Patent ID: 8427850
Filing Date: 2013-04-23
Classification: H02M

Abstract:
1. A switching power supply device comprising: a power transformer including a primary winding, a secondary winding, and an auxiliary winding; a switching element which is connected to said primary winding and performs a switching operation to supply and stop supplying a first direct-current voltage to said primary winding; an output voltage generation circuit which converts, into a second direct-current voltage, an alternating-current voltage induced in said secondary winding through the switching operation of said switching element, and supplies the second direct-current voltage to a load; a transformer reset detection circuit which monitors a voltage signal of said auxiliary winding and generates a transformer reset signal according to a decrease in the voltage signal; a secondary-side on-time signal generation circuit which generates a secondary-side on-time signal indicating a secondary-side on-time that is a time period from when said switching element is turned off to when the transformer reset signal is generated; a feedback control circuit which generates a feedback signal corresponding to a voltage level of the second direct-current voltage; a switching control circuit which controls the switching operation of said switching element according to the feedback signal so that the second direct-current voltage falls within a normal operation specified range; and an overvoltage detection circuit which generates an overvoltage detection signal when the secondary-side on-time indicated by the secondary-side on-time signal becomes smaller than a set value, wherein the set value is smaller than the secondary-side on-time resulting when the second direct-current voltage is controlled to be within the normal operation specified range, and said switching control circuit controls the switching operation of said switching element so that the second direct-current voltage becomes equal to or below an overvoltage specified value when said overvoltage detection circuit generates the overvoltage detection signal.