Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2002-01-03
2003-03-11
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S056120, C363S088000
Reexamination Certificate
active
06532160
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a switching power supply. In particular, the present invention relates to a switching power supply having a transformer which includes an input winding and an output winding, wherein a DC input voltage supplied to the input winding of the transformer is switched by a switching circuit.
BACKGROUND ART
Heretofore, various types of switching power supplies have been proposed and put to practical use. In a dominant type among them, an inputted DC voltage is switched by a switching operation of a switching circuit connected to an input winding of a power conversion transformer and the resulting switching output is taken out to an output winding of the power conversion transformer. A voltage appearing at the output winding based on the switching operation of the switching circuit is rectified by a rectifier circuit and then converted into DC by a smoothing circuit to be output.
In the switching power supply, an output rectifier diode is connected in series with a power transfer line. Thus, reducing the loss arising from this output rectifier diode may effectively contribute to an enhanced efficiency of the switching power supply.
The reduced loss of the output rectifier diode may be simply obtained by use of a diode having a low forward voltage drop. However, the diode having a low forward voltage drop involves in an insufficiently low reverse withstand voltage. Thus, particularly when the diode having a low forward voltage drop is used as the output rectifier diode, it is necessary to restrain the reverse voltage.
In this kind of switching power supplies, a most important consideration on the reverse voltage is a surge voltage arising from parasitic elements based on the switching operation of the switching circuit. The surge voltage is applied as a reverse voltage to the output rectifier diode. A snubber circuit has been known as means for restraining the surge voltage applied to the output rectifier diode. One conventional snubber circuit is described in
FIG. 3
of Japanese Patent Laid-Open Publication No. Hei 6-54531 as a prior art. This known snubber circuit comprises a series circuit of a capacitor and a resistance wherein the series circuit is connected in parallel with an output rectifier diode. In order to restrain the surge voltage, this snubber circuit is adapted to consume unnecessary energy including the surge voltage by the resistance through the capacitor.
However, this conventional snubber circuit may not sufficiently restrain the surge voltage applied to the output rectifier diode when it is turned off. Thus, it is undesirably forced to use an output rectifier diode having a high withstand voltage. In addition, since the conventional snubber circuit is configured to consume the energy of the surge voltage by the resistance, an associated resistor is increased in size, resulting in undesirable heat and power loss in the resistor. Thus, this conventional snubber circuit involves a drawback in achieving a downsized and highly efficient switching power supply.
The aforementioned Japanese Patent Laid-Open Publication No. Hei 6-54531 also shows a snubber circuit using no resistor in
FIG. 1
thereof. This snubber circuit disclosed in this Patent Laid-Open Publication comprises a series circuit composed of a capacitor and a diode and connected to both ends of an output rectifier diode, and an inductor connected between a connection point, at which the capacitor is connected to the diode, and an output side of an output smoothing choke coil. According to the snubber circuit having the above construction, unnecessary surge energy may be absorbed by the capacitor, and the absorbed energy may be regenerated at the output side of the switching power supply through the inductor.
However, in the aforementioned conventional snubber circuit, one end of the inductor is connected to the output side of the choke coil. Accordingly, the series circuit composed of the diode in the snubber circuit and the inductor is connected in parallel with the output smoothing choke coil. Thus, when the switching circuit is turned on, the current rectified by the rectifier circuit flows into not only the choke coil but also the diode of the snubber circuit and the inductor which are connected in parallel with the choke coil. This causes the loss due to a forward voltage drop in the diode of the switching circuit and the loss due to the DC resistance of the inductor. Further, this Patent Laid-Open Publication discloses the technique applicable only for single-diode type forward converter, and does not disclose any application to the switching power supply having a center tap type rectifier circuit. Assuming that the technique described in this prior art is employed in the switching power supply having a center tap type rectifier circuit as-is with departing from the disclosure therein, the demand for downsizing may not be satisfied due to the resultingly increased number of components.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a switching power supply having an enhanced efficiency yielded by reducing the loss due to a diode and inductor which form a snubber circuit.
It is another object of the present invention to provide a switching power supply capable of employing a small size, low power element to form a snubber circuit.
It is still another object of the present invention to provide a highly efficient, low noise, small size switching power supply, in a switching power supply having a center tap type rectifier circuit.
It is yet another object of the present invention to provide a highly efficient switching power supply capable of employing an output rectifier diode having a low forward voltage drop, in a switching power supply having a center tap type rectifier circuit.
It is yet still another object of the present invention to provide a switching power supply including a highly efficient, low noise snubber circuit capable of sufficiently restraining a surge voltage applied to a output rectifier diode, in a switching power supply having a center tap type rectifier circuit.
It is another further object of the present invention to provide a switching power supply including a low-energy-consumption, highly efficient snubber circuit, in a switching power supply having a center tap type rectifier circuit.
It is still a further object of the present invention to provide a highly efficient switching power supply having a snubber circuit capable of regenerating energy, in a switching power supply having a center tap type rectifier circuit.
It is additional object of the present invention to provide a small size switching power supply having a small number of components, in a switching power supply having a center tap type rectifier circuit.
In order to achieve the above and other objects, according to the present invention, there is provided a switching power supply comprising a transformer, a switching circuit, an output rectifier circuit, an output smoothing circuit, and a snubber circuit. The transformer includes an input winding and an output winding. The switching circuit switches a DC input voltage supplied through the input winding of the transformer. The output rectifier circuit includes at least one output rectifier diode. One electrode of the output rectifier diode is connected to one end of the output winding of the transformer. The input side of the output smoothing circuit is connected to the other electrode of the output rectifier diode.
The snubber circuit includes a snubber capacitor, a snubber diode, and a snubber inductor. The snubber capacitor and the snubber diode are connected with each other at each one end thereof. The other end of the snubber capacitor is led to the output winding, while the other end of the snubber diode is led to the other of the electrode of the output rectifier diode. One end of the snubber inductor is connected to a connection point between the snubber capacitor and the snubber diode, while the other end of the snubber inductor is connected to the input side of the output s
Hirokawa Masahiko
Murai Yasuhiro
Yamada Tomomi
Cohen & Pontani, Lieberman & Pavane
Han Jessica
TDK Corporation
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