To provide a switching power supply circuit with one switching element for narrowing the frequency variation range.
In the switching power supply circuit having a primary voltage resonance circuit and a secondary resonance circuit, the secondary circuit is formed to make additive polarity connection of the primary winding N1 and the secondary winding N2. Resonance frequency of the secondary parallel resonance circuit is set substantially equal to 2/3 of the resonance frequency of the primary parallel resonance circuit or below and the time ratio, i.e. the ratio of on time and off time, of a switching element Q1 is set at 1 or above. Consequently, the control range of switching frequency can be narrowed and the current can be made small at the moments in time of turn on and turn off.
Nobuo Suzuki
Wachi Shigeaki
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