| 摘要 |
PROBLEM TO BE SOLVED: To improve the mobility in power source input of a cold-cathode tube and the lighting property at low temperature thereof by detecting the current carried to the cold-cathode tube to which a lighting voltage is inputted, controlling the oscillating frequency of a voltage control oscillating circuit, and protecting a piezoelectric transformer for lighting voltage in conformation to the change of load impedance. SOLUTION: When a DC voltage Vcc is inputted to an inverter 10, the secondary output of a piezoelectric transformer 11 is applied to a cold-cathode tube 50, and a cold-cathode tube current detecting circuit 15 detects the current as voltage signal. The detected current is fed back to a voltage control oscillating circuit, in which the oscillating frequency is controlled so as to be close to the resonance frequency of the piezoelectric transformer 11. When the step-up ratio of the piezoelectric transformer 11 is raised, and the cold-cathode tube 50 starts to discharge, the current is suddenly increased, and the oscillating frequency of the voltage control oscillating circuit 12 is stabilized in the resonance frequency of the piezoelectric transformer 11. A primary current of the piezoelectric transformer 11 is monitored by a protecting circuit 30, and when an excessive current is detected, a stop signal is outputted to temporarily stop the output of the voltage control oscillating circuit 12. |
