Abstract:
A switch mode power supply (SMPS) response to a disturbance is improved by using a hysteretic control in combination with a fixed frequency, pulse-width modulated (PWM) controller for providing robust control and optimizing the response to disturbances in buck or buck derived switch mode power supply (SMPS) system topologies.

Description:
TECHNICAL FIELD 
     The present disclosure relates to switch mode power supplies, and, more particularly, to improving large signal transient response of a buck switch-mode power supply (SMPS) by using hysteretic control in combination with pulse-width modulation (PWM) control of the SMPS. 
     BACKGROUND 
     The optimum response to a disturbance in any negative feedback control system is achieved with a hysteretic control. However, many systems cannot tolerate the variable frequency or increased ripple present in a strictly hysteretic controlled system. Prior technology has implemented control techniques such as dual edge modulation, or increasing the bandwidth of the control system, but the response to a large disturbance is not optimized using these control techniques. In addition, using these control techniques during normal operation of the system generally results in reduced phase margin and stability concerns. What is needed is a better way to improve a SMPS system&#39;s response to a disturbance. 
     SUMMARY 
     Therefore it is desirable to optimize the SMPS system&#39;s response to a disturbance by using a hysteretic control in combination with a fixed frequency, pulse-width modulated (PWM) control for providing robust control and optimizing the response to disturbances in buck or buck derived SMPS system topologies. 
     According to a specific example embodiment of this disclosure, a switch mode power supply (SMPS) controller using hysteretic control in combination with pulse-width modulation (PWM) control comprises: a hysteretic control circuit having a first input coupled to a reference voltage, a second input coupled to a feedback voltage representing a load side output voltage of a switch mode power supply (SMPS), a first output for controlling a high side hysteretic demand, and a second output for controlling a low side hysteretic disable; a PWM error generator having a third input coupled to the reference voltage, a fourth input coupled to the feedback voltage, a fifth input coupled to a current sense circuit, and a third output for controlling PWM demand; and a power circuit driver having a high side (HS) power control output, and a low side (LS) power control output, the HS and LS power control outputs are adapted to drive power switches of a switch mode power supply (SMPS), the third output of the PWM error generator is coupled to the power circuit driver for control thereof, and the first and second outputs of the hysteretic control circuit are coupled to the power circuit driver for further control thereof; the PWM error generator third output controls the power circuit driver to make the feedback voltage substantially equal to the reference voltage unless a difference between the feedback voltage and the reference voltage is greater than at least a predetermined value, wherein if the feedback voltage is greater than the reference voltage by at least the predetermined value, then the hysteretic control circuit forces the HS and LS power control outputs to an off-state, if the feedback voltage is less than the reference voltage by at least the predetermined value, then the hysteretic control circuit controls a duty cycle of the HS and LS power control outputs so as to control the difference between the reference voltage and the feedback voltage to less than the predetermined value, and thereafter the PWM error generator will additionally control the HS and LS power control outputs so as to force the difference between the reference voltage and the feedback voltage to substantially zero over a certain time period. 
     According to another specific example embodiment of this disclosure, a method for controlling a switch mode power supply (SMPS) using hysteretic control in combination with pulse-width modulation (PWM) control comprises: determining a voltage difference between a reference voltage and a feedback voltage with a hysteretic control circuit and a PWM error generator, the feedback voltage being representative of an output voltage of a switch mode power supply (SMPS); controlling power switches of the SMPS with the PWM error generator when the voltage difference is less than a predetermined value, and controlling the power switches of the SMPS with the hysteretic control circuit when the voltage difference is equal to or greater than the predetermined value until the PWM error generator has time to reduce the voltage difference to less than the predetermined value. Wherein the step of controlling the power switches of the SMPS with the hysteretic control circuit and the PWM error generator comprises; a) determining whether the feedback voltage is greater than the reference voltage by at least the predetermined value, and if so b) forcing the power switches of the SMPS to an off-state with the hysteretic control circuit; c) determining whether the feedback voltage is less than the reference voltage by at least the predetermined value, and if so d) controlling a duty cycle of the power switches with the hysteretic control circuit so that the difference between the reference voltage and the feedback voltage is maintained at less than the predetermined value, and thereafter e) additionally controlling the power switches with the PWM error generator so as to force the difference between the reference voltage and the feedback voltage to substantially zero over a certain time period; and f) returning to step a). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  illustrates a schematic block diagram of a basic regulator system; 
         FIG. 2  illustrates a more detailed schematic block diagram of the general power regulator shown in  FIG. 1 ; 
         FIG. 3  illustrates a schematic block diagram of a control circuit, according to the teachings of this disclosure; 
         FIG. 4  illustrates a schematic diagram of a detailed implementation of the control circuit shown in  FIG. 3 , according to a specific example embodiment of this disclosure; 
         FIG. 5  illustrates a schematic diagram of a synchronous buck SMPS power circuit controlled by the control circuit shown in  FIGS. 3 and 4 , respectively, according to the teachings of this disclosure; 
         FIG. 6  illustrates a graph of a SMPS response using only a PWM control loop to a disturbance comprising a significant increase in load; 
         FIG. 7  illustrates a magnified time axis of the graph shown in  FIG. 6 ; 
         FIG. 8  illustrates a graph of a SMPS response using a fast outer hysteretic control in combination with the PWM control loop to a disturbance comprising a significant increase in load; 
         FIG. 9  illustrates a magnified time axis of the graph shown in  FIG. 8 ; 
         FIG. 10  illustrates a graph of a SMPS response using only a PWM control loop to a disturbance comprising a significant decrease in load; 
         FIG. 11  illustrates a magnified time axis of the graph shown in  FIG. 10 ; 
         FIG. 12  illustrates a graph of a SMPS response using a fast outer hysteretic control in combination with the PWM control loop to a disturbance comprising a significant decrease in load; and 
         FIG. 13  illustrates a magnified time axis of the graph shown in  FIG. 12 . 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Referring now to the drawing, the details of specific example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     In a general sense, a power converter can be defined as a device which converts one form of energy into another on a continuous basis. Any storage or loss of energy within such a power system while it is performing its conversion function is usually identical to the process of energy translation. There are many types of devices which can provide such a function with varying degrees of cost, reliability, complexity, and efficiency. 
     The mechanisms for power conversion can take many basic forms, such as those which are mechanical, electrical, or chemical processing in nature. The focus of herein will be on power converters which perform energy translation electrically and in a dynamic fashion, employing a restricted set of components which include inductors, capacitors, transformers, switches and resistors. How these circuit components are connected is determined by the desired power translation. Resistors introduce undesirable power loss. Since high efficiency is usually an overriding requirement in most applications, resistive circuit elements should be avoided or minimized in a main power control path. Only on rare occasions and for very specific reasons are power consuming resistances introduced into the main power control path. In auxiliary circuits, such as sequence, monitor, and control electronics of total system, high value resistors are common place, since their loss contributions are usually insignificant. 
     Referring to  FIG. 1 , depicted is a schematic block diagram of a basic regulator system. A power system  102 , e.g., a basic switch-mode power converter where an input of an uncontrolled source of voltage (or current, or power) is applied to the input of the power system  102  with the expectation that the voltage (or current, or power) at the output will be very well controlled. The basis of controlling the output is some form of reference, and any deviation between the output and the reference becomes an error. In a feedback-controlled system, negative feedback is used to reduce this error to an acceptable value, as close to zero required by the system. It is desirable, typically, to reduce the error quickly, but inherent with feedback control is the trade-off between system response and system stability. The more responsive the feedback network is, the greater becomes the risk of instability. 
     At this point, it should be mentioned that there is another method of control—feed forward. With feed forward control, a control signal is developed directly in response to an input variation or perturbation. Feed forward is less accurate than feedback since output sensing is not involved, however, there is no delay waiting for an output error signal to be developed, and feed forward control cannot cause instability. It should be clear that feed forward control typically is not adequate as the only control method for a voltage regulator, but it is often used together with feedback to improve a regulator&#39;s response to dynamic input variations. 
     Referring to  FIG. 2 , depicted is a more detailed schematic block diagram of the general power regulator shown in  FIG. 1 . The power system  102  has been separated into two blocks: the power circuit  206  and the control circuit  208 . The power circuit  206  handles the power system load current and is typically large, robust, and subject to wide temperature fluctuations. Its switching functions are by definition, large-signal phenomenon, normally simulated in most stability analyses as just a two-state switch with a duty cycle. The output filter (not shown) is also considered as a part of the power circuit  206 , but can be considered as a linear block. The control circuit  208  will normally be made up of a gain block, an error amplifier, and a pulse-width modulator, used to define the duty cycle for the power switches. According to the teachings of this disclosure, the control circuit  208  is optimized to respond to a disturbance in the power system  102 . 
     Referring now to  FIGS. 3 and 4 , depicted in  FIG. 3  is a schematic block diagram of a control circuit, according to the teachings of this disclosure, and depicted in  FIG. 4  is a schematic diagram of a detailed implementation of the control circuit shown in  FIG. 3 , according to a specific example embodiment. A traditional peak current mode PWM control is combined with a fast outer hysteretic control for operating a synchronous buck SMPS (see  FIG. 5 ). In the presence of a disturbance that significantly deviates the feedback from the set-point or reference, the fast hysteretic control contains the deviation to a predefined level. The slower PWM control loop then drives the error or deviation to zero in a normal fashion. 
     An output voltage feedback signal at node  320  and a reference voltage at node  326  are presented to a fixed frequency PWM error generator  312 . A control voltage from the output of operational amplifier  436  results and which establishes a peak current demand necessary to produce a substantially zero-volt differential between the signals at nodes  320  and  326 . This control voltage is presented to the inverting input of the PWM comparator  434 . The non-inverting input of the PWM comparator  434  is presented with the summation (via summer  440 ) of the current sense signal received at node  328  and a slope compensation ramp signal  456 . A clock oscillator signal  454  from a fixed frequency oscillator (not shown) establishes the switching frequency, maximum duty cycle, and start of the energy transfer cycle. The output from the PWM comparator  434  terminates the PWM demand on a cycle-by-cycle basis. Flip-flop  442  holds the demand off until the start of the next cycle established by the clock oscillator signal  454 . The PWM error generator  312  produces a fixed frequency PWM demand of a traditional closed loop, negative feedback control system. In steady-state operation or during small perturbations of the SMPS system, the PWM demand directly controls the high side (HS) drive and low side (LS) drive outputs to nodes  322  and  324 , respectively (PWM demand is the percent of time the HS drive and/or LS drive outputs are in the on-state). The output voltage feedback signal at node  320  and the reference voltage at node  326  are also presented to a fast, robust outer hysteretic control block  310 . During dynamic conditions, e.g., a significant change in load current and/or source voltage, the differential voltage between the signals received at nodes  320  and  326  will deviate from zero. 
     The PWM error generator  312  requires a finite amount of time to drive this differential voltage back to zero. If the voltage feedback at node  320  drops below the reference voltage at node  326  by a predetermined amount, the fast hysteretic control will asynchronously control the duty cycle of the HS drive output at node  322  and the LS drive output at node  324 . As a result, the differential voltage is limited to a certain controlled level. After a finite amount of time, the PWM error generator  312  drives this deviation back to zero. If the voltage feedback at node  320  rises above the reference voltage at node  326  by a predetermined amount, the fast hysteretic control  310  asynchronously forces both the HS drive output at node  322  and the LS drive output at node  324  to off-states. By forcing the HS drive output at node  322  into the off-state, no additional or unwanted energy is stored in the inductor  542  (see  FIG. 5 ). By forcing the LS drive output at node  324  into the off-state, the forcing function or voltage across the inductor  542  is increased, thereby causing a faster decay in any energy stored therein. After a finite amount of time, the PWM error generator  312  will drive the deviation back to substantially zero. 
     Referring to  FIG. 5 , depicted is a schematic diagram of a synchronous buck SMPS power circuit controlled by the control circuit shown in  FIGS. 3 and 4 , according to the teachings of this disclosure. A synchronous buck SMPS power circuit, generally represented by the numeral  206 , may comprise a power source  540 , e.g., battery, a series pass switch  544 , e.g., power field effect transistor; a shunt switch  546 , e.g., power field effect transistor; a power inductor  542 , a load capacitor  556  for smoothing alternating current (AC) ripple from the desired direct current (DC) output, a current sensor  548 , and load voltage divider resistors  552  and  554 . Power source commons or grounds  550  are also indicated in  FIG. 5 . 
     Operation begins when a voltage feedback signal at node  320  is below a reference voltage at node  326 . The voltage feedback signal at node  320  represents the value of the regulated output voltage. When this condition is true, operation is enabled. Load current may be determined with the current sensor resistor  548 , e.g., a current-to-voltage converter. 
     In accordance with the teachings of this disclosure, during steady-state operation, or during small perturbations of the system, switches  544  and  546  are modulated by a fixed frequency PWM controller, e.g., PWM error generator  312  (see  FIGS. 3 and 4 ). During dynamic conditions, switches  544  and  546  are asynchronously controlled by the fast hysteretic control  310 . This fast hysteretic control contains the deviation to a predefined level. The slower PWM control loop then drives the error or deviation to substantially zero. 
     Referring to  FIGS. 6 and 7 , depicted is a graph of a SMPS response using only a PWM control loop to a disturbance comprising a significant increase in load, and a magnified time axis thereof. Specifically, the disturbance depicted is an increase in load current. The load step occurs at a rate faster than the bandwidth of the PWM control loop. As prior technology suggests, increasing the bandwidth of the control loop can marginally improve the deviation, but the SMPS system will loose phase margin and increase the risk of instability. In addition, it can be argued that dual edge modulation techniques inherently can not react at twice the rate of a traditional PWM control loop as suggested because both edges are controlled by a single error signal. 
     Referring to  FIGS. 8 and 9 , depicted is a graph of a SMPS response using a fast outer hysteretic control in combination with the PWM control loop to a disturbance comprising a significant increase in load, and a magnified time axis thereof. Specifically, the disturbance depicted is an increase in load current. The load step occurs at a rate faster than the bandwidth of the PWM control loop. The peak deviation from the set-point is reduced by approximately fifty percent. 
     Referring to  FIGS. 10 and 11 , depicted is a graph of a SMPS response using only a PWM control loop to a disturbance comprising a decrease in load current, and a magnified time axis thereof. The load step occurs at a rate faster than the bandwidth of the PWM control loop. 
     Referring to  FIGS. 12 and 13 , depicted is a graph of a SMPS response using a fast outer hysteretic control in combination with the PWM control loop to a disturbance comprising a decrease in load, and a graph having a magnified time axis thereof. The load step occurs at a rate faster than the bandwidth of the PWM control loop. In this instance, the fast outer hysteretic control forces both switches off (see  FIG. 5 , transistors  544  and  546 ) in the presence of a positive deviation on the feedback signal. This increase has two effects. 
     First, by forcing the HS drive off, no additional or unwanted energy is stored in the inductor. Second, by forcing the LS drive off, the forcing function or voltage across the inductor is increased causing a faster decay in any stored energy. The peak deviation from the set point is reduced by 50%, approximately. Therefore, the combination of a traditional peak current mode PWM control in paralleled with a fast outer hysteretic control minimizes deviation from a set-up in the presence of an outside disturbance. 
     While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.