Patent Publication Number: US-6218891-B1

Title: Integrated circuit including a driver for a metal-semiconductor field-effect transistor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND PATENTS 
     This application is related to the following U.S. patents and applications: 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Reference 
                   
                   
                   
               
               
                 No. 
                 Title 
                 Inventor(s) 
                 Date 
               
               
                   
               
             
            
               
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                 Synchronous Rectifier 
               
               
                   
                 in a Clamped-mode 
               
               
                   
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                 Zero-Voltage Switching 
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                 Lossless Synchronous 
               
               
                   
                 Rectifier Gate Drive 
               
               
                 5,291,382 
                 Pulse Width Modulated 
                 Cohen 
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                 DC/DC Converter with 
                   
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                 patent) 
                 Reduced Ripple Current 
               
               
                   
                 Component Stress and 
               
               
                   
                 Zero Voltage Switching 
               
               
                   
                 Capability 
               
               
                 5,303,138 
                 Low Loss Synchronous 
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                 Rectifier for 
                   
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                 Application to Clamped- 
               
               
                   
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                 Fixed Frequency 
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                 5,528,482 
                 Low Loss Synchronous 
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                 Mode Power Converters 
               
               
                 5,541,828 
                 Multiple Output 
                 Rozman 
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                 Continuous Power 
               
               
                   
                 Transfer to an Output 
               
               
                   
                 and with Multiple 
               
               
                   
                 Output Regulation 
               
               
                 5,590,032 
                 Self-Synchronized Drive 
                 Bowman, et 
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                 Circuit for a 
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                 1996 
               
               
                 patent) 
                 Synchronous Rectifier 
               
               
                   
                 in a Clamped-Mode 
               
               
                   
                 Power Converter 
               
               
                 5,625,541 
                 Low Loss Synchronous 
                 Rozman 
                 April 29, 
               
               
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                 Rectifier for 
                   
                 1997 
               
               
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                 Application to Clamped- 
               
               
                   
                 Mode Power Converters 
               
               
                 08/872,250 
                 A Micromagnetic Device 
                 Kossives, 
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                 And Method of 
               
               
                   
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                 Rozman 
                 Feb. 9, 
               
               
                 (‘299 
                 for Damping Ringing in 
                   
                 1999 
               
               
                 patent) 
                 Self-driven Synchronous 
               
               
                   
                 Rectifiers 
               
               
                 5,920,475 
                 Circuit and Method for 
                 Boylan, et 
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                 patent) 
                 Synchronous Rectifier 
               
               
                   
                 Converter 
               
               
                 5,940,287 
                 Controller for a 
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                 Synchronous Rectifier 
                   
                 1999 
               
               
                 patent) 
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                 Employing the same 
               
               
                 5,956,245 
                 Circuit and Method for 
                 Rozman 
                 Sept. 21, 
               
               
                 (‘245 
                 Controlling a 
                   
                 1999 
               
               
                 patent) 
                 Synchronous Rectifier 
               
               
                   
                 Converter 
               
               
                 6,002,597 
                 Synchronous Rectifier 
                 Rozman 
                 Dec. 14, 
               
               
                 (‘597 
                 having Dynamically 
                   
                 1999 
               
               
                 patent) 
                 Adjustable Current 
               
               
                   
                 Rating and Method of 
               
               
                   
                 Operation Thereof 
               
               
                 6,011,703 
                 Self-synchronized Gate 
                 Boylan, et 
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                 patent) 
                 Converter Employing 
               
               
                   
                 Self-driven Synchronous 
               
               
                   
                 Rectifier and Method of 
               
               
                   
                 Operation Thereof 
               
               
                 RE 36,571 
                 Low Loss Synchronous 
                 Rozman 
                 Feb. 15, 
               
               
                 (‘571 
                 Rectifier for 
                   
                 2000 
               
               
                 patent) 
                 Application to Clamped- 
               
               
                   
                 mode Power Converters 
               
               
                   
               
            
           
         
       
     
     The above-listed patents and applications are incorporated herein by reference as if reproduced herein in their entirety. 
    
    
     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 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a schematic diagram of an embodiment of a power converter constructed according to the principles of the present invention; 
     FIG. 2 illustrates a schematic diagram of an integrated circuit constructed according to the principles of the present invention; 
     FIG. 3 illustrates voltage waveforms of the integrated circuit of FIG. 2 demonstrating the principles of the present invention; and 
     FIG. 4 illustrates a diagram of an embodiment of an integrated circuit constructed according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, illustrated is a schematic diagram of an embodiment of a power converter  100  constructed according to the principles of the present invention. The power converter  100  has an input couplable to a source of electrical power  105  having an input voltage Vin and an output that provides an output voltage Vout to a load  190 . The power converter  100  includes an inverter with an integrated circuit (e.g., a power switch integrated circuit)  115  coupled to the input. In the illustrated embodiment, the power switch integrated circuit  115  includes a power metal-semiconductor field-effect transistor (MESFET)  116  and a power MESFET driver  117  coupled to the input of the power converter  100 . 
     A transformer T 1  of the power converter  100  includes a primary winding S 1  coupled to the power switch integrated circuit  115  and a secondary winding S 2  coupled to a rectifier  130 . The rectifier  130  includes first and second integrated circuits (e.g., first and second rectifier switch integrated circuits)  131 ,  135 , which cooperate to perform together as a synchronous rectifier. The first rectifier switch integrated circuit  131  includes a first rectifier MESFET  132  and a first rectifier MESFET driver  133 , and the second rectifier switch integrated circuit  135  includes a second rectifier MESFET  136  and a second rectifier MESFET driver  137 . 
     The rectifier  130  is coupled to the secondary winding S 2  and rectifies a periodic waveform supplied by the secondary winding S 2 . The power converter  100  further includes an output filter  170  that is coupled between the rectifier  130  and the load  190 . The output filter  170  has a filter inductor LF and a filter capacitor CF, that filters the rectified waveform to provide the output voltage Vout at the output of the power converter  100 . The power converter  100  still further includes a control circuit  180 , coupled to the power switch integrated circuit  115 , that monitors the output voltage Vout and adjusts the switching cycles of the power MESFET  116  to regulate the output voltage Vout despite variations in the input voltage Vin or the load  190 . The control circuit  180  is further coupled to the first and second rectifier switch integrated circuits  131 ,  135  of the rectifier  130  and functions to controllably switch and rectify the periodic waveform supplied by the secondary winding S 2 . Of course, the control circuit  180  may monitor other control points within the power converter  100  as required. 
     In operating the power converter  100 , the control circuit  180  periodically switches the power MESFET  116  of the power switch integrated circuit  115  to apply the input voltage Vin across the primary winding S 1 . During steady-state operation, the power MESFET  116  is ON (conducting) for a primary duty cycle D to apply the input voltage (e.g., a DC input voltage) Vin across the primary winding S 1 . The first rectifier MESFET  132  and the second rectifier MESFET  136  of the rectifier  130  are also periodically switched ON and OFF in a complementary manner to deliver an appropriate rectified voltage to the output filter  170 . 
     While the embodiment illustrated and described depicts a forward power converter topology, the principles of the present invention are equally applicable to other topologies such as a buck power converter or a buck-boost power converter, including but not limited to a full bridge, half bridge, push-pull, flyback, etc. Additionally, those skilled in the art will realize that the principles of the present invention may be employed with a wide variety of switching topologies, including those not specifically described herein. 
     Turning now to FIG. 2, illustrated is a schematic diagram of an embodiment of an integrated circuit  200  constructed according to the principles of the present invention. The integrated circuit  200  includes a MESFET  210  having a control terminal  220  and a driver  215  coupled between a driver supply voltage Vd and the control terminal  220 . A gate-to-source voltage Vgs is illustrated at the control terminal  220  of the MESFET  210 . 
     The MESFET  210  is an N-channel device having a nominal gate-to-source intrinsic capacitance Cgs. Additionally, the MESFET  210  has an equivalent gate-to-source, intrinsic body diode Dgs. Both the intrinsic capacitance Cgs and the intrinsic body diode Dgs are drawn explicitly in FIG. 2 to facilitate a discussion of the operation of the integrated circuit  200 . The driver  215  includes a bias capacitor Cbias having a bias voltage Vbias and first, second, third, fourth and fifth driver diodes D 1 , D 2 , D 3 , D 4 , D 5 , which are collectively designated as the driver diodes D 1 -D 5 . Both the bias capacitor Cbias and the driver diodes D 1 -D 5  are preferably integrated with the MESFET  210 . In the illustrated embodiment, the MESFET  210  is a Gallium Arsenide MESFET (GaAsMESFET). 
     The MESFET  210 , being an N-Channel device, requires a negative gate-to-source voltage Vgs to bias the MESFET  210  into a non-conduction state. Alternatively, the MESFET  210  requires a positive gate-to-source voltage Vgs to fully enhance the channel into a conduction state. The driver  215  cooperates with the intrinsic components of the MESFET  210  to provide both a negative bias for the non-conduction state and a positive bias for the conduction state. The driver  215  thereby eliminates the need for an external negative bias supply for the MESFET  210 . Additionally, the driver  215  may be appropriately rearranged as to provide the same functionality for a MESFET having an opposite polarity (e.g., a P-Channel instead of an N-Channel as shown). 
     When the MESFET  210  is in the conduction state, the intrinsic body diode Dgs limits the gate-to-source voltage Vgs to about 0.5-0.8 volts. During this period, the driver supply voltage Vd is in a HIGH state providing a positive voltage to the driver  215 . A minimum condition for the difference in voltage between the HIGH state and the LOW state of the driver supply voltage Vd is partially determined by the number of diodes included in the driver diodes D 1 -D 5 . In the illustrated embodiment, five diodes are chosen to provide a value of about 3.4 volts (0.68 volts per diode times five diodes) for the bias voltage Vbias across the bias capacitor Cbias at the required forward current for the driver diodes D 1 -D 5 . Of course, other methods of providing the bias capacitor voltage could be used. For example, a reverse biased zener diode may be used in place of one or more of the driver diodes D 1 -D 5 . Additionally, a silicon band gap reference could be used, or perhaps a resistor. Alternative devices used in place of or in addition to driver diodes D 1 -D 5  to generate the bias voltage Vbias are within the spirit and scope of the present invention. The minimum differential HIGH-to-LOW value of the driver supply voltage Vd may be computed to be the bias voltage Vbias (3.4 volts) plus the gate-to-source voltage Vgs (0.5 volts), which is about 3.9 volts. This value of the bias voltage vbias is chosen to assure that the MESFET  210  will be in the conducting state when the driver  215  is in the high condition, and in the non-conduction state when the driver supply voltage Vd traverses to its LOW condition. 
     The value of the bias capacitor Cbias is dependent on the value of the intrinsic capacitance Cgs of the MESFET  210 . In the illustrated embodiment, the intrinsic capacitance Cgs is about 5 picofarads and the bias capacitor Cbias is selected to be about 20 picofarads. Typically, the value of the bias capacitor Cbias is chosen to be several times the value of the intrinsic capacitance Cgs, up to perhaps ten times the value or more. Choosing the bias capacitor Cbias to be several times the value of the intrinsic capacitance Cgs allows the bias capacitor Cbias to maintain a substantially DC voltage throughout the switching action. The larger the ratio of the bias to intrinsic capacitance, the more stable the DC voltage on the bias capacitor Cbias. This value of bias capacitor Cbias along with the minimum differential HIGH-to-LOW value for the driver supply voltage Vd of 3.9 volts provides a negative bias value of about 3.4 volts for the gate-to-source voltage Vgs when the driver supply voltage Vd shifts from the HIGH to the LOW state. This assures that the MESFET  210  is in a non-conduction state, during this period. 
     Turning now to FIG. 3, illustrated are voltage waveforms  300  of the integrated circuit  200  of FIG. 2 demonstrating the principles of the present invention. The voltage waveforms  300  include a first waveform  305  and a second voltage waveform  310 . The first voltage waveform  305  corresponds to the driver supply voltage Vd and the second voltage waveform  310  corresponds to the gate-to-source voltage Vgs. The driver supply voltage Vd is seen to initially rise from a LOW state of about zero volts to a HIGH state of about 3.9 volts. This action causes the gate-to-source voltage Vgs also to rise from about —0.8 volts to about 0.5 volts thereby placing the MESFET  210  of FIG. 2 into the conduction state. This state is maintained until the driver supply voltage Vd falls to its LOW state of about zero volts again, and the gate-to-source voltage Vgs again moves to a value of about —0.8 volts placing the MESFET  210  into the non-conduction state. This sequence repeats on a periodic basis. Of course, the aforementioned values are submitted for illustrative purposes only. 
     Turning now to FIG. 4, illustrated is a diagram of an embodiment of an integrated circuit (e.g., a power switch integrated circuit)  400  constructed according to the principles of the present invention. The power switch integrated circuit  400  includes a Schottky diode  405 , a Metal-Insulator-Metal (MIM) capacitor  410  and a power GaAsMESFET  415 . In the illustrated embodiment, the Schottky diode  405  and the MIM capacitor  410  are coupled to provide a driver circuit for the power GaAsMESFET  415 , wherein only one exemplary Schottky diode is shown. 
     The power switch integrated circuit  400  may be constructed according to the following process starting with providing a semi-insulating GaAs substrate. Implanted N+ contact regions are formed in the substrate to provide contacts for the Schottky diode  405  and the power GaAsMESFET  415 . A channel implant region is constructed for the power GaAsMESFET  415 . Isolation implant regions are provided to underlie all passive devices and surround all active devices. An ohmic metal (such as Germanium/Gold/Nickel) is evaporated over the N+ contact regions and the ohmic metal is alloyed, which melts the ohmic metal into the N+ contact region thereby creating a low contact resistance. A gate metal (such as Titanium/Platinum/Gold) is evaporated to form a gate of the power GaAsMESFET  415 . 
     A Metal 0 (such as Titanium/Platinum/Gold) is deposited and patterned by lift off on the ohmic contacts and the gate metal. The Metal 0 is also used as the bottom plate for the MIM capacitor. A MIM capacitor dielectric is formed and a MIM capacitor top plate metal (such as Titanium/Platinum/Gold) is deposited and patterned by lift-off. A second dielectric material is then deposited and cured. Vias are etched through to the MIM capacitor top metal and the Metal 0 layers. The vias are coated with a thin field metal (such as Titanium/Gold) to provide electrical contact for plating, and the vias are plated with gold (i.e., Metal 1). The Metal 1 provides a top side contact for the Metal 0 and the MIM capacitor top metal. A Metal 2 is formed to provide an interconnect from the Schottky diode  405  to the top plate of the MIM capacitor  410  and the gate of the power GaAsMESFET  415 . 
     Although the present invention and its advantages have been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.