Design principles and techniques for small transformers

Small transformers refer to power transformers and audio transformers below 2kva. The design principles and techniques of small transformers are as follows:

1 Determination of transformer cross-sectional area

The core cross-sectional area a is determined according to the total power p of the transformer. When designing, if the load is basically constant, the core cross-sectional area can be taken as the theoretical value calculated usually, that is, a=1.25. If the load changes greatly, such as some equipment, some audio, power amplifier power supply, etc., the cross-sectional area of ​​the transformer should be appropriately larger than the ordinary theoretical calculated value, so as to ensure sufficient power output capacity.

2 Determination of turns per volt

The number of turns of the transformer is mainly determined by the cross-sectional area of ​​the core and the quality of the silicon steel sheet. Experiments have shown that the value of the number of turns per volt should be 10% to 15% lower than the value of the counting formula given in the book. For example, a 35W power transformer is usually calculated (8500 gauss for the middle sheet) to have 72 turns per volt, but in reality only 6 turns per volt are needed, so the no-load current of the wound transformer is about 25ma. Usually, after appropriately reducing the number of turns, the wound transformer can not only reduce the internal resistance, but also avoid the trouble of not being able to wind the silicon steel sheet of ordinary specifications, and save costs, thereby improving the cost performance.

3 Determination of wire diameter of enameled wire

The wire diameter should be determined according to the load current. Since the current difference of enameled wire in different environments is large, the range of determining the wire diameter is also large. Generally, when the heat dissipation conditions are not ideal and the ambient temperature is relatively high, the current density of the enameled wire should be 2a/mm2 (wire diameter). If the load current of the transformer is basically unchanged during continuous operation, but the heat dissipation conditions are good, and the ambient temperature is not high, the current density of such enameled wire is 2?5a/mm2 (wire diameter). If the working current of the transformer is only 1/2 of the maximum working current, the current density of such enameled wire is 3~3.5a/mm2 (wire diameter). The current density of the enameled wire of the audio transformer can be 3?5~4a/mm2 (wire diameter). In this way, taking materials according to the situation can not only ensure quality but also greatly reduce costs.

4 Conclusion

In summary, in order to design a transformer with a high cost performance ratio, the cross-sectional area of ​​the core can only be larger, not smaller; appropriately reduce the number of turns per volt; analyze the load conditions in detail; and reasonably select the specifications of the enameled wire. Only through repeated practice and careful consideration can we truly master the design principles and techniques of the transformer.