Step-up transformer, low-voltage winding put the middle, that is, low-high;
Step-down transformer, medium voltage winding in the middle, that is, low high.
For the step-up transformer, the input is low and the output is high or medium; the low-lying intermediate makes it possible to minimize the distance in the low-high, low-, thereby reducing the distance traveled by the magnetic energy, Loss, reduced low - high, low - in the leakage resistance, thereby reducing the low - high, low - between the short - circuit voltage percentage, thereby reducing the reactance, thereby improving the energy conversion efficiency of the transformer.
For buck transformer, the input is high, the output is low or medium, similar to the step-up transformer, the optimal arrangement should be low, high, medium. However, due to insulation safety problems, the high voltage side must be placed at the outermost end. So the arrangement of sub-optimal way to low, medium and high.
2, from the manufacturing cost considerations, the transformer core is grounded. Low voltage coil inside, the coil on the core of the insulation distance can be smaller. So that the sum of the main insulation dimensions of the three coils will be smaller. The weight of the entire transformer, the volume is relatively lower than the - low - high arrangement to be smaller.
But from the operating and manufacturing costs, low - medium - high arrangement, more reasonable.
1, from the operation to consider: step-down transformer, low pressure side is often for the distance of the load. And the voltage level is low, the protection level is lower. So we hope that the low-high impedance is bigger as well. While the pressure is often for the far load, we hope that the impedance between them is smaller as well. This arrangement, just to meet this requirement.
In theory, these two arrangements can meet the needs of the operation, manufacturing the same difficulty. If it is a step-down transformer. Low - medium - high permutations are commonly used, while mid - low - high permutations rarely encounter.
