POWER SUPPLY CIRCUIT

PROBLEM TO BE SOLVED: To provide a step-down type power supply circuit capable of supplying constant power to an LED even when a power supply voltage fluctuates.SOLUTION: The power supply circuit comprises: a step-down type power supply circuit connected between an input power supply terminal and a GND terminal, and having a series circuit of an LED constituting a load, an energy storing coil L1, and a switching element Q1, and a regenerative diode connected in parallel with the coil L1 and the LED constituting a load, the anode terminal of which is connected to a connection point of the coil L1 and the switching element Q1 and the cathode terminal is connected to an input power supply terminal, the step-down type power supply circuit stepping down the power supply voltage of a commercial AC power supply and supplying the resulting power to the LED constituting a load; an ON detection circuit for detecting that a current flowing in the coil L1 becomes zero; an OFF detection circuit for detecting that a current flowing in the coil L1 exceeds a reference value; and a drive control circuit for controlling on/off of the switching element Q1 on the basis of the result of detection by the ON detection circuit or the OFF detection circuit.