The voltage of alternating current (AC) can be changed with a transformer. The electrical power of 1 megavolt from a power plant is first transmitted to the substation and pole-mounted power transformer to lower the voltage to 100 V, which is then supplied to individual homes. Indeed, the initial voltage is reduced to 1/10,000.

Alternating current: Its voltage can be changed with a transformer.

 By contrast, the voltage of direct current (DC) cannot be changed with a transformer.

 In DC-DC converter, a DC voltage is first converted to an AC voltage using semiconductors, the resulting AC voltage is changed to a certain AC voltage with a transformer, which is then converted to a required DC voltage again.

 There is an advantage that if the frequency of AC in the midway process is increased the size of a transformer used can be reduced.

 Though a DC-DC converter seems like changing DC to DC but in fact AC conversion is involved in the converter.