The design of high-frequency transformers includes: the design of coil parameters, the selection of magnetic core materials, the selection of magnetic core structures, the design of magnetic core parameters, and the selection of assembly structures. Detailed introduction to the calculation and selection of high-frequency transformer coil parameters, selection of magnetic core materials, selection of magnetic core structures, design of magnetic core parameters, and selection of assembly structures.
(1) Calculation and Selection of High Frequency Transformer Coil Parameters
The coil parameters of high-frequency transformers include:
Number of turns, wire cross-section (diameter), wire form, winding arrangement, and insulation arrangement.
The number of turns in the original winding is determined by the applied excitation voltage or the excitation inductance (stored energy) of the original winding, and the number of turns cannot be too many or too few. If there are too many turns, it will increase leakage inductance and winding time; If the number of turns is too small, when the applied excitation voltage is relatively high, it may increase the inter turn voltage drop and inter layer voltage drop, and insulation must be strengthened [5]. The number of turns in the secondary winding is determined by the output voltage. The cross-sectional area (diameter) of a wire is determined by the current density of the winding. It should also be noted that the size of the wire cross-section (diameter) is also related to the leakage inductance.
The winding arrangement of high-frequency transformers 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 arranged close to the magnetic core, followed by the feedback winding, with the primary winding arranged on the outer layer, which is beneficial for the insulation arrangement of the primary winding on the magnetic core;
② If you want to increase the coupling between the primary and secondary windings, you can use a winding arrangement where half of the primary winding is close to the magnetic core, followed by the feedback winding and secondary winding, and half of the primary winding is wound on the outer layer. This is beneficial for reducing leakage inductance.
In addition, when the original winding is a high-voltage winding, the number of turns cannot be too small, otherwise, the voltage difference between turns or layers will be large, which will cause local short circuits.
For insulation arrangements, the first thing to note is that the insulation material grade of the electromagnetic wires and insulation components used should match the allowable working temperature of the magnetic core and winding. Low grade cannot meet the heat resistance requirements, while high grade will increase unnecessary material costs. Secondly, for coils wound on cylindrical magnetic circuits, a coil skeleton is used to ensure insulation and simplify the winding process. In addition, insulation should be strengthened between the outer and inner layers of the coil, as well as between the high and low voltage windings. If only one layer of insulation film is used for general insulation, 2-3 layers of insulation film should be used for reinforced insulation.
(2) Selection of Magnetic Core Materials for High Frequency Transformers
High frequency transformer magnetic cores generally use soft magnetic materials. Soft magnetic materials have high magnetic permeability, low coercivity, and high electrical resistivity. High magnetic permeability, at a certain number of coil turns, can achieve high magnetic induction intensity with a small excitation current, and the coil can withstand high external voltage. Therefore, with a constant output power, the volume of the magnetic core can be reduced. If the coercivity of the magnetic core is low and the hysteresis loop area is small, then the iron loss is also low. If the resistivity is high, the eddy current is small and the iron loss is also small.
Ferrite materials are composite oxide sintered bodies. Like other soft magnetic core materials, the advantages of soft magnetic ferrite are high electrical resistivity, low AC eddy current loss, low price, and easy processing into various shapes of magnetic cores. The disadvantages are low working magnetic flux density, low magnetic permeability, large magnetostriction, and sensitivity to temperature changes. It is suitable for use at high frequencies, so high-frequency transformers generally use ferrite materials as magnetic cores.
(3) Selection of magnetic core structure for high-frequency transformers
The basic structure of magnetic core includes:
① Stacking, usually made of silicon steel or nickel steel thin sheets punched and cut into shapes such as E, I, F, O, etc., and stacked into a core.
② Circular iron core, made of O-shaped thin sheets stacked together, can also be wound from narrow and long silicon steel or alloy steel strips.
③ C-shaped iron core, which eliminates the difficulty of winding circular iron cores, is composed of two C-shaped iron cores connected together.
④ Can shaped iron core is a structural form where the magnetic core is on the outside and the copper coil is on the inside, eliminating the inconvenience of circular coils and reducing EMI. The disadvantage is poor heat dissipation of the internal coil and high temperature rise.
When designing a high-frequency transformer, factors to consider when choosing a magnetic core structure include reducing leakage and inductance, increasing coil heat dissipation area, facilitating shielding, easy coil winding, and convenient assembly and wiring.
In the design of high-frequency transformer magnetic core structure, the size of the window area should be determined after considering various factors comprehensively. In order to prevent electromagnetic interference from the inside out and outside in of high-frequency power transformers, some magnetic core structures have enclosed and semi enclosed shells outside the window. The enclosed shell has good electromagnetic interference shielding effect, but it is inconvenient for heat dissipation and wiring, and must have wiring holes and exhaust holes. A semi enclosed shell is used to shield electromagnetic interference in enclosed areas, while an unsealed area is used for wiring and heat dissipation. If the window is completely open, wiring and heat dissipation are convenient, but the shielding effect against electromagnetic interference is poor.
(4) Selection of High Frequency Transformer Core Parameters Δ B
When selecting the magnetic core parameters for high-frequency transformers, it is necessary to pay attention to the fact 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.
For the working mode of unidirectional variation of magnetic flux: Δ B is limited by both saturation magnetic flux density and loss.
For the working mode of bidirectional variation of magnetic flux: the area enclosed by the hysteresis loop is much larger than that of the local loop, and the loss is also much larger. Δ B is mainly limited by the loss, and attention should also be paid to the DC bias problem that may occur.
For the power transmission method of inductors, the magnetic permeability is the equivalent magnetic permeability with an air gap, which is generally smaller than the magnetic permeability measured by the magnetization curve.
(5) Selection of assembly structure for high-frequency transformers
The assembly structure of high-frequency transformers is divided into two types: horizontal and vertical. If flat magnetic cores, chip magnetic cores, and thin film magnetic cores are selected, they all adopt a horizontal assembly structure with relatively large upper and lower surfaces, which is conducive to heat dissipation; All others adopt a vertical structure. In addition, standard components such as clamps and wiring terminals used in the assembly structure should be used as much as possible to facilitate outsourcing and reduce costs.
(6) Determination of working point for high-frequency transformer
For newly purchased magnetic cores, due to the inaccurate magnetic induction intensity values provided by the manufacturer, it is generally necessary to roughly test them first. The specific method is to connect the voltage regulator to the primary coil, observe the output voltage waveform of the secondary coil with an oscilloscope, and gradually increase the input voltage of the primary coil from small to large until the waveform displayed on the oscilloscope undergoes a strange change. At this point, the magnetic core is saturated. According to the formula: U=4.44fN1 Φ m, it can be inferred that the value is at the value of Φ m.