Patent ID: 7027313

Claim:
A constant voltage output control method for a switching power supply circuit having a transformer provided with a primary winding and a secondary output winding; an oscillating switching device which is connected in-series with the primary winding to a direct current power supply that excites the primary winding; a switching control circuit that executes ON/OFF control of the oscillating switching device; and a rectifying smoothing circuit that rectifies and smoothes an output of the secondary output winding; this constant voltage output control method executing a constant voltage control of an output voltage V 2b that is generated in the secondary output winding by changing an ON time T 1 of the oscillating switching device of the switching power supply circuit, comprising the steps of: deriving a set voltage V 2aset based on equation (1), V 2aset =Np÷Ns×V 2bset (1) where, V 2bset represents an output voltage of the secondary output winding that is subject to the constant voltage control, Np represents a number of turns of the primary winding, and Ns represents a number of turns of the secondary output winding; comparing the set voltage V 2aset with a flyback voltage V 2a of the primary winding; and executing at least one of a time shortening control and a time lengthening control such that, in the case that the flyback voltage V 2a is more than the set voltage V 2aset , the time shortening control is executed that shortens the ON time T 1 of the oscillating switching device in accordance with a difference voltage dV between the set voltage V 2aset and the flyback voltage V 2a , and in the case that the flyback voltage V 2a is less than the set voltage V 2aset , the time lengthening control is executed that lengthens the ON time T 1 of the oscillating switching device in accordance with the difference voltage dV.