A power-supply apparatus is provided which includes a transformer, a primary semiconductor unit, a secondary semiconductor unit, and a secondary electronic device. Each of the primary semiconductor unit and the secondary semiconductor units has a plurality of semiconductor devices installed therein. The transformer, the primary semiconductor unit, the secondary semiconductor unit, and the secondary electronic device are electrically joined through connecting conductors. The transformer is laid on the primary semiconductor unit to make a first stack. Similarly, the secondary electronic device is laid on the secondary semiconductor unit. This permits the power-supply apparatus to be reduced in overall size thereof and minimizes adverse effects of electromagnetic noise to ensure the high efficiency in power supply operation.