Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-02-07
2001-11-20
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S040000, C363S056120
Reexamination Certificate
active
06320765
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power circuit adapted for use as a power supply in various electronic apparatus.
There are widely known switching power circuits of a type employing a switching converter such as a flyback converter or a forward converter. Since such a switching converter performs its switching operation with rectangular waves, there exists a limit in suppression of switching noise. And it is also obvious that, due to the operating characteristic thereof, some restriction is unavoidable in improving the power conversion efficiency.
In view of the points mentioned above, a variety of switching power circuits employing various resonant converters have already been proposed by the present applicant. A resonant converter is capable of attaining a high power conversion efficiency with facility and realizing low noise as the switching operation is performed with sinusoidal waves. And it is further possible to achieve another merit that the circuit can be constituted of a relatively small number of component parts.
FIG. 6
is a circuit diagram showing a conventional switching power circuit of a configuration based on the known invention filed previously by the present applicant.
In the power circuit shown in this diagram, a full-wave rectifier circuit consisting of a bridge rectifier Di and a smoothing capacitor Ci is provided as a rectifier smoothing circuit for obtaining a DC input voltage from a commercial alternating power supply (alternating input voltage VAC), wherein a rectified smoothed voltage Ei corresponding to one-fold level of the alternating input voltage VAC is generated.
As a switching converter for intermittently turning on and off the input rectified smoothed voltage Ei (DC input voltage), there is provided a voltage resonant converter which comprises a switching element Q
1
of one transistor to perform its switching operation in a single end form.
The voltage resonant converter employed here adopts a separately excited structure, and the switching element Q
1
consists of a MOS-FET for example. The drain of this switching element Q
1
is connected to the positive terminal of the smoothing capacitor Ci via a primary winding N
1
of the insulating converter transformer PIT, and its source is connected to a primary-side ground.
A parallel resonance capacitor Cr is connected between the drain and source of the switching element Q
1
. The capacitance of this parallel resonance capacitor Cr and a leakage inductance obtained in the primary winding N
1
of the insulating converter transformer PIT constitute a primary parallel resonance circuit. And a resonance action is caused by the parallel resonance circuit in accordance with the switching operation of the switching element Q
1
, so that such switching operation of the switching element Q
1
becomes a voltage resonance type.
Further a clamp diode DD consisting of a body diode is connected in parallel between the drain and source of the switching element Q
1
, thereby forming a path of a clamp current which flows during the off-time of the switching element.
In this case, the drain of the switching element Q
1
is connected to an oscillation circuit
41
in a switching driver
10
B which will be described next. The drain output supplied to the oscillation circuit
41
is used for variably controlling the switching on-time in control of the switching frequency as will be mentioned later.
The switching element Q
1
is driven by the switching driver
10
B which is integrally equipped with the oscillation circuit
41
and a drive circuit
42
, and the switching frequency is variably controlled for execution of constant voltage control. The switching driver
10
B in this case is provided as a single integrated circuit (IC) for example.
The switching driver
10
B is connected to the line of a rectified smoothed voltage Ei via a start resistor Rs, and at a power supply start time for example, a source voltage is impressed via the start resistor Rs to thereby start the switching driver
10
B.
The oscillation circuit
41
in the switching driver
10
B performs oscillation to thereby generate an oscillation signal and then outputs the same. Subsequently in the drive circuit
42
, this oscillation signal is converted into a driving voltage and then is outputted to the gate of the switching element Q
1
. Thus, the switching element Q
1
performs its switching operation based on the oscillation signal generated in the oscillation circuit
41
. Therefore, the switching frequency of the switching element Q
1
and the on/off duty ratio in one switching period are determined depending on the oscillation signal generated in the oscillation circuit
41
.
The oscillation circuit
41
performs its operation of changing the oscillation signal frequency (switching frequency fs) on the basis of the level of a secondary DC output voltage EO which is supplied via a photo coupler
30
as will be mentioned later. And simultaneously with the operation of changing the switching frequency fs, the oscillation circuit
41
further serves to control the oscillation signal waveform in such a manner that the on-time TON (conduction angle) of the switching element Q
1
is changed while the off-time TOFF of the switching element Q
1
is maintained constant. Consequently, the secondary DC output voltage EO can be stabilized due to such operation of the oscillation circuit
41
, as will be described later.
The insulating converter transformer PIT transmits the switching output of the switching element Q
1
to the secondary side.
As shown in
FIG. 8
, the insulating converter transformer PIT has an EE-shaped core where E-shaped cores CR
1
and CR
2
composed of ferrite for example are combined with each other in such a manner that magnetic legs thereof are opposed mutually, and the primary winding N
1
and the secondary winding N
2
thereof are coiled in a split state respectively by the use of a split bobbin B with regard to the center magnetic leg of the EE-shaped core. And a gap G is formed to the center magnetic leg as shown in the diagram, whereby coarse coupling is attained with a required coupling coefficient.
The gap G can be formed by shaping the center magnetic leg of each of the E-shaped cores CR
1
and CR
2
to be shorter than the two outer magnetic legs thereof. The coupling coefficient k is set as, e.g., k≈0.85 suited to attain coarse coupling, hence avoiding a saturated state correspondingly thereto.
As shown in
FIG. 6
, the end of the primary winding N
1
of the insulating converter transformer PIT is connected to the drain of the switching element Q
1
, while the beginning of the primary winding N
1
is connected to the positive terminal (rectified smoothed voltage Ei) of the smoothing capacitor Ci. Therefore, when the switching output of the switching element Q
1
is supplied to the primary winding N
1
, there is generated an alternating voltage of a period corresponding to the switching frequency.
On the secondary side of the insulating converter transformer PIT, an alternating voltage induced by the primary winding N
1
is generated in the secondary winding N
2
. In this case, a secondary parallel resonance capacitor C
2
is connected in parallel to the secondary winding N
2
, so that a parallel resonance circuit is formed by a combination of the leakage inductance L
2
of the secondary winding N
2
and the capacitance of the secondary parallel resonance capacitor C
2
. And the alternating voltage induced in the secondary winding N
2
by this parallel resonance circuit is a resonance voltage, i.e., a voltage resonance action is caused on the secondary side.
More specifically, this power circuit has, on its primary side, a parallel resonance circuit for turning the the switching operation into a voltage resonance type, and also has, on its secondary side, another parallel resonance circuit for producing a voltage resonance action. In this specification, the switching converter of a configuration equipped with resonance circuits on its primary and secondary sides as mentioned above
Frommer William S.
Frommer & Lawrence & Haug LLP
Han Jessica
Sony Corporation
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