Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
2000-03-24
2001-02-13
Riley, Shawn (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
C323S289000
Reexamination Certificate
active
06188207
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply adopting a synchronous rectification method, and more particularly to a step-up switching power supply that achieves an improvement in the power conversion efficiency by reducing the loss at the rectifying device.
2. Discussion of Background
Switching power supplies, achieving a high degree of power conversion efficiency and can be constituted as compact units, are adopted in a wide range of applications - as power sources in various types of industrial apparatuses and consumer apparatuses including computers and office automation apparatuses. At present, as battery driven apparatuses such as portable information terminals and communication apparatuses are used by a greater than ever number of people, step - up switching power supplies need to achieve a further improvement in the efficiency.
As a means for meeting this requirement, switching power supplies adopting a synchronous rectification method have been proposed. Such switching power supplies adopt a synchronous rectification method and are developed based upon the principle that the saturation voltage between the collector and the emitter of a transistor in an ON state is lower than the forward descending voltage at a diode. The diode is normally used as a rectifying device in a rectifier circuit in the prior art. The switching power supply achieves an improvement in the switching source efficiency by replacing a diode with a transistor to reduce the power loss occurring at the rectifying device.
An example of a switching power supply adopting the synchronous rectification method is disclosed in Japanese Unexamined Patent Publication No. 289534/1996. The switching power supply disclosed in this publication is a step-up/step-down switching power supply adopting the synchronous rectification method. The switching power supply is provided with a choke coil to which a DC input voltage is applied, a switching element that cuts off the voltage intermittently, and a rectifier/smoothing circuit that rectifies and smooths the switching output. A synchronous rectifying device at the rectifier/smoothing circuit is constituted of a bipolar PNP transistor. The voltage manifesting at the two ends of the choke coil is used as a drive signal for the PNP transistor. Thus, compared to switching power supplies that employ a diode as the rectifying device, the power loss can be reduced. Furthermore, since the voltage manifesting at the two ends of the choke coil is directly utilized as a drive signal for the PNP transistor, an advantage is achieved in that the drive circuit is realized through a simple structure.
However, the saturation voltage between the collector and the emitter of a bipolar PNP transistor is high, at approximately 0.2V compared to the saturation voltage in a bipolar NPN transistor which is normally approximately 0.1V. Thus, it is difficult to make the most of the advantages of the bipolar transistor in a switching power supply adopting the synchronous rectification method that employs a PNP transistor for a current control element.
In addition, a bipolar high frequency power transistor, which is ideal in applications in this type of switching power supply, is constituted of silicon, and the relative difficulty in manufacturing PNP silicon transistors compared to NPN silicon transistors presents a stumbling block in achieving an inexpensive switching power supply.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an inexpensive switching power supply adopting a synchronous rectification method that achieves a higher degree of power conversion efficiency.
In order to achieve the object described above, the switching power supply according to the present invention comprises a choke coil, a switching element and a rectifier/smoothing circuit.
The choke coil is in series at one of a pair of source supply lines to which a DC input voltage is supplied. The switching element, which is connected between the pair of source supply lines, intermittently cuts off the DC input voltage applied to the choke coil.
The rectifier/smoothing circuit includes a synchronous rectifying device, a smoothing capacitor and a base drive circuit, and has a voltage input. This voltage is achieved by superimposing a flyback voltage onto the DC input voltage. The flyback voltage is generated at the choke coil while the switching element is in an OFF state.
The synchronous rectifying device is constituted of an NPN transistor. The smoothing capacitor has a first end and a second end, with the first end being connected to the synchronous rectifying device. The smoothing capacitor outputs a stepped up DC voltage at the first and second end of the smoothing capacitor.
The base drive circuit includes an auxiliary winding, a DC blocking capacitor and a reset circuit. The auxiliary winding is magnetically coupled with the choke coil. The DC blocking capacitor is provided between the auxiliary winding and the base of the NPN transistor.
The reset circuit is connected between the base of the NPN transistor and any one of the first end and the second end of the smoothing capacitor.
Since the switching power supply according to the present invention employs an NPN transistor as its synchronous rectifying device as described above, the advantage achieved through the low level of the saturation voltage between the collector and the emitter of the bipolar transistor can be made the most of to provide a switching power supply that achieves high power conversion efficiency. In addition, since the price of an NPN transistor is lower than that of a PNP transistor and a base current is supplied simply by adding one winding to the coil, an inexpensive switching power supply can be provided.
REFERENCES:
patent: 5691632 (1997-11-01), Otake
patent: 5929615 (1999-07-01), D'Angelo et al.
patent: 5994882 (1999-11-01), Ma
patent: 5-284734 (1993-10-01), None
patent: 8-289534 (1996-11-01), None
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Riley Shawn
TDK Corporation
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