Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-03-02
2002-04-02
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021160, C363S021020
Reexamination Certificate
active
06366480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply apparatus in which output produced by switching D.C. input on the primary side is rectified in a rectification circuit, and is supplied to a load, and more particularly, to an energy-storage type switching power supply apparatus in which a rectification diode and a switching element in parallel to the rectification diode are connected in the rectification circuit.
2. Description of the Related Art
Conventionally, for switching power supply apparatus which can satisfy demands for high quality and can be formed at relatively low expenditure, the RCC (ringing choke converter) as disclosed in (1) Japanese Unexamined Utility Model Publication No. 63-100993 has been employed. In such ringing choke converters and flyback converters, a diode is connected to a rectification circuit on the secondary side of a transformer. That is, when a switching element on the primary side of the transformer is on, input voltage is applied to the primary winding, allowing current to flow, so that energy is stored in the transformer. When the switching element is off, the energy stored in the transformer is released from the secondary winding in the form of current. The current is rectified and smoothed to obtain an output voltage. The output voltage is stabilized by control of the on-time of the switching element.
(2) A switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2-261053 has the following constitution. A switching means is connected in parallel to a rectification diode on the secondary side of a transformer. An output voltage obtained by rectification in the rectification circuit and smoothing is applied via the switching means to the secondary winding of the transformer, and energy is regenerated in the primary.
(3) In a switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 9-271167, a synchronous rectifier comprising a MOSFET is connected to a rectification circuit on the secondary side of a transformer. When energy stored in the transformer is released in the form of current, the MOSFET is turned on, whereby the rectification loss is reduced. Moreover, the arrangement is such that the MOSFET is driven by means of a drive winding provided in the transformer.
However, the above-mentioned switching power supply apparatus of (1) to (3) have the following inconveniences.
(1) Japanese Unexamined Utility Model Publication No. 63-100993.
In the method using only the diode as the rectifying means, the forward voltage drop of the diode is large (about 0.6V). This causes the problem that a loss generated by the voltage drop reduces the efficiency and increases the temperature of the diode itself.
(2) Japanese Unexamined Patent Application Publication No. 2-261053.
In this switching power supply apparatus, rectification on the primary side of the transformer is controlled with the switching means connected in parallel to the diode, so that the output voltage is stabilized. Therefore, the magnetic flux change range is constant, irrespective of the output power, and the maximum exciting energy is stored in the transformer at all times. Thus, the conduction loss and the core loss of the transformer, and moreover, a loss caused by the regeneration of current is large, and especially, the efficiency at light-loading and non-loading is reduced. Moreover, described is an example, in which the current to be regenerated in the secondary winding is made constant, so that the magnetic flux amplitude at light-loading is decreased. In this case, it is necessary to provide a circuit for detecting current to be returned to the secondary winding, and the power loss caused by the detection circuit is problematic.
(3) Japanese Unexamined Patent Application Publication No. 9-271167.
In this switching power supply apparatus, the MOSFET, which is the synchronous rectifier, is driven only by the drive winding provided in the transformer. Accordingly, the on-time of the MOSFET is determined by the relation between the generation voltage in the drive winding and the threshold voltage at the gate of the MOSFET. In general, generation voltage in a drive winding is determined by a turns ratio, so that it is difficult for the generation voltage to have an optional value. Thus, adjustment of the on-time of the MOSFET is difficult. Moreover, the threshold voltage of MOSFET is not constant and is varied in some range in general. The on-time of the MOSFET is considerably changed, depending on the threshold voltage, and it is very difficult to determined the optimum on-time of the MOSFET. Furthermore, since the MOSFET is turned off by use of a voltage drop caused by spontaneous discharging of the gate terminal voltage of the MOSFET, the turn-off speed of the MOSFET is low, the switching loss is large, and the efficiency is low. The MOSFET is heated.
SUMMARY OF THE INVENTION
In view of the forgoing, the present invention has been devised. It is an object of the present invention to provide a switching power supply apparatus with which a high efficiency, high stabilization, and small-size and light-weight can be realized.
To solve the above-described problems, the switching power supply apparatus comprises a transformer having a primary winding and a secondary winding, a first switching element connected in series with the primary winding, a first control circuit for controlling the on-time of the first switching element whereby the output is controlled, and a rectification circuit for rectifying the output from the secondary winding, whereby input voltage is applied to the primary winding when the first switching element is on and causes current to flow therein so that energy is stored in the transformer, and the energy stored in the transformer is released as electric current from the secondary winding when the first switching element is off, and the current is rectified in the rectification circuit to obtain an output, wherein the rectification circuit comprises a rectification diode, a second switching element connected in parallel to the rectification diode, a second switching element drive winding provided in the transformer to generate a voltage which causes the second switching element to be on, and a second control circuit for turning on the second switching element with a voltage from the second switching element drive winding, and turning off the second switching element after a time determined by a predetermined time constant by use of a further switching element connected to the control terminal of the second switching element.
The switching power supply apparatus in accordance with the present invention is an RCC (ringing choke converter) type switching power supply apparatus, in which energy stored in the transformer when the first switching element is on is released in the form of current to the secondary side when the first switching element is turned off. That is, when the first switching element turns on, an input voltage is applied to the primary winding, allowing current to flow, so that energy is stored in the transformer. When the first switching element turns off, the energy stored in the transformer is released as electric current from the secondary winding. The current is rectified and smoothed to obtain an output voltage. In this case, when the first switching element is turned off, voltage is generated in the second switching element drive winding provided in the transformer, and the voltage is applied to the control terminal of the second switching element, so that the second switching element is turned on to conduct. Then, the current from the secondary winding flows through the rectification diode, and the second switching element connected in parallel to the rectification diode. However, since the second switching element is selected so that the voltage drop of the second switching element is smaller than that in the forward direction of the rectification diode, most of the output current flows in t
Hosotani Tatsuya
Okamoto Yasuji
Takemura Hiroshi
Murata Manufacturing Co.
Ostrolenk Faber Gerb & Soffen, LLP
Riley Shawn
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