Switching power circuit and insulating converter transformer

Details Switching power circuit and insulating converter transformer Switching power circuit and insulating converter transformer

2000-09-28

2002-06-25

Donovan, Lincoln (Department: 2832)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C336S170000, C336S180000

Reexamination Certificate

active

06411528

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an insulating converter transformer, and also to a switching power circuit equipped with an insulating converter transformer 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 resonance type converters have already been proposed by the present applicant. A resonance type 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 smaller number of component parts.
FIG. 7
is a circuit diagram showing an exemplary switching power circuit of a configuration based on the invention filed previously by the present applicant. This power circuit is equipped with a voltage resonance type converter which consists of a switching element Q
1
of one transistor and performs its self-excited switching operation in a single end form.
In the power circuit shown in this diagram, there is provided a full-wave rectifying circuit which comprises a bridge rectifying circuit Di and a smoothing capacitor Ci to serve as a rectifying and 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, e.g., one-fold level of the alternating input voltage VAC is generated. In this rectifying and smoothing circuit, a rush current limiting resistor Ri is inserted in its rectified current path so as to suppress a rush current which flows into the smoothing capacitor Ci when the power supply is turned on for example.
The voltage resonance type switching converter in this power circuit adopts a self-excited structure equipped with a switching element Q
1
of one transistor. In this case, the switching element Q
1
consists of a high withstand voltage bipolar transistor (BJT: junction transistor).
The base of the switching element Q
1
is connected to the positive side of the smoothing capacitor Ci (rectified smoothed voltage Ei) via a starting resistor RS, so that a base current at the start is obtained from a rectifying and smoothing line. And a series resonance circuit for self-excited oscillation driving, which consists of a series connection circuit of a driving coil NB, a resonance capacitor CB and a base current limiting resistor RB, is connected between the base of the switching element Q
1
and a primary-side ground.
A path of a clamp current flowing during the off-time of the switching element Q
1
is formed by a clamp diode DD inserted between the base of the switching element Q
1
and the negative terminal (primary-side ground) of the smoothing capacitor Ci. Meanwhile, the collector of the switching element Q
1
is connected to one end of the primary winding N
1
of an insulating converter transformer PIT, and the emitter thereof is grounded.
A parallel resonance capacitor Cr is connected in parallel to the collector-emitter of the switching element Q
1
. This parallel resonance capacitor Cr constitutes a primary parallel resonance circuit of the voltage resonance type converter by the self capacitance thereof and a leakage inductance L
1
of the primary winding N
1
of the undermentioned insulating converter transformer PIT. Although a detailed description is omitted here, a voltage Vcp obtained across the resonance capacitor Cr due to the action of this parallel resonance circuit is actually composed of a sine-wave pulse during the off-time of the switching element Q
1
, so that the operation is performed in a voltage resonance mode.
An orthogonal control transformer PRT shown in this diagram is a saturable reactor where a detection coil ND, a driving coil NB and a control coil NC are wound. This orthogonal transformer PRT is provided for driving the switching element Q
1
and also for executing constant voltage control.
In the structure of this orthogonal control transformer PRT, although not illustrated, two double U-shaped cores having four magnetic legs form a solid core where the ends of the respective magnetic legs are mutually joined. And a detection coil ND and a driving coil NB are wound around two predetermined magnetic legs of the solid core in the same direction, and further a control coil NC is wound orthogonally to the detection coil ND and the driving coil NB.
In this case, the detection coil ND of the orthogonal control transformer PRT is inserted in series between the positive terminal of the smoothing capacitor Ci and the primary winding N
1
of the insulating converter transformer PIT, so that the switching output of the switching element Q
1
is transferred to the detection coil ND via the primary winding N
1
. In the orthogonal control transformer PRT, the switching output obtained in the detection coil ND is induced in the driving coil NB through transformer coupling, hence generating an alternating voltage as a driving voltage in the driving coil NB. This driving voltage is delivered as a driving current from the series resonance circuit (NB, CB), which constitutes a self-excited oscillation driving circuit, to the base of the switching element Q
1
via the base current limiting resistor RB. Consequently, the switching element Q
1
performs its switching operation at a switching frequency determined by the resonance frequency of the series resonance circuit (NB, CB).
The insulating converter transformer PIT transfers 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 windings N
2
(and N
2
A) 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 loose 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 loose coupling, hence avoiding a saturated state correspondingly thereto.
Referring now to FIGS.
10
and
11
,.a description will be given on the primary winding N
1
and the secondary windings N
2
(and N
2
A) coiled around the split bobbin B of the insulating converter transformer PIT.
FIG. 10
is a diagram typically showing how the primary winding N
1
and the secondary windings N
2
(and N
2
A) are coiled around the split bobbin B.
The split bobbin B has split areas for coiling the primary winding N
1
and the secondary windings N
2
(and N
2
A) respectively. This diagram represents an example where the primary winding N
1
coiled around the split bobbin B has an intra-bobbin winding width K
1
, and the secondary windings N
2
(and N
2
A) coiled around the split bobbin B have an intra-bobbin winding width K
2
.
In this case, the primary winding N
1
is coiled in a fixed direction from a predetermined start position N
1
S. And when the primary winding N
1
thus coiled has reached the end of the intra-bobbin winding width K
1
, it is coiled on the preceding primary winding N
1
in the reverse direction with respect to the preceding pri

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