Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 12/556,991, entitled BUCK DC-TO-DC CONVERTER AND METHOD, filed on Sep. 10, 2009, which in turn is a continuation of U.S. patent application Ser. No. 11/558,797, filed Nov. 10, 2006, entitled BUCK DC TO DC CONVERTER AND METHOD, which in turn claims priority of U.S. Provisional Application Ser. No. 60/735,679, filed Nov. 11, 2005, entitled INTEGRATED, FAST-DISCHARGE BUCK CONVERTER, all of which are hereby incorporated by reference in their entirety for all purposes. 
     
    
     BACKGROUND 
       [0002]    The present teachings relate generally to the field of buck converters, and more particularly, to switching control thereof. 
         [0003]    These teachings relate to switching power supplies (switching converters). These devices are used to efficiently transform voltage and currents at one level to voltage and currents at a different level. Switching converters are particularly important when either high power or battery operation require high efficiency. Switching converters are pervasive throughout many consumer products such as cell phones, PDAs, personal computers, etc. A key feature of the switching power supply is its small size and low cost, which is achieved thru efficient design. 
         [0004]    One of the ways to improve the size and cost of switching converters is to optimize the size of the external passive components. This is achieved by optimizing power device topology. 
         [0005]    With the advent of deep sub-micron CMOS, power supplies with very low voltage, high tolerance and high currents are required. As a result, passive filter components have to be scaled to a very low impedance, and in particular the output capacitor is selected to be of high quality and large value. This capacitor dominates the size and cost of the switching converters for sub-micron CMOS. In general, a smaller the capacitor means lower cost. 
         [0006]    There is a need for power device topologies that allow the output capacitor to be reduced. 
         [0007]    Buck DC-to-DC converters are frequently used to provide lower operating voltages as required in various devices that use integrated circuits. Typically a much larger supply voltage is used with a narrow duty cycle to charge an output capacitor through an inductor, with accumulated inductor current maintaining the output voltage when not connected to the higher voltage supply during the off portion of the duty cycle. 
         [0008]    This arrangement of having a higher supply voltage for charging the inductor current and a much smaller output voltage for discharging the inductor current causes the step load recovery to be asymmetrical. When the load current steps from the small value to near its maximum value, the voltage across the inductor is the supply voltage minus the output voltage. In this case, the relatively large supply voltage allows the inductor to be charged quickly, and the corresponding output voltage droop is minimized. 
         [0009]    When the load current steps from near its maximum value to near its minimum value, the voltage across the inductor is just the output voltage. This relatively small voltage discharges the inductor slowly. This has the unfortunate result of allowing a large output voltage overshoot. This is an inherent limitation of the fundamental buck converter structure. 
         [0010]    One method to avoid this problem employs discrete MOSFETs to allow the bulk diode of the synchronous rectifier FET to turn on for a short time. This increases the discharge voltage by the turn-on voltage of the bulk diode. Unfortunately this method is not typically available in the integrated circuit form. In integrated circuit power devices, it is desirable not to turn on the bulk diodes for latch-up reasons. Latch-up is self destructive in most integrated circuits and efforts are made to avoid it. In addition, the improvement in the discharge rate is only increased by the addition of a diode forward drop. 
         [0011]    For the above reasons, it would be beneficial to provide improved methods and devices for discharging built up inductor current in buck converters when the converter experience is a substantial drop in load current being drawn there from, without using a larger capacitor to store the extra charge. 
       SUMMARY 
       [0012]    The needs of the teachings set forth above as well as further and other needs and advantages of the present teachings are achieved by the embodiments of the teachings described herein below. 
         [0013]    In one embodiment, a DC-to-DC buck converter comprises an inductor coupled between an input terminal and one end of a capacitor to conduct an inductor current to the capacitor to produce an output voltage across the capacitor and provide a varying range of load current; a first switch adapted to connect the input terminal to a voltage source; a second switch adapted to connect the input terminal to ground; and a control circuit adapted to control the first and second switches to provide a normal operating sequence to alternatively connect the input terminal to either the voltage source or to ground in response to the output voltage to provide the varying range of load current, wherein the control circuit is adapted to open both the first and second switches together to reduce an increase in the output voltage caused by a substantial reduction in the load current. 
         [0014]    The control circuit may be adapted to keep the first switch off and to modulate the second switch to reduce an increase in the output voltage caused by the substantial reduction in the load current. 
         [0015]    The converter may further comprise a reverse bias bypass circuit connected across the second switch and adapted to limit voltage across the second switch when the input terminal exhibits a reversed or negative voltage while current through the inductor adjusts to the substantial reduction in the load current. The bypass circuit may include a serially opposed diode and zener diode, wherein the diode is reversed biased during the normal operating sequence and forward biased to a reversed biased zener diode when the inductor terminal exhibits a reversed or negative voltage while current through the inductor is adjusting to the substantial reduction in the load current. 
         [0016]    The second switch may be a MOSFET having an isolated bulk, a MESFET having an isolated bulk, an IGBT, a MESFET, a MOSFET, a bipolar transistor or a self isolated switch that does not have a bulk/body connection, such as a III-V FET, for example, but not limited to, a gallium nitrite (GaNi) FET. The converter may further comprise a separate diode connected from each source and drain electrode of the MOSFET to the bulk dielectric and adapted to prevent forward bias of bulk semiconductor junctions in the MOSFET. Each of the separate diodes may be a Schottky diode. The second switch may be a p-bulk MOSFET having an n-well isolated bulk region. 
         [0017]    The first and second switches and the control circuit may be constructed as part of the same integrated circuit. The second switch may be constructed as part of a separate integrated circuit from the controller circuit to enhance heat dissipation from the second switch. The second switch may include a pair of series connected N-well MOSFETS having commonly connected source and p-well terminals. 
         [0018]    In another embodiment, a method for converting a DC voltage to a lower DC voltage includes the steps of: conducting current from an input terminal, through an inductor to charge a capacitor connected to the inductor at an output terminal and to provide a varying range of load current from the output terminal; alternately switching the input terminal between a supply voltage and a ground potential to produce a desired voltage at the output terminal that is lower than the supply voltage, while providing the varying range of load current; disconnecting the input terminal from both the supply voltage and the ground potential to reduce an increase in voltage at the output terminal caused by a substantial reduction in the load current, sensing voltage at the input terminal and reconnecting the input terminal to ground if the voltage at the input terminal exceeds a predetermined amount. 
         [0019]    The step of alternately switching may include the steps of switching the input terminal to ground using a MOSFET having a isolated bulk and protecting semiconductor junctions with the isolated bulk within the MOSFET from becoming forward biased by reverse voltage at the input terminal including bypassing the semiconductor junctions with individual Schottky diodes having a lower forward bias threshold voltage than the semiconductor junctions of the isolated bulk. 
         [0020]    In yet another embodiment, the method for converting a DC voltage to a lower DC voltage includes the steps of: conducting current from an input terminal, through an inductor to charge a capacitor connected to the inductor at an output terminal and to provide a varying range of load current from the output terminal; alternately switching the input terminal between a supply voltage and a ground potential to produce a desired voltage at the output terminal that is lower than the supply voltage, while providing the varying range of load current, disconnecting the input terminal from both the supply voltage and the ground potential to reduce an increase in voltage at the output terminal caused by a substantial reduction in the load current and limiting the amount of reverse voltage that can be produced at the input terminal while inductor current is adjusting in response to the substantial reduction in load current, the limiting the amount of reverse voltage includes the step of providing, to a semiconductor switching device used to connect the input terminal to the ground potential, a gate voltage lower than an input terminal voltage. 
         [0021]    The control circuit may be adapted to open the first and second switches to reduce an increase in the output voltage caused by the substantial reduction in load current. The converter may further comprise a reverse bias bypass circuit connected across the second switch and adapted to limit reverse voltage across the second switch when the input terminal exhibits a reversed or negative voltage. The bypass circuit may include a serially opposed diode and zener diode, wherein the diode is reversed biased during the normal operating sequence and forward biased to a reversed biased zener diode when the inductor terminal exhibits a reversed or negative voltage. 
         [0022]    The second switch may be a MOSFET having an isolated bulk. The converter may further comprise a separate diode connected from each source and drain electrode of the MOSFET to the bulk dielectric and adapted to prevent forward bias of bulk semiconductor junctions in the MOSFET. The second switch may be a p-bulk MOSFET having an n-well isolated bulk region. 
         [0023]    The second switch (and also the first switch) may also be a self isolated switch that does not have a bulk/body connection, such as a III-V FET, for example, but not limited to, a gallium nitrite (GaNi) FET. 
         [0024]    The first and second switches and the control circuit may be constructed as part of the same integrated circuit. The second switch may be constructed as part of a separate integrated circuit from the controller circuit to enhance heat dissipation from the second switch. The second switch may include a pair of series connected N-well MOSFETS having commonly connected source and p-well terminals 
         [0025]    In still another embodiment, a method for converting a DC voltage to a lower DC voltage, comprises the steps of: conducting current from an input terminal, through an inductor to charge a capacitor connected to the inductor at an output terminal and to provide a varying range of load current from the output terminal; alternately switching the input terminal between a supply voltage and a ground potential to produce a desired voltage at the output terminal that is lower than the supply voltage, while providing the varying range of load current including the step of allowing a reverse to voltage to form on the input terminal to reduce an increase in voltage at the output terminal caused by a substantial reduction in the load current. 
         [0026]    The method may further comprise the step of keeping the input terminal disconnected from the supply voltage while inductor current adjusts in response to the substantial reduction in load current. 
         [0027]    The may further comprise limiting the amount of reverse voltage that can be produced at the input terminal while inductor current is adjusting in response to the substantial reduction in load current. The method may further comprise the step of bypassing a semiconductor switching device used to connect the input terminal to the ground potential with one or more components adapted to limit reverse voltage produced at the input terminal with respect to the ground potential. 
         [0028]    The step of alternately switching may include the steps of switching the input terminal to ground using a MOSFET having a isolated bulk and protecting semiconductor junctions with the isolated bulk within the MOSFET from becoming forward biased by reverse voltage at the input terminal including bypassing the semiconductor junctions with individual Schottky diodes having a lower forward bias threshold voltage than the semiconductor junctions of the isolated bulk. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    For a better understanding of the present teachings, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 
           [0030]      FIG. 1  is a circuit schematic diagram of a DC-to-DC buck converter constructed in accordance with the prior art; 
           [0031]      FIG. 2  is a plot of various voltages and an inductor current which occur within the buck converter of  FIG. 1 , both during normal mode operation and during a substantial reduction in load current; 
           [0032]      FIG. 3  is a plot of various voltages and inductor current produced within a buck converter functioning in accordance with one embodiment of the present teachings; 
           [0033]      FIG. 4  is a plot of output voltage versus inductor current over two cycles of a substantial reduction in the load current, with the first cycle not implementing the present teachings and the second. cycle implementing an embodiment of the present teachings and exhibiting less voltage overshoot the than of the first cycle; 
           [0034]      FIG. 5  is a schematic diagram illustrating one embodiment of the present teachings; 
           [0035]      FIG. 6  is a schematic diagram illustrating another embodiment of the present teachings; 
           [0036]      FIG. 7  is a schematic diagram illustrating yet another embodiment of the present teachings; 
           [0037]      FIG. 8  is a schematic diagram illustrating a modification of the embodiment of  FIG. 7 ; 
           [0038]      FIG. 9  is a schematic cross section of a MOSFET switch constructed in accordance with the embodiment of  FIG. 5 ; 
           [0039]      FIG. 10  is a schematic diagram of another modification of the embodiment of  FIG. 7 ; 
           [0040]      FIG. 11  is a schematic cross section of a GaNi FET utilized in these teachings; and 
           [0041]      FIG. 12  is a schematic diagram illustrating a further embodiment of the present teachings. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]      FIG. 1  shows a classic prior art buck converter  10  generally including an inductor  12  connected between an input terminal  14  and an output terminal  15 , a capacitor  16  connected between output terminal  15  and ground  18  and a controller integrated circuit  20 . Also shown are a supply voltage source  22  and a nominally represented load  24 . Integrated circuit  20  includes a pair of switches  26 ,  28  and a switching controller  29 . Switch  26  is connected to selectively couple input terminal  14  to voltage supply  22 , and is referred to herein as supply voltage or supply source switch  26 . Switch  28  is connected to selectively couple input terminal  14  to ground  18  and is referred to herein as synchronous rectifier switch  28 . Switches  26 ,  28  are, in one embodiment, MOSFETS having a bulk connection to their respective source terminals and, in other embodiments, MESFETS having a bulk connection to their respective source terminals, or an IGBT, MESFET, MOSFET, a bipolar transistor or self isolated switch that does not have a bulk/body connection, such as a III-V FET, for example, but not limited to, a gallium nitrite (GaNi) FET. Switching controller  29  determines the duty cycle for controlling switches  26 ,  28  in response to the output voltage at terminal  15 . Various different switching controller topologies and methodologies exist for this purpose. Different approaches may be used to depending upon the load current demands placed upon the converter, such as a steady state load current versus a significant load current range, wherein the actual current can change rapidly. 
         [0043]      FIG. 2  shows the operation of buck converter  10  by plotting various voltages and the inductor current. The various plots  30  include time frame  31 , output terminal voltage  32 , inductor current  34 , supply source switch modulation  36 , synchronous rectifier switch modulation  38  and duty cycle  40 . Plots  30  are shown over time to include different operating conditions of converter  10 . Time segment  42  shows normal mode operating conditions for converter  10 , with source switch modulation  36  and rectifier switch modulation  38  showing alternate switching of input terminal  14  between supply voltage  22  and ground  18 . Correspondingly, inductor current  34  rises briefly  42   a,  while supply voltage switch  26  is conducting and drops  42   b  while rectifier switch  28  is conducting. 
         [0044]    Time point  43  represents a substantial decline in load current being drawn by load  24 , which results in an increase in voltage produced at output terminal  15  from the stored energy in inductor  12 . Inductor current  34  gradually declines over time period  44 , as rectifier switch  28  remains closed  44   a  and output terminal voltage  32  remains high  44   b.  Duty cycle plot  40  represents the operation of a normal duty cycle controller and essentially flat-lines during time interval  44  while inductor current is still slowly adjusting to the substantial reduction in load current. 
         [0045]      FIG. 3  shows plots  50  of the operation of a DC-to-DC buck converter  10  in accordance with one embodiment of the present teachings. This method of operation is programmed into controller  29 . In one distance, the controller  29  includes one or more processors and one or more computer usable media, where the computer usable media has computer readable code embodied therein, the computer readable code cause them to one or more processors to execute the methods of these teachings. The plots shown include time  52 , output voltage  54 , inductor current  56 , supply source switch modulation  58 , synchronous rectifier switch modulation  60  and duty cycle  62 . Time period  64  shows normal mode rectifier operation, with modulation plots  58 ,  60  indicating that input terminal  14  is being alternately switched between supply voltage  22  and ground  18 . Inductor current plot  56  shows the same variation as previously described during normal mode converter operation. 
         [0046]    Time point  65  represents a substantial reduction in load current, in response to which inductor current begins to decline and output terminal voltage increases. Synchronous rectifier modulation plot  60  shows switch  28  being turned on during periods  66 ,  67  to connect input terminal  14  to ground  18 . At time point  67   a,  switch  28  is turned off and remains off, which allows the voltage at input terminal  14  to go negative, which causes reverse conduction in switch  28  and thus an increased voltage drop between terminal  14  and ground  18 . The negative or reversed voltage causes a significant increase in negative inductor current to draw down the voltage at output terminal  15  more rapidly. This lowers the maximum value of the output terminal voltage thereby affording some over-voltage protection to load  24 . In one instance, the semiconductor switching device  28  used to connect the input terminal to the ground potential is provided a gate voltage lower than the input terminal voltage. The rectifier switch modulation plot  60  shows further modulation of switch  28  at its time  67   b.  Such additional modulation may be programmed into controller  29  in accordance with the known inductor current decline characteristics of the load for which the converter is intended. Once the inductor current in plot  56  has declined to a suitable level  68  in response to the reduced load current, the normal mode modulation of switches  26 ,  28  can begin over period  70 . In one instance, the voltage at the input terminal is sensed and compared to a predetermined voltage, where the predetermined voltage is indicative of declined (and almost extinguished) inductor current, and the input terminal is reconnected to ground (through switch  28 ) if the voltage at the input terminal exceeds the predetermined voltage. The duty cycle plot  62  shows active duty cycle control being generated while inductor current is adjusting to the reduced load current. 
         [0047]      FIG. 4  shows a plot  72  of output voltage  74 , inductor current  76  and duty cycle activity  75 , over time  78  including two sequential time periods  79 ,  80  which use two different control methods. Period  79  shows a sharp increase in load current  79   a  followed by a substantial decline in load current  79   b.  At point  79   c,  the output voltage, or output terminal voltage in plot  74  exhibits a significant overshoot due to the reduction in load current. Time period  80  also shows a load current increase at time point  80   a  followed by a substantial reduction in load current at point  80   b.  In time period  80  however, the switch method described in reference to  FIG. 3  is employed resulting in a smaller increase  80   c  in the output terminal voltage of plot  74 . Duty cycle plot  75  shows a mathematical saturation at zero at point  75   a.  Duty cycle plot  75  also shows the controller producing negative numbers at  75   b  thereby commanding the new behavior in switch  28  while using the method of the present teachings. 
         [0048]    The determination of a substantial drop in load current may be made by any suitable method. There are a wide variety of known methodologies for controlling duty cycles in buck converters, which generally include monitoring the output voltage over time, generally because of the great variety of load current demands for which such circuits are designed. For this reason, the determination of when a substantial drop in load current exists can have some variety of specific definitions. The present teachings are not intended to be limited to any specific definition of substantial reduction in load current. 
         [0049]      FIG. 5  shows another DC-to-DC buck converter  84  having a different switching and control semiconductor  86 . Semiconductor  86  includes a pair of MOSFET switches  88 ,  90  for alternately connecting input terminal  92  between a supply voltage and ground. However, MOSFET switches  88 ,  90  have P-bulk terminals  88   a,    90   a,  which are allowed to float or are otherwise controlled to maintain the semiconductor junctions with the bulk in each MOSFET in a reverse bias condition when the voltage at input terminal  92  is reversed, or driven negative, from its normal mode operating values. In extreme cases of larger reductions in load current, accumulated energy or current in the inductor can cause significant negative or reverse voltage to appear at input terminal  92  while terminal  92  is not connected to ground in accordance with one embodiment of the present teachings. This voltage can forward bias semiconductor junctions with the MOSFET switch  28  bulk and result in significant forward bias current in rectifier switch  28 . This can cause overheating and damage in integrated circuit  86 . Controlling the biasing of bulk terminals  88   a,    90   a  is one way of preventing forward biasing of the MOSFET bulk junctions and resulting overheating. Since a self isolated switch, such as a III-V FET, does not have a bulk/body connection, the above technique is not applicable to a self isolated switch, such as a III-V FET. 
         [0050]      FIG. 6  shows a modified converter schematic  84   a  having the same basic components as converter  84  of  FIG. 5  with the addition of a forward bias bypass circuit  96  connected between input terminal  92  and ground  99 . Bypass circuit  96  includes a series connected Zener diode  97  and regular diode  98  having reversed polarity and coupled to input terminal  92  to be reversed biased and non-conducting during normal mode operation of converter  84   a  and to be forward biased and conducting with a predetermined voltage drop when negative or reversed voltages are created at input terminal  92 . The predetermined voltage drop is set by the reversed bias Zener voltage plus the forward biased normal diode voltage drop, and limits the maximum negative or reversed voltage that can be formed on input terminal  92  by declining inductor current. This predetermined voltage is used in conjunction with the floating P-bulk terminals to prevent forward biasing of MOSFET  90  by negative or reversed voltage on input terminal  92 . When the predetermined negative or reverse voltage is reached at input terminal  92 , bypass circuit conducts current from input terminal  92  to ground  99 , thereby providing a negative or reversed voltage bias at input terminal  92  to reduce the output terminal voltage and also bypass switch  90  and avoid overheating of integrated circuit  86 . 
         [0051]      FIG. 7  shows another configuration of a buck converter  100  having semiconductor MOSFETS  102 ,  103  for switching of input terminal  104  in response to controller  106 . MOSFETS  102 ,  103  are constructed in separate semiconductor substrate from controller  106 , thus preventing forward bias bulk junction current flow from overheating controller  106 . In this case, the bulk terminals  102   a,    103   a  are connected to their respective source terminals to allow forward biasing of the bulk junctions by negative or reversed voltage on input terminal  104 . In this configuration the negative or reverse voltage on input terminal  104  is limited to the bulk junction forward bias voltage drop. Since a self isolated switch, such as a III-V FET, does not have a bulk/body connection, the above technique is not applicable to a self isolated switch, such as a III-V FET. 
         [0052]      FIG. 8  shows a further modification of converter  106  using MOSFETS  102 ,  103  constructed in a separate substrate  108  from controller  110 . MOSFET  103  further includes bypass diodes  112 ,  114 . Diodes  112 ,  114  are Shottky diodes having faster switching and lower forward bias voltage drop than normal diode junctions, including the bulk junctions of MOSFET  103 . They are connected to MOSFET  103  to be reversed biased during normal mode operation of converter  100  and to be forward biased by negative or reverse voltage appearing on input terminal  104 . Their lower forward bias voltage drop bypasses the bulk conjunctions and prevents overheating in MOSFET  103 . Since a self isolated switch, such as a III-V FET, does not have a bulk/body connection, the above technique is not applicable to a self isolated switch, such as a III-V FET. A self isolated switch, such as a III-V FET, does not have a bulk/body connection, could not have bulk/body-drain diode that could be forward biased. 
         [0053]      FIG. 9  shows a schematic cross section of MOSFET  103  used in the embodiment of  FIG. 8 . Schottky diodes  112 ,  114  are shown connected to MOSFET  103  for the purpose of biasing the isolated P-well  116 , in which the gate, source and drain components are constructed. When drain terminal  118  becomes negatively biased by input terminal  104  of  FIG. 8 , Schottky diode  112  becomes forward biased and Schottky diode  114  becomes reverse biased thereby pulling P-well  116  below ground  124  and preventing the natural forward biasing between N+ material  120  and P-well  116 . During normal mode operation, source terminal  122  is connected to ground  124  and the accumulation of positive charge on P-well  116  is limited to the forward voltage drop of Schottky diode  114 . Capacitor  126  may optionally be used to keep P-well  116  close to ground during transient events. 
         [0054]      FIG. 10  shows a schematic diagram of a converter  130  representing a modification of the converter  100  of  FIG. 7 . An additional MOSFET  132  is shown connected between the source P-well terminal  103   a  of MOSFET  103  and ground  134 . This series connection of MOSFETs  103  and  132  allows the voltage at terminal  136  to become more negative when Inductor discharge current flows into input terminal  136  towards MOSFETs  103  and  132 . This causes a higher discharge current yielding faster discharge time. The result is significantly reduced output voltage overshoot over the single synchronous rectifier switch  103  of  FIG. 7 . 
         [0055]      FIG. 11  shows a schematic diagram illustrating a III-V FET structure.  FIG. 11  can be compared to  FIG. 9  in order to illustrate the differences between a self isolated switch, such as the GaNi FET shown in  FIG. 11 , and the MOSFET shown in  FIG. 9 . 
         [0056]      FIG. 12  shows a schematic diagram of a modification of the embodiment of  FIG. 1  in which the MOSFETs are replaced by GaNi FETs. It should be noted that in the above described embodiments utilizing MOSFETs as the switches, except for the embodiment involving body/bulk connections, such as those shown in  FIGS. 5 and 8 , the MOSFETs can be replaced by a self isolated switch that does not have a bulk/body connection, such as a III-V FET, for example, but not limited to, a gallium nitrite (GaNi) FET. It should be noted that, although the examples shown in  FIGS. 11 and 12  utilize a gallium nitrite (GaNi) FET, these teachings are not limited only to embodiments utilizing a gallium nitrite (GaNi) FET, and embodiments in which other group III elements, such as, but not limited to, Aluminum (Al), boron (B) or Indium (In) or other group V elements are utilized in the III-V switch are also within the scope of these teachings. 
         [0057]    Various modifications and changes may be made by persons skilled in the art to the embodiments described above without departing from the scope of the teachings as defined in the appended claims.

Technology Category: 5