The paintings of the switching strength deliver is roughly: rectification --- thru the switching circuit --- into high-frequency AC ---transformation thru the high-frequency transformer --- rectification, filtering --- 直流. The mains strength we typically use is 50hz, and 50hz AC strength desires to be converted thru a transformer, however the low frequency results in an extended transformer winding, which makes the transformer cumbersome and calls for lots of wires. After turning into a high-frequency alternating current, a exceptionally small transformer may be used to step down, saving value and space.