Patent ID: 8729827

Claim:
A semiconductor light emitting element drive device, comprising: a direct-current power supply circuit configured to output direct-current power; a converter circuit which has a transformer and a first switching device, said transformer having a primary winding and first- and second-secondary windings, said first- and second-secondary windings being same polarity with each other, and said first switching device being connected to said primary winding, wherein said converter circuit being configured to be input the power outputted from said direct-current power supply circuit and to supply a load current to a light source unit comprising light emitting elements from said first-secondary winding, said converter circuit being configured to control the load current by variably controlling an ON width of said first switching device; a current regulation circuit which has a second switching device connected in series with said second-secondary winding, said current regulation circuit being configured to decrease an energy supplied to said first-secondary winding from said primary winding so as to decrease the load current supplied to said light source unit as the increase of an ON width of said second switching device; and a control unit configured to control ON and OFF of said first and second switching devices, wherein said control unit is configured, (a-1) if increasing a dimming ratio of said light source unit in a range where the dimming ratio of said light source unit is higher than a first ratio, to increase the ON width of said first switching device with said second switching device kept off, (a-2) if decreasing the dimming ratio of said light source unit in a range where the dimming ratio of said light source unit is higher than the first ratio, to decrease the ON width of said first switching device with said second switching device kept off, and (b) in a range where the dimming ratio of said light source unit is lower than the first ratio, to regulate the ON width of said second switching device with the ON width of said first switching device held at a lower limit, thereby controlling the dimming ratio of said light source unit.