Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-10-18
2001-03-13
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S131000
Reexamination Certificate
active
06201713
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply unit having a main-switching element and one or more sub-switching elements, which perform ON/OFF operations in synchronism with the ON/OFF operations of the main-switching element or opposite to.
2. Description of the Related Art
In general, in electronic equipment such as electronic calculators and communication devices, a switching power supply unit has been widely used for supplying a stabilized DC voltage from a commercial AC power supply. Such a switching power supply unit may comprise in each circuit system such a forward converter or a flyback converter, in which a main-switching element, which is connected in series to a primary winding of a transformer, is repeatedly turned on/off, and then an input voltage is intermittently applied to the transformer so as to obtain a DC output through a rectifying/smoothing circuit connected to a secondary winding. In contrast with such switching power supply units, there are provided switching power supply units, in which improvement in circuit characteristics can be achieved by adding a circuit which includes a sub-switching element performing ON/OFF operations in synchronism with or opposite to the ON/OFF operations of a main-switching element.
A description will be given of a structure of a conventional switching power supply unit having such a sub-switching element, referring to the drawings.
First, a conventional unit disclosed in Japanese Unexamined Patent Publication No. 8-317647 will be illustrated, referring to FIG.
26
.
In this figure, numeral
50
represents a switching power supply unit, which has a partial-resonance converter circuit
51
and a drive circuit
52
. Of these components, the partial-resonance converter circuit
51
comprises capacitors C
51
, C
52
, C
53
and C
54
, diodes D
51
, D
52
and D
53
, a transformer T
51
, a main-switching element S
51
and a sub-switching element S
52
.
In addition, the drive circuit
52
comprises an output control circuit
53
, comparators
54
and
55
, an inverter
56
, an isolating circuit
57
, a triangle-wave oscillator
58
, a light-emitting-side photocoupler Pa, a light-receiving-side photocoupler Pb, a transistor Q
51
, and resistors R
51
, R
52
, and R
53
.
In the switching power supply unit having the above-described structure, the sub-switching element S
52
performs ON/OFF operations in reverse to the ON/OFF operations of the main-switching element S
51
.
Next, another conventional circuit disclosed in Japanese Unexamined Patent Application No. 8-37777 will be illustrated with reference to FIG.
27
.
In this figure, numeral
60
represents a switching power supply unit, in which an FET disposed on the secondary side of a transformer is used for rectification. This is referred to as the so-called synchronous rectification system. The switching power supply unit
60
comprises a transformer T
61
, an input capacitor C
61
, an FET Q
61
as a main-switching element, an FET Q
62
as a sub-switching element, similarly, an FET Q
63
as another sub-switching element, a choke coil L
61
, an output capacitor C
62
, a light-emitting-side photocoupler PA, a light-receiving-side photocoupler PB, comparators
61
,
62
and
63
, a triangle-wave oscillator
64
, isolating circuits
65
and
66
, an inverter
67
, a control circuit
68
, and a control-signal output circuit
69
. Of these constituent parts, the control-signal output circuit
69
comprises transistors Q
64
and Q
65
, and resistors R
61
to R
65
.
In the switching power supply unit
60
having the above-described structure, the FET Q
62
performs ON/OFF operations in synchronism with the ON/OFF operations of the FET Q
61
, whereas the FET Q
63
performs ON/OFF operations in reverse to the ON/OFF operations of the FET Q
61
.
In each of the above-described switching power supply units, however, the circuit driving the sub-switching element is formed of an IC. Additionally, since a ground level is different between the main-switching element and the sub-switching element, the installation of an IC and an isolating circuit comprising a photocoupler is necessary. As a result, the use of an IC, a pulse transformer, or the like, leads to complications in the circuit structure and increase in production cost. Furthermore, such an increase in the number of parts can be a hindrance to miniaturization and weight reduction.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a switching power supply unit capable of achieving reduction in cost, size and weight by simply forming a circuit for controlling a sub-switching element performing ON/OFF operations in synchronism with or oppositely to the ON/OFF operations of a main-switching element without using an IC and an isolating circuit.
To this end, the present invention provides a switching power supply unit capable of providing a DC output, which includes a DC power supply, a transformer having a primary winding and a secondary winding, a main-switching element for connecting in series to the primary winding, one or more sub-switching elements for performing ON/OFF operations in synchronism with or oppositely to the ON/OFF operations of the main-switching element a sub-switching-element drive winding for generating voltage turning the sub-switching element on, a switching unit for turning the sub-switching element off, and a time-constant circuit for controlling the switching unit.
Additionally, according to the present invention, the switching unit includes a transistor, whose emitter or collector is connected to the control terminal of the sub-switching element, whereas the base of the transistor is connected to the time-constant circuit.
Additionally, according to the present invention, the time-constant circuit includes a first impedance circuit and a first capacitor charged/discharged by voltage generated in the sub-switching-element drive winding.
Additionally, according to the present invention, the impedance value of the first impedance circuit changes in accordance with the DC output or in response to a signal.
Additionally, according to the present invention, the control terminal of the sub-switching element is connected to an end of the sub-switching-element drive winding through a second impedance circuit.
Additionally, according to the present invention, the second impedance circuit includes a second capacitor.
Furthermore, the above second impedance circuit includes an inductor.
Furthermore, the impedance value of the first or second impedance circuit varies with a direction of current flowing through the impedance circuit.
In addition, the switching power supply unit of the present invention further includes a voltage-stabilizing circuit for stabilizing voltage applied to the time-constant circuit.
In addition, the above-described voltage-stabilizing circuit has a Zener diode.
According to the switching power supply unit of the present invention, adjustment of a time constant of the time-constant circuit permits the ON time of the sub-switching element to be set arbitrarily. Moreover, the second impedance circuit permits the timing for turning on the sub-switching element to be adjusted. In this arrangement, both the main-switching element and the sub-switching element can be turned on/off by inserting a deadtime in which both of the switching elements are turned off between their on/off operations. There are thus no losses and no breakdowns of the elements caused when both of them are simultaneously turned on.
Furthermore, since the sub-switching element is driven by the voltage generated in the sub-switching-element drive winding of the transformer, the installation of an IC and an isolating circuit comprising photoelectric devices such as a pulse transformer and a photocoupler is not necessary. As a result, the number of components, size, weight, and production cost can be reduced.
Furthermore, when the transistor as the switching unit is turned on, the sub-switching element is rapidly turned off. This accel
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Sterrett Jeffrey
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