It is aimed at decreasing losses not only in a synchronous rectifier
circuit provided at a secondary side of a DC-DC converter, but also in a
full-bridge switching circuit provided at a primary side thereof. The
DC-DC converter comprises a transformer for voltage conversion, a
synchronous rectifier circuit at the secondary side, and a full-bridge
switching circuit at the primary side. The DC-DC converter performs
synchronous rectifier control which uses switch transistors to change
paths of currents flowing through the secondary coil in synchronization
with switching operations at the primary side. The DC-DC converter
detects currents flowing through a load at the secondary side,
primary-side currents varying with the load currents, or primary-side
input voltages to dynamically control off-timings of a synchronous
rectification transistor at the secondary side. In addition, the DC-DC
converter detects primary-side input voltages and currents flowing
through the secondary-side load to dynamically control on-timings of a
transistor in the primary-side switching circuit.