Abstract:
A half-bridge power circuit comprises a first gallium nitride field effect transistor (GaN FET); a first driver coupled to a gate of the first GaN FET; an anode of a capacitor coupled to an output of the driver and a source of the first GaN FET; a diode having a cathode coupled to the cathode of the capacitor; and a bootstrap capacitor clamp (BCC) controller, including: a field effect transistor (FET) coupled to an anode of the diode, and a comparator coupled to a gate of the FET, the comparator configured to receive as inputs: a) a signal representative of an input voltage (VDRV) applied to the FET; b) a ground; c) a boot signal representative of a voltage at the anode of the capacitor (Boot); and d) a signal representative of a voltage at the source of the first GaN FET (SW).

Description:
TECHNICAL FIELD 
       [0001]    This Application is directed, in general, to a voltage clamping circuit and, more specifically, to a voltage clamping circuit for a Gallium Nitride Field Effect Transistor (GaN FET). 
       BACKGROUND 
       [0002]    Generally, Gallium Nitride technologies are enabling fabrication of power field effect transistors (FETs) with lower gate capacitance (C g ) and gate charge (Q g ), compared to state-of-the-art silicon FETS, such as metal oxide semiconductor FETs (MOSFETs) for a same resistance of a FET in saturation (rds on ). 
         [0003]    Currently Gallium Nitride FETs (GaN FETs) can be four to five times better than a MOSFET (i.e. these various values are ¼ to ⅕ that of silicon FETs), and it is believed than GaN FETS can be potentially 100s of times better than MOSFETs. This means that GaN FETs can be switched at a much higher switching frequency with an equivalent power loss. Equivalently, it means that GaN FETs can aid in reaching a higher efficiency in a power circuit, if the GaN FETs are used instead of MOSFETs without a change in operation frequency. 
         [0004]    While GaN FETs have been available for some time, a recent breakthrough in their manufacturing in 2010 has resulted in GaN FETs implemented on silicon substrates, which has caused industry to believe that GaN FETs can be adopted instead of MOSFETs in at least a fraction of uses in the next few years. 
         [0005]    For more information regarding GaN FETs, please see “Application Note: Fundamentals of Gallium Nitride Power Transistors” by Stephen L. Colino, et al, Efficient Power Conversion Corporation, Copyright 2011, which is hereby incorporated by reference in its entirety. Also, please see “Enhancement-Mode GaN MIS-HEMTs With n-GaN/i-AlN/n-GaN Triple Cap Layer and High-k Gate Dielectrics”, by M. Kanamura., et al, IEEE Electron Device Letters, Vol. 31. No. 3, March 2010, pages 189-191, which is also incorporated by reference in its entirety. 
         [0006]    However, there are drawbacks associated with GaN FETs. Although GaN FETs have higher performance than silicon MOSFETs, they are also more sensitive and demanding in their usage requirements. One example of this sensitivity is that of the GaN FETs&#39; gate and source (Vgs) sensitivity to voltage excursions. For example, efficient power conversion (EPC) enhancement-mode GaN FETs typically require a 5 Volt drive signal to achieve saturation, but the drive Voltage should not exceed 6 Volts under any condition, since it will cause a “soft damage” (increase of rds on ) of the GaN FET. To make matters, worse, unlike silicon MOSFETs, GaN FETs do not have a body diode, and, therefore, when the GaN FETs are off, if V ds  goes negative, a GaN FET turns on at −3 Volts or −4 Volts difference between drain and source, instead of a body diode drop voltage as would occur in the case of a MOSFET. 
         [0007]    Therefore, there is a need in the art to address at least some of the issues associated with GaN FETs. 
       SUMMARY 
       [0008]    A first aspect provides an apparatus, comprising: a first gallium nitride field effect transistor (GaN FET); a first driver coupled to a gate of the first GaN FET; an anode of a capacitor coupled to an output of the driver and a source of the GAN; a diode having a cathode coupled to the cathode of the capacitor. The first aspect further provides a bootstrap capacitor clamp (BCC) controller, including: a field effect transistor (FET) coupled to an anode of the diode; and a comparator coupled to a gate of the FET, the comparator configured to receive as inputs: a) a signal representative of an input voltage (VDRV) applied to the FET; b) a ground; c) a boot signal representative of a Voltage at the anode of the capacitor (Boot); and d) a signal representative of a Voltage at the source of the first GaN FET. The BCC controller is configured to compare: a) a difference of: i) the VDRV and the GND, to generate a first comparison signal, to b) a difference of ii) the Boot and the source of the GaN FET, to generate a second comparison signal; wherein the BCC controller is further configured to maintain a relationship between the first comparison signal and the second comparison signal base on the comparison, and wherein the BCC controller is further configured to drive a gate output signal to the GaN FET to the maintain this relationship. 
         [0009]    A second aspect provides a system, comprising: a) a GaN FET; b) a bootstrap capacitor clamp (BCC) controller coupled to a gate of the GaN FET. The BCC controller comprises: a comparator; a FET, a gate of which is coupled to an output of the comparator. The BCC controller further comprises a first isolation switch coupled to a positive input of the comparator, a drain of the first isolation switch coupled between a first resistor and a second resistor, wherein the first resistor is also coupled to an anode of a capacitor, and wherein the second resistor is coupled to a ground; and a second isolation switch coupled to a negative input of the comparator, a drain of the second isolation switch coupled between a third resistor and a fourth resistor, wherein the third resistor is also coupled to a signal representative of a signal voltage (VDRV) and wherein the fourth resistor is also coupled to a source of the GaN FET. 
         [0010]    A third aspect provides a system comprising: A system, comprising: a first gallium nitride field effect transistor (GaN FET); a first driver coupled to a gate of the first GaN FET; an anode of a capacitor coupled to an output of the driver and a source of the GaN FET; a diode having a cathode coupled to the cathode of the capacitor; and a bootstrap capacitor clamp (BCC) controller. In the third aspect, the BCC controller includes a field effect transistor (FET) coupled to an anode of the diode; and a comparator coupled to a gate of the FET, the comparator configured to receive as inputs: a) a signal representative of an input voltage (VDRV) applied to the source of the FET; b) a ground; c) a boot signal representative of a voltage at the anode of the capacitor; and d) a signal representative of a voltage at the source of the first GaN FET. In the third aspect, the bootstrap capacitor clamp is configured to compare: a) a difference of: i) the VDRV and the GND to generate a first comparison signal; to b) a difference of ii) the Boot and the SW to generate a second comparison signal; wherein the BCC controller is configured to maintain a relationship between the first comparison signal and the second comparison signal base on the comparison, and wherein the BCC controller is further configured to drive a gate output signal to the a drain of a second GaN FET coupled to a source of the first GaN FET; an inductor coupled to a source of the first GaN FET; and a drain of a second GaN FET coupled to a source of the first GaN FET. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Reference is now made to the following descriptions: 
           [0012]      FIG. 1  illustrates a prior art half bridge power stage that employs both a low side MOSFET and a high side GaN FET; 
           [0013]      FIG. 2  illustrates a prior art GaN FET driver system that employs transformers; 
           [0014]      FIG. 3  illustrates an example of a half bridge power stage that employs both low side and high side GaN FETs with driver circuitry constructed according to principles of the present Application; 
           [0015]      FIG. 4  is an example of a bootstrap capacitor clamp (BCC) controller for a GaN FET, such as employable with the half bridge power amplifier of  FIG. 3 ; and 
           [0016]      FIG. 5  is an illustration of a transient input voltage, and various resulting voltages across at different nodes of the half-bridge power stage that employs both low side and high side GaN FETs with driver circuitry of  FIG. 3  and the BCV circuitry of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Turning to  FIG. 1 , illustrated is a prior art half bridge power stage  100  that employs both a low-side MOSFET  110  and a high-side GaN FET  120 . A (SW) switch node  130  of the power stage  100  is coupled to both a source of the GaN FET  120  and a drain of the MOSFET  110 . An inductor  140  is coupled to the SW node  130 . A VIN is coupled to a drain of the GaN FET  120 , and a GND is coupled to both the source and the gate of the MOSFET  110 . In  FIG. 1 , MOSFET  110  is a negative channel FET (NFET). 
         [0018]    This ensures that the NFET  110  can conduct drain to source when SW node  130  is more than one Voltage drop above GND through its body diode (not illustrated). However, NFET  110  will not conduct when SW node  130  is greater than GND, as the low side NFET  110  intrinsic body diode is oriented drain to source, so it will not conduct. 
         [0019]    An anode of a boot capacitor  150  is coupled to the SW node  130 . An output of a driver  160  is coupled to the gate of the GaN FET  120 , with the negative rail of the driver  160  also coupled to the SW node  130 . A positive rail of the driver  160  is coupled to a cathode of the b capacitor  150 . A cathode of a high voltage diode  170 , e.g., from 50V to 600V handling capacity, is coupled to the cathode of the capacitor  150 . A source reference voltage, such as 5.7 Volts, is coupled to the anode of the high voltage diode  170 . 
         [0020]    In the circuit  100 , the VIN Voltage is used as an upper rail for the SW node  130 , and the 5.7V is used as VDR. Generally, in the circuit  100 , bootstrapping is to provide the gate of NMOS  120  with a voltage higher than VIN. VDR is charged into the capacitor, and then is booted up to provide VDR+VIN to the gate of high side FET  120 . For more information on bootstrapping, please see “ Application Note AN -6076:  Design and Application Guide of Bootstrap Circuit for High - Voltage Gate - Drive IC. ” By Fairchild Corporation, Rev 1.0.0, Copyright 2008, which is incorporated by reference in its entirety. 
         [0021]    Regarding  FIG. 1A , a half bridge power stage such as the half bridge power stage  100 , can be used with a number of power circuits, such as a buck circuit, a boost circuit, and isolated buck circuit. However, in a current limit condition, i.e., when a controller detects a high current is passing through the system and brings half-bridge to a “Tri-state” or similar fault condition, a half-bridge can go to a tri-state and, if there is still current in the conductor going out of the half bridge  100 , current is drawn from the low side NFET  110 . This can be through the intrinsic body diode of the NFET  110 . For more information on “tri-state”, please see Wikipedia, The Free Encyclopedia, “Three-stage logic”, as of Feb. 2, 2012, which is incorporated by reference in its entirety 
         [0022]    However, there are problems associated with a “mixed” system, such as the half bridge power stage  100  of  FIG. 1 , which employs both a low-side MOSFET  110  and a high-side GaN FET  120 . For example, the NFET  110  constrains operating characteristics of the half bridge power stage  100  due to such factors of its higher rds on  when compared to the GaN FET  120  at a same operating frequency, thereby not availing the operation of a half bridge circuit  100  to many of the various advantages of employment of GaN FET technologies. 
         [0023]    Generally, as is understood by the present inventor, one problem with mixed design is that not all of possible benefits of GaN can be used. In other words, switching losses happen on both FETs. If only one FET with GaN in a system operating at high frequency, this is inefficient. Moreover, there are significant integration issues. 
         [0024]      FIG. 2  illustrates a prior art floating transformer system  200  that employs GaN FETs  210 ,  220 ,  230 ,  240 , to drive an output Voltage across a capacitor  250 . However, the transformer system  200  employs a transformer  260  to help achieve these ends. Transformers are typically large, bulky, expensive, can not typically be integrated within an integrated circuit, and tend to have significant variability in performance parameters. 
         [0025]      FIG. 3  illustrates an example of a half bridge power stage  300  that employs both a low side GaN FET  310  and a high side GaN FETs  320  with driver circuitry, including a bootstrap capacitor clamp (BCC) controller  380 . 
         [0026]    A SW node  330  of the power stage  300  is coupled to both a source of the GaN FET  320  and a drain of the GaN FET  310 . An inductor  340  is coupled to the SW node  330 . A VIN is coupled to a drain of the GaN FET  320 , and a GND is coupled to both the source of the GaN FET  110 . 
         [0027]    An anode of a boot capacitor  350  is coupled to the SW node  330 . An output of a driver  360  is coupled to the gate of the GaN FET  320 , with the negative rail of a high-side driver  360  also coupled to the SW node  330 . A positive rail of the high-side driver  360  is coupled to a cathode of the capacitor  150 . A cathode of a high voltage diode  370  is coupled to the cathode of the capacitor  350 . 
         [0028]    In the half bridge power stage  300 , a bootstrap capacitor clamp (BCC) controller  380  is coupled to an anode of the diode  370  at a boot node  355 . The BCC controller  380  includes both a comparator  383  and a MOSFET  386 , such as a PFET. The drain of the MOSFET  386  receives a reference Voltage VDRV at its source. Generally, VDRV can be regarded as an “ideal” high voltage rail (minus the drop across diode  370 ) for the driver  360 , and hence the gate of the high side GaN FET  320 . 
         [0029]    The BCC controller  380  includes at the input of the comparator  383  the VDRV, the GND, a signal from boot node  355 , and a signal from the SW node  330 . An output of the comparator  383  is coupled to a gate of the MOSFET  386 . The BCC controller  380  also employs the LS_ON signal, as will be explained in further detail regarding  FIG. 4 . 
         [0030]    The half bridge power stage  300  further includes a voltage level shifter (VLS)  392  coupled to an input of the driver  360  and a VLS  398  coupled to an input of a low-side driver  395 . An output of the low-side driver  395  is coupled to a gate of the low-side GaN FET  310 . A high voltage rail of the low-side driver  395  is coupled to VDRV through a high voltage diode  397 , and a low voltage rail of the low-side driver  395  is coupled to GND. In some approaches, PGND and GND are connected on single points to minimize switching noise effects. 
         [0031]    In the circuit  300 , a deadtime control circuit  399  drives the driver  360  when the high side GaN FET  320  is to be on through a HS_ON signal, and the driver  395  when the low side GaN FET  320  is to be on through an LS_ON signal. 
         [0032]    In the half bridge power stage  300 , the VIN voltage is used as an upper voltage range of the SW node  330 , and the BCC  380  is used to power up the boot capacitor  150 . Generally, regarding employment of the low side GaN FETs, instead of a low side MOSFET of circuit  100 , there is no body diode within the GaN FET to conduct current. In a current limit condition or similar fault condition, a half-bridge can go to a tri-state, if there is still current in the inductor going out of the half bridge, current would be drawn from a low side GaN FET. 
         [0033]    However, this creates problems in prior art configurations because an enhancement mode (EM) GaN FET has no body diode, and an EM GaN FET would start conducting source to drain when the drain, coupled to SW node, is at −3 Volts or −4 Volts, depending upon the current. During this time of this voltage, which can be several switching periods of half bridge and full bridge that can take as much as few hundreds of milliseconds. 
         [0034]    Generally, a bootstrap capacitor is charged to the difference of drive voltage VD and most negative voltage the switch pin sees during low-side FET off period. For example for driver voltage of 5 volts and a SW node that can go to −3V in GaN case, the bootstrap capacitor is charge to 5−(−3V)=8V. In next cycle the bootstrap capacitor voltage is used to turn on the high-side GaN FET stressing it with 8V, which can damage the device. Therefore, a voltage drop across the GaN FET from drain to source under discussion is 8 volts. 
         [0035]    Even more problematically, however, is that, when a high side driver of a prior art configuration receives a high side on (HS_ON) signal, the high side GaN FET is driven to the upper rail, which is the voltage of the cathode of the bootstrap capacitor, such as can be 5 Volts. However, the SW node is at perhaps −3 Volts or −4 Volts, which means that 8 or 9 Volts are being applied as Vgs across the high side GaN FET, in excess of the GaN FET ability to handle without significant reliability issues occurring within the high side GaN FET. These problems can include an increased rds on  resistance, shorted life of the GaN FET, perhaps causing the GaN FET to cease to function entirely. 
         [0036]    The circuit  300  addresses at least some of these disadvantages, and advantageously measures a low voltage limit of the SW node  330 , thereby avoiding high side GaN FET  320  gate source voltage overdriving. In on aspect, the BCC controller  380  is configured to keep a voltage difference between a first input voltage of the amplifier  385  and a second input of the amplifier  385  substantially constant. 
         [0037]    Generally, the BCC controller  380  senses the voltage of the VDRV, the GND, the boot node  355 , and the SW node  330 , and turns off the MOSFET  386  if a certain threshold is reached, or a relationship is reached, among the VDRV, the GDN, the SW node  330 , and the boot node  355 . In one example implementation, the comparator  383  within the BCC controller  380  turns of the MOSFET  380  if the voltage of the boot node  355  minus voltage at SW node  330  is greater than the VDRV voltage minus the GND voltage. This helps to prevent overcharging of a voltage difference between the boot node  355  and the SW node  330 , or in other words, between the gate of the high side GaN FET  320  and the source of the high side GaN FET  320 , thereby mitigating or even avoiding completely problems with Vgs GaN FET  320  overcharging. 
         [0038]    In one example usage of the half bridge power stage  300 , if an overcurrent condition, or other condition, starts to occur, and the SW node  330  starts to drop voltage, and otherwise the low side GaN FET  310  starts to drop, reaching or approaching the −3 or −4 reverse conduction voltage at the SW node  330 , the BCC controllers  380  opens the MOSFET  386 , which allows the voltage at capacitor to  350  to float with the drop of voltage at the SW node  330 , so as SW node  330  drops, so does the upper rail of the upper driver  360 , as the upper drive  360  upper rail is coupled to the boot node  355 . As the upper rail of the upper driver  360  drops, so does the driven gate voltage of the upper side GaN FET  360 . 
         [0039]    Therefore, the drive voltage of the gate of the GaN FET  320  does not exceed the specified relationship, such as boot node  355  voltage minus the SW node  330  voltage, thereby mitigating or even avoiding completely problems with Vgs GaN FET  320  overcharging. 
         [0040]    Employment of the BCC controller  380  in conjunction with the boot capacitor  350 , offers a number of advantages over various prior art approaches, such at the floating transformer system  200  of  FIG. 2  or use of a zener diode directly between the gate and source of a high side GaN FET instead of bootstrapping to create a high side drive. 
         [0041]    In the half bridge controller  300 , the implementation can be lower cost compared to the diode, especially for low voltage to medium voltage range (e.g. medium range: 60 Volt-100 Volts) implementations. Moreover, employment of the BCC controller  380  in conjunction with the boot capacitor  350  enables a generate a wide choices of “protection” (i.e. Vgs) voltages for the high side GaN FET  320  rather than having to design around the various intrinsic reverse bias voltages of various zener diodes), and is more power efficient, since the BCC controller  380  turns of its FET  386  to prevent overcharging of the Vgs of the high side of the GaN FET  320 , whereas the zener is always consuming power and more, may cause a low side driver of a low side GaN FET to go to current limit. 
         [0042]    Moreover, in high voltage applications with VIN of 60 to 100V, a local supply supplies 5-10V to drive the gates of transistors (a 60/100V transistors has a V dsmax  of 60/100, but V gsmax  is 5-10V), therefore a low dropout (LDO) or a buck regulator is typically employed to create 5-10V. Usually, the drive requirements are few tens of milliamps, and LDO or buck that is needed are very small bucks with max current limit of 50-100 mA. However, if a clamp capacitor were to be employed with prior art MOSFET circuitry, it may cause more current than LDO/buck current limit to come from these circuits and causing their output voltage to drop. 
         [0043]    Advantageously, the half bridge controller  300  and BCC controller  380  can be embodied within a single integrated circuit, which can lead to lower cost, and smaller area. 
         [0044]      FIG. 4  illustrates the BCC  380  controller in more detail. Within the BCC controller  380 , a first resistor  401  is coupled to the boot node  355 . The first resistor  401  is also coupled to a second resistor  402  at a node  405 , and the second resistor  402  is coupled to GND. Within the BCC controller  380 , a third resistor  403  is coupled to the VDRV. The third resistor  403  is also coupled to a fourth resistor  404  at a node  410 , and the fourth resistor  410  is coupled to the SW node  330 . 
         [0045]    The node  405  and the node  410  are coupled to a high voltage isolating switch  420 ,  425 , respectively, wherein high voltage MOS switches to cascade low voltage circuits against high voltages seen in SW and BOOT pins. The first and second isolating switches  420 ,  425  are coupled into the positive and negative inputs of the comparator  383 . The output of the comparator  383  is coupled to the MOSFET  386 . In the illustrated example, the switches  420 ,  425  are driven on when LS_ON signal is received at these switches to create a first comparison signal and a second comparison signal, respectively. 
         [0046]    The BCC controller  380  can work as follows. 
         [0047]    Depending upon the proportion of values of R  401  and  402 , which compares Boot voltage and Ground voltage, a weighted average between these two is generated to generate a weighted value proportional to a target boot voltage, hypothetical voltage being applied to the gate of the high side GaN FET  320 . Depending upon the proportion of values of R  403  and  404 , which compares VDRV voltage and SW voltage, a weighted average between these two is generated to generate a weighted value proportional to an upper range VDRV target voltage, voltage being applied to the gate of the GaN FET  320 . 
         [0048]    Within the comparator  383 , if the boot node is slightly lower than the VDRV node, but the GND and SW are the same, then the comparator  383  applies a negative or ground voltage to the gate of the MOSFET  386 , which in the illustrated aspect is a PFET. Therefore, the PFET would on, as the Vgs would meet the threshold values. However, if the boot node becomes higher than the ground, then the comparator  383  turns off the MOSFET  386 , as it would be a positive signal applied to a gate of a PFET, thereby allowing the boot node  355  voltage to float, protecting the gate of the high side GaN FET  320 . In the BCC controller  380 , both the VDRV and the GND are substantially fixed, so as the boot voltage goes up, the MOSFET  386  is turned off. Moreover, as the SW node  330  voltage goes down past its set point, the MOSFET  386  is also turned off. This in turn protects the Vgs of the high side GaN FET  320  of  FIG. 3  through allowing boot node  355  to float. 
         [0049]    In a further aspect, the isolating switches  420 ,  425  are employed. These isolating switches are enabled when the LS_ON signal is received from the deadtime control  399  of  FIG. 3 . Therefore, the comparator  383  of the BCC  380  only changes its outputs when the LS_ON signal is received as a high. When the LS_ON signal is not high, the isolation switches  420 ,  425  continue to output their last values. 
         [0050]    However, in this aspect, even should the MOSFET  386  be on and VDRV is applied to the high voltage diode  370 , and hence the boot  355 , minus the voltage drop across the high voltage diode  370 , this is not problematic, as the deadtime control  399  command the driver  360  to output a low signal to the gate of the GaN FET  320 , thereby helping to ensure that the Vgs of the GaN FET  320  is nonetheless within tolerance parameters. 
         [0051]      FIG. 5  illustrates three graphs  510 ,  520 ,  530  of simulations of simultaneous voltages various nodes of the circuit  300  under various conditions. 
         [0052]    In  FIG. 5 ,  510  is the voltage across boot capacitor  350 .  520  shows SW and BOOT node of  FIG. 3 . A top graph of  530  is HS OFF (equivalent to LS_ON); a bottom of graph  530  is gate of PFET  386 . 
         [0053]    In the illustrated example of  FIG. 5 , VDRV is set to about 6V. In this experiment; VSW can goes to −1.4V when high side FET is off (Low Side GaN FET  310  is always off in this example waveform, showing a fault condition). The  510  waveform shows boot capacitor  350  voltage discharges to turn on High side GaN FET  320  when HS_OFF becomes zero. When HS_OFF goes to “1” again, the gate of PMOS  396  goes to “zero” for 20 nanoseconds, allowing the boot capacitor  350  to charge to 6V VDRV-GND target. Once the boot capacitor  350  is charged to 6V, PMOS  380  turns off and prevents overcharging to 7.5V that could occur if the circuit  300 , or other circuits employing the principles of the present Application, were not used. 
         [0054]    Those skilled in the art to which this Application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.