Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2000-06-23
2001-05-29
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S132000, C363S017000
Reexamination Certificate
active
06239993
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to a circuit for reducing losses associated with an isolated power converter, a method of operating the circuit and a power converter employing the circuit or the method.
BACKGROUND OF THE INVENTION
The development of high-efficiency power supplies in combination with a requirement of higher power density is a continuing goal in the field of power electronics. A switched-mode power converter is a frequently employed component of a power supply that converts an input voltage waveform into a specified output voltage waveform. There are several types of switched-mode power converters including, for instance, an asymmetrical half-bridge power converter.
A conventional asymmetrical half-bridge power converter includes two power switches coupled to a controller, at least one isolation transformer, a voltage balancing capacitor, a rectifier and a filter. The asymmetrical half-bridge power converter generally operates as follows. The first and second power switches conduct current in a complimentary manner, with generally unequal duty cycles, to convert an input DC voltage into an AC voltage to be applied across the isolation transformer. Any DC component of the voltage applied to a primary winding of the isolation transformer is blocked by the voltage balancing capacitor coupled in series with the primary winding of the isolation transformer. The rectifier then rectifies a secondary voltage from the isolation transformer and the filter smooths and filters the rectified voltage to develop an output voltage for delivery to a load. The controller monitors the output voltage of the asymmetrical half-bridge power converter and adjusts the duty cycle of the power switches to ultimately control the output voltage. The output voltage may be maintained at a relatively constant level despite relative fluctuations in the input voltage and the load.
The asymmetrical half-bridge power converter is a well known power circuit topology that, when operating in a flyback mode, may be capable of zero voltage switching (ZVS) operation. A high magnetizing current, usually exceeding twice the load current, however, may be required to attain ZVS operation. Further, one or more of the power switches may be subject to current spikes induced therein by the reverse recovery of the rectifier.
In U.S. Pat. No. 5,402,329, entitled, “Zero Voltage Switching Pulse Width Modulated Power Converters, incorporated herein by reference, Wittenbreder suggests placing an inductor in series with the primary winding of the isolation transformer. One of the ZVS transitions is driven by a combination of the magnetic energy stored in the transformer and the magnetizing energy stored in the inductor. The other ZVS transition is driven by energy stored in the inductor. While the series inductance may allow for ZVS operation with lower magnetizing currents, the inductor may cause spurious voltage spikes across the rectifier during reverse recovery.
Accordingly, what is needed in the art is a circuit, employable with a variety of power converter topologies, that reduces voltage spikes across a rectifier of the power converter during a reverse recovery phenomenon and advantageously recovers energy from the inductor to increase an efficiency of the power converter.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides, a circuit for reducing losses associated with a power converter, a method of operating the circuit and power converter employing the circuit or the method. The power converter has a primary switching circuit coupled to a tapped primary winding of an isolation transformer and a rectifier coupled to a secondary winding of the isolation transformer. The circuit is coupled across a tapped portion of the tapped primary winding. In one embodiment, the circuit includes (1) an inductor, configured to reduce current spikes in the primary switching circuit caused by a reverse recovery phenomenon associated with the rectifier and to effect substantially zero voltage switching of a power switch of the primary switching circuit; and (2) a diode, coupled to the inductor via the tapped portion, configured to clamp a voltage across the rectifier. The tapped portion is configured to enable energy from the inductor to be recovered within the power converter.
The present invention introduces, in one aspect, a circuit capable of reducing current spikes in the primary switching circuit and voltage spikes across the rectifier associated with the reverse recovery phenomenon and recovering energy associated therewith to increase an efficiency of the power converter.
In one embodiment of the present invention, the power converter is an asymmetric half-bridge power converter. Of course, other power converter topologies are well within the broad scope of the present invention. In a related embodiment, the tapped portion is configured to deliver the energy from the inductor to a capacitor of the asymmetrical half-bridge power converter. The energy may thus be recuperated within the power converter to increase an efficiency thereof.
In one embodiment of the present invention, the inductor is coupled to an end tap of the tapped primary winding, while the diode is coupled to an intermediate tap of the tapped primary winding. In an alternative embodiment, the diode is coupled to the end tap of the tapped primary winding, while the inductor is coupled to the intermediate tap of the tapped primary winding. Of course, other circuit configurations may be possible and are well within the scope of the present invention.
In one embodiment of the present invention, the power converter further includes a controller configured to control conduction intervals of the power switch. In a related embodiment, the controller is configured to monitor an output voltage of the power converter and to control the power switch in response thereto. The output voltage of the power converter may thus be regulated despite variations in the input voltage or the load. Of course, the controller may monitor other characteristics associated with the power converter as desired and control the power switch as a function thereof.
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: 5303137 (1994-04-01), Peterson
patent: 5402329 (1995-03-01), Wittenbreder, Jr.
patent: 5448467 (1995-09-01), Ferreira
patent: 5612860 (1997-03-01), Meszlenyi
“A Simple Clamped ZVS PWM Converter” by Ernest H. Wittenbreder; 1993 IEEE; pp. 715-721 No month.
Fraidlin Simon
Frolov Andrey
Louzanov Sergey
Polikarpov Anatoliy
Lucent Technologies - Inc.
Riley Shawn
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