In a power supply of a synchronous rectification type, the self-turn on phenomenon
of MOSFET is suppressed without increase of the drive loss to thereby improve the
power efficiency. In a synchronous rectifier circuit, a threshold value of a commutation
MOSFET is made higher than that of a rectification MOSFET and particularly a threshold
value of a commutation MOSFET 3 is made 0.5V or more higher than that of
a rectification MOSFET 2. The threshold value of the rectification MOSFET
2 is lower than 1.5V and the threshold of the commutation MOSFET 3
is higher than 2.0V.