Transformer Design
SMPS transformers run at high frequency. Most of the cost savings (and space savings) in off-line power supplies result from the smaller size of high frequency transformer compared to the 50/60 Hz transformers formerly used. There are additional design tradeoffs.
The terminal voltage of a transformer is proportional to the product of the core area, magnetic flux, and frequency. By using a much higher frequency, the core area (and so the mass of the core) can be greatly reduced.
However, higher frequency also means more energy lost during transitions of the switching semiconductor. Furthermore, more attention to the physical layout of the circuit board is required, and the amount of electromagnetic interference will be more pronounced.
Core losses increase at higher frequencies. Cores use ferrite material which has a low loss at the high frequencies and high flux densities used. The laminated iron cores of lower-frequency (<400 Hz) transformers would be unacceptably lossy at switching frequencies of a few kilohertz.
Read more about this topic: Switched-mode Power Supply
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