Patent ID: 9331593
Filing Date: 2016-05-03
CPC Classification: H02M,Y02B

Claim Text:
1. A switching power supply device comprising: a transformer for power conversion; at least one main switching element connected in series to a primary coil of the transformer; a primary control circuit to turn on or off the main switching element; a synchronous rectifying switching element connected in series to a secondary coil of the transformer; and a secondary control circuit to turn on or off the synchronous rectifying switching element, wherein: the secondary control circuit comprises: an ON-timing detector circuit to detect an ON timing of the synchronous rectifying switching element through monitoring of a terminal voltage of the synchronous rectifying switching element, and an OFF-timing detector circuit to detect an OFF timing of the synchronous rectifying switching element through comparison of the terminal voltage of the synchronous rectifying switching element with a threshold voltage, a voltage obtained by adding a predetermined offset voltage to a reference voltage is applied as the threshold voltage to the OFF-timing detector circuit, before elapse of a predetermined period since activation of the synchronous rectifying switching element, the reference voltage is applied as the threshold voltage to the OFF-timing detector circuit, after the elapse of the predetermined period and before subsequent activation of the synchronous rectifying switching element, and the reference voltage and the offset voltage are set such that the terminal voltage of the synchronous rectifying switching element when a current therethrough flows in a forward direction is lower than the voltage obtained by adding the offset voltage to the reference voltage.