Abstract:
A method for converting power includes charging an inductor by coupling the inductor to a voltage source for a predetermined amount of time. Thereafter, the inductor is discharged by coupling the inductor to a ground until the current flowing through the inductor equals zero. A method for detecting a zero current flowing through the inductor includes coupling the inductor to a transistor and comparing the output of that transistor to a transistor coupled to ground.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 09/978,125, filed Oct. 15, 2001, which will issue as U.S. Pat. No. 6,507,175 on Jan. 14, 2003, and which claims the benefit of provisional patent application Ser. No. 60/240,340, filed Oct. 13, 2000. 
     
    
     
       FIELD OF INVENTION  
         [0002]    This application relates generally to electronic circuits and more particularly to an electronic circuit for detecting a zero current condition, where such a circuit can be used in voltage regulators and switching power converters (“SPC”), including multiphase power converters.  
         BACKGROUND OF THE INVENTION  
         [0003]    Power regulators are often used in electronic equipment to supply power at a predetermined voltage to a system. For example, a typical desktop computer may contain a power supply that converts alternating current (“AC”) from a wall socket, to direct current (“DC”) with a voltage that is usable by the various components of the computer system. With continued reference to computer systems, a hard disk drive may require a 12 volt (“V”) power input, while various integrated circuit components may require, for example, power at 5.0 V, 3.3 V, or 1.5 V. A power supply must thus contain power regulators to generate the required voltage levels.  
           [0004]    Buck power regulators are often used to generate power outputs for microelectronic devices because they are relatively efficient and provide high current slewing (di/dt) capability. When providing a microprocessor with a regulated input voltage, di/dt and response time are very important considerations. The output inductor value of the regulator determines the di/dt capability of the regulator and also the boundary between continuous conduction mode (“CCM”) (when the inductor current is continuous) and discontinuous conduction mode (“DCM”) (when the inductor current is not continuous, but drops to zero until the transistor is turned ON; DCM typically occurs when a low load resistance is coupled to the buck power regulator.)  
           [0005]    With reference to FIG. 1, an exemplary buck (step-down) power regulator  100 , which converts a DC voltage to a lower voltage, is presented. A supply voltage, V s , is input into transistor  102 , which is coupled to a diode  104  that, in turn, is coupled to ground. Coupled to the junction of transistor  102  and diode  104  is an LC circuit comprising an inductor  106  and a capacitor  108 . A load  1   10  thus receives power at the required voltage, where the voltage is determined by the duty cycle of transistor  102  (i.e., the percentage of time when transistor  102  is turned on).  
           [0006]    When transistor  102  is on, inductor  106  is being charged and the supply voltage supplies the output current. When transistor  102  is turned off, inductor  106  “freewheels” through diode  104  and supplies the energy to load  110 . The purpose of the diode is not to rectify, but to re-direct current flow in the circuit and to ensure that there is a path for the current from the inductor to flow. Capacitor  108  serves to reduce the ripple content in the voltage, while inductor  106  smoothes the current passing through it.  
           [0007]    A problem of the buck power regulator is that, as low voltage outputs are required, the voltage drop of diode  104  leads to various consequences. For example, the circuit becomes less efficient because of the voltage drop of approximately 0.7 volt across the diode. Such inefficiencies become less tolerable when devices run on battery power as opposed to AC power.  
           [0008]    In response to the above deficiencies, buck power regulator  200 , detailed in FIG. 2, was developed. As can be seen, buck power regulator  200  is similar to buck power regulator  100 , with a transistor  204  replacing diode  104 . Transistor  204  may be configured to have a low on resistance. Transistor  102  is usually termed the high-side switch and transistor  204  is the low-side switch. In addition, drivers  222  and  224  control the operation of transistors  102  and  104 , respectively. By controlling the on and off cycles of transistors  102  and  204 , drivers  222  and  224  are able to more efficiently control the output voltage, V out , that is present at load  110 , and supply the desired amount of current.  
           [0009]    In normal operation of a power converter, there is a ripple in the output current, due to the charging and discharging of inductor  106 . One method of reducing the ripple of the output current is the use of a multiphase power supply. Instead of having, for example, a single source supplying a 20 amp output, there may be four phases, each of which supply 5 amps. An exemplary multiphase buck power converter is shown in FIG. 12.  
           [0010]    In multiphase power converter  1200 , it is desired to convert an input voltage at terminal  1202  to an output voltage at terminal  1204  across a load  1206 . In a manner similar to that described above with respect to FIG. 2, transistors  1212  and  1214  are each coupled to the input voltage  1202 . Coupled to the junction  1211  of transistors  1212  and  1214  is inductor  1216 . Similarly, transistors  1222  and  1224  are each coupled to the input voltage  1202 . Coupled to the junction  1221  of transistors  1222  and  1224  is inductor  1226 . Similarly, transistors  1232  and  1234  are each coupled to the input voltage  1202 . Coupled to the junction  1231  of transistors  1232  and  1234  is inductor  1236 . Similarly, transistors  1242  and  1244  are each coupled to the input voltage  1202 . Coupled to the junction  1241  of transistors  1242  and  1244  is inductor  1246 . Each of the transistor pairs is coupled to capacitor  1208  to provide the output needed at output  1204 . Because of the presence of four power converters, each converter is only responsible for one-fourth of the total current needed, resulting in smaller transistors and inductors and a corresponding reduction in cost. In addition, the ripple in the output current is reduced because each of the converters is only responsible for a portion of the output current. The phases are slightly offset from each other such that the peak current of each individual phase do not coincide with each other. This is shown in FIG. 15, which shows the individual output currents for each phase as well as the total output current. As can be readily seen, the ripple in the output current is substantially reduced from the ripple in the current of each individual phase, and the period of the ripple is approximately one-fourth of the ripple of each individual phase.  
           [0011]    [0011]FIG. 3 presents a plot of the inductor current of an exemplary buck power regulator. Axis  302  represents the passage of time, while axis  304  details the current flowing through inductor  106 . The current flowing through inductor  106  rises for the time period Ton when transistor  102  is on and the current falls during time period T off , when transistor  102  is off. The period, T, is T on  plus T off . The output voltage would be the input voltage times T on .  
           [0012]    Problems may arise, however, when buck power regulator  200  is required to produce a voltage through a smaller load. An exemplary resulting current plot is shown in FIG. 4. It can be seen that the current through inductor  106  becomes negative during a portion of the cycle, i.e., the current through inductor  106  reverses direction and flows into the ground. This behavior is undesirable because of the various inefficiencies that occur because the inductor is basically wasting power that would ideally remain in the system. Such a problem may not be present in buck power regulator  100  of FIG. 1, as diode  104  automatically “turns off” when the polarity of the inductor current changes.  
           [0013]    It is desirable to develop a method and apparatus for converting voltage that alleviate the above and other problems that may be present in the prior art.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention uses a Zero Current Detection (“ZCD”) circuit to determine the direction of current flow in various circuits, such as a switch of a switching power converter (“SPC”). In such a manner, once zero current is detected, the operation of the circuit can be changed such that inefficiencies are reduced.  
           [0015]    In one embodiment, the ZCD circuit may comprise a pair of current mirrors supplying current to a matched pair of transistors. One of the transistors is coupled to ground while the other transistor is coupled to the node of interest. The outputs of the matched pair are input into a comparator. When the non-inverting input voltage exceeds the inverting input voltage, the comparator changes state.  
           [0016]    In one embodiment, the ZCD circuit may be used in a SPC that is configured as a buck converter having Field Effect Transistors (“FETs”) used as power switches. The ZCD signal may be used to maximize the efficiency of the system by controlling the operation of the FETs during DCM operation. In such a manner, the current flow through the inductor is prevented from becoming negative.  
           [0017]    In another embodiment, the ZCD circuit may be used in a multiphase power converter in a tri-state mode to decrease the switching time when transients occur.  
           [0018]    The result is increased system efficiency and faster transient response. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, where like reference numbers refer to similar elements throughout the Figures, and:  
         [0020]    [0020]FIG. 1 illustrates an exemplary buck power regulator of the prior art;  
         [0021]    [0021]FIG. 2 illustrates an improved buck power regulator;  
         [0022]    [0022]FIG. 3 shows the inductor current of an exemplary buck power regulator supplying a high voltage;  
         [0023]    [0023]FIG. 4 shows the inductor current of an exemplary buck power regulator supplying a low voltage;  
         [0024]    [0024]FIG. 5 presents a further improvement to a buck power regulator;  
         [0025]    [0025]FIG. 6 illustrates an exemplary embodiment of the zero current detector;  
         [0026]    [0026]FIG. 7 shows a voltage/time curve of an embodiment of the present invention;  
         [0027]    [0027]FIG. 8 shows an alternative embodiment of a buck power regulator using the zero current detector;  
         [0028]    [0028]FIG. 9 shows the output current when a transient occurs;  
         [0029]    [0029]FIGS. 10 and 11 shows the operation of the switches in a buck power regulator;  
         [0030]    [0030]FIG. 12 illustrates an exemplary multiphase buck power converter;  
         [0031]    [0031]FIG. 13 illustrates an exemplary multiphase buck power converter with a zero current detector;  
         [0032]    [0032]FIG. 14 shows the output voltage present during a load transient;  
         [0033]    [0033]FIG. 15 shows the output current when using a multiphase power converter;  
         [0034]    and  
         [0035]    [0035]FIG. 16 shows the output current when using a multiphase power converter using a zero current detector. 
     
    
     DETAILED DESCRIPTION  
       [0036]    The present invention may be described herein in terms of various functional components and various processing steps. It should be appreciated that such functional components may be realized by any number of hardware or structural components configured to perform the specified functions. For example, the present invention may employ various integrated components comprised of various electrical devices, e.g., resistors, transistors, capacitors, diodes, inductors, and the like, whose values may be suitably configured for various intended purposes. In addition, the present invention may be practiced in any integrated circuit application where a detection of a zero current flow is desired. Such general applications that may be appreciated by those skilled in the art in light of the present disclosure are not described in detail herein. Further, it should be noted that while various components may be suitably coupled or connected to other components within exemplary circuits, such connections and couplings can be realized by direct connection between components, or by connection through other components and devices located therebetween.  
         [0037]    [0037]FIG. 5 presents an exemplary embodiment of a buck power regulator. A transistor  502  is coupled to a transistor  504  at node  511 . Coupled to the junction  511  of transistors  502  and  504  are inductor  506 , capacitor  508 , and load  510 . A driver  522  is coupled to transistor  502  and a driver  524  is coupled to transistor  504 . Drivers  522  and  524  serve to control when transistors  502  and  504 , respectively, are conducting and when they are off. In addition, there is also a zero current detector  512  coupled to junction  511 . The output of zero current detector  512  is coupled to a controller  514 , which is coupled to both drivers  522  and  524  to control the conduction of transistors  502  and  504 . The output of the regulator is at node  520 .  
         [0038]    With reference to FIG. 4 and FIG. 5, it can be seen that the current through inductor  506  is identical to the current at junction  511 . When the current at junction  511  reaches a level of zero amps, current detector  512  outputs a signal to controller  514 , which then sends a signal to driver  524 , which turns transistor  504  off. With transistor  504  turned off, current no longer flows from inductor  506  into ground. The energy in the inductor also becomes zero and capacitor  508  cannot discharge into ground because switch  504  is closed, resulting in a more efficient power regulation than a buck power regulator with a diode.  
         [0039]    With reference to FIG. 6, an exemplary zero current detection circuit  600  is disclosed. A current source  602  supplies current to transistor  604 . Transistors  606  and  610  act as a current mirror and supply substantially identical current to transistors  608  and  612 , each of which may be configured in diode-connected fashion, as illustrated in FIG. 6. In a preferred embodiment, transistors  606  and  610  are matched to within a tight tolerance of each other. Transistors  608  and  612  are also matched with to within a tight tolerance of each other. The source of transistor  608  is coupled to ground, while the source of transistor  612  is coupled to junction  511  (see FIG. 5). Both transistors  608  and  612  are coupled to inputs of comparator  620 . Comparator  620  is coupled to controller  514 . Comparator  620  is configured such that, when both inputs to comparator  620  are equal, the output of comparator  620  changes, as detailed in FIG. 8. Comparator  620  is preferably a low-offset, high-speed comparator.  
         [0040]    Both transistor  608  and transistor  612  are coupled to the same amount of current, via current mirrors  606  and  610 , respectively. Because the source of transistor  608  is coupled to ground and the source of transistor  612  is coupled to junction  511 , transistors  608  and  612  produce an equal voltage at their respective drains when the input to the source of transistor  608  is equal to the input of the source of transistor  612 . In other words, when junction  511  is equal to ground (i.e., the voltage is zero), the voltages at the drains of transistors  608  and  612  are equal. Thus, transistors  608  and  612  serve to level shift the inputs into the common-mode range of comparator  620 . It can be readily shown that the voltage at junction  511  is zero only when there is no current flowing through junction  511 . Transistors  614  and  618  serve to prevent an excessive voltage level at comparator  620  by directing excessive voltage to ground.  
         [0041]    The voltage at junction  511  is ideally shown in FIG. 7. It can be seen that the voltage at junction  511  is at a peak when transistor  502  first turns on and inductor  506  is being charged by the power supply. The voltage at junction  511  drops below zero voltage when transistor  502  turns off, as inductor  506  pulls charge from ground through transistor  504 , resulting in a negative voltage potential at junction  511 . The voltage reaches zero when the current through inductor  506  begins to flow in the opposite direction, from inductor  506 , through transistor  504 , to ground. Thus, it can be seen that, by sensing the voltage at junction  511 , the zero current detect circuit is able to determine when the current is zero by detecting when the voltage at junction  511  is zero.  
         [0042]    Thus, zero current detection circuit  600  disclosed in FIG. 6 can be used in place of element  512  of FIG. 5 to detect when the current through inductor  506  begins to flow in the negative direction. Once it is determined that a zero current condition is present, driver  524  can be configured to turn off transistor  504  to prevent voltage from flowing from inductor  506  to ground. This results in increased efficiency as the amount of energy lost to ground is drastically reduced.  
         [0043]    An alternative embodiment of the power regulator is presented in FIG. 8. Power regulator  800  features a voltage source  830  that feeds a switch  802 . It should be understood that switches  802  and  804  may suitably be replaced with a transistor switch and diode  814  is shown parallel to switch  804  to demonstrate an FET switch. A load inductance  832  and a capacitor  808  and a load  810  is also present in the circuit. Also illustrated in FIG. 8 are parasitic inductances  836 ,  834 , and  838 . The output of the circuit can be taken at node  820 .  
         [0044]    Voltage is sensed at both sides of load  810 . The measurement taken at the high side of the line, at node  840 , is termed V sense +. The measurement taken on the low side, from node  842 , is termed V sense −. The two voltage measurements are input to controller  812 , which operates switches  802  and  804 . The two voltage measurements serve to provide a more accurate reading, to controller  812 , of when a change in the load is encountered. It should be understood that a zero detect circuit, although not illustrated, may also be present in power regulator  800 . Such a zero detect circuit may be coupled to node  811  to sense a zero current condition. The presence of the zero current condition can be forwarded to controller  812  to more accurately control switches  802  and  804 .  
         [0045]    The operation of the circuit may be described more fully with respect to FIGS.  9 - 11 . FIG. 9 illustrates a graph of the current through load  810  in exemplary operation. As can be seen at the left end of the graph, when load  810  presents a low load (high impedance), the current through load  810  is also low. However, when the impedance is decreased, current through load  810  rises to a high value, as can be seen at the right end of FIG. 9. The time period during the transient from the steady-state operation at low load and the steady-state operation at high load is depicted as region  902  and may be termed the hysteretic mode.  
         [0046]    During the steady-state modes, the operation of switches  802  and  804  are periodic, as depicted in FIG. 10 for switch  802  and FIG. 11 for switch  804 . During those periods, switch  802  and  804  may operate in a mutually exclusive manner, as shown in FIGS. 10 and 11. In other words, when switch  802  is on, switch  804  is off and when switch  802  is off, switch  804  is on. The ratio between the on time and off time of the switches determines the output voltage of the regulator. However, during the hysteretic mode, switch  802  may pulse on and off to set the current through load  810  to the appropriate level. Once the appropriate current level is established, operation of the switches continues as before.  
         [0047]    The result is that, in a relatively small amount of time, circuit  800  is able to react to a change in the load and supply the correct amount of current to the load.  
         [0048]    In a multiphase power converter, with reference to FIG. 13, the configuration of the circuit is as follows. In multiphase power converter  1300 , it is desired to convert an input voltage at node  1302  to an output voltage at node  1304  across a load  1306 . In a manner similar to that described above with respect to FIG. 2, transistors  1312  and  1314  are each coupled to the input voltage  1302 . Coupled to the junction  1311  of transistors  1312  and  1314  is inductor  1316  and zero current detector  1315 . Similarly, transistors  1322  and  1324  are each coupled to the input voltage  1302 . Coupled to the junction  1321  of transistors  1322  and  1324  is inductor  1326  and zero current detector  1325 . Similarly, transistors  1332  and  1334  are each coupled to the input voltage  1302 . Coupled to the junction  1331  of transistors  1332  and  1334  is inductor  1336  and zero current detector  1335 . Similarly, transistors  1342  and  1344  are each coupled to the input voltage  1302 . Coupled to the junction  1341  of transistors  1342  and  1344  is inductor  1346  and zero current detector  1345 . Each of the transistor pairs is coupled to capacitor  1308  to provide the output needed at output  1304 .  
         [0049]    The use of the zero current detector has a profound effect on the operation of the power converter. It is understood that, when the load to a power converter increases, there is a corresponding increase in the current. Typically, when such an increase in the current occurs, there is a corresponding decrease in the voltage at the load. With reference to FIG. 14, the load voltage/time curve of an exemplary power converter of the prior art is shown. The voltage begins at a level of approximately 1.15 volts. When a load transient occurs and more current is being drawn from the power converter, the voltage at the load decreases to approximately 0.85 volts and remains lower than required for a certain time period, while the power converter is adapting to the change in current. Once the power converter has adapted, the output voltage is back at the specified 1.15 volts. Modern electronics require a very steady supply voltage in order to operate correctly. Such a prolonged droop in the voltage can be very detrimental to the operation of certain electronic components.  
         [0050]    As described above, the typical configuration of switches in a power converter switches the high side switch and the low side switch simultaneously, such that only one of the switches is on at one time. During transients, however, there may be an occasion when both switches are off at one time, with the high side switch pulsing, in order to supply more current to the load. In addition, as described above, when a zero current condition is detected, both switches may be off, to prevent current from flowing into ground. Thus, it can be seen that, in order to supply more current to the load, the low-side transistor (transistors  1314 ,  1324 ,  1334 , and  1344 ) is turned off.  
         [0051]    One reason for the voltage droop is because, if the low-side switch is on, it must be turned off before the current to the load can be increased. However, with the combination of the zero current detection circuit and the multiphase power converter, it can be seen that there is a greater likelihood of the low-side switches being off, resulting in a faster transient response. With reference to FIG. 16, the operation of the multiphase power converter with the zero current detector will be graphically described.  
         [0052]    [0052]FIG. 16 shows the current/time graph of the 4-phase, multiphase power converter, along with the individual inductor currents. During region  1602  of the graph, one of the individual phases is at zero current, forcing off both the low side and high side switches. As described above, when the current through an individual inductor is rising, the high-side switch is on and the low-side switch is off. When the current through an individual inductor is falling, the high-side switch is off and the low-side switch is on. It can be seen that, during region  1602 , of the four different phases, only one or two of the other phases simultaneously have falling inductor current. Therefore, only one or two low-side switches are on at once. Thus, during a load transient, there is a lesser necessity to turn off low-side switches to meet the higher current requirements. This results in a faster response to transients due to increased load.  
         [0053]    It should also be understood that such an improved transient response time is also present in the embodiment shown in FIG. 5, for the same reason.  
         [0054]    The above description presents exemplary modes contemplated in carrying out the invention. The techniques described above are, however, susceptible to modifications and alternate constructions from the embodiments shown above. Other variations and modifications of the present invention will be apparent to those of ordinary skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. For example, the present invention may be used in a multiphase power converter with multiple low-side switches. The multiple low-side switches may be switched off (e.g., using a tri-state mode of operation) to decrease the response time of the converter. While the zero current detection circuit was described with respect to a buck power regulator, it can be used in various other applications. For example, the zero current detection circuit can be used in a highly-phased power regulation system such as those used in low-voltage conversion applications such as for microprocessor loads. The ZCD may be used as part of a power IC to detect when a switching element is at ground. When zero current is detected, the operation of the power regulation system may be changed to minimize various inefficiencies that may be present due to high RMS currents.  
         [0055]    Consequently, it is not the intention to limit the invention to the particular embodiments disclosed. On the contrary, the invention is intended to cover all modifications and alternate constructions falling within the scope of the invention, as expressed in the following claims when read in light of the description and drawings. No element described in this specification is necessary for the practice of the invention unless expressly described herein as “essential” or “required.”