Patent ID: 9674923
Date: 2017-06-06
CPC Classifications: H02M,H05B,Y02B

Claim:
1. A lighting circuit receiving power supply from an AC power source and lighting a light source, comprising: a rectifying and smoothing circuit configured to rectify and smooth AC current supplied from the AC power source; a switching element connected to the rectifying and smoothing circuit; a control unit configured to perform control of switching the switching element between an on state and an off state; a transformer including a primary coil and a secondary coil, the primary coil connected in series to the switching element, a voltage being induced across both ends of the secondary coil by current flowing through the primary coil; a first voltage supply circuit configured to supply a voltage that occurs across both ends of the primary coil to the light source; a zener diode connected across both ends of the secondary coil; and a second voltage supply circuit connected between the secondary coil and a power source terminal of the control unit and configured to supply the voltage induced in the secondary coil to the power source terminal when the switching element is in the on state, wherein the control unit includes a current detection circuit that detects a current flowing through the zener diode, and, upon detection of a current exceeding a reference value by the current detection circuit, the control unit maintains the control of the switching element in a deactivated state, and a turns ratio N of the primary coil to the secondary coil satisfies where X1 V denotes a voltage that the AC power source supplies, Y V denotes the maximum value of a voltage range of a power source voltage of the control unit at which normal operation of the control unit is guaranteed, and Z V denotes the minimum value of the voltage range of the power source voltage of the control unit at which normal operation of the control unit is guaranteed.