Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-10-27
2001-07-17
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06262897
ABSTRACT:
BACKGROUND OF THE INVENTION
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 Q
1
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 L
2
of a secondary winding N
2
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 EO
1
corresponding to the level almost equal to the AC voltage excited at the secondary winding N
2
. Thus, in this case, it becomes necessary that the winding count of the primary winding N
1
and the winding count of the secondary winding N
2
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 N
1
and the secondary winding N
2
, 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 I
3
and I
4
flowing through rectifier diodes D
01
and D
02
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 D
01
and D
02
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 Q
1
performs an abnormal operation as described later.
FIGS. 13A
to
13
C 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 Q
1
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 Q
1
is turned off, a collector current Icp flows in a short time to the collector of the switching element Q
1
as shown in FIG.
13
B.
Besides, the waveform of the secondary side resonant current I
2
flowing through the secondary winding N
2
of the insulating converter transformer PIT becomes a waveform as shown in FIG.
13
C.
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 Q
1
, the switching element Q
1
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 Q
1
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 Q
1
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 Q
1
has some voltage level and current level, electric power loss in the switching element Q
1
is increased. Thus, it becomes necessary to enlarge a heat radiation plate to suppress heat generation of the switching element Q
1
.
SUMMARY OF THE INVENTION
In view of the foregoing problems, the present invention has an object to provide a switching power supply circuit in which ringing noise is not superimposed on a secondary side resonant current flowing through a rectifier diode provided at a secondary side, and an operation of a switching element becomes a ZVS operation even in an intermediate load state.
A switching power supply circuit of the present invention includes switching means provided with a switching element and for intermittently outputting an inputted DC input voltage, an insulating converter transformer for transmitting output of the switching means to a secondary side, a primary side voltage resonant circuit inserted to make an operation of the switching means a voltage resonant type, and a secondary side resonant circuit constituted by a combination of a secondary side parallel resonant circuit formed by connecting a secondary side parallel resonant capacitor in parallel with a secondary winding of the insulating converter transformer, and a secondary side series resonant circuit formed by connecting a secondary side series resonant capacitor in series with the secondary winding of the insulating converter transformer.
Further, the switching power supply circuit includes DC output voltage generation means for obtaining a secondary side DC output voltage by inputting an AC voltage obtained in the secondary winding of the insulating converter transformer and by performing a rectification operation, and constant voltage control means for performing constant voltage control by varying a switching frequency of the switching element in accordance with a level of the secondary side DC output voltage.
According to the above structure, by providing the secondary side resonant circuit constituted by the combination of the secondary side series resonant circuit and the secondary side parallel resonant circuit to the secondary winding of the insulating converter transformer, the secondary side resonant current flowing through the secondary winding N
2
of the insulating converter transformer can be made almost a sinusoidal shape by the resonant operation of the secondary side parallel resonant circuit. By this, since conduction angles of resonant currents flowing through the rectifier diodes provided at the secondary side become almost equal to each other, the ringing noise comes not to be superimposed on the resonant currents flowing through the rectifier diodes.
Since the constant voltage control of the secondary side DC output voltage becomes combination control of controlling the switching frequency and the conduction angle of the switching current flowing through the switching element, even in the case where load is varied, enlargement of the period in which the switching element is t
Frommer William S.
Frommer Lawrence & Haug LLP.
Sony Corporation
Sterrett Jeffrey
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