Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-01-03
2001-11-20
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06320763
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switching power supply units, and more specifically, the invention relates to switching power supply units which supply DC stabilized voltages.
2. Description of the Related Art
FIG. 9
shows a view illustrating a switching power supply unit described in Japanese Unexamined Patent Application Publication No. 11-187664. First, referring to
FIG. 9
, a description will be given of the main structure of a conventional switching power supply unit. A first switching circuit S
1
is formed by a parallel circuit composed of a switching element Q
1
, a diode D
1
, and a capacitor C
1
, and a second switching circuit S
2
is formed by a parallel circuit composed of a switching element Q
2
, a diode D
2
, and a capacitor C
2
. The second switching circuit S
2
, the capacitor C, and an inductor L form a series resonance circuit. Control circuits
11
and
12
are disposed in such a manner that the first and second switching elements Q
1
and Q
2
are alternately switched on/off between periods during which both switching elements are turned off, and a rectifying element Ds is connected parallel to a capacitor Cs to place the period of a resonance produced by the capacitor Cs.
In the switching unit having such a structure, when the first switching element Q
1
is switched off, a voltage is generated at a bias winding T
4
as a driving winding of the second switching element, and the second switching element Q
2
is thereby turned on. Then, a control transistor is switched on in a specified time determined by a time constant circuit inside the control circuit
12
so as to turn off the second switching element Q
2
. In this situation, if the rectifying diode Ds on the secondary side is in a conducting state, at a timing in which the diode Ds is brought into a non-conducting state, and if the diode Ds is in a non-conducting state, at a timing in which the second switching element Q
2
is turned off, that is, at a timing in which the second switching element Q
2
and the rectifying diode Ds are brought into a non-conducting state, a voltage is generated at a bias winding T
3
as a driving winding of the first switching element Q
1
to turn on the first switching element Q
1
.
In this way, the first switching element Q
1
and the second switching element Q
2
are alternately turned on/off between the periods in which both switching elements Q
1
and S
2
are turned off, and, energy stored in the primary winding T
1
of the transformer T during a period in which the first switching element Q
1
is turned on is output as electrical energy from the secondary winding T
2
during a period in which the first switching element Q
1
is turned off. The output energy is rectified by the rectifying element Ds, and is smoothed by a smoothing capacitor Co. Then, a DC voltage Vo is applied to a load L
1
via a detection circuit
14
.
In the switching power supply unit having such a structure, a part of energy once stored in the primary winding or the inductor L during a period in which the first switching element Q
1
is turned on is stored in a resonance capacitor C on the primary side when the first switching element Q
1
is turned off, and then, the energy is fed back to an input voltage during a period in which the second switching element Q
2
is turned on. As a result, since a circulation current circulating in the circuit exists, a ratio of the circulation current with respect to an excitation current for supplying an output current increases more as the load becomes lighter, and under no load, only the circulation current exists. When the circulation current increases, conduction losses of the transformer T, and the first and second switching elements Q
1
and Q
2
increase, and the efficiency of the switching power supply thereby decreases.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a switching power supply unit in which such a feedback current is decreased, and, under light load and under no load, conduction losses of the transformer and the first and second switching elements are significantly reduced so that high efficiency of the switching power supply unit can be obtained and reduction of size and weight thereof can be achieved.
The switching power supply unit according to the present invention comprises: a transformer having a primary winding and a secondary winding; a capacitor; a first switching circuit connected in series with a series circuit of the primary winding and an input power, the first switching circuit comprising a parallel circuit of a first switching element, a first diode and a first capacitor; a second switching circuit connected in parallel with the primary winding, the second switching circuit comprising a parallel circuit of a second switching element, a second diode and a second capacitor; first and second switching controlling circuits connected to the first and second switching circuits, respectively for controlling the first and second switching elements so as to turn on/off alternately with interposing of a period in which the first and second switching elements turn off, a rectifying and smoothing circuit connected to the secondary winding; and a voltage detection and voltage reduction circuit having a control signal input terminal and connected to the rectifying and smoothing circuit, the voltage detection and voltage reduction circuit detecting an output voltage of the rectifying and smoothing circuit and outputting a feedback signal to the first switching controlling circuit such that an output voltage of the rectifying and smoothing circuit is stabilized at either a first voltage or a second voltage lower than the first voltage based on a signal applied to the control signal input terminal.
According to the present invention, by decreasing an output voltage at no load or at light load, the inclination of a reset current determined by the inductor of the secondary winding of the transformer and the output voltage is reduced and a feedback current can thereby be reduced. With this arrangement, conduction losses of the transformer and the switching elements can be greatly reduced, with the result that high efficiency, miniaturization, and weight reduction can be achieved.
In addition, since an operational mode can be changed by decreasing the output voltage on the secondary-side circuit, it is not necessary to control the switching elements by transmitting a control signal to the primary-side control circuit from the secondary side, as performed in conventional cases. Thus, reduction of size, weight, and cost by reducing the number of components can also be achieved.
It is preferable that at least one of the first and second switching elements is formed by a field-effect transistor. In the case, since the parasitic diode and parasitic capacitance of a field-effect transistor can be used as the diode and the capacitor forming the switching circuit, reduction of size, weight, and cost can be achieved by reducing the number of components.
The transformer may include a first driving winding for generating a voltage bringing the first switching element into conduction and a second driving winding for generating a voltage driving the second switching element to perform a self-excited oscillation.
In this case, since the self-excited oscillation of the switching circuit is performed by a voltage generated from each of the primary and secondary driving windings of the transformer, it is thereby unnecessary to dispose any oscillation circuit comprising an IC or the like. As a result, reduction of size, weight, and cost can be achieved by reducing the number of components.
The transformer may include a leakage inductor disposed between the primary winding and the secondary winding or an inductor connected in series to the transformer; and the inductor and the capacitor form a resonance circuit.
In this case, a resonance circuit is formed by both the inductor disposed between the primary and secondary windings of the transformer or the leakag
Murata Manufacturing Co. Ltd.
Nguyen Matthew
Ostrolenk Faber Gerb & Soffen, LLP
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