FIG. 1 is a circuit diagram illustrating a configuration of a conventional switching power-supply apparatus. For example, the switching power-supply apparatus shown in FIG. 1 inputs a direct-current voltage as a direct-current input voltage Vin, which is generated by rectifying and smoothing an alternating-current voltage from a commercial power-supply, in a system called as a current resonance type switching power-supply apparatus. A switching element (first switching element) Q11 consisting of MOSFET and a switching element (second switching element) Q12 consisting of MOSFET are connected in series to both ends of a direct-current power-supply Vin for supplying the direct-current input voltage Vin.
A voltage resonance capacitor Cv1 and a first resonance circuit which includes a resonance reactor Lr1, a primary winding Np1 of a transformer T1 and a current resonance capacitor Ci1, are connected between a drain and a source of the switching element Q12 (they may be connected between a drain and a source of the switching element Q11). For example, a leakage inductance of the transformer T1 is substituted for the resonance reactor Lr1.
A diode D1 is connected between the drain and the source of the switching element Q12. A diode D2 is connected between the drain and the source of the switching element Q11. The diodes D1 and D2 may be parasitic diodes of the switching elements Q11 and Q12.
Also, secondary windings Ns11 and Ns12, each of which is wound to have a reversed phase with respect to the other winding, are connected in series at a secondary side of the transformer T1. Voltages generated in the secondary windings Ns11 and Ns12 are rectified by diodes D11 and D12, smoothed by an output smoothing capacitor Co1, and output as an output voltage Vo1.
Gate signals each of which has a dead time for preventing the switching elements Q11 and Q12 from being turned on simultaneously, are alternately input into gates of the switching elements Q11 and Q12 from a control circuit 10, with the same turn-on width.
When the switching elements Q11 and Q12 are alternately turned on/off, resonance currents Q11i and Q12i shown in FIG. 2 flow in the switching elements Q11 and Q12, which flows sinusoidal resonance currents D11i and D12i through the diodes D11 and D12 at the secondary side of the transformer T1.
The output voltage Vo1 is returned to the control circuit 10 located at a primary side via an insulation means such as a photo coupler not shown. Switching frequencies of the switching elements Q11 and Q12 are controlled such that the output voltage Vo1 has a certain value using the control circuit 10.
In this current resonance type switching power-supply apparatus, as shown in FIG. 2, when the respective switching elements Q11 and Q12 are turned on, switching loss does not occur because respective currents flow in a minus direction (respective currents flowing through the diodes D11 and D12). Also, when the respective switching elements Q11 and Q12 are turned off, a surge voltage does not occur because resonance working is carried out. Thus, low-voltage switching elements can be employed and it is an extremely-effective system for configuring a highly-efficient power-supply.
However, in the current resonance type switching power-supply apparatus shown in FIG. 1, the currents D11i and D12i have discontinuous changes because the sinusoidal resonance currents D11i and D12i alternately flow at the secondary side. Thus, a ripple current Co1i which flows in the output smoothing capacitor Co1, is about fifty to seventy percents of the output current, and is large in comparison with a forward converter in which a current has a continuous change. An electrolytic capacitor which is generally employed as the output smoothing capacitor Co1, has a regulation about an allowable ripple current. In order to meet the regulation, a plurality of electrolytic capacitors must be connected in parallel. This brings a problem that a cost and a mounting area increase.
In order to resolve this problem, Patent Literature 1 discloses a method for reducing a ripple current of an electrolytic capacitor by connecting a plurality of circuits in parallel and working the respective circuits while shifting phases of the respective circuits.