Patent ID: 7034800

Claim:
A driving circuit for a piezoelectric transformer, comprising: a first series connection body connected to both terminals of a DC power source, in which first and second switching members are turned on/off alternately by first and second driving signals and are connected in series; a second series connection body connected in parallel with the first series connection body, in which third and fourth switching members are turned on/off alternately by third and fourth driving signals respectively having the same frequency and duty ratio as those of the first and second driving signals and are connected in series; a piezoelectric transformer outputting a voltage, which is input from a primary side electrode, from a secondary side electrode after stepping-up or down the voltage by using a piezoelectric effect; a third series connection body connected between a connection point of each switching member of the first series connection body and a connection point of each switching member of the second series connection body, including an inductor and a pair of input electrodes of the piezoelectric transformer; a fourth series connection body connected between a pair of output electrodes of the piezoelectric transformer, in which a cold-cathode fluorescent tube and a current detecting resistor are connected in series; a first driving portion generating the first and second driving signals; a second driving portion generating the third and fourth driving signals; a feedback portion returning a voltage signal generated at the current detecting resistor after rectifying it; and a control portion receiving the voltage signal from the feedback portion to output first and second control signals respectively to the first and second driving portions, the control portion comprising: an A/D converter converting the voltage signal from the feedback portion to current detecting data having a digital value; a comparator comparing the current detecting data from the A/D converter with reference data; and a phase controller changing a phase difference between the first and second control signals based on a comparison result from the comparator, wherein at a commencement of lighting of the cold-cathode fluorescent tube, the control portion fixes frequencies of the first and second control signals at a predetermined frequency, and controls the phase difference between the first and second control signals so that an output power of the piezoelectric transformer becomes from substantially zero to a predetermined output power, and after lighting of the cold-cathode fluorescent tube, the control portion controls the phase difference between the first and second control signals so that the current detecting data substantially is matched with the reference data.