transformer include:

 

   Turns, conductor section(diameter), conductor form, winding arrangement and insulation arrangement.

 

The number of turns of the primary winding depends on the applied excitation voltage or the excitation inductance of the primary winding(Stored energy)To decide, the number of turns cannot be too much or too little. If there are too many turns, leakage inductance and winding hours will be increased;If the number of turns is too small, the inter turn voltage drop and inter layer voltage drop may increase when the external excitation voltage is relatively high, and the insulation must be strengthened[5]。 The number of turns of the secondary winding is determined by the output voltage. Conductor section(diameter)It depends on the current density of the winding. Also note the wire section(diameter)The size of is also related to the leakage inductance.

 

      The winding arrangement of high frequency transformer includes:  

 

      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 winding arrangement of the primary winding in the outermost layer is conducive to the insulation arrangement of the primary winding to the magnetic core  ;  

 

      ② If you want to increase the coupling between the primary and secondary windings, you can use the winding arrangement in which half of the primary winding is close to the magnetic core, then the feedback winding and secondary winding are wound, and the outermost layer is wound around half of the primary winding, which is conducive to reducing 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 turns or layers will be large, which will cause local short circuit.  

 

      For insulation arrangement, first of all, pay attention to the insulation material grade of the used electromagnetic wire and insulating parts to match the allowable working temperature of the magnetic core and winding. The grade is low, which can not meet the heat resistance requirements. If the grade is too high, it will increase the unnecessary material cost. Secondly, for the coil wound on the cylindrical magnetic circuit, it is best to use the coil skeleton, which can not only ensure the insulation, but also simplify the winding process. In addition, insulation shall be strengthened between the outermost and innermost layers of the coil, high-voltage and low-voltage windings. If the general insulation is only padded with one layer of insulating film, the reinforced insulation shall be padded  two  ～  three  An insulating film.  

 

      (2)  Selection of magnetic core material for high frequency transformer  

 

    The magnetic core of high-frequency transformer generally uses soft magnetic materials. Soft magnetic materials have high permeability and low coercivity, High resistivity. The magnetic permeability is high. At a certain number of coil turns, the coil can have a higher magnetic induction intensity through a small exciting current, and the coil can withstand a higher applied voltage. Therefore, under the condition of a certain output power, the volume of the magnetic core can be reduced. If the core coercivity is low and the hysteresis loop area is small, the iron consumption is also small. If the resistivity is high, the eddy current is small and the iron consumption is small.

 

   Ferrite material is a composite oxide sintered body. Like other soft magnetic core materials, soft magnetic ferrite has the advantages of high resistivity, small AC eddy current loss, low price and easy to be processed into magnetic cores of various shapes. Its disadvantages are low working magnetic flux density, low permeability, large magnetostriction and sensitive to temperature changes. It is suitable for use at high frequency, so high frequency transformers generally use ferrite material as the magnetic core.

 

      (3)  Selection of magnetic core structure of high frequency transformer  

 

      The basic structure of magnetic core includes:  

 

      ① Lamination, usually made of silicon steel or nickel steel sheet  E  、  I  、  F  、  O  Equal shape, stacked into an iron core.  

 

      ② Annular core, made of  O  It can also be wound by narrow and long silicon steel and alloy steel strip.  

 

      ③  C  This kind of iron core can avoid the disadvantage of difficult winding of annular iron core  C  It is formed by butt joint of type I iron core.  

 

      ④ The can shaped iron core is a structural form in which the magnetic core is outside and the copper coil is inside, which avoids the inconvenience of the ring coil and can reduce  EMI  。 The disadvantage is that the internal coil has poor heat dissipation and high temperature rise.  

 

      Factors to be considered when selecting the magnetic core structure in the design of high-frequency transformer: reduce magnetic leakage and inductance, increase the coil heat dissipation area, which is conducive to shielding and easy coil winding,  Convenient assembly and wiring, etc.  

 

    In the magnetic core structure design of high frequency transformer, the size of window area should be determined after comprehensive consideration of various factors. In order to prevent electromagnetic interference of high-frequency power transformer from inside to outside and from outside to inside, some magnetic core structures have closed and semi closed shells outside the window. The closed shell has good shielding effect against electromagnetic interference, but it is inconvenient for heat dissipation and wiring. Wiring holes and air outlets must be reserved. Semi enclosed enclosure, the enclosed place plays the role of shielding electromagnetic interference, and the unsealed place is used for wiring and heat dissipation. If the window is completely open, the wiring and heat dissipation are convenient, and the shielding effect of electromagnetic interference is poor.

 

      (4)  Magnetic core parameters of high frequency transformer Δ  B  Choice of  

 

      When selecting the magnetic core parameters of high frequency transformer, it must be noted that the working magnetic flux density is not only limited by the magnetization curve, but also limited by the loss, but also related to the working mode of power transmission.  

 

      For working mode with unidirectional variation of magnetic flux:     ， Δ  B  It is limited by both saturated flux density and loss.  

 

      For the working mode of flux change in both directions:     , the area surrounded by the working hysteresis loop is much larger than that of the local loop, and the loss is much larger, Δ  B  Mainly limited by loss, but also pay attention to the problem of DC bias.  

 

      For the power transmission mode of inductor, the permeability is the equivalent permeability with air gap, which is generally smaller than the permeability measured by magnetization curve.  

 

      (5)  Selection of assembly structure of high frequency transformer  

 

      The assembly structure of high-frequency transformer is divided into horizontal and vertical. If plane magnetic core, chip magnetic core and film magnetic core are selected, they all adopt horizontal assembly structure, and the upper and lower surfaces are relatively large, which is conducive to heat dissipation  ;  Others adopt vertical structure. In addition, standard parts shall be used as far as possible for clamps and wiring terminals used in the assembly structure, so as to facilitate outsourcing processing and reduce costs.  

 

      (6)  Determination of working point of high frequency transformer  

 

      For the newly purchased magnetic core, because the magnetic induction intensity 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 original coil, observe the output voltage waveform of the auxiliary coil with an oscilloscope, and slowly increase the input voltage of the original coil from small to large until the waveform displayed by the oscilloscope changes strangely. At this time, the magnetic core is saturated. According to the formula:  U=4.44fN1  Φ  m  It can be inferred that Φ  m  Value.