Abstract:
An apparatus for and method of achieving current balancing among phases of a multi-phase power supply by reducing and controlling the temperature variation among packages disposed within each phase. Each package contains a dc-to-dc converter (e.g. a buck converter), a temperature sensor and may also contain a driver, which supplies a pulse train for driving the converter. By controlling the temperature among phases to within ±X % a balancing of currents among phases to within ±X/2% is achieved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates in general to power supplies. More specifically, the present invention relates to current sharing in multiphase switching power supplies.  
           [0002]    As microprocessors and their supporting components become larger and more complex, the load presented by these devices becomes heavier. As a consequence, the power supply, which powers the microprocessor and its supporting components, must be capable of providing higher currents.  
           [0003]    One way of increasing the current capability of a power supply is to use two or more phases or multi-phase switching mode converters to drive the load. Typically, these multi-phase supplies are designed with phase interleaving in order to reduce ripple in the converted signal.  
           [0004]    A recognized problem associated with multi-phase power supplies relates to an uneven distribution of load currents carried among the various phases of the supply. If this uneven distribution is substantial, a single phase may be burdened with supplying most of the load current. This problem not only limits the output capability of the supply, it also affects its reliability. Accordingly, methods of equalizing the distribution of load current among phases have been sought and developed. These methods have often been referred to in the art as “current sharing” methods.  
           [0005]    One “current sharing” approach measures the voltage dropped across precision power resistors placed in the load current path of each of the power supply phases. This approach is undesirable for at least two reasons. First, use of precision power resistors is costly, especially as current demands continue to rise, since the resistors must be made physically larger. Second, because precision power resistors dissipate wasted power, the efficiency of the power supply is compromised.  
           [0006]    To avoid reliance on highly dissipative and costly precision resistors, an alternative current sharing approach measures the voltage drop across switching elements in one or more phases of the supply. Because these voltage drops are proportional to the currents supplied by the corresponding power phases, they can be used to direct current sharing among power stages. Whereas this approach may be an improvement, in that it does not require use of precision power resistors, a problem is that it is not a very accurate method. For example, individual phase currents, derived from the measured voltages, can produce inaccuracies of up to 60%.  
           [0007]    Accordingly, there exists a need for a simple, inexpensive and accurate current sharing method, which can be used in multi-phase power supplies.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    According to a first aspect of the invention, a current sharing multi-phase power supply comprises a first phase unit, having a first package containing a first dc-to-dc converter. The dc-to-dc converter of the first phase unit supplies a first portion of a load current to a load. The first phase unit further includes a first pulse width modulator coupled to the first converter. This first pulse width modulator functions to provide a first periodic pulse train to the first converter. The multi-phase supply also has at least a second phase unit, which includes a second dc-to-dc converter and a second pulse width modulator coupled to the second converter. The second pulse width modulator functions to provide a second periodic pulse train to the second converter and the second dc-to-dc converter supplies a second and remaining portion of the load current to the load. The supply further comprises a temperature control bus, which is coupled to the first and second phase units. The temperature control bus has a bus temperature related to temperatures of the first and second packages. Duty cycles of the first and second pulse trains are adjusted in a feedback operation, the extent of the adjustments depending on a relationship among the temperatures of the first and second packages and the bus temperature. Depending on the application, the bus voltage on the temperature control bus represents either an average or a peak of the measured temperatures of the first and second packages.  
           [0009]    In a second aspect of the invention, a multi-phase power supply having a plurality of phase blocks is disclosed. Each phase block comprises a package containing a dc-to-dc converter, a temperature sensor and a temperature-to-voltage converter. Each phase block also includes a pulse width modulator disposed within a feedback loop and coupled between an output of the temperature-to-voltage converter and an input of the converter.  
           [0010]    In a third aspect of the invention, a method of promoting current sharing among phases of a multi-phase power supply comprises the steps of, measuring a temperature of a package containing a dc-to-dc converter for each phase and, for each phase, employing a feedback control means, disposed between an output of the supply and a control input of the converter, to control and reduce the temperature variation and current variation among packages.  
           [0011]    A further understanding of the nature and the advantages of the inventions disclosed herein may be realized by reference to the remaining portions of the specification and the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows an exemplary single-phase power supply, which houses a converter and a temperature sensor within a single package, according to an embodiment of the present invention;  
         [0013]    [0013]FIG. 2 shows an exemplary multi-phase power supply, which employs an average temperature control function to achieve current sharing among phases of the supply, according to an embodiment of the present invention;  
         [0014]    [0014]FIG. 3 shows an alternative, exemplary single-phase power supply, which houses a converter and a temperature sensor within a single package, according to an embodiment of the present invention; and  
         [0015]    [0015]FIG. 4 shows an exemplary multi-phase power supply, which employs a peak temperature control function to achieve current sharing among phases of the supply, according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring first to FIG. 1, there is shown an exemplary single-phase power supply  10 , which houses a converter  100  and a temperature sensor  102  within a single package  110 , according to an embodiment of the present invention. Converter  100  may comprise a buck converter having one or more switching elements, which may be, for example, metal-oxide-semiconductor field effect transistors (MOSFETs), bipolar junction transistors (BJTs), silicon controlled rectifiers (SCRs), or the like. If a buck converter, inductor  104  and capacitor  106  actually comprise part of the converter and function as current and voltage filters for reducing ripple in the current and voltage signals at the output of converter  100 .  
         [0017]    In the single-phase embodiment shown in FIG. 1, converter  100 , temperature sensor  102  and a temperature-to-voltage converter  108  are contained in a single, tightly-coupled package  110 . Temperature sensor  102  is coupled to package  110 , preferably in close proximity to a switching element of converter  100 , to measure and determine the temperature of the converter  100 . The measured temperature is converted to a voltage, V C , by temperature-to-voltage converter  108 , which may or may not be included within package  110 . Temperature-to-voltage converter  108  may comprise, for example, a diode having a temperature dependent voltage, or a commercially available monolithic integrated circuit, which integrates both the temperature sensor  102  and temperature-to-voltage converter  108  in a single package.  
         [0018]    Once the temperature of converter  100  is determined by temperature sensor  102 , the difference between a temperature-dependent voltage, V BUS , on a temperature control bus  112  and V C  are input into an operational amplifier (op-amp)  114 . The output of op-amp  114  is represented by a voltage, V, which has a positive value if (V BUS −V C )&gt;0 and a negative value if (V BUS −V C )&lt;0. Similarly, the output voltage increment, ΔV, of the error amplifier  116  in FIG. 1 has a positive value, if an input reference voltage, V REF , is greater than the voltage drop, V L , across the load  118  and has a negative value if V REF &lt;V L .  
         [0019]    As shown in FIG. 1, V and ΔV are summed by a summer  115  and then input into pulse width modulator (PWM)  120 . Another input to PWM  120  is a periodic sawtooth waveform, V SAW . From (V+ΔV) and V SAW , PWM  120  generates a periodic, on-off switching signal, V SWITCH , which controls the switching elements of converter  100 , via a driver  122 . It should be noted that, while driver  122  in the embodiment shown in FIG. 1 is located outside package  110 , in an alternative embodiment, driver  122  may be packaged within package  110 .  
         [0020]    The duty cycle of V SWITCH  is dependent upon and modified by the magnitude of the sum of (V+ΔV). If (V BUS −V C )&gt;0 and (V REF −V L )&gt;0, then (V+ΔV) is positive and PWM  120  operates to increase the duty cycle of V SWITCH . The increase in duty cycle causes the average load voltage to increase and, consequently, the temperature of converter  100  rises in response. On the other hand, if (V BUS −V C )&lt;0 and (V REF −V L )&lt;0 then (V+ΔV) is negative and the PWM  120  operates to decrease the duty cycle of V SWITCH . The decrease in duty cycle causes the average load voltage to decrease and, consequently, the temperature of converter  100  drops. In either case, the feedback operation continues, until the temperature of converter  100  and, consequently, package  110  becomes approximately equal to the temperature of temperature control bus  112 .  
         [0021]    Referring now to FIG. 2, there is shown an exemplary multi-phase power supply  20 , which employs an average temperature control function to achieve current sharing among phases of the supply, according to an embodiment of the present invention. Multiphase power supply  20  comprises N phases, where N is an integer, which is greater than or equal to one, and represents the maximum number of phases in the supply. If N=1, multi-phase power supply  20  reduces to the single-phase power supply  10  shown in FIG. 1. However, if N&gt;1, then each phase of multi-phase power supply  20  shares a single error amplifier  216  and is coupled to a single temperature control bus  212 , as shown in FIG. 2.  
         [0022]    Like the single-phase embodiment shown in FIG. 1, the phases of the multi-phase power supply shown in FIG. 2 contain single, tightly-coupled packages  210 - 1  through  210 -N. Packages  210 - 1  through  210 -N comprise converters  200 - 1  through  200 -N, temperature sensors  202 - 1  through  202 -N and temperature-to-voltage converters  208 - 1  through  208 -N, respectively. Temperature sensors  202 - 1  through  202 -N are coupled to their respective packages  210 - 1  through  210 -N, preferably in close proximity to one or more  10  switching element to measure the temperatures of their corresponding packages  210 - 1  through  210 -N. The measured temperatures are converted to voltages V C-1  through V C-N  by temperature-to-voltage converters  208 - 1  through  208 -N. Although temperature-to-voltage converters  208 - 1  through  208 -N are shown as being included within packages  210 - 1  through  210 -N, in an alternative embodiment they are located outside packages  210 - 1  through  210 -N.  
         [0023]    The converted voltages V C-1  through V C-N  of phases 1 through N are coupled to the inverting inputs of op-amps  214 - 1  through  214 -N, respectively. A common temperature control bus  212  is coupled to the non-inverting inputs of the op-amps  214 - 1  through  214 -N. Each of op-amps  214 - 1  through  214 -N also has a resistor R coupled across its inputs. These resistors preferably have the same resistance and have voltage drops across them of (V BUS −V C-1), (V   BUS −V C-2 ) . . . (V BUS -V C-N ). Using a Thevenin or Norton transformation of these voltage drops across the resistors R (or by applying some other appropriate circuit analysis), it can be shown that V BUS  represents the average of the converted voltages V C-1  through V C-N , or, in other words, V BUS (avg)=(V C-1 +V C-2 + . . . V C-N )/N. Accordingly, since the converted voltages V C-1  through V C-N  represent the temperatures of phases 1 through N, the temperature on temperature control bus  212 , as represented by V BUS (avg), is an average of the temperatures of packages  210 - 1  through  210 N.  
         [0024]    The outputs of op-amps  214 - 1  through  214 -N are labeled as voltages V 1  through VN, respectively. The polarity of each of voltages V 1  through VN is determined by whether the converted voltages V C-1  through V C-N  are greater than or less than V BUS (avg). Each of V 1  through VN are individually summed with the output voltage increment, ΔV, of common error amplifier  216 , by summers  215 - 1  through  215 -N, to provide input signals V 1 +ΔV through VN+ΔV, which are coupled to a pulse width correction input of PWMs  220 - 1  through  220 -N, respectively. Each of PWMs  220 - 1  through  220 -N has a second input for a sawtooth signal, similar to what was described for the single-phase supply. (To enhance clarity, these sawtooth inputs and signals are not shown in FIG. 2.) Preferably, the sawtooth signals among phase 1 through N are interleaved, i.e., have predetermined phase differences among the phases. Interleaving reduces ripple on the output signal, V L , dropped across load  218  and also allows for a smaller filter capacitor  206  to be used.  
         [0025]    PWMs  220 - 1  through  220 -N generate periodic, on-off switching signals, V SWITCH-1  through V SWITCH-N . These switching signals V SWITCH-1  through V SWITCH-N  control the switching elements of converters  200 - 1  through  200 -N, via drivers  222 - 1  through  222 -N, respectively. It should be noted that, while drivers  222 - 1  through  222 -N are shown as being located outside packages  210 - 1  through  210 -N, in an alternative embodiment, drivers  222 - 1  through  222 -N are located within packages  210 - 1  through  210 -N, respectively.  
         [0026]    The duty cycles of on-off switching signals V SWITCH-1  through V SWITCH-N  depend upon the magnitudes of the sums of (V 1 +ΔV) through (VN+ΔV). For example, focusing on phase 1 of supply  20 , if (V Bus (avg)−V C-1 )&gt;0 and (V REF −V L )≧0, then (V 1 +ΔV) is positive and PWM  220 - 1  operates to increase the duty cycle of V SWITCH-1 . The increase in duty cycle of V SWITCH-1  causes the portion of the load current supplied by converter  200 - 1  to increase and, consequently, the temperature of converter  200 - 1  rises in response. On the other hand, if (V Bus (avg)−V C-1 )&lt;0 and (V REF −V L )≦0, then (V 1 +ΔV) is negative and PWM  220 - 1  operates to decrease the duty cycle of V SWITCH-1 . The decrease in duty cycle of V SWITCH-1  causes the portion of the load current supplied by converter  200 - 1  to decrease and, consequently, the temperature of converter  200 - 1  drops in response. In either case, the feedback operation continues, until the temperature of converter  200 - 1  and, consequently, package  210 - 1  becomes approximately equal to the average temperature on temperature control bus  212 . The feedback operation for the remaining phases of supply  20  is similar to that of phase 1. Accordingly, the feedback operation causes all phases 1 through N to converge to the average temperature on temperature control bus  212 .  
         [0027]    Each of packages  210 - 1  through  210 -N are, preferably, tightly-coupled and include a substantially similar heat sink. Under these conditions, the temperature rise ΔT of each package is equal to the product of the power dissipated by the heat-sinked package (i.e. P diss ) and the thermal resistance of the heat sink, R θ . Accordingly, since P diss  is proportional to I 2 , ΔT is also proportional to I 2 . From these relationships, it is seen that the variation in current from phase to phase of supply  20  can be balanced by the temperature control operation described above. In fact, it can be shown that by controlling the temperature among phases within a range of ±X %, a current balance of ±X/ 2 % among phases is achieved.  
         [0028]    Referring now to FIG. 3, there is shown an alternative, exemplary single-phase power supply  30 , which houses a converter  300  and a temperature sensor  302  within a single package  310 , according to another embodiment of the present invention. The primary physical difference between the embodiments shown in FIGS. 1 and 3 relates to the component coupled between the inputs of the op-amp coupled to the temperature-to-voltage converter. Whereas the component in FIG. 1 is a resistor, the component in FIG. 3 is a diode (labeled “D”) having an anode coupled to the inverting input and a cathode coupled to the non-inverting input of op-amp  314 . As explained below, the single-phase supply  30  can be used in a multi-phase power supply to control the temperature among phases of the multiphase supply to achieve current balancing among phases.  
         [0029]    Converter  300  of the single-phase power supply  30  may comprise a buck converter having one or more switching elements, which may be, for example, metal-oxide-semiconductor field effect transistors (MOSFETs), bipolar junction transistors (BJTs), silicon controlled rectifiers (SCRs), or the like. If a buck converter, inductor  304  and capacitor  306  actually comprise part of the converter and function as current and voltage filters for reducing ripple in the current and voltage signals at the output of converter  300 .  
         [0030]    In the single-phase embodiment shown in FIG. 3, converter  300 , temperature sensor  302  and a temperature-to-voltage converter  308  are contained in a single, tightly-coupled package  310 . Temperature sensor  302  is coupled to package  310 , preferably in close proximity to a switching element of converter  300 , to measure and determine the temperature of the converter  300 . The measured temperature is converted to a voltage, V C , by temperature-to-voltage converter  308 , which may or may not be included within package  310 . Temperature-to-voltage converter  308  may comprise, for example, a diode having a temperature dependent voltage, or a commercially available monolithic integrated circuit, which integrates both the temperature sensor  302  and temperature-to-voltage converter  308  in a single package.  
         [0031]    Once the temperature of converter  300  is determined by temperature sensor  302 , the difference between a temperature-dependent voltage, V BUS , on a temperature control bus  312  and V C  are input into an operational amplifier (op-amp)  314 . The output of op-amp  314  is represented by a voltage, V, which is substantially equal to zero, if V C =V BUS , and is positive if V C &lt;V BUS . The output voltage increment, ΔV, of the error amplifier  316  has a positive value, if an input reference voltage, V REF , is greater than the voltage drop, V L , across the load  318  and has a negative value if V REF &lt;V L .  
         [0032]    As shown in FIG. 3, V and ΔV are summed by a summer  315  and then input into pulse width modulator (PWM)  320 . Another input to PWM  320  is a periodic sawtooth waveform, V SAW . From (V+ΔV) and V SAW , PWM  320  generates a periodic, on-off switching signal, V SWITCH , which controls the switching elements of converter  300 , via a driver  322 . It should be noted that, while driver  322  in the embodiment shown in FIG. 3 is located outside package  310 , in an alternative embodiment, driver  322  may be packaged within package  310 .  
         [0033]    The duty cycle of V SWITCH  is dependent upon and modified by the magnitude of the sum of (V+ΔV). If V BUS &gt;V C  and V REF ≧V L , then (V+ΔV) is positive and PWM  320  operates to increase the duty cycle of V SWITCH . The increase in duty cycle causes the average load voltage to increase and, consequently, the temperature of converter  300  rises in response. The feedback operation continues, until the temperature of converter  300  and, consequently, package  310  becomes equal to the temperature on temperature control bus  312 , after which the feedback operation stops until V BUS  once again becomes larger than V C .  
         [0034]    Referring now to FIG. 4, there is shown an exemplary multi-phase power supply  40 , which employs a peak temperature control function to achieve current sharing among phases of the supply, according to an embodiment of the present invention. Multiphase power supply  40  comprises N phases, where N is an integer, which is greater than or equal to one, and represents the maximum number of phases in the supply. If N=1, multiphase power supply  40  reduces to the single-phase power supply  30  shown in FIG. 3. However, if N&gt;1, then each phase of multi-phase power supply  40  shares a single error amplifier  416  and is coupled to a single temperature control bus  412 , as shown in FIG. 4.  
         [0035]    Like the single-phase embodiment shown in FIG. 3, the phases of the multiphase power supply shown in FIG. 4 contain single, tightly-coupled packages  410 - 1  through  410 -N. Packages  410 - 1  through  410 -N comprise converters  400 - 1  through  400 -N, temperature sensors  402 - 1  through  402 -N and temperature-to-voltage converters  408 - 1  through  408 -N, respectively. Temperature sensors  402 - 1  through  402 -N are coupled to their respective packages  410 - 1  through  410 -N, preferably in close proximity to one or more switching elements to measure the temperatures of their corresponding packages  410 - 1  through  410 -N. The measured temperatures are converted to voltages V C-1  through V C-N  by temperature-to-voltage converters  408 - 1  through  408 -N. Although temperature-to-voltage converters  408 - 1  through  408 -N are shown as being included within packages  410 - 1  through  410 -N, in an alternative embodiment they are located outside packages  410 - 1  through  410 -N.  
         [0036]    The converted voltages V C-1  through V C-N  of phases 1 through N are coupled to the inverting inputs of op-amps  414 - 1  through  414 -N, respectively. A common temperature control bus  412  is coupled to the non-inverting inputs of the op-amps  414 - 1  through  414 -N. Each of op-amps  414 - 1  through  414 -N also has a diode (labeled “D”) coupled across its inputs, the anode coupled to the inverting input and the cathode coupled to the non-inverting input. In this embodiment, the temperature on temperature control bus  412  is represented by V BUS (peak), which represents the highest temperature of the converted voltages V C-1  through V C-N . The diodes, V BUS (peak), and the feed back operation within phases 1 through N together ensure that the temperature of any of the packages  410 - 1  through  410 -N does not exceed the temperature on temperature control bus  412 , which is characterized by V BUS (peak).  
         [0037]    The outputs of op-amps  414 - 1  through  414 -N are labeled as voltages VI through VN, respectively. Any of V 1  through VN are nonzero if the corresponding converted voltages V C-1  through V C-N  are less than V BUS (peak). Here it is assumed that the diodes are ideal. Accordingly, a diode presents itself as a short circuit if a particular V C  is greater than V BUS  (peak) and presents itself as an open circuit if V C  is less than V BUS  (peak). Whereas it has been assumed and it is preferred that the diodes in this embodiment are ideal, it is to be understood that they need not necessarily be.  
         [0038]    Each of V 1  through VN are individually summed with the output voltage increment, ΔV, of common error amplifier  416 , by summers  415 - 1  through  415 -N, to provide input signals V 1 +ΔV through VN+ΔV. V 1 +ΔV through VN+ΔV are coupled to a pulse width correction input of PWMs  420 - 1  through  420 -N, respectively. Each of PWMs  420 - 1  through  420 -N has a second input for a sawtooth signal, similar to what was described for the single-phase supply shown in FIG. 3. (To enhance clarity, these sawtooth signals are not shown in FIG. 4.) Preferably, the sawtooth signals among phase 1 through N are interleaved, i.e., have predetermined phase differences among the phases. Interleaving reduces ripple on the output signal, V L , dropped across load  418  and also allows for a smaller filter capacitor  406  to be used.  
         [0039]    PWMs  420 - 1  through  420 -N generate periodic, on-off switching signals, V SWITCH-1  through V SWITCH-N . These switching signals V SWITCH-1  through V SWITCH-N  control the switching elements of converters  400 - 1  through  400 -N, via drivers  422 - 1  through  422 -N, respectively. It should be noted that, while drivers  422 - 1  through  422 -N are shown as being located outside packages  410 - 1  through  410 -N, in an alternative embodiment, drivers  422 - 1  through  422 -N are located within packages  410 - 1  through  410 -N, respectively.  
         [0040]    The duty cycles of on-off switching signals V SWITCH-1  through V SWITCH-N  depend upon the magnitudes of the sums of (V 1 +ΔV) through (VN+ΔV). For example, focusing on phase 1 of supply  40 , if (V BUS (peak)−V C-1 )&gt;0 and (V REF −V L )&gt;0, then (V 1 +ΔV) is positive and PWM  420 - 1  operates to increase the duty cycle of V SWITCH-1 . The increase in duty cycle of V SWITCH-1  causes the portion of the load current supplied by converter  400 - 1  to increase and, consequently, the temperature of converter  400 - 1  rises in response. On the other hand, if (V BUS (peak)−V C-1 )&lt;0, V 1  is equal to zero and, depending on the sign of ΔV, PWM  420 - 1  operates to either increase or decrease the duty cycle of V SWITCH-1 . Under these conditions, the feedback operation continues ensuring that the temperature of converter  400 - 1  and, consequently, package  410 - 1  remain less than the peak temperature on temperature control bus  412 , which is characterized by V BUS (peak). The feedback operation for the remaining phases of supply  40  is similar to that of phase 1. Accordingly, the feedback operation causes the temperatures on all phases 1 through N to remain below the peak temperature on temperature control bus  412 .  
         [0041]    Each of packages  410 - 1  through  410 -N are, preferably, tightly-coupled and include a substantially similar heat sink. Under these conditions, the temperature T of each package is equal to the product of the power dissipated by the heat-sinked package (i.e. P diss ) and the thermal resistance of the heat sink, R θ . Accordingly, since P diss  is proportional to I 2 , T is also proportional to I 2 . From these relationships, it is seen that the variation in current from phase to phase of supply  20  can be balanced by the temperature control operation described above. In fact, it can be shown that by controlling the temperature among phases within a range of ±X %, a current balance of ±X/2% among phases can be achieved.  
         [0042]    While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. For example, the feedback control loops of the various phases of the multi-phase power supply embodiments shown in FIGS. 2 and 4 need not exactly comprise the exact sequence and combination of an op-amp, a summer, a shared error amplifier, a PWM and a driver. Indeed, the feedback control loop, as conceived by the inventor, may comprise any circuitry, which operates to feedback a signal for adjusting the duty cycle provided by the PWM, based on the relative temperatures of the phase&#39;s package and the temperature control bus. For this and other reasons, therefore, the above description should not be taken as limiting the scope of the invention, which is defined by the appended claims.