Switching power supply device

摘要

In a switching power supply device having low loss in synchronous rectifying switches, exhibiting high power efficiency, and not causing troubles by reverse current in the switches. A secondary control circuit includes a reference voltage circuit to generate a reference voltage having a predetermined potential, and an ON-timing detector circuit to detect an ON timing of a synchronous rectifying switch through monitoring of a terminal voltage of the switch, an OFF-timing detector circuit to detect an OFF timing of the switch, and a timer circuit to be turned on at the ON timing and measure a predetermined period. The threshold voltage consisting of the reference voltage generated by the reference voltage circuit and an offset voltage is applied to the OFF-timing detector circuit during the measurement of the timer circuit, and the threshold voltage consisting of the reference voltage is applied to the OFF-timing detector circuit during the non-measurement.

主权项

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.