1. Field of the Invention
The present invention relates to a switching power supply circuit provided as a power supply in various electronic instruments.
2. Description of the Related Art
As switching power supply circuits, various switching power supply circuits with various resonant converters have been proposed in view of suppression of switching noise and high electric power conversion efficiency. In the resonant converter, a switching operation waveform becomes a sinusoidal shape, so that low noise can be realized. Besides, it has also a merit that it can be constructed by a relatively small number of parts.
A circuit diagram of FIG. 8 or FIG. 10 shows an example of a switching power supply circuit provided with a voltage resonant converter as prior art. This power supply circuit includes one switching element Q1 and performs a self-excited switching operation through a so-called single end system.
In the power supply circuit shown in FIG. 8, when a rectifier diode constituting a bridge rectifier circuit DBR is turned on, high frequency oscillation current (hereinafter referred to as a ringing noise) as shown in FIGS. 9B and 9C is superimposed on currents 13 and 14 flowing through the rectifier diodes of the bridge rectifier circuit DBR by a leak inductance component L2 of a secondary winding N2 of an insulating converter transformer PIT and electrostatic junction capacitance (several pF) of the respective rectifier diodes constituting the bridge rectifier circuit DBR.
Such a ringing noise is radiated as a power supply noise from four rectifier diodes constituting the bridge rectifier circuit DBR. Thus, in the case where the power supply circuit shown in FIG. 8 is actually constituted, the number of parts is increased because of such measures that a ferrite bead inductor or ceramic capacitor is added to the secondary side of the insulating converter transformer PIT.
Besides, in the power supply circuit shown in FIG. 8, the bridge rectifier circuit DBR is provided at the secondary side, so that secondary side DC output voltage is obtained. That is, there is generated DC output voltage EO1 corresponding to the level almost equal to the AC voltage excited at the secondary winding N2. Thus, in this case, it becomes necessary that the winding count of the primary winding N1 and the winding count of the secondary winding N2 are almost equal to each other.
Thus, for example, it is difficult to miniaturize a split bobbin of the insulating converter transformer PIT in which Litz wires are wound as the primary winding N1 and the secondary winding N2, and therefore, it has been impossible to miniaturize and lighten the insulating converter transformer PIT.
Besides, in a power supply circuit shown in FIG. 10, with respect to resonant currents I3 and I4 flowing through rectifier diodes D01 and D02 of a double voltage rectifier circuit provided at the secondary side, as shown in FIG. 1B and 11C, the ringing noise is rather low at the time when the rectifier diodes D01 and D02 are turned on.
However, in the power supply circuit shown in FIG. 10, as shown in FIG. 12, in a region where load electric power becomes an intermediate load state of a range of, for example, 50 W to 120 W, a switching element Q1 performs an abnormal operation as described later.
FIGS. 13A to 13C are waveform views showing operation waveforms in the intermediate load state of the power supply circuit shown in FIG. 10.
Also in this case, the switching element Q1 performs a switching operation by a series resonant circuit (NB, CB) as a self-excited oscillation driving circuit, so that a primary side parallel resonant voltage Vcp as shown in FIG. 13A is obtained. However, in this case, in a period T1 immediately before the end of a period TOFF in which the switching element Q1 is turned off, a collector current Icp flows in a short time to the collector of the switching element Q1 as shown in FIG. 13B.
Besides, the waveform of the secondary side resonant current I2 flowing through the secondary winding N2 of the insulating converter transformer PIT becomes a waveform as shown in FIG. 13C.
In this case, as shown in FIGS. 13A and 13B, in the period T1 immediately before the end of the off period TOFF of the switching element Q1, the switching element Q1 is in a conduction state, which is deviated from the so-called ZVS (Zero Voltage Switching) operation of a resonant type basic operation, that is, the switching operation is performed when the primary side resonant voltage Vcp supplied between the collector and emitter of the switching element Q1 becomes a zero level.
In the power supply circuit shown in FIG. 10, the abnormal operation like this occurs since the period TOFF in which the switching element Q1 is turned off is increased with the decrease of load electric power Po. In the period T1 in which the abnormal operation like this occurs, since switching is performed in the state where the switching element Q1 has some voltage level and current level, electric power loss in the switching element Q1 is increased. Thus, it becomes necessary to enlarge a heat radiation plate to suppress heat generation of the switching element Q1.