Agree, but normally it is a 1:1 ratio transformer to keep the volume of the PSU small (in some cases 2:1 ratio transformer is used).Normally a small High frequency transformer is used in switch mode PSU's to maintain isolation between the secondary and primary. Without this isolation there might be safety issues as well as tripping of Earth Leakage breakers (GFI).
Actually, a 1:1 ratio transformer is smaller than a step down transformer with the same wattage.I have never checked ratios, but wattage and core size would be constant no matter what the ratio.
I would have thought a step down ratio would be used otherwise the high frequency rectifier diodes on the secondary would have to be rated at over 350volts to cope with peak voltage, increasing expense..??? .. Also, if the output cap/filter failed would we get 350 volt DC pulses on the output...?
Efficiency increases with frequency and correct core selection, and that has more to do with physical size especially when you consider most SM PSUs operate anywhere up to 300kHz.
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by Jake Hertz