Switching power supply having a frequency limiting circuit

摘要

A switching power supply device comprises: a transformer having a primary winding, a secondary winding and a feedback winding; a switching element connected in series with the primary winding; a controlling element connected to a control terminal of the switching element; a controlling circuit provided between the controlling element and the feedback winding; a rectifying circuit connected to the secondary winding; and a frequency limiting circuit including a charging circuit and a kick voltage absorbing circuit. The charging circuit is adapted to be charged by a voltage reverse in polarity, produced in the feedback winding. The kick voltage absorbing circuit absorbs a current developed by a kick voltage in the feedback winding, by discharge of the charging circuit, whereby the frequency limiting circuit reducing the switching frequency of the switching element by prolongation of the off-state time-period of the switching element in correspondence to a light load signal from a load when the load is light.