Electric power conversion systems – Current conversion – Including automatic or integral protection means
Utility Patent
2000-04-21
2001-01-02
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S126000
Utility Patent
active
06169671
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to a snubber circuit for a power switch and rectifying diode and a power converter employing the same.
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 boost power converter converts the input voltage to an output voltage that is greater than the input voltage. Typically, the boost power converter is employed in off-line applications wherein power factor correction is required and a stable regulated voltage is desired at the output of the power converter. For instance, boost power converters are employed in the telecommunications and computer industries as a segment of the power management system.
A non-isolated boost power converter generally includes an energy storage device (e.g., an inductor) coupled between the input voltage and a power switch. The power switch is then coupled to a rectifier (e.g., a rectifying diode) and an output capacitor. The load is connected in parallel to the capacitor. Again, the output voltage (measured at the load) of the boost power converter is always greater than the input voltage. When the power switch is conducting, the diode is reverse biased thereby isolating the output stage. During this period, the input voltage supplies energy to the inductor. When the power switch is not conducting, the output stage receives the energy stored in the inductor for delivery to the load coupled to the output of the converter.
Analogous to all types of power converters, a boost power converter is subject to inefficiencies that impair the overall performance thereof. More specifically, the rectifying diode suffers from a reverse recovery condition thereby producing excessive power losses in both the rectifying diode and the power switch and oscillations in both current and voltage therefrom. The effect of the reverse recovery condition is more severe in non-isolated converters, such as the boost power converter, due to the low impedance across the voltage source during the commutation of the rectifying diode. The reverse recovery condition can also detrimentally affect the longevity of the components, especially the rectifying diode and power switch, of the boost power converter.
Furthermore, the power switch [e.g., a metal-oxide semiconductor field-effect transistor (MOSFET)] is subject to losses when substantial voltage and current are simultaneously imposed on the power switch during the transition periods thereof. The losses associated with the power switch and rectifier increase linearly as the switching frequency of the boost power converter escalates. Therefore, efforts to minimize the losses associated with the boost power converter and, more specifically, the losses associated with the power switch and rectifier will improve the overall efficiency of the boost power converter.
A traditional manner to reduce the losses associated with the power switch and rectifying diode is to introduce a snubber circuit coupled thereto. Snubber circuits are generally employed for various functions including to minimize overcurrents and overvoltages across a device during conduction and non-conduction periods and to shape the device switching waveforms such that the voltage and current associated with the device are not concurrently high. For instance, with respect to rectifying diodes, a snubber circuit may be employed to minimize oscillations in both voltage and current and power losses associated therewith due to reverse recovery current resulting from a snap-off of the rectifying diode during a transition from a conduction to non-conduction mode of operation. In a more specific example, U.S. Pat. No. 5,923,547, (“the '547 patent”) entitled “A Snubber Circuit for a Power Converter and Method of Operation Thereof,” issued Jul. 13, 1999, to Hengchun Mao, commonly assigned with the present invention and incorporated herein by reference, discloses a snubber circuit that achieves a substantially zero current turn-off for the power switch of the converter and further reduces the reverse recovery currents of the rectifier over a wide range of power applications.
While the snubber circuit disclosed in the '547 patent and other related circuits specifically address the losses associated with the power switch and rectifier, there are other considerations that are germane to the overall performance of the power converter. As an example, lower voltage rectifying diodes are becoming more responsive and, depending on the design specifications, at times it makes sense to cascade a plurality of low-voltage rectifying diodes in the power converter. A limitation with the present snubber circuits is that such circuits do not account for voltage sharing between the plurality of rectifying diodes. Additionally, enhancing an overall efficiency associated with snubber circuits is always a continuing goal of circuit designers.
Accordingly, what is needed in the art is snubber circuit that improves the operation associated with the present snubber circuits and, in specific applications, can account for voltage sharing between a plurality of rectifying diodes in a power converter such as a boost power converter.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides for use in a power converter having a power switch subject to switching losses and a rectifying diode subject to a reverse recovery condition, a snubber circuit and related method to mitigate adverse effects associated with the switching losses and the reverse recovery condition. In one embodiment, the snubber circuit includes a recovery circuit, having first and second energy storage devices and a snubber diode, coupled to the rectifying diode. The snubber circuit further includes a third energy storage device, coupled across the rectifying diode, configured to cooperate with the recovery circuit to mitigate adverse effects associated with the reverse recovery condition and reduce a rate of change of a voltage across the power switch to reduce the switching losses.
The present invention introduces, in one aspect, the pervasive concept of using an energy storage device coupled across the rectifier of a power converter to enhance the advantages associated with the snubber circuit. The enhancement manifests itself in further reducing losses of the power converter and thereby increasing overall converter efficiency.
In one embodiment of the present invention, the power converter further includes a second rectifying diode series-coupled to the rectifying diode. The snubber circuit further includes a fourth energy storage device coupled across the second rectifying diode. In a related, but alternative embodiment, the snubber circuit further includes first and second damping resistors respectively coupled across the third and fourth energy storage devices. The energy storage devices provide enhance energy management during transition times and the damping resistors reduce ringing of the voltage waveforms.
In one embodiment of the present invention, the snubber circuit further includes a resistor series-coupled to the snubber diode and configured to enhance a current flow to the second energy storage device. In a related, but alternative embodiment, the snubber circuit further includes a second snubber diode coupled to the snubber diode and configured to facilitate reduction of energy losses experienced by the snubber diode and the resistor. These snubber diodes increase the effectiveness of the snubber circuit.
In one embodiment of the present invention, the power switch is selected from the group consisting of a metal oxide semiconductor field effect transistor and a bipolar junction transistor. Of course, the power switch may be selected from any current or fut
Han Jessica
Lucent Technologies - Inc.
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