Abstract:
A control system including a DC/DC converter and a control module. The DC/DC converter includes a first inductor and a second inductor. The DC/DC converter is configured to i) receive a first DC voltage and ii) output a second DC voltage. The control module is configured to, during a first operation mode, charge the first inductor while discharging the second inductor, and, during a second operation mode, one of i) charge the first inductor while charging the second inductor and ii) discharge the first inductor while discharging the second inductor. The control module is further configured to initiate the second operation mode in response to detecting a current transient in the DC/DC converter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/710,384 (now U.S. Pat. No. 7,679,347), filed on Feb. 23, 2007, which is a continuation of U.S. patent application Ser. No. 10/890,491 (now U.S. Pat. No. 7,190,152), filed on Jul. 13, 2004, and relates to U.S. patent application Ser. No. 10/621,058 (now U.S. Pat. No. 7,161,342), filed on Jul. 15, 2003, entitled “Low Loss DC/DC Converter”, U.S. patent application Ser. No. 10/754,187, filed on Jan. 8, 2004, which is a continuation-in-part of U.S. patent application Ser. No. 10/693,787, filed on Oct. 24, 2003, and U.S. patent application Ser. No. 10/810,452, filed on Mar. 26, 2004. The disclosures of the above applications are all hereby incorporated by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to DC/DC converters, and more particularly to digital control systems for DC/DC converters. 
     BACKGROUND OF THE INVENTION 
     DC/DC converters are electronic devices that employ inversion and/or rectification to transform DC voltage at a first level into DC voltage at a second level. For example, a DC/DC converter may step-up DC voltage, step-down DC voltage, or may be capable of both stepping up and stepping down DC voltage. DC/DC converters typically include one or more inductors. Inductors are circuit elements that operate based on magnetic fields. The source of the magnetic field is charge that is in motion, or current. If current varies with time, the magnetic field that is induced also varies with time. A time-varying magnetic field induces a voltage in conductors that are linked by the magnetic field. 
     Referring to  FIG. 1A , a DC/DC converter  10  includes an inductor  12 . Inductors  12  in DC/DC converters  10  typically communicate with at least one switch and at least one capacitor. For example, the switch may be a transistor and the capacitor may be an output capacitor that filters an output voltage of the DC/DC converter  10 . A control module may communicate with the switch to control when the inductor  12  charges or discharges. For example, when the switch is on, the input current may flow through the switch and inductor  12  to the capacitor while building up the magnetic field of the inductor  12 . When the switch is off, the inductor  12  opposes the drop in current and supplies current to the capacitor. 
     Referring now to  FIGS. 1B and 1C , one or more conductors form coupled inductor circuits  14  and  16 , respectively. In  FIG. 1B , first and second conductors pass through the same magnetic core and exhibit mutual coupling with a coupling coefficient that is approximately equal to 1. In  FIG. 1C , a single conductor passes through the magnetic core two or more times and exhibits mutual coupling with a coupling coefficient that is approximately equal to 1. Those skilled in the art can appreciate that still other inductor circuits may be employed. In  FIGS. 1B and 1C , the coupled inductor circuits  14  and  16  are implemented in DC/DC converters  18  and  20 , respectively. DC/DC converters  18  and  20  that employ coupled inductor circuits  14  and  16  have a fast response with small voltage ripple and high efficiency. 
     Control modules in DC/DC converters generate control signals to turn the switches on an off and to adjust a rate at which the inductors charge and discharge. The control signals typically have fixed frequencies and duty cycles to obtain predetermined output voltages. However, when the control module maintains control signals at a fixed frequency and duty cycle, the control module is unable to adapt to changing circuit conditions. 
     SUMMARY OF THE INVENTION 
     A closed-loop control system for a DC/DC converter according to the present invention includes a DC/DC converter that receives a first DC voltage and that generates a second DC voltage. The DC/DC converter includes first and second inductances. A control module communicates with the DC/DC converter, receives the second DC voltage, and generates at least one control signal to one of charge or discharge the first and second inductances. The control module has first and second modes. During the first mode the control module alternately charges one of the first and second inductances and discharges the other of the first and second inductances. During the second mode the control module one of charges or discharges both of the first and second inductances. 
     In other features, the control module initiates the second mode when a transient condition occurs in the DC/DC converter. The control module detects the transient condition when the second DC voltage is one of greater than a first predetermined voltage or less than a second predetermined voltage. During the second mode the control module initiates the first mode when the second DC voltage is both less than the first predetermined voltage and greater than the second predetermined voltage. The control module discharges both of the first and second inductances when the second voltage is greater than the first predetermined voltage. The control module charges both of the first and second inductances when the second DC voltage is less than the second predetermined voltage. 
     In still other features of the invention, the DC/DC converter includes an output capacitance. The control module detects the transient condition when current through the output capacitance is one of greater than a first predetermined current or less than a second predetermined current. During the second mode the control module initiates the first mode when the current is less than the first predetermined current and greater than the second predetermined current. The control module charges both of the first and second inductances when the current is less than the second predetermined current. The control module discharges both of the first and second inductances when the current is greater than the first predetermined current. The control module determines the current based on a rate of change of the second DC voltage. The current is one of greater than the first predetermined current or less than the second predetermined current when the rate of change is greater than a predetermined rate of change. The control module determines the current based on an average value of the second DC voltage during a predetermined time period. 
     In yet other features, during the second mode the control module initiates the first mode after a predetermined time period. The DC/DC converter includes an output capacitance. The output capacitance discharges when the control module discharges both of the first and second inductances. The DC/DC converter includes an output capacitance. The output capacitance charges when the control module charges both of the first and second inductances. 
     In still other features of the invention, the DC/DC converter includes first, second, third, and fourth switches. Second terminals of the first and third switches communicate with first terminals of the second and fourth switches, respectively. First terminals of the first and third switches communicate. Second terminals of the second and fourth switches communicate. A first end of the first inductance communicates with the second terminal of the third switch and the first terminal of the fourth switch. A first end of the second inductance communicates with the second terminal of the first switch and the first terminal of the second switch. Second ends of the first and second inductances communicate. A capacitance has a first end that communicates with the second ends of the first and second inductances and a second end that communicates with the second terminals of the second and fourth switches. 
     In yet other features, the first, second, third, and fourth switches comprise transistors. The control module generates first, second, third, and fourth control signals that communicate with control terminals of the first, second, third, and fourth switches, respectively. The control module asserts the third and fourth control signals to charge the first inductance and the first and second control signals to charge the second inductance. The first DC voltage is input to the first terminals of the first and third switches. The second DC voltage is referenced from the first end of the capacitance. The DC/DC converter includes a current source. A first end of the current source communicates with the second end of the first and second inductances and the first end of the capacitance and a second end of the current source communicates with the second terminals of the second and fourth switches and the second end of the capacitance. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1A  is a functional block diagram and electrical schematic of an inductor implemented in an exemplary DC/DC converter according to the prior art; 
         FIG. 1B  is a functional block diagram and electrical schematic of a coupled inductor circuit with two conductors implemented in an exemplary DC/DC converter according to the prior art; 
         FIG. 1C  is a functional block diagram and electrical schematic of a coupled inductor circuit with one conductor implemented in an exemplary DC/DC converter according to the prior art; 
         FIG. 2  is a functional block diagram and electrical schematic of a coupled-inductor DC/DC converter with a control module that implements an open-loop control system according to the present invention; 
         FIG. 3  is a timing diagram that illustrates the control signal waveforms generated by the control module of  FIG. 2  including alternating charging and discharging of the first and second inductors; 
         FIG. 4  is a functional block diagram of a closed-loop control system for a DC/DC converter; 
         FIG. 5  is a graph showing the output voltage of the DC/DC converter in  FIG. 4  as a function of time; 
         FIG. 6  is a timing diagram that illustrates the control signal waveforms generated by the control module of  FIG. 4  including an overlap of the charging pattern for the first and second inductors; 
         FIG. 7  is an electrical schematic of the closed-loop DC/DC control system of  FIG. 4 ; 
         FIG. 8  is a flowchart illustrating steps performed by the control module of  FIGS. 4 and 7  including initiating same-phase operation of the first and second inductors for a predetermined time period; and 
         FIG. 9  is a flowchart illustrating steps performed by the control module of  FIGS. 4 and 7  including initiating same-phase operation of the first and second inductors while a variable is outside of a predetermined range. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     Referring now to  FIG. 2 , an open-loop control system  28  for a DC/DC converter  30  includes a control module  32 . The DC/DC converter  30  includes first, second, third, and fourth transistors  34 ,  36 ,  38 , and  40 , respectively. Sources (or second terminals) of the first and third transistors  34  and  38 , respectively, communicate with drains (or first terminals) of the second and fourth transistors  36  and  40 , respectively. Drains of the first and third transistors  34  and  38 , respectively, communicate and sources of the second and fourth transistors  36  and  40 , respectively, communicate. 
     First and second inductors  42  and  44 , respectively, form a coupled inductor circuit  46 . A first end of the first inductor  46  communicates with the source of the third transistor  38  and the drain of the fourth transistor  40 . A first end of the second inductor  44  communicates with the source of the first transistor  34  and the drain of the second transistor  36 . Second ends of the first and second inductors  42  and  44 , respectively, communicate. A first end of a capacitor  48  communicates with the second ends of the first and second inductors  42  and  44 , respectively. 
     A second end of the capacitor  48  communicates with the sources of the second and fourth transistors  36  and  40 , respectively. A first end of a current source  50  communicates with the first end of the capacitor  48  and the second ends of the first and second inductors  42  and  44 , respectively. A second end of the current source  50  communicates with the sources of the second and fourth transistors  36  and  40 , respectively, and the second end of the capacitor  48 . A input DC voltage  52  (V in ) of the DC/DC converter  30  communicates with the drains of the first and third transistors  34  and  38 , respectively. An output DC voltage  54  (V out ) of the DC/DC converter  30  in referenced from the second ends of the first and second inductors  42  and  44 , respectively, the first end of the capacitor  48 , and the first end of the current source  50 . 
     The control module  32  generates first, second, third, and fourth control signals U 2 , D 2 , U 1 , and D 1  that communicate with gates (or control terminals) of the first, second, third, and fourth transistors  34 ,  36 ,  38 , and  40 , respectively. The control module  32  charges the first inductor  42  by setting the third and fourth control signals U 1  and D 1 , respectively, high (or low) and discharges the first inductor  42  by setting the third and fourth control signals U 1  and D 1 , respectively, low (or high). 
     The control module  32  charges the second inductor  44  by setting the first and second control signals U 2  and D 2 , respectively, high (or low) and discharges the second inductor  44  by setting the first and second control signals U 2  and D 2 , respectively, low (or high). Based on a frequency and duty cycle of the control signals, the DC/DC converter  30  transforms the input DC voltage  52  into the output DC voltage  54 , which is at a different level than the input DC voltage  52 . 
     Referring now to  FIG. 3 , signal waveforms of the third and fourth control signals U 1  and D 1 , respectively, indicated by  62 , and of the first and second control signals U 2  and D 2 , respectively, indicated by  64 , are shown as square waveforms. The control module  32  maintains the signal waveforms for the first, second, third, and fourth control signals U 2 , D 2 , U 1 , and D 1 , respectively, at a predetermined frequency and duty cycle so that the DC/DC converter  30  generates a desired voltage. Signal waveforms  62  of the third and fourth control signals U 1  and D 1 , respectively, are complementary (or 180 degrees out-of-phase) to the signal waveforms  64  of the first and second control signals U 2  and D 2 , respectively. Therefore, when the first inductor  42  charges, the second inductor  44  discharges. Likewise, when the first inductor  42  discharges, the second inductor  44  charges. 
     An advantage of the open-loop control system  28  of  FIG. 2  is that the DC/DC converter  30  has a high efficiency and generates a small voltage ripple. The DC/DC converter  30  also has a relatively fast response, which allows the capacitor  48  to be smaller in size. Additionally, the control module  32  maintains the signal waveforms of the first, second, third, and fourth control signals U 2 , D 2 , U 1 , and D 1 , respectively, at a fixed frequency and duty cycle. Therefore, no additional control is required for the open-loop control system  28  during normal operations. 
     However, there are advantages to allowing the phases of the signal waveforms for the third and fourth control signals U 1  and D 1 , respectively, and the first and second control signals U 2  and D 2 , respectively, to overlap for a controlled period of time. For example, allowing same-phase operation of the control signals for a controlled period of time reduces the size of the effective inductor and produces a much faster response in the DC/DC converter  30 . This allows the capacitor  48  to be even smaller in size. However, if same-phase operation of the control signals continues for too long, too much current may be charged in the first and second inductors  42  and  44 , respectively, which adversely affects performance of the DC/DC converter  30 . Therefore, it is necessary to determine under which conditions same-phase operation of the control signals is initiated and for how long. 
     Referring now to  FIG. 4 , a closed-loop control system  72  for the DC/DC converter  30  according to the present invention is shown. An input of the control module  32  receives the output DC voltage  54  of the DC/DC converter  30 . The control module  32  also optionally receives voltage signals V x     1    and V x     2    from the first and second inductors  42  and  44 , respectively. For example, the control module  32  may perform a current estimation based on voltage signals V x     1    and V x     2    to sense a balance of the first and second inductors  42  and  44 , respectively. 
     The control module  32  ensures that the phases of the signal waveforms for the third and fourth control signals U 1  and D 1 , respectively, are complementary to the phases of the signal waveforms for the first and second control signals U 2  and D 2 , respectively, during normal operations. The control module  32  initiates same-phase operation of the control signals when a large voltage or current transient is detected in the DC/DC converter  30  based on the output DC voltage  54 . 
     In an exemplary embodiment, the control module  32  initiates same-phase operation of the control signals when a value of the output DC voltage  54  is outside of a predetermined range. For example, the control module  32  sets the signal waveforms of the control signals low (or high) when the value of the output DC voltage  54  is greater than a first predetermined voltage. This allows both the first and second inductors  42  and  44 , respectively, to discharge. The control module  32  sets the signal waveforms of the control signals high (or low) when the value of the output DC voltage  54  is less than a second predetermined voltage. This allows both the first and second inductors  42  and  44 , respectively, to charge. 
     The control module  32  may revert back to complementary operation of the control signals when the value of the output DC voltage  54  is back within the predetermined range. Alternatively, the control module  32  may revert back to complementary operation of the control signals after a predetermined time period. In an exemplary embodiment, the predetermined time period is a function of one or more circuit conditions such as a current or voltage magnitude within the DC/DC converter  30 . 
     In the event that the output DC voltage  54  is within the predetermined range, the current, I c , flowing through the capacitor  48  may still be too high or too low. Therefore, in another exemplary embodiment, the control module  32  initiates same-phase operation of the control signals when a value of the current flowing through the capacitor  48  is outside of a predetermined range. For example, the control module  32  sets the signal waveforms of the control signals low (or high) when the current flowing through the capacitor  48  is greater than a first predetermined current. This allows both the first and second inductors  42  and  44 , respectively, to discharge. 
     The control module  32  sets the signal waveforms of the control signals high (or low) when the current flowing through the capacitor  48  is less than a second predetermined current. This allows both the first and second inductors  42  and  44 , respectively, to charge. As in the case of the voltage threshold, the control module  32  may revert back to complementary operation of the control signals when the current flowing through the capacitor  48  is back within the predetermined range. Alternatively, the control module  32  may revert back to complementary operation of the control signals after a predetermined time period. 
     Referring now to  FIG. 5 , the control module  32  estimates the current, I c , flowing through the capacitor  48  based on the output DC voltage  54 , V out . The current flowing through the capacitor  48  is proportional to the rate of change of the output DC voltage  54 . Therefore, the control module  32  computes the amount of time, T cross , that it takes for the output DC voltage  54 , indicated by  80 , to increase or decrease from a first predetermined voltage (V L     2    or V L     1   ) to a second predetermined voltage (V L     1    or V L     2   ). In the exemplary embodiment illustrated in  FIG. 5 , the output DC voltage  54  decreases from a first predetermined voltage (V L     1   ), indicated by  82 , to a second predetermined voltage (V L     2   ), indicated by  84 . 
     As the value of T cross  decreases, the slope of V out  increases, which corresponds to an increase in the current flowing through the capacitor  48 . Likewise, as the value of T cross  increases, the slope of V out  decreases, which corresponds to a decrease in the current flowing through the capacitor  48 . Therefore, by comparing T cross  to a predetermined time period, the control module  32  determines when the current flowing through the capacitor  48  is outside of the predetermined range. Alternatively, the control module  32  may estimate the current flowing through the capacitor  48  based on an average value of V out  during a predetermined time period. 
     Referring now to  FIG. 6 , the phase of the signal waveforms for the third and fourth control signals U 1  and D 1 , respectively, indicated at  92 , overlaps the phase of the signal waveforms for the first and second control signals U 2  and D 2 , respectively, indicated at  94 , for a controlled period of time, T overlap . The T overlap  period identifies when the control module  32  maintains same-phase operation of the control signals. Before and after the T overlap  period, the control module  32  maintains complementary operation of the control signals. 
     Referring now to  FIG. 7 , the control module  32  and the DC/DC converter  30  are illustrated in further detail. Similar reference numbers are used to identify elements as in  FIG. 2 . The control module  32  includes a voltage compare module  102  and a control signal generator  104 . The control module  32  also optionally includes a current detection module  106 . A first input of the voltage compare module  102  receives the output DC voltage  54  from the DC/DC converter  30 . A second input of the voltage compare module  102  receives a predetermined voltage. The voltage compare module  102  compares the output DC voltage  54  and the predetermined voltage to determine when the output DC voltage  54  is greater than or less than the predetermined voltage. 
     The voltage compare module  102  outputs the result to the control signal generator  104 . Inputs of the optional current detection module  106  receive the voltage signals V x     1    and V x     2    from the first and second inductors  42  and  44 , respectively. The current detection module  106  computes the difference between V x     1    and V x     2    and transmits the difference to the control signal generator  104 . The control signal generator  104  generates the first, second, third, and fourth control signals U 2 , D 2 , U 1  and D 1 , respectively, based on values of the control signals from the voltage compare module  102  and/or the current detection module  106 . The control signal generator  104  transmits the first, second, third, and fourth control signals U 2 , D 2 , U 1  and D 1 , respectively, to the gates of the first, second, third, and fourth transistors  34 ,  36 ,  38 , and  40 , respectively, in the DC/DC converter  30 . 
     Referring now to  FIG. 8 , a first closed-loop control algorithm begins in step  114 . In step  116 , control reads the value of the output DC voltage  54  from the DC/DC converter  30 . In step  118 , control determines whether the output DC voltage  54  is greater than a first predetermined voltage plus a threshold. If true, control proceeds to step  120 . If false, control proceeds to step  122 . In step  120 , control initiates same-phase operation of the control signals by setting the signal waveforms of the control signals low (or high). In step  124 , control resets a timer. In step  126 , control determines whether the timer has expired. If false, control loops to step  126 . If true, control proceeds to step  128 . 
     In step  128 , the control module  32  reverts back to complementary operation of the control signals and control ends. In step  122 , control determines whether the output DC voltage  54  is less than a second predetermined voltage minus a threshold. For example, the thresholds in steps  118  and  122  may be equal and/or the first and second predetermined voltages may be equal. If true, control proceeds to step  130 . If false, control proceeds to step  132 . In step  130 , the control module  32  initiates same-phase operation of the control signals by setting the signal waveforms of the control signals high (or low) and control proceeds to step  124 . 
     In step  132 , the control module  32  estimates the current flowing through the capacitor  48  in the DC/DC converter  30 . In step  134 , control determines whether the current flowing through the capacitor  48  is greater than a first predetermined current. If true, control proceeds to step  120 . If false, control proceeds to step  136 . In step  136 , control determines whether the current flowing through the capacitor  48  is less than a second predetermined current. For example, the second predetermined current may be equal in magnitude to the first predetermined current and have an opposite polarity. If true, control proceeds to step  130 . If false, control ends. 
     Referring now to  FIG. 9 , a second closed-loop control algorithm begins in step  144 . In step  146 , control reads the value of the output DC voltage  54  from the DC/DC converter  30 . In step  148 , control determines whether the output DC voltage  54  is greater than a first predetermined voltage plus a threshold. If true, control proceeds to step  150 . If false, control proceeds to step  152 . In step  150 , the control module  32  initiates same-phase operation of the control signals by setting the signal waveforms of the control signals low (or high). In step  154 , control determines whether the output DC voltage  54  is less than the first predetermined voltage plus the threshold. Is false, control loops to step  154 . If true, control proceeds to step  156 . In step  156 , the control module  32  reverts back to complementary operation of the control signals and control ends. 
     In step  152 , control determines whether the output DC voltage  54  is less than a second predetermined voltage minus a threshold. For example, the thresholds in steps  148  and  152  may be equal and/or the first and second predetermined voltages may be equal. If true, control proceeds to step  158 . If false, control proceeds to step  160 . In step  158 , the control module  32  initiates same-phase operation of the control signals by setting the signal waveforms of the control signals high (or low). In step  162 , control determines whether the output DC voltage  54  is greater than the second predetermined voltage minus the threshold. If false, control loops to step  162 . If true, control proceeds to step  156 . In step  160 , the control module  32  estimates the current flowing through the capacitor  48  in the DC/DC converter  30 . 
     In step  164 , control determines whether the current flowing through the capacitor  48  is greater than a first predetermined current. If true, control proceeds to step  166 . If false, control proceeds to step  168 . In step  166 , the control module  32  initiates same-phase operation of the control signals by setting the signal waveforms of the control signals low (or high). In step  170 , control determines whether the current flowing through the capacitor  48  is less than the first predetermined current. If false, control loops to step  170 . If true, control proceeds to step  156 . 
     In step  168 , control determines whether the current flowing through the capacitor  48  is less than a second predetermined current. For example, the second predetermined current may be equal in magnitude to the first predetermined current and have an opposite polarity. If false, control ends. If true, control proceeds to step  172 . In step  172 , the control module  32  initiates same-phase operation of the control signals by setting the signal waveforms of the control signals high (or low). In step  174 , control determines whether the current flowing through the capacitor  48  is greater than the second predetermined current. If false, control loops to step  174 . If true, control proceeds to step  156 . 
     The present invention allows for closed-loop digital control of a coupled-inductor DC/DC converter  30 . However, the methods of the present invention may also be employed to control other electronic circuits of a similar nature. By utilizing an output voltage feedback path, the control module  32  is capable of detecting large voltage or current transients in the circuitry of the DC/DC converter  30 . Therefore, the previous constraint of constant complementary operation of the control signals is relaxed. This allows the DC/DC converter  30  to achieve an even faster response and requires an even smaller output capacitor  48  than DC/DC converters that employ open-loop control systems. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and the following claims.