Patent ID: 6867986

Claim:
A constant current 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 connected in-series with the primary winding to connect the primary winding to an energizing direct current power supply; a switching control circuit to execute ON/OFF control of the oscillating switching device; and a rectifying smoothing circuit to rectify and smooth an output of the secondary output winding; the constant current output control method for changing an ON/OFF time of the oscillating switching device of the switching power supply circuit, and executing a constant current output control of an output current I 2o of the rectifying smoothing circuit comprising: deriving an OFF adjustment time T 3 based on equation (1), T 3 =T 2 ×( Np÷Ns×Ip ref ÷2 ÷I 2oset −1)− T 1 (1) where, I 2oset represents a set output current of the rectifying smoothing circuit for which the constant current output control is to be executed, Np represents a number of turns of the primary winding, Ns represents a number of turns of the secondary output winding, T 1 being a fixed time representing an ON time of the oscillating switching device to excite the primary winding during an oscillation period T, Ip ref representing a reference peak current flowing in the primary winding when the ON time T 1 completely elapses, and T 2 representing an output time during which output is generated in the rectifying smoothing circuit; controlling an OFF time of the oscillating switching device during the oscillation period T wherein the OFF time is equal to the sum of the output time T 2 and the OFF adjustment time T 3 ; and setting the output current I 2o of the rectifying smoothing circuit to the set output current I 2oset .