Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
2000-07-28
2001-04-17
Riley, Shawn (Department: 2838)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C257S280000, C363S021030, C363S025000, C363S089000, C363S127000
Reexamination Certificate
active
06218891
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to integrated circuits and, more specifically, to an integrated circuit including a driver for a metal-semiconductor field-effect transistor and a power converter employing the integrated circuit.
BACKGROUND OF THE INVENTION
A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC-DC converters are frequently employed to advantage. DC-DC converters generally include an inverter, a transformer having a primary winding coupled to the inverter and a rectifier coupled to a secondary winding of the transformer. The inverter generally includes a switching device that converts the DC input voltage to an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the DC-DC converter.
Conventionally, the switching device used in the inverter is a controllable switch such as a metal-oxide semiconductor field-effect transistor (MOSFET). The controllable switch in the inverter is modulated by periodically being driven into conduction and non-conduction states to maintain a required output voltage for the power converter. The rectifier may include passive rectifying devices that conduct the load current only when forward-biased in response to the input waveform to the rectifier. Passive rectifying devices, however, generally cannot achieve forward voltage drops that are low enough to provide a desired conversion efficiency of the DC-DC converter. To achieve a higher level of efficiency, DC-DC converters may therefore use synchronous rectifiers.
A synchronous rectifier replaces the passive rectifying devices of the conventional rectifier with a controllable switch. This controllable switch is also periodically driven into conduction and non-conduction states in synchronism with the periodic waveform of the AC voltage. The rectifier switches typically exhibit resistive-conductive properties and may thereby avoid the higher forward voltage drops inherent in the passive rectifying devices.
A metal-semiconductor field-effect transistor (MESFET) may be used as a controllable switch. The MESFET consists of a conducting channel positioned between a source and drain contact region. A carrier flowing from the source to the drain is controlled by a Schottky metal gate. Control of the channel is accomplished by varying the depletion layer width underneath the metal contact which modulates the thickness of the conducting channel and thereby the current.
A key advantage of the MESFET is the higher mobility of the carriers in the channel as compared to a MOSFET. The higher mobility leads to a higher current, transconductance and transit frequency for the device. A higher transit frequency makes the MESFET of particular interest for higher frequency applications. The use of a Gallium-Arsenide metal-semiconductor field-effect transistor (GaAsMESFET) rather than a Silicon MESFET provides additional advantages in that the room temperature mobility is more than five times larger and the saturation velocity is about twice that of Silicon. These qualities make the GaAsMESFET particularly attractive for use as a switching device in high speed applications requiring low losses. For a better understanding of Gallium-Arsenide devices see “Optimum Silicon and GaAs Power Field-Effect Transistors for Advanced High-Density, High Frequency Power Supply Applications,” by K. Shenai, C. Korman, and B. Baliga, HFPC 1989, and “10 MHz PWM Converters With GaAs VFETs”, by R. Kollman, G. Collins, and D. Plumton, APEC 1996, both of which are incorporated herein by reference.
Unlike the MOSFET in switching applications, however, the MESFET structure contains the Schottky metal gate. The Schottky metal gate limits the forward bias voltage on the gate to the turn-on voltage of a Schottky diode, which may be about 0.7 volts for Gallium-Arsenide. Therefore, the gate of a MESFET responds as a forward biased diode when the MESFET is used as a switch and in its conducting state. Additionally, the MESFET requires a bias voltage of an appropriate polarity to force it into a non-conducting state, since the MESFET conducts for a gate-to-source voltage of zero volts. As a result of these characteristics, it is difficult to use the MESFET as a controllable switch in many power converters since driver circuits and sources of appropriate bias supply voltages are often complex and more difficult to construct.
Accordingly, what is needed in the art is a driver for a MESFET that resolves the deficiencies and reduces the complexity associated with the prior art driver circuits.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides an integrated circuit including a metal-semiconductor field-effect transistor (MESFET) having a nominal intrinsic capacitance and requiring a negative voltage to bias the MESFET into a non-conduction state, a method of driving the MESFET and a power converter employing the integrated circuit and method. In one embodiment, the integrated circuit includes a driver including a bias capacitor integrated with the MESFET. The driver is configured to apply a positive voltage to bias the MESFET into a conduction state, and apply the negative voltage to bias the MESFET into the non-conduction state without employing an external negative bias source.
The present invention introduces, in one aspect, the concept of eliminating an external bias supply that would otherwise be required to drive the MESFET. Elimination of the external bias supply is highly beneficial in that the MESFET may be more advantageously employed in situations that would otherwise require complicated bias circuits. The driver including the bias capacitor to provide the required bias voltage on a “dynamic” basis resolves a long felt need in the application of MESFETs. Although the cited voltage polarities are directed toward an N-Channel MESFET device, the principles of the present invention may be applied to a P-Channel MESFET as well.
In one embodiment of the present invention, the driver is coupled to a control terminal of the MESFET and further includes at least one diode. In a related, but alternative embodiment, the at least one diode is parallel-coupled to the bias capacitor. The quantity and placement of the diode(s) regulate the amount of bias voltage that the driver may supply to the MESFET and allow tailoring of the bias voltage to specific MESFET requirements.
In one embodiment of the present invention, the MESFET is a Gallium-Arsenide metal-semiconductor field-effect transistor (GaAsMESFET). Use of the driver with a GaAsMESFET is particularly advantageous in that the switching speeds normally encountered in GaAsMESFET applications are very high. Since the driver provides a dynamic bias voltage to the GaAsMESFET that may otherwise deteriorate over an extended time period, high speed switching applications are particularly well suited for the application of GaAsMESFETs in accordance with the principles of the invention.
In one embodiment of the present invention, the integrated circuit forms a portion of a power converter. In a related, but alternative embodiment, the power converter is selected from the group consisting of a boost power converter, a buck power converter and a buck-boost power converter. The present invention is equally applicable to isolated and non-isolated power converter topologies. Of course, use in or with other power converter topologies or other applications is 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 shoul
Lotfi Ashraf W.
Rozman Allen F.
Tan Jian
Tang Wei
Lucent Technologies - Inc.
Riley Shawn
LandOfFree
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