Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-04-21
2001-02-13
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S040000
Reexamination Certificate
active
06188586
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to an asymmetrical half-bridge power converter having reduced input ripple and a method of manufacturing the asymmetrical half-bridge power converter.
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 control circuit, at least one input/output 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 control circuit 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. The input current to a conventional asymmetrical half-bridge power converter is a discontinuous waveform that can create noise problems. The noise problems often require substantial filtering to meet specifications for conducted and radiated noise from the power converter.
Part of this discontinuity in the input current waveform occurs due to currents in various magnetizing and output inductances being switched by the first and second power switches, thereby generating AC components in the input current. In addition, the output current reflected to the primary winding and alternately switched by the first and second power switches generates a significant component of the input ripple current. To address the potentially deleterious input current ripple, a magnetizing inductance of the transformer may be made larger by minimally gapping the transformer core, consistent with the requirement to prevent core saturation by a DC component of the magnetizing flux. Additionally, an output inductor may be made as large as practical, consistent with design trade-offs. Although helpful, attention to these considerations does not sufficiently reduce the input ripple current.
The input ripple current also varies in magnitude as a function of the duty cycles of the power switches. At a duty cycle of 50 percent, the input ripple current is typically at a minimum, and is primarily composed of the switched current in the transformer magnetizing inductance. However, the asymmetrical half-bridge power converter (and many other types of power converters) is not typically operated at a 50 percent duty cycle, thereby resulting in input ripple currents that are higher than desired.
Accordingly, what is needed in the art is an improved way to reduce input ripple current in an asymmetrical half-bridge power converter.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides an asymmetrical half-bridge power converter and a method of manufacturing the same. In one embodiment, the asymmetrical half-bridge power converter includes: (1) first and second power switches configured to be controlled by complementary drive signals having nominal first and second duty cycles of D and 1-D, respectively and (2) first and second capacitors, having intrinsic capacitance values proportional to 1-D and D, respectively, and intrinsic equivalent series resistance (ESR) values proportional to D and 1-D, respectively, configured to reduce input ripple current associated with the asymmetrical half-bridge power converter.
The present invention introduces, in one aspect, the broad concept of selecting the capacitance and ESR values of the first and second capacitors of the asymmetrical half-bridge power converter such that they relate to the duty cycle of the complementary first and second drive signals that control the power switches. Those skilled in the pertinent art understand that ESR characterizes the intrinsic resistance of a capacitor and, in the context used herein, includes resistance in the circuit providing the interconnection with the capacitor. The ESR is considered to be in series with the capacitance of the capacitor. Once optimized, these values allow input ripple current to be reduced, even if the duty cycle of the complementary drive signals vary from their nominal values.
In one embodiment of the present invention, at least one external resistor is added to at least one of the first and second capacitors such that the total series resistance of each of the first and second capacitors is proportional to D and 1-D, respectively. In some cases, the ESR of the first and second capacitors cannot be selected to be equivalent to D and 1-D, respectively. In such cases, it may be advantageous to employ one or more external resistors to supplement the ESR of the first and second capacitors.
In a related embodiment of the present invention, a voltage across the external resistor is employable to sense conduction of the first and second power switches. In another embodiment, a voltage across the external resistor is signed and scaled to be indicative of a direction and magnitude of current in the first and second power switches. The use of the voltage across the external resistor to sense the current in the first and second switches avoids the need to provide a separate current sensing process, which often necessitates the use of a current sensing transformer.
In one embodiment of the present invention, first and second R-C time constants of the first and second capacitors are equivalent. The equivalency of the R-C time constants of the first and second capacitors forms a fundamental principle for the reduction of input ripple current in asymmetrical half-bridge power converters.
In one embodiment of the present invention, the complementary drive signals vary from the nominal duty cycles. The complementary drive signals may vary depending on changes in either the input voltage or the load. In either case, the intrinsic capacitance values and ESR values continue to cooperate to advantageously reduce input ripple current.
In one embodiment of the present invention, the asymmetrical half-bridge power converter includes an isolation transformer. The first and second power switches are configured to alternately switch to apply power from an input of the asymmetrical half-bridge power converter to the isolation transformer. The isolation transformer advantageously provides isolation between the input and the output of the asymmetrical half-bridge power converter.
In one embodiment of the present invention, the asymmetrical half-bridge power converter includes a controller, coupled to the first and second power switches, configured to provide the complementary drive signals to the first and second power switches. The controller operates the first and second power switches to regulate the output voltage of the asymmetrical half-bridge power converter.
In one embodiment of the present invention, the asymmetrical half-bridge power
Farrington Richard
Jacobs Mark E.
Liu Rui
Thottuvelil Vijayan J.
Laxton Gary L.
Lucent Technologies - Inc.
Wong Peter S.
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