Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-10-15
2002-10-08
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C070S071000
Reexamination Certificate
active
06462965
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates a switching regulator, especially a switching regulator which comprises a synchronous rectifier circuit that performs synchronous rectification with a main switching element by using rectifying switching elements as rectifier elements.
2. Description of the Related Art
In this type of switching regulators, relatively large loss is normally caused by the forward voltage of rectifier diodes if such rectifier diodes are used in a rectifier circuit for rectifying a voltage induced in a secondary winding of a transformer. As one representative of prior art for overcoming such drawback, a synchronous rectifier circuit is known in which rectifying switching elements such as Field Effect Transistors (MOS FET), for example, are used as a substitute for the rectifier diodes.
In Japanese un-examined patent publication number 7-115766, for example, is disclosed a flyback type switching regulator, using a certain synchronous rectifier circuit in which when the turn-off of a main switching element is followed by the current flowing through a current transformer via a body diode of a rectifying switching element that comprises MO-SFETs, a voltage is generated according to the current flowing through a resistor connected to a secondary winding of the current transformer, and thus an output of a comparator comparing the voltage with a reference voltage becomes “H”(High) level so that the rectifying switching element turns on through a drive circuit; and thereafter, energy stored in the transformer is transferred to an output side and the current flowing through the rectifying switching element becomes zero so that the output of the comparator becomes “L”(Low) level and the rectifying switching element is made to turn off. In this case, when the current ceases to flow through the rectifying switching element, a control signal to turn off the rectifying switching element is given, whereby the polarity of voltage across an auxiliary winding is reversed so that a signal to make the main switching element turn on is generated.
Also in Japanese registered patent publication number 2976180 is disclosed a synchronous rectifier circuit wherein a current flowing through a rectifying switching element is detected by a current transformer so that the rectifying switching element may be turned on or off. More specifically, the synchronous rectifier circuit is a circuit where a primary winding of a current transformer is connected in series to a parallel circuit of diode and MOS FET that is a rectifying switching element; a resistor is connected in parallel to the secondary winding of the current transformer; and a buffer amplifier lowering an output impedance of the current transformer is connected to a path between the resistor and the gate of the MOS-FET, whereby when the exciting energy of the transformer that comprises a flyback converter finishes discharging the energy as an electrical energy, the polarity of the output signal of the current transformer is reversed and the rectifying switching element is turned off; and thus, a reverse current is prevented from flowing when an output current becomes so small that a period of the current interruption is produced, so that the regulator efficiency is improved.
As is apparent from the foregoing, the above-mentioned synchronous rectifier circuit is one where the rectifying switching element is not only turned on synchronously with the main switching element but also is forcedly turned off in order to block the reverse current flowing through the rectifying switching element.
In the above-mentioned conventional circuits, however, higher efficiency cannot be attained as the rectifying switching element is turned on after detecting the current flowing through the current transformer and thus the rise time is too slow. Further, whilst the above-mentioned registered patent publication number 2976180 teaches that an output voltage is applied to the gate of the MOS-FET via transistors that construct a part of a buffer amplifier when a current flows through the current transformer, it is not possible to drive the rectifying switching element by lower output voltages such as DC 2 V and 3.3 V when such lower output voltages are strongly requested recently.
Also, in the case of a switching regulator in which component regulators including such synchronous rectifier circuit perform a parallel running, the unbalance in the output currents between the component regulators has a serious effect on the regulator as a whole. That is: as a MOS-FET serving as a rectifying switching element has a characteristic that a current is permitted to flow bidirectionally between a drain and a source (bidirectional conductibility), the other circuitry elements in the regulator are likely to be damaged by the reverse current flowing through the rectifying switching element from other component regulators while the rectifying switching element is on.
With regard to the problem, in the Japanese un-examined patent publication number 7-75336 is proposed a regulator wherein even though a higher voltage than an output voltage of a certain regulator itself is applied from output terminals of other regulators, a main switching element is switched by on-pulse of minimum width in each cycle, thus preventing the continuous flow of direct current over two cycles in the rectifying switching element. In this case, however, there must be provided an additional function to keep the main switching element “on” by on-pulse of a minimum width under any condition, thus leading to likelihood that the circuit topology becomes too complicated.
SUMMARY OF THE INVENTION
To eliminate the above-mentioned problem, it is, therefore, an object of the present invention to provide a switching power supply that can operate by lower voltage, while preventing a reverse current from flowing.
It is another object of the present invention to provide a switching power supply with a simple circuit topology that can prevent harmful effects caused by the reverse currents flowing from other component regulators in a parallel running.
According to a first aspect of the present invention, a switching power supply has a rectifying switching element used as a rectifier element that is connected to a secondary winding of a transformer, and includes a synchronous rectifier circuit. The switching power supply also includes a current transformer for detecting a current flowing through the rectifying switching element, with an auxiliary winding wound in the transformer. A drive circuit is included for feeding a voltage generated in the auxiliary winding to the rectifying switching element as an “on” signal. A turn-off circuit is also included for turning off the rectifying switching element before the current flowing through the rectifying switching element reverses, following a detection signal generated in a secondary winding of the current transformer.
When a current starts flowing through the rectifying switching element, the rectifying switching element is built up abruptly by the voltage that is induced. The current is not induced in the current transformer due to it having a high impedance but in the auxiliary winding which has a low impedance, so that the rectifying switching element turns on immediately when the current approaches close to a maximum value. Consequently, the resistance of the rectifying switching element in an “on” state can be reduced as compared with that in the conventional drive circuit by a current transformer so that not only the rectifying switching element but also the power supply itself can have improved efficiency.
Further, as the voltage induced in the auxiliary winding is fed directly to the rectifying switching element as a drive signal, voltage sufficient to turn on the rectifying switching element can be fed merely by suitably changing the number of turns of the auxiliary winding without being influenced by the output voltage so that the needs for a lower output voltage in power supplies can be met. Besides, the re
Akerman & Senterfitt
Densei-Lambda Kabushiki Kaisha
Riley Shawn
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