Switching power supply, luminaire, and control method for the luminaire

A luminaire comprising:

• a switching power supply;
• and
• a lighting load connected as a load circuit of the switching power supply,
• the switching power supply including:
• a first inductor;
• a switching element configured to supply, if the switching element is on, a power supply voltage to the first inductor and feeds an electric current;
• a constant current element connected in series to the switching element and configured to turn off the switching element if the electric current of the switching element exceeds a predetermined upper limit;
• a rectifying element connected in series to the switching element or the constant current element and configured to feed the electric current of the first inductor if the switching element is turned off;
• a second inductor magnetically coupled to the first inductor and configured to have induced therein potential for turning on the switching element if the electric current of the first inductor increases, have induced therein potential for turning off the switching element if the electric current of the switching element decreases, and supply the induced potential to a control terminal of the switching element;
• and
• a control circuit configured to supply the potential to a control terminal of the constant current element and output, if an average of the potential is lower than a lower limit for enabling the switching element to continue switching, pulse-like potential that repeats a period in which a potential value equal to or higher than the lower limit is output and a period in which a potential value lower than the lower limit is output.