A power conversion circuit capable of varying an output voltage within a range from a negative voltage lower than a ground voltage to a positive voltage higher than a supply voltage, and a driving method and a drive unit are provided. A power conversion circuit includes a transformer with a 1:1 ratio between the primary winding and secondary winding, a voltage outputting capacitor, and four switches. The power conversion circuit can be operated as a DC-DC converter of a step-up type, a step-up-and-down type, a step-down type, an inverted-output step-up-and-down type, or an inverted-output step-up type by selecting two switches used for control from among the four switches and alternately turning the two switches on. By switching the operating modes of the power conversion circuit, the output voltage can be varied within a range from a negative voltage to a positive voltage higher than a supply voltage.