PURPOSE: To prevent the unbalance of voltage when a pair of loads are unbalanced in a half-bridge type AC/DC converter.

CONSTITUTION: A reactor L is connected between a power supply 1 and a connecting center point 9 of a pair of transistors Q1 and Q2. Diodes D1 and D2 are respectively connected in parallel to a pair of the transistors Q1 and Q2. A pair of capacitors C1 and C2 are connected in series between the first and second DC output terminals 5 and 6. A connecting center point of a pair of capacitors C1 and C2 is connected to ground terminals 3 and 7. In order to detect a voltage unbalance between a pair of capacitor C1 and C2 due to the unbalance of the first and second load R1 and R2 connected between the first and second DC output terminals 5 and 6 and the ground terminal 7, detecting resistors Ra and Rb having the same resistance value is connected between the first and second DC output terminals 5 and 6 are connected. Electric potential at the center point of the detecting resistors Ra and Rb is used as unbalance potential for changing the width of control pulses of the transistors Q1 and Q2.