Power adapters, also known as external power supplies, are devices that convert the power supply for small portable electronic equipment and electronic appliances. Typically composed of a housing, a power transformer, and a rectifying circuit, they are categorized by their output type into AC output and DC output models; and by their connection method into wall-mounted and desktop types. They are widely used with devices such as telephone systems, game consoles, language recorders, portable music players, laptops, and cellular phones.

Power adapters are extensively used in consumer electronics, with most providing output power of 100 watts or less, and there is a rapid increase in market demand. Applications include electric bike battery chargers, small-sized LCD TVs, laptops, printers, DVD players, and set-top boxes. Early power adapters predominantly used linear transformers, but as consumer electronics require more efficient power adapters with wider input voltage ranges, and due to rising costs of copper, iron, and labor, these linear transformers in the power adapters have been gradually replaced by switching power supplies.