PROBLEM TO BE SOLVED: To simplify a drive circuit and to prevent a switching element from being broken down due to a malfunction such as a simultaneous conduction or the like in a bridge inverter. SOLUTION: A bridge inverter is constituted of FET's 1, 2, 3, 4, and square- wave outputs are generated at output terminals 7, 8. When a pulse signal source 5 is at an L-level, a transistor 210 is turned off, the FET 2 is forward-biased, and a capacitor 103 is charged by a DC power supply 6. At this point, a diode 404 is conducted, and the gate of the FET 4 is short-circuited. Then, a current flows in a route consisting of a DC power supply 10, an inductor 9, the FET 3, a load 11, a diode 101, the FET 2 and the DC power supply 10 in this order, and a capacitor 303 is charged simultaneously. When the pulse signal source 5 is at an H-level, a current flows in a route of the FET 1 and the FET 4. By following a fundamental arm and the continuity state of the FET 2, the driving signal of another arm is supplied, and a malfunction such as a simultaneous conduction or the like is not generated.
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