The influence of the specification of the power adapter mechanism on the size of the transformer

　　In order to meet the insulation and creep distance requirements of the power adapter, that is, to meet the requirements of UL and VDE specifications, it is difficult for the power adapter to achieve a smaller transformer size at high frequencies. The specified creep distance is 4~8mm (the minimum distance between the primary winding and the secondary winding in offline applications), and this value must remain unchanged even in high-frequency transformers. This leads to poor window utilization and increased leakage inductance, especially when using small cores. These specifications make the magnetic core size to be selected much larger than that required when only considering the electrical and temperature rise requirements.

　　Calculation of the number of turns of the primary winding

　　Once the size of the magnetic core is selected, the number of turns of the primary winding must be determined to obtain the best efficiency. In order to minimize copper losses, there is a tendency to use as few winding turns as possible. However, assuming that the frequency and voltage have been kept constant, the smaller the number of turns of the primary winding, the larger the magnetic flux density swing required by the core material. In extreme cases, the core will saturate. The second effect of reducing the number of winding turns and increasing the swing of magnetic flux density is to increase the magnetic loss to a certain point, and at this time, the magnetic loss accounts for most of the loss.

　　As previously explained, the efficiency can be optimized only when the copper loss and the magnetic loss are equal or almost equal. When the push-pull compressor works at high frequencies, in order to achieve the optimal efficiency requirements, the maximum magnetic flux density swing and the minimum number of primary winding turns must be determined. This type of design is called a magnetic loss limiting design.

　　In the low frequency range, especially for single-ended converters, the magnetic loss is much smaller than the copper loss, and the factor that can limit the minimum number of primary windings can prevent the core from saturation. This type of design is called saturation limit design.

　　In any case, the saturation of the magnetic core must be avoided. When the primary winding is in the saturation region, its impedance will decrease and approach the DC winding resistance of the winding. The low resistance makes the destructive high current flow into the primary winding of the transformer, which inevitably causes the burning of the primary switching element.

　　Under steady-state conditions, each cycle is the same, and working parameters can be defined according to a single cycle. It can be seen from the above formula that the number of turns N of the primary winding is proportional to the primary winding voltage V and the time t of the voltage applied to the primary winding, and is proportional to the magnetic flux density swing △B and the core cross-sectional area A. Inversely proportional.

　　It seems that, as long as an appropriate constant is inserted in the formula, it is quite easy to determine the number of turns of the primary winding. However, more complicated factors emerged in the selection of constants.

　　In some voltage control converter circuits, under startup or instantaneous operation, the maximum voltage of the primary winding may occur at the point of maximum conduction time. If a converter of this type of topology is used, in order to prevent the saturation of the magnetic core, the maximum primary winding voltage value and the maximum on-time must be used when calculating the number of turns of the primary winding using the formula.

　　If a current control converter is used, the saturation of the magnetic core at the beginning can be controlled, and the primary voltage value is the smallest at the maximum on-time point, so that the maximum on-time and minimum primary voltage value are used when calculating the primary winding . This makes the number of turns obtained by the saturation limit design method less.

　　In some duty cycle control systems, compensation before the primary input voltage, fast current limiting of the primary winding or conversion ratio control are used. In these cases, the same conditions as current-mode control can be used in transformer design.

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