| 摘要 |
<p>A switching power supply circuit comprises a rectifying and smoothing means, a switching means, a switching-driving means, an isolated converter transformer formed by at least winding a primary winding and a secondary winding. The switching power supply circuit further comprises a primary side parallel resonant circuit, a secondary side series resonant circuit, a secondary side direct-current output voltage generating means and a constant-voltage control means. A coupling coefficient between a primary side and a secondary side of said isolated converter transformer is set so as to obtain zero voltage switching in a predetermined load condition range of medium load at least, and a resonant frequency of said primary side parallel resonant circuit and a resonant frequency of said secondary side series resonant circuit are set so as to obtain a certain power conversion efficiency and higher under predetermined load conditions at least. An abnormal operation in which a current flows through a switching device before the end of an off period of the switching device at medium load is inhibited by setting the coupling coefficient of an isolated converter transformer to about 0.7 or lower. A primary side parallel resonance frequency and a secondary side series resonance frequency are set so as to obtain favorable power conversion efficiency characteristics.
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