Electrical isolation by means of transformers is needed in switch-mode dc power supplies for three reasons:
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SAFETY:
It is necessary for the low-voltage dc output to be isolated from the utility supply to avoid the shock hazard.
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DIFFERENT
REFERENCE POTENTIALS:
The dc supply may have to operate at a different potential, for example, the dc supply to the gate drive for the upper MOSFET in the power-pole is referenced to its Source.
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VOLTAGE
MATCHING:
If the dc-dc conversion is large, then to avoid requiring large voltage and current ratings of semiconductor devices, it may be economical and operationally more suitable to use an electrical transformer for conversion of voltage levels.
