Abstract: A switching power supply circuit, which is small, lightweight, and highly efficient in power conversion, includes an insulating converter transformer, which is formed by a core with no gap and a primary and secondary winding wound on the core such that the mutual inductance between the windings exhibits an additive mode. A half-wave rectifier circuit is provided on the secondary side of the circuit and performs a rectification operation in the additive mode to obtain a secondary side DC output voltage. In a constant voltage control circuit system for stabilizing the secondary side output voltage, the switching frequency of a switching element is varied in response to the secondary side output voltage level to control the resonance impedance of a primary side parallel resonance circuit and the continuity angle of the switching element compositely.