Patent ID: 7339801

Claim:
A switching power supply circuit comprising: a main switching element that is supplied with a direct-current (DC) voltage and implements switching; a switching drive unit that switch-drives the main switching element; a first inductor that is provided in series to a path through-which the DC voltage is supplied to the main switching element; a primary-side parallel resonant capacitor that is connected in parallel to the main switching element so that at least an inductance of the first inductor and a capacitance of the primary-side parallel resonant capacitor form a primary-side parallel resonant circuit having a certain first resonant frequency; a second inductor; a primary-side series resonant capacitor that is connected in series to the second inductor so that at least an inductance of the second inductor and a capacitance of the primary-side series resonant capacitor form a primary-side series resonant circuit having a certain second resonant frequency regarded as equivalent to the first resonant frequency, and a series circuit of the second inductor and the primary-side series resonant capacitor is connected in parallel to the main switching element; a converter transformer around which the second inductor as a primary winding and a secondary winding are wound, an output switching obtained in the primary winding inducing an alternating voltage in the secondary winding, a coupling coefficient of the converter transformer being set so that such a requisite total coupling coefficient between a primary side and a secondary side as to offer loose coupling therebetween is obtained; a secondary-side DC output voltage generating unit that is supplied with the alternating voltage induced in the secondary winding of the converter transformer, and implements rectifying operation to thereby generate a secondary-side DC output voltage; and a constant-voltage control unit that implements constant-voltage control of the secondary-side DC output voltage by controlling the switching drive unit according to a level of the secondary-side DC output voltage and thereby varying a switching frequency of the main switching element.