Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
1999-08-26
2001-07-10
Wong, Peter S. (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
C323S282000, C323S290000, C363S053000
Reexamination Certificate
active
06259235
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to an active clamp for a 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 waveform into a specified output voltage waveform. A buck converter is one example of a 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 non-isolated buck converter generally includes a power switch couplable to a source of input voltage. The power switch intermittently switches to provide an output voltage to a load couplable to an output of the buck converter. A controller regulates the output voltage by varying a duty cycle of the power switch. 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.
The controller typically switches the power switch at a high switching frequency, such as one beyond the audible range, to reduce the size and weight of inductive components employed and, therefore, to reduce the cost, as well as the size and weight, of the buck converter. Conventional buck converters, therefore, typically include a low pass output filter having a filter inductor and a filter capacitor. The comer frequency of the output filter may be set sufficiently lower than the switching frequency of the power switch to minimize the output ripple.
Since the power switch is coupled in series with the filter inductor, turning off the power switch may result in a high voltage thereacross unless an alternative path is provided for the inductor current. A freewheeling diode may, therefore, be coupled between common and a node between the power switch and the filter inductor to provide a path for the inductor current while the power switch is off. During a conduction interval of the power switch, the freewheeling diode is reversed biased. Then, during a non-conduction interval of the power switch, the inductor current flows through the freewheeling diode, transferring some of its stored energy to the load. The buck converter, like other switched-mode power converters, preferably includes at least two semiconductor switches, the power switch and the freewheeling diode.
Analogous to other types of power converters (e.g., a boost converter), the buck converter is subject to inefficiencies that impair its overall performance. More specifically, the power switch and freewheeling diode may be subject to conduction losses that reduce the efficiency of the converter. Additionally, the power switch [e.g., a metal-oxide semiconductor field-effect transistor (MOSFET)] is subject to switching losses that occur, in part, when a charge built-up in a parasitic capacitance of the power switch is dissipated during turn-on. Furthermore, the freewheeling diode may also be subject to a reverse recovery condition (when the power switch is turned on) that induces 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 and switching losses associated with the freewheeling diode and power switch will improve the overall efficiency of the 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 condition and further reduces the switching losses 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, for use with a power converter couplable to a source of electrical energy, the converter having a power switch that conducts intermittently to transfer energy from the source to an inductive element, and a freewheeling diode that alternately conducts with the power switch to transfer energy to an output of the converter, an active clamp and a method of operating the active clamp. In one embodiment, the active clamp includes: (1) an inductor, coupled in series with the freewheeling diode, and (2) a series-coupled capacitor and clamping switch, coupled in parallel with the inductor, that cooperate therewith to mitigate adverse effects of reverse recovery currents associated with the freewheeling diode and enable substantially zero voltage switching of the power and clamping switches.
The present invention, in one aspect, introduces the broad concept of an active clamp employable with switched-mode power converter topologies having a freewheeling diode subject to reverse recovery currents. The active clamp is capable of reducing the reverse recovery losses associated with the freewheeling diode and is further capable of reducing the switching losses associated with the power switch of the power converter.
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. Consequently, the amount of energy stored in the capacitor is sufficient to enable 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, the freewheeling diode and a body diode of the clamping switch are similarly oriented. Of course, an external diode may be employed as required.
In one embodiment of the present invention, the converter is selected from the group consisting of a non-isolated boost converter, a non-isolated buck converter, a non-isolated buck-boost converter, a non-isolated capacitive-coupled buck-boost converter, a non-isolated Sepic converter, and a non-isolated Zeta converter. Those skilled in the pertinent art understand, however, that other 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, those skilled in the pertinent art understand that other control points within the converter, or power supply as a whole, are 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: 4931716 (1990-06-01), Jovanovic et al.
patent: 4959764 (1990-09-01), Basset
patent: 5066900 (1991-11-01), Bassett
patent: 5262930 (1993-11-0
Fraidlin Simon
Polikarpov Anatoliy
Tyco Electronics Logistics AG
Vu Bao Q.
Wong Peter S.
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