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 Ql 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 Ql of one transistor. In this case, the switching element Ql consists of a high

withstand voltage bipolar transistor (BJT: junction
transistor).

The base of the switching element Ql 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 Ql and a primary-side ground.
A path of a clamp current flowing during the off-time of the switching element Ql is formed by a clamp diode DD inserted between the base of the switching element Ql and the negative terminal (primary-side ground) of the smoothing capacitor Ci. Meanwhile, the collector of the switching element Ql is connected to one end of the primary winding Nl 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 Ql. 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 LI of the primary winding Nl 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 Ql, 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 Ql 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 Nl of the insulating converter transformer PIT, so that the switching output of the switching element Ql is transferred to the detection coil ND via the primary winding Nl. 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 Ql via the base current limiting resistor RB. Consequently, the switching element Ql 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 Ql to the secondary side.
As shown in Fig. 8, the insulating converter transformer PIT has an EE-shaped core where E-shaped cores CR1 and CR2 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 Nl and the secondary windings N2 (and N2A) 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 CR1 and CR2 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 Nl and the secondary windings N2 (and N2A) coiled around the split bobbin B of the insulating converter transformer PIT.
Fig. 10 is a diagram typically showing how the primary winding Nl and the secondary windings N2 (and
N2A) are coiled around the split bobbin B.
The split bobbin B has split areas for coiling the primary winding Nl and the secondary windings N2 (and N2A) respectively. This diagram represents an example where the primary winding Nl coiled around the split bobbin B has an intra-bobbin winding width Kl, and the secondary windings N2 (and N2A) coiled around the split bobbin B have an intra-bobbin winding width K2.
In this case, the primary winding Nl is coiled in a fixed direction from a predetermined start position N1S. And when the primary winding Nl thus coiled has reached the end of the intra-bobbin winding width Kl, it is coiled on the preceding primary winding Nl in the reverse direction with respect to the preceding primary winding Nl. The primary winding Nl is coiled in this manner until an end position N1E of the primary winding Nl after a predetermined number of turns thereof.
In Fig. 10, there is typically shown an example where the start position N1S of the primary winding Nl is at the center (lower portion) of the split bobbin B and is spaced apart most (left position) from the secondary winding N2 . The primary winding Nl is coiled from the start position N1S while its direction is alternately changed as rightward → leftward → rightward → ... up to
the end position N1E which is at the left of the outside (upper portion) of the split bobbin B.
Similarly to the primary winding Nl mentioned above, each of the secondary windings N2 (and N2A) is coiled from a predetermined start position N2S within the intra-bobbin winding width K2 by a predetermined number of turns while the direction thereof is changed alternately. In this case, however, the secondary winding N2 is formed to partially serve as the secondary winding N2A as will be described later, so that the secondary winding N2 is once led out as a center tap CT, and then the secondary winding N2A is coiled thereon.
In this case where the output level of the secondary winding N2A is, for example, about 1/10 of the output level of the secondary winding N2, the number of turns of the secondary winding N2A is approximately 1/6 to 1/10 of that of the secondary winding N2, and therefore it is less than 5 turns.
The example of Fig. 10 typically shows that the secondary winding N2 coiled from the start position N1S is led out as a center tap CT at the end position N2E, and then is coiled, as the secondary winding N2A, from its start position N2AS up to its end position N2AE.
The ends of the primary winding Nl and secondary
windings N2 (and N2A) led out respectively from the start positions N1S, N2S and the end positions N1E, N2AE, and the center tap CT thereof, are bound and soldered respectively to predetermined pin terminals P, P ... provided on, e.g., the top surface of the split bobbin B. As shown in Fig. 11, the primary winding Nl coiled within the intra-bobbin winding width Kl of the split bobbin B and the secondary windings N2 (and N2A) coiled within the intra-bobbin winding width K2 are arrayed without any gap, i.e., in a state where the mutually adjacent primary windings Nl are in contact with each other.
One end of the primary winding Nl of the insulating converter tremsformer PIT is connected to the collector of the switching element Ql, while the other end thereof is connected to the positive terminal (rectified smoothed voltage Ei) of the smoothing capacitor Ci via a series connection of the detection coil ND, as shown in the diagram.
On the secondary side of the insulating converter transformer PIT, an alternating voltage induced by the primary winding Nl is generated in the secondary winding N2. In this case, since a secondary parallel resonance capacitor C2 is connected in parallel to the secondary
winding N2, a parallel resonance circuit is formed by leakage inductances L2 (and L2A) of the secondary windings N2 (and N2A) and a capacitance of the secondary parallel resonance capacitor C2. Due to this parallel resonance circuit, the alternating voltage generated in the secondary winding N2 (and N2A) is obtained as a resonance voltage. That is, the operation on the secondary side is performed in a voltage resonance mode.
More specifically, this power circuit has, on its primary side, a parallel resonance circuit to execute the switching operation in a voltage resonance mode, and also has, on its secondary side, another parallel resonance circuit to perform half-wave rectification (voltage resonance operation). In this specification, the switching converter of a configuration equipped with resonance circuits on its primary and secondary sides as mentioned above will be referred to as "composite resonance type switching converter".
In the secondary parallel resonance circuit formed as described, a center tap is provided for the secondary winding N2, and rectifying diodes D01, D02 and smoothing capacitors C01, C02 are connected as shown in the diagram, thereby providing a first half-wave rectifying circuit 2 which consists of a combination of "rectifying diode D01
and smoothing capacitor C01", and a second half-wave rectifying circuit 3 which consists of a combination of "rectifying diode D02 and smoothing capacitor C02".
The first half-wave rectifying circuit 2 receives an input resonance voltage supplied from the secondary parallel resonance circuit and generates a DC output voltage E01. Similarly, the second half-wave rectifying circuit 3 receives an input resonance voltage supplied from the secondary parallel resonance circuit and generates a DC output voltage E02.
In this case, the DC output voltage E01 and the DC output voltage E02 are branched and delivered to the control circuit 1 as well. In the control circuit 1, the DC output voltage E01 is used as a detection voltage, and the DC output voltage E02 is used as an operating power for the control circuit 1.
In the insulating converter transformer PIT, the mutual inductance M regarding the inductance L1 of the primary winding Nl and the inductances L2 and L2A of the secondary windings N2 and N2A becomes either +M or -M depending on the relation of the polarities (winding directions) of the primary winding Nl and the secondary windings N2 and N2A to the connection of the rectifying diodes DO (D01, D02) .
For example, if the connection is in a state of Fig, 9A, the mutual inductance becomes +M. Meanwhile, if the connection is in a state of Fig. 9B, the mutual inductance becomes -M.
Applying the above to the secondary operation of the power circuit shown in Fig. 7, when the alternating voltage obtained in the secondary winding N2 is positive for example in the first half-wave rectifying circuit 2, it is supposed that the operation with the rectified current flowing in the rectifying diode D01 is performed in the +M (forward) mode. Meanwhile, when the alternating voltage obtained in the secondary winding N2 is negative contrary to the above, the rectifying diode DO is turned off so that none of the rectified current flows therein. That is, in this power circuit, the mutual inductance relative to the primary winding Nl and the secondary winding N2 executes rectification in the + M mode.
In this configuration, the power is supplied to the load increased by the action of the primary parallel resonance circuit and the secondary parallel resonance circuit, so that the power supplied to the load is also increased correspondingly thereto to consequently enhance the increase rate of the maximum load power.
Such correspondence to the load condition can be
realized due to the improved situation where a saturated state is not reached readily because of the loose coupling attained by a required coupling coefficient with the gap G formed in the insulating converter transformer PIT, as explained previously with reference to Fig. 8. For example, in case the gap G is not existent in the insulating converter transformer PIT, the operation will be abnormal with a high probability as the insulating converter transformer PIT is placed in its saturated state during the flyback, whereby proper execution of the aforementioned half-wave rectification is rendered considerably difficult.
In the control circuit 1, the level of the control current (DC) caused to flow in the control coil NC is varied in accordance with a change of the secondary DC output voltage level E01, thereby varying under control the inductance LB of the driving coil NB wound in the orthogonal control transformer PRT. Consequently, such action changes the resonance condition of the series resonance circuit in the self-excited oscillation driving circuit formed for the switching element Ql inclusive of the inductance LB of the driving coil NB. As will be described next with reference to Fig. 7, this operation varies the switching frequency of the switching element
Ql to eventually stabilize the secondary DC output voltage E01.
When the switching frequency is varied in Fig. 7 where there is provided the orthogonal control transformer PRT of a structure capable of varying under control the inductance LB of the driving coil NB, the on-time TON of the switching element Ql is varied under control while the off-time TOFF thereof is maintained fixed. That is, in this power circuit, the constant voltage control is performed to vary the switching frequency under control to thereby control the resonance impedance with regard to the switching output, and simultaneously the conduction angle control (PWM control) of the switching element in the switching period is also performed. Such composite control operation is realized in a single set of control circuitry.
More specifically, in this power circuit, the secondary DC output voltage E01 delivered from the first half-wave rectifying circuit 2 is supplied as a detection voltage to the control circuit 1 to thereby vary under control the resonance voltage level obtained from the secondary winding N2 of the insulating converter transformer PIT, hence keeping constant the secondary DC output voltage E01. However, no action is executed to
keep constant the secondary DC output voltage E02 which is delivered from the second half-wave rectifying circuit 3 and is supplied as an operating voltage to the control circuit 1.
Figs. 12A to 12F graphically represent examples of operating waveforms in individual sections of the power circuit shown in Fig. 7, wherein the secondary output waveform is shown principally.
The output waveforms in Figs. 12A to 12F are obtained when the power circuit of Fig. 7 is so formed as to achieve the optimal driving conditions with the maximum load power POMAX = 217W. In an exemplary case where the alternating input voltage VAC is 100V, the insulating converting transformer PIT is so constructed that, for obtaining 135V of the regulated secondary DC output voltage E01 and 15V of the non-regulated secondary DC output voltage E02, 38 turns of coils are provided as the secondary winding N2, and 5 turns of coils are provided as the secondary winding N2A.
In such circuit configuration, the switching element Ql performs its switching operation with the series resonance circuit (NB, CB) serving as a self-excited oscillation driving circuit, so thas obtt a primary parallel resonance voltage Vcp of Fig. 12A iained
across the parallel connection circuit of the switching element Ql and the parallel resonance capacitor Cr by the action of the parallel resonance circuit. As shown in the diagram, this parallel resonance voltage Vcp has a waveform being at a zero level during the on-time TON of the switching element Ql and becoming a sine-wave pulse during the off-time TOFF thereof, correspondingly to the operation in a voltage resonance mode.
The switching output is transferred to the secondary side of the insulating converter transformer PIT by the on/off operation of the switching element Ql, so that a secondary resonance current 12 of a waveform shown in Fig. 12B flows in the connection end of the secondary winding N2 of the insulating converter transformer PIT and the rectifying diode D01, whereby a secondary resonance voltage V2 of a waveform shown in Fig. 12C is generated between such connection end and the secondary-side ground.
In this case, the rectifying diode D01 is turned on when the secondary resonance voltage V2 of Fig. 12C becomes higher than the level of the secondary DC output voltage E01, so that a secondary rectified current 13 of a waveform shown in Fig. 12D flows into the rectifying diode D01.
Meanwhile a secondary resonance voltage V3 of a waveform shown in Fig. 12E is generated across the secondary winding N2A. In this case, the waveform of the secondary resonance voltage V3 is similar to that of the secondary resonance voltage V2 shown in Fig. 12C, and the similarity ratio therebetween is equivalent to the ratio between the total number of coils (38 turns) of the secondary winding N2 and the number of coils (5 turns) of the secondary winding N2A. That is, the voltage V3 becomes, for example, 5/38 times the secondary resonance voltage V2.
In this case also, the rectifying diode D02 shown in Fig. 7 is turned on when the secondary resonance voltage V3 becomes higher than the level of the secondary DC output voltage E02, so that a secondary rectified current 14 flows during a period (6 Msec) shown in Fig. 12F, and its peak value is 4.5Ap for example.
The operating waveform of the secondary resonance voltage V2 shown in Fig. 12C arid the operating waveform of the secondary resonance voltage V3 shown in Fig. 12E are similar to each other, but the secondary rectified current 13 of Fig. 12D flowing in the rectifying diode D01 and the secondary rectified current 14 of Fig. 12F flowing in the rectifying diode D02 are different in
waveform from each other.
The above is supposed to result from that the secondary winding N2A coiled around the split bobbin B of the insulating converter transformer PIT is in a state of loose coupling to the primary winding Nl and the secondary winding N2. Since the number of turns of the secondary winding N2A is less than 5 as mentioned, in case the secondary winding N2A is coiled in alignment within the intra-bobbin winding width K2 of the split bobbin B, the secondary winding N2A is coiled in a partially unbalanced state such as on the right side alone of the intra-bobbin winding width K2, as shown in Fig. 11 for example.
If the secondary winding N2A is coiled, as described above, in a partially unbalanced state to the intra-bobbin winding width K2 of the split bobbin B where the secondary winding N2 is to be coiled, when the load power supplied by the non-regulated secondary DC output voltage E02 exceeds 10W for example, the output current obtained from the secondary winding N2A is a combination of the +M operation mode (forward converter operation) and the -M operation mode (flyback converter operation) superposed thereon.
As a result, the secondary rectified current 13 is
superposed on the secondary rectified current 14, and therefore the waveform of the secondary rectified current 14 becomes such that, as shown in Fig. 12F, its peak value is in the latter half of the conduction angle of the rectifying diode D02 .
Thus, the heat generated in the rectifying diode D02 increases in accordance with a rise of the peak value of the secondary rectified current 14 flowing in the rectifying diode D02, hence increasing the power loss in the rectifying diode D02 and causing impairment of the reliability due to the temperature rise derived from the generated heat.
Fig. 13 graphically shows the relationship between the secondary DC output voltages E01, E02 and the load current IL2 supplied from the second half-wave rectifying circuit 3 in the power circuit of Fig. 7.
In Fig. 13, a straight line denoted by a white circle "O" represents the relationship between the secondary DC output voltage E02 and the load current IL2 in case the power circuit of Fig. 7 is so formed as to attain optimal driving conditions with the maximum load power POMAX = 217W.
And a waveform denoted by a white triangle "" represents the relationship between the secondary DC
output voltage E02 and the load current IL2 in case the power circuit of Fig. 7 is so formed as to attain optimal driving conditions with the maximum load power POMAX = 82W.
As obvious from Fig. 13, when the load current IL2 is changed from OA to 1.OA in the power circuit formed correspondingly to POMAX = 217W, the voltage variation
level AE02 of the secondary DC output voltage E02 becomes approximately 6.6V.
Similarly, when the load current IL2 is changed from OA to 1.OA in the power circuit formed correspondingly to POMAX = 82W, the voltage variation
level AE02 of the secondary DC output voltage E02 becomes approximately 6.4V. That is, in any of the above circuit configurations, the secondary DC output voltage level E02 is varied widely in response to the variation of the load current IL2 outputted from the second half-wave rectifying circuit 3, hence deteriorating the cross regulation.
Therefore, in a configuration where a 12V local regulator for example is connected to the output of the second half-wave rectifying circuit 3 and a regulated constant voltage is to be obtained from such a local regulator, it is necessary to maintain the secondary DC
output voltage E02 above a predetermined level even when the secondary DC output voltage E02 has lowered in response to an increase of the load current IL2. In this case, however, the secondary DC output voltage E02 rises with a level reduction of the load current IL2, thereby increasing the power loss in the local regulator in response to a rise of the voltage level.
Also in the case of any change in the maximum load power PO of the load which is supplied with the secondary DC output voltage E01 delivered from the first half-wave rectifying circuit 2, there occurs a variation in the level of the secondary DC output voltage E02 delivered from the second half-wave rectifying circuit 3.
As shown in Fig. 13 for example, the relationship between the level of the secondary DC output voltage E02 and the load current IL2 is different depending on whether the maximum load power Po = 217W or PO = 82W of the load supplied with the secondary DC output voltage E01. This signifies that the level of the secondary DC output voltage E02 is varied even in response to any variation of the load connected to the regulated secondary DC output voltage E01, and it has been customary heretofore that cross regulation is deteriorated by such level variation of the secondary DC
output voltage E02 .
SUMMARY OF THE INVENTION
An object of the present invention is to provide a switching power circuit and an insulating converter transformer which are capable of reducing the power loss on a secondary sidev
To achieve the above object, according to a first aspect of the present invention, there is provided a switching power circuit having the following configuration. That is, the switching power circuit includes a rectifying and smoothing means for generating a rectified smoothed voltage out of an input commercial AC power and outputting the same as a DC input voltage, and an insulating converter transformer where a primary winding is coiled on its primary side while at least a first secondary winding and a second secondary winding, whose number of turns is less than a predetermined number of turns, are coiled on its secondary side. The transformer has a gap in cores thereof to obtain a required coupling coefficient for loose coupling with regard to the primary winding and the first secondary winding, and serves to transfer the output of the primary side to the secondary side.
The switching power circuit also includes a switching means having a switching element to deliver the DC input voltage through on/off control thereof to the primary winding of the insulating converter transformer, and a primary resonance circuit for actuating the switching means in a resonance mode. The resonance circuit consists at least of a leakage inductance component including the primary winding of the insulating converter transformer, and a capacitance of a primary resonance capacitor.
The switching power circuit further includes a secondary resonance circuit consisting of a leakage inductance component of one secondary winding of the insulating converter transformer and the capacitance of the secondary resonance capacitor to thereby form a resonance circuit, wherein the secondary resonance capacitor is connected to at least one of the secondary windings of the insulating converter transformer, a first DC output voltage generating means so formed as to obtain a first secondary DC output voltage from the alternating voltage induced in the first secondary winding, a second DC output voltage generating means so formed as to obtain a second secondary DC output voltage from the alternating voltage induced in the second secondary winding, and a

Claim;
1. A switching power circuit comprising:
a rectifying and smoothing means for generating a rectified
smoothed voltage out of an input commercial AC power and
outputting the same as a DC input voltage;
an insulating converter transformer where a primary winding is
coiled on its primary side while at least a first secondary winding
and a second secondary winding, whose number of turns is less
than a predetermined number of turns, are coiled on its secondary
side, said transformer having a gap in cores thereof to obtain a
required coupling coefficient for loose coupling with regard to said
primary winding and said first secondary winding, and serving to
transfer the output of the primary side to the secondary side;
a switching means having a switching element to deliver the DC
input voltage through on/off control thereof to the primary winding
of said insulating converter transformer;
a primary resonance circuit for actuating said switching means in
a resonance mode, said primary resonance circuit consisting at
least of a leakage inductance component including the primary
winding of said insulating converter transformer, and a
capacitance of a primary resonance capacitor;
a secondary resonance circuit consisting of a leakage inductance
component of one secondary winding of said insulating
converter transformer and the capacitance of said secondary
resonance capacitor to thereby from a resonance circuit, wherein
said secondary resonance capacitor is connected to at least one of
the secondary windings of said insulating converter transformer;
a first DC output voltage generating means so formed as to obtain
a first secondary DC output voltage from the alternating voltage
induced in the first secondary winding;
a second DC output voltage generating means so formed as to
obtain a second secondary DC output voltage from the alternating
voltage induced in the second secondary winding; and
a constant voltage control means for executing constant voltage
control of the first secondary DC output voltage by varying the
switching frequency of said switching element in accordance with
the level of the first secondary DC output voltage.
Wherein the second secondary winding of said insulating converter
transformer is coiled to form tight coupling with respect to the
primary winding and said first secondary winding.
2. A switching power circuit as claimed in claim 1, wherein the said
second secondary winding of said insulating converter transformer
is coiled independently of the first secondary winding and around a
split bobbin where coiled areas of the windings are split on the
primary side and the secondary side, so that a fixed equal winding
pitch is kept between adjacent turns in the upper or lower coiled
portion of the first secondary winding.
3. A switching power circuit as claimed in claim 1, wherein the said
second secondary winding of said insulating converter transformer
is connected at one end thereof to the first secondary winding via a tap and is coiled around a split bobbin where coiled areas of the windings are split on the primary side and the secondary side, so that a fixed equal winding pitch is kept between adjacent turns in the upper or lower coiled portion of the first secondary winding.
4. A switching power circuit substantially as herein described with reference to and illustrated in the accompanying drawings.