A switch power supply having
active power factor correction is provided, including a
power factor correction (PFC) circuit, a filter circuit, a main power stage, a first
capacitor (C1) and an indicating circuit. An alternating-current input (IN) is connected to the first
capacitor through the PFC circuit and is connected to the filter circuit through the indicating circuit. The indicating circuit consists of a
light emitting diode (LED) having unidirectional
conductivity and an
inductor (L); and the current direction of power supplied from the PFC circuit to the main power stage through the
inductor is opposite to the conduction direction of the LED. The LED may also be a
light emitter of a photocoupler. The filter circuit at least comprises an
electrolytic capacitor (C2). When the
electrolytic capacitor is normal, an exciting current of the main power stage basically does not appear in the
inductor and the LED does not emit light. When the
equivalent series resistance (ESR) of the
electrolytic capacitor sharply rises, the exciting current appears in the inductor. When a power
transistor in the main power stage is switched off, the exciting current in the inductor cannot be abruptly changed; instead, the exciting current passes through the LED for freewheeling and enables the LED to emit light, thereby notifying a user or a circuit that the electrolytic
capacitor may have the risk of failure. The switch power supply has low additional cost and unchanged efficiency and is easy to implement.