Current-fed dc/dc converter with multilevel transformer and...

Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter

Reexamination Certificate

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C323S255000

Reexamination Certificate

active

06317336

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application is related to U.S. patent application, Ser. No. 09/539,690, entitled “REGULATION CIRCUIT FOR A POWER CONVERTER AND METHOD OF OPERATION THEREOF,” filed concurrently herewith, commonly assigned with the present application and incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power converters and, more specifically, to a regulation circuit for a current-fed converter, a method of operating the regulation circuit and a power converter employing the circuit or the method.
BACKGROUND OF THE INVENTION
A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC/DC converters are frequently employed to advantage. DC/DC converters generally include an inverter, a transformer, a rectifier on a secondary side of the transformer and an output filter. The inverter generally includes switching devices (power switches), such as field effect transistors (FETs), that convert a DC input voltage to an AC voltage. The transformer transforms the AC voltage to another value, and the rectifier generates the desired DC voltage to the output filter. Conventionally, the rectifier comprises a plurality of rectifying diodes that conduct the load current only when forward-biased in response to the input waveform to the rectifier. Finally, the output filter eliminates large fluctuations in the output voltage to provide an essentially constant DC voltage at the output of the converter.
A conventional current-fed DC/DC converter is often used for off-line power factor correction and battery applications due to its voltage step-up capability and its continuous input current characteristics. This conventional current-fed converter typically employs two power switches coupled to equal and opposite polarity windings on the primary side of a transformer. A traditional form of power switch control operates the two power switches in a 180 degree interleaved fashion with a duty cycle equal to or greater than 50 percent. When both power switches are on, the input voltage is applied to an input inductor causing its current to increase. Then, when one of the power switches is turned off, the current in the input inductor continues to flow through the connected primary winding thereby transferring power to an output load.
This causes the conventional current-fed converter to transfer power only during a portion of a switching cycle, which becomes even smaller as the input voltage becomes smaller. This characteristic of the conventional current-fed converter causes the output current to be discontinuous and to pulsate strongly. A discontinuous and strongly pulsating output current greatly increases the difficulty in designing both the transformer and the output filter, typically forcing them to be physically larger. Additionally, overall converter efficiency is also lower due to the discontinuous operating characteristics.
Accordingly, what is needed in the art is a way to improve the operating characteristics, overall efficiency of a current-fed converter, while maintaining or reducing component size at the same time.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides a regulation circuit for a current-fed power converter having a power switch adapted to transfer power to a transformer. In one embodiment, the regulation circuit includes a switching network coupled to a tapped winding of the transformer and operable to vary a turns ratio of the transformer thereby regulating an output voltage of the power converter.
The present invention introduces, in one aspect, the broad concept of using a switching circuit to vary a turns ratio of a transformer to regulate an output voltage of the converter. Additionally, conversion efficiency is improved, magnetic components may be reduced in size and component stresses may be minimized. The current-fed power converter allows the transformer to continuously deliver energy to the output thereby utilizing the transformer more efficiently and reducing output pulsating current at the same time.
In one embodiment of the present invention, the switching network includes at least one regulation switch, coupled to the tapped winding. The regulation switch may be selected from the group consisting of (1) a metal oxide semiconductor field-effect transistor, (2) a bipolar junction transistor, (3) an insulated gate bipolar transistor; and (4) a gate turn-off thyristor. Of course, other controllable switches are well within the broad scope of the present invention.
In one embodiment of the present invention, the switching network includes a diode coupled to the tapped winding. The diode may thus prevent reverse current flow through the tapped winding. Of course, those skilled in the art recognize that an active switch may be employed in place of the diode.
In one embodiment of the present invention, the power converter further has a rectifier coupled to the tapped winding, wherein the switching network includes first and second diodes coupled to opposing ends of the tapped winding. A regulation switch is coupled between the first and second diodes and an output of the power converter.
In one embodiment of the present invention, the tapped winding is a first tapped primary winding wherein the transformer has a second tapped primary winding. In an embodiment to be illustrated and described, the switching network includes a diode and a regulation switch coupled to the first tapped primary winding, and a second diode coupled to the second tapped primary winding.
In one embodiment of the present invention, the power converter is a boost power converter. Conventional power converters, such as the boost power converter, are familiar to those skilled in the art. Of course, the present invention is not limited to the power converter topologies disclosed herein.
In one embodiment of the present invention, the power converter further has a second power switch, wherein the power switch and the second power switch operate in an interleaved manner.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.


REFERENCES:
patent: 5408171 (1995-04-01), Eitzmann et al.
patent: 5602462 (1997-02-01), Stich et al.
patent: 5825164 (1998-10-01), Williams
patent: 6014326 (2000-06-01), Koch
U.S. Patent Application Serial No. 09/539,690, filed on Mar. 31, 2000, entitled “Current-Fed DC/DC Converter With Multilevel Transformer and Method of Operation Thereof” by Yimin Jiang, et al.

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