Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-05-09
2001-02-20
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S082000, C363S090000
Reexamination Certificate
active
06191960
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to an active clamp for an isolated power converter, a method of operating the active clamp and a power converter employing the active clamp or the method.
BACKGROUND OF THE INVENTION
A power converter is a power processing circuit that converts an input voltage or current waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage into a specified output voltage. A flyback converter is one example of an isolated switched-mode power converter that is typically employed in applications wherein a stable, regulated voltage is desired at the output of the power converter.
A flyback converter generally includes a switching circuit having at least one power switch, an isolation transformer, a rectifier having a freewheeling diode, and an output filter. A controller is included and employed to control the power switch. The conventional flyback converter operates as follows. The power switch is intermittently switched to apply the input voltage across a primary winding of the isolation transformer. The rectifier is coupled to a secondary winding of the isolation transformer and regulates the voltage received therefrom. The output filter smooths and filters the rectified voltage to provide a substantially constant output voltage at the output of the flyback converter. The controller monitors the output voltage and adjusts the duty cycle of the power switch to maintain the output voltage at a constant level despite variations in the input DC voltage and the load current. Depending on the duty cycle of the power switch, the output voltage may be regulated to any desired voltage between zero and the input voltage.
Analogous to other types of power converters (e.g., forward converter, half-bridge converter), the flyback converter is subject to inefficiencies that impair its overall performance. More specifically, the power switch, which may be a metal-oxide semiconductor field-effect transistor (MOSFET), has a finite switching speed and an intrinsic parasitic capacitance that affect its performance. A charge built up in the parasitic capacitance of the power switch may be dissipated during turn-on of the power switch, resulting in switching loss. Further, the freewheeling diode also has a finite switching speed and is, therefore, subject to a reverse recovery phenomenon that may induce a substantial current spike through both the power switch and the freewheeling diode. The losses associated with the power switch and the freewheeling diode increase linearly as the switching frequency of the converter is increased. Therefore, minimizing the reverse recovery phenomenon and the switching loss associated with the freewheeling diode and power switch will improve the overall efficiency of the flyback converter.
Accordingly, what is needed in the art is an active clamp, employable with a variety of power converter topologies, that reduces the losses associated with the reverse recovery phenomenon and further reduces the switching loss associated with the power switch of a power converter.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides, a secondary active clamp for a power converter, a method of actively clamping energy of the power converter and a power converter employing the clamp or the method. The power converter has a primary switching circuit coupled to a primary winding of an isolation transformer and a rectifier coupled to a secondary winding of the isolation transformer. In one embodiment, the clamp includes (1) an inductor coupled in series with a freewheeling diode of the rectifier and (2) a series-coupled capacitor and clamping switch coupled in parallel with the inductor. The series-coupled capacitor and clamping switch cooperate with the inductor to mitigate adverse effects of a reverse recovery phenomenon associated with the rectifier and to effect substantially zero voltage switching of a power switch of the primary switching circuit.
The present invention introduces, in one aspect, the concept of an active clamp employable on a secondary side of an isolation transformer. The active clamp is capable of reducing losses due to the reverse recovery phenomenon associated with the freewheeling diode of the rectifier and is further capable of reducing the switching losses associated with the power switch of the primary switching circuit.
In one embodiment of the present invention, the clamping switch conducts to couple the capacitor across the inductor, thereby enabling the capacitor to discharge through the inductor. The amount of energy stored in the capacitor is sufficient to effect substantially zero voltage switching of the power and clamping switches.
In an embodiment to be illustrated and described, the clamping switch is a metal oxide semiconductor field-effect transistor (MOSFET). Those skilled in the pertinent art will understand, however, that the present invention fully encompasses all controllable switches, whether conventional or later-developed. In a related embodiment, wherein the clamping switch includes a body diode, the freewheeling diode and a body diode of the clamping switch are similarly oriented. An external diode may be employed as required to supplement the body diode of the clamping switch. Of course, an external diode may be employed if the clamping switch does not include a body diode.
In one embodiment of the present invention, the converter is selected from the group consisting of a flyback converter, a forward converter and a half-bridge converter. Those skilled in the pertinent art understand, however, that other isolated converter topologies are well within the broad scope of the present invention.
In one embodiment of the present invention, the converter further includes a controller coupled to the power and clamping switches. The controller controls conduction intervals of the power and clamping switches. In an embodiment to be illustrated and described, the controller monitors the output voltage of the converter and controls the power and clamping switches in response thereto. Of course, the controller may monitor other points within the power converter and remain well within the broad scope of the present invention.
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: 4447866 (1984-05-01), Reeves
patent: 4811187 (1989-03-01), Nakajima et al.
patent: 5003454 (1991-03-01), Bruning
patent: 5006975 (1991-04-01), Neufeld
patent: 5075839 (1991-12-01), Fischer et al.
patent: 5289361 (1994-02-01), Vinciarelli
patent: 5489837 (1996-02-01), Arakawa
Fraidlin Simon
Polikarpov Anatoliy
Lucent Technologies - Inc.
Vu Bao Q.
Wong Peter S.
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