Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-04-26
2002-03-12
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06356465
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply circuit to be used as a power supply for various electronic apparatus.
Switching converters such as flyback converters and forward converters are widely known as switching power supply circuits. These switching converters form a rectangular waveform in switching operation, and therefore there is a limit to suppression of switching noise. It is also known that because of their operating characteristics, there is a limit to improvement of power conversion efficiency. Hence, various switching power supply circuits formed by resonance type converters have been proposed. A resonance type converter makes it possible to readily obtain high power conversion efficiency, and to achieve low noise because the resonance type converter forms a sine-wave waveform in switching operation. The resonance type converter has another advantage of being able to be formed by a relatively small number of parts.
FIG. 5
is a circuit diagram showing an example of a prior art switching power supply circuit. The power supply circuit shown in
FIG. 5
is supplied with a commercial alternating-current input voltage VAC, and then a rectified and smoothed voltage Ei is generated by a full-wave rectifier circuit comprising a bridge rectifier circuit Di and a smoothing capacitor Ci.
A self-excited voltage resonance type converter circuit that includes a switching device Q
1
and performs switching operation by a so-called single-ended system is provided as a switching converter for interrupting the rectified and smoothed voltage Ei inputted from the full-wave rectifier circuit. A BJT (Bipolar Junction Transistor), which is a high withstand voltage bipolar transistor, is employed as the voltage resonance type converter in this case.
An isolating converter transformer PIT transmits switching output of the switching device Q
1
to the secondary side of the switching power supply circuit. As shown in
FIG. 6
, the isolating converter transformer PIT has an E-E-shaped core formed by combining E-shaped cores CR
1
and CR
2
made for example of a ferrite material in such a manner that magnetic legs of the core CR
1
are opposed to magnetic legs of the core CR
2
. A primary winding N
1
and a secondary winding N
2
are wound around a central magnetic leg of the E-E-shaped core in a state divided from each other by using a dividing bobbin B. Also, a gap G is formed in the central magnetic leg, as shown in
FIG. 6
, to provide loose coupling at a required coupling coefficient. The gap G can be formed by making the central magnetic leg of each of the E-shaped cores CR
1
and CR
2
shorter than two outer magnetic legs of each of the E-shaped cores CR
1
and CR
2
. The coupling coefficient is set at 0.85, for example, to provide a loosely coupled state, whereby a saturated state is not readily obtained.
As shown in
FIG. 5
, an ending point of the primary winding N
1
of the isolating converter transformer PIT is connected to a collector of the switching device Q
1
, while a starting point of the primary winding N
1
is connected to a positive electrode of the smoothing capacitor Ci via a resonance current detecting winding ND. Hence, the primary winding N
1
is supplied with the switching output of the switching device Q
1
, whereby an alternating voltage whose cycle corresponds to the switching frequency of the switching device Q
1
occurs in the primary winding N
1
.
An alternating voltage induced by the primary winding N
1
occurs in the secondary winding N
2
on the secondary side of the isolating converter transformer PIT. In this case, a secondary-side parallel resonant capacitor C
2
is connected in parallel with the secondary winding N
2
, and therefore leakage inductance L
2
of the secondary winding N
2
and capacitance of the secondary-side parallel resonant capacitor C
2
form a parallel resonant circuit. The parallel resonant circuit converts the alternating voltage induced in the secondary winding N
2
into a resonance voltage, whereby voltage resonance operation is obtained on the secondary side.
The power supply circuit is provided with a parallel resonant circuit to convert switching operation into voltage resonance type operation on the primary side, and the parallel resonant circuit to provide voltage resonance operation on the secondary side. The switching converter provided with resonant circuits on the primary side and the secondary side as described above is referred to as a “complex resonance type switching converter.”
As for secondary-side operation of the isolating converter transformer PIT, mutual inductance M between inductance L
1
of the primary winding N
1
and inductance L
2
of the secondary winding N
2
becomes +M or −M, depending on winding direction of the primary winding N
1
and the secondary winding N
2
, a connecting relation of a rectifier diode D
0
, and change in polarity of the alternating voltage induced in the secondary winding N
2
. For example, an equivalent of a circuit shown in
FIG. 7A
has a mutual inductance of +M, while an equivalent of a circuit shown in
FIG. 7B
has a mutual inductance of −M. This will be applied to the secondary-side operation of the isolating converter transformer PIT shown in FIG.
5
. When the alternating voltage obtained at the secondary winding N
2
has a positive polarity, an operation that causes rectified current to flow in the bridge rectifier circuit DBR can be considered a +M operation mode, or forward operation, whereas when the alternating voltage obtained at the secondary winding N
2
has a negative polarity, an operation that causes rectified current to flow in the bridge rectifier diode DBR can be considered a −M operation mode, or flyback operation. Every time the alternating voltage obtained at the secondary winding N
2
becomes positive or negative, the operation mode of the mutual inductance becomes +M or −M, respectively.
With such a configuration, power increased by effects of the primary-side parallel resonant circuit and the secondary-side parallel resonant circuit is supplied to a load side, and accordingly the power supplied to the load side is increased as much, thereby improving a rate of increase of maximum load power. This is achieved because as described with reference to
FIG. 5
, the gap G is formed in the isolating converter transformer PIT to provide loose coupling at a required coupling coefficient, and thereby a saturated state is not readily obtained.
A base of the switching device Q
1
is connected to a positive electrode side of the smoothing capacitor Ci via a base current limiting resistance RB and a starting resistance RS, so that base current at the start of power supply is taken from a line of the rectified and smoothed voltage. A clamp diode DD inserted between the base of the switching device Q
1
and a primary-side ground forms a path of clamp current that flows during the off period of the switching device Q
1
. The collector of the switching device Q
1
is connected to one end of the primary winding N
1
of the isolating converter transformer PIT, while an emitter of the switching device Q
1
is grounded.
A parallel resonant capacitor Cr is connected in parallel with the collector and emitter of the switching device Q
1
. Also in this case, capacitance of the parallel resonant capacitor Cr and leakage inductance L
1
of the primary winding N
1
side of the isolating converter transformer PIT form a primary-side parallel resonant circuit of the voltage resonance type converter.
An orthogonal type control transformer PRT shown in
FIG. 5
is a saturable reactor provided with the resonance current detecting winding ND, a driving winding NB, and a control winding NC. The orthogonal type control transformer PRT is provided to drive the switching device Q
1
and effect control for constant voltage. The structure of the orthogonal type control transformer PRT is a cubic core, not shown in the figure, formed by connecting two cores each having four magnetic legs with each other a
Berhane Adolf Deneke
Maioli Jay H.
Sony Corporation
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