Abstract:
A method and apparatus for dynamically adjusting operation of a converter device to improve conversion efficiency. The converter device includes an inverter and a synchronous rectifier, and is driven by a plurality of drive signals having a duty cycle. The method comprises the steps of: (a) varying timing of a first drive signal a first amount; (b) observing the duty cycle of the conversion device; (c) further varying the first drive signal appropriately to alter the duty cycle toward an extremum; and (d) continuing to operate the converter device with the duty cycle proximate the extremum. In its most preferred embodiment, the method of the present invention includes the further step of: (e) periodically effecting steps (a)-(c) varying a drive signal other than the first drive signal. The comprises: (a) a drive varying means for varying timing of selected individual drive signals of the plurality of drive signals; and (b) a measuring means connected with the drive varying means for measuring the duty cycle. In its most preferred embodiment, the apparatus further comprises: (c) a control means for controlling the drive varying means. The control means is connected with the measuring means and with the drive varying means, and effects the controlling to operate the converter device proximate an extremum for the duty cycle.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to electrical power supplies, and especially to direct current—to—direct current (DC-DC) power supplies, also referred to as power converters, or converters. Prior art control of DC-DC power converters has largely been effected using analog control techniques and circuitry. Such analog approaches involve integrated circuits or discrete circuit elements, and require a product designer to select a priori a preferred control parameter or parameters. The designer thus needed to desensitize the product to anticipated ordinary variations in circuitry parameters caused by component tolerances and operating conditions during the process of optimizing the design. Designers employing such prior art control techniques thus had to predict the environment their product would encounter—such as, temperature values, electrical parameter values, and load values. Such pertinent parameters had to be estimated in order to choose control parameter values to be monitored and their acceptable variations, reactions to control parameter variances beyond predetermined limits, and to which parameters the circuitry must be desensitized for acceptable or optimal performance. 
     Digital control of converters has been implemented on a limited basis. At present, analog controls for converters remain faster and less expensive than digital controls. Nevertheless, digital control techniques and circuitry for DC-DC power converters have advantages in that they provide opportunities for real-time adjustment of operational and control parameters. A controller based on a microprocessor or a digital signal processor (DSP) offers a circuit designer access to adaptable control processes limited mainly by software execution speeds. 
     A direct design approach to optimizing efficiency of an operating DC-DC power converter using a digital controller (e.g., using a microprocessor or a DSP) would be to measure output power and input power, and to adjust various controllable parameters to maximize the power ratio. This approach is straightforward and logical because it measures the very parameters that make up the calculation for efficiency of a power supply: output power and input power. However, such a direct approach is complex and costly to construct and implement because of additional components required to accomplish the required measurements. Converters are commonly employed in products with the converter output voltage regulated to a specified value. Additionally, converters often have a substantially constant input voltage. With those two values presenting little variation during normal power supply operation, advantage can be taken of the duty cycle of the power train, which depends upon the ratio of the output voltage and the input voltage, remaining relatively constant, particularly for continuous current mode (CCM) operation. Duty cycle, as described below, presents an indicator for real-time optimization of the efficiency of the power train by adjusting controllable parameters. 
     There have been some attempts to adaptively operate power supply devices to improve efficiency. U.S. Pat. No. 5,742,491 of Apr. 21, 1998, to Bowman et al for “Power Converter Adaptively Driven”, discloses a drive circuit for a power converter. The Bowman invention provides an apparatus and method for adjusting the timing for driving a power supply circuit with respect to the primary switch employed in the device. Variations of drive timing are achievable in response to varying operating conditions experienced by the power supply device. The Bowman invention is intended to maximize efficiency of the power supply while keeping stresses on individual components of the power supply within acceptable limits. According to Bowman, the optimum drive timing for one set of operating conditions is different from optimum drive timing for another set of operating conditions. As an example, a synchronous rectifier drive timing that produces maximum efficiency at a first load condition may produce excessive voltage stress on the rectifier switch at a second, lesser load condition. Conversely, when the timing is changed to lower the voltage stress at the second load condition, a loss of efficiency is liable to occur at the first load condition. Bowman&#39;s apparatus and method provide for the designer an a priori adaptation of the delay between drive waveforms supplied to the inverter and synchronous rectifier of a power supply device as a function of an operating condition of the converter to allow the converter to operate efficiently in distinct operating environments over a range of operating conditions. 
     Bowman, therefore, succeeds in improving operation of a power supply device over a wider range of conditions. Bowman builds a representative test device and measures predetermined parameters associated with that test device in a laboratory environment. Bowman provides a delay circuit constructed for use in production devices as though the production devices will operate the same as the laboratory test devices that are the basis for Bowman&#39;s determinations in designing the delay circuit. That is, Bowman&#39;s does not provide dynamic real-time efficiency adjustment capability. 
     Prior art approaches to controlling DC-DC power converters during operation have relied upon fixed designs based upon engineering analysis or laboratory data to optimize efficiency for design-anticipated conditions. Some provisions for after-design adjustment of operating parameters have been attempted, but they have provided only coarse adjustment with less than ideal accommodation of changing conditions. There has been no facility for continuous realtime adjustment of parameters over a range to improve efficiency based upon real-time observation of extant parameters. 
     There is a need for an apparatus and method for providing fine adjustments of efficiency of a DC-DC power converter during normal operation to enable accommodation of varying operating conditions and device parameter variaton without adding significant cost or complexity to the converter design. 
     SUMMARY OF THE INVENTION 
     A method and apparatus or dynamically adjusting operation of a converter device to improve conversion efficiency is described. The converter device includes an inverter (and may include a synchronous rectifier) and is driven by a plurality of drive signals having a duty cycle. Each individual drive signal has a leading edge and a lagging edge. The method involves adjusting a controllable parameter and observing, or measuring, the duty cycle of a conversion device. In its preferred embodiment, the method comprises the steps of: (a) varying timing of a first drive signal a first amount; (b) observing the duty cycle of the conversion device; (c) further varying the first drive signal appropriately to alter the duty cycle toward an extremum; and (d) continuing to operate the converter device with the duty cycle proximate the extremum. In its most preferred embodiment, the method of the present invention includes the further step of: (e) periodically effecting steps (a)-(c) varying a drive signal other than the first drive signal. The apparatus of the present invention is an apparatus for dynamically altering operation of a converter device to improve conversion efficiency. The converter device includes an inverter and a synchronous rectifier (or, at least two actively controlled switches). The converter device is driven by a plurality of drive signals having a duty cycle. The apparatus comprises: (a) a drive varying means for varying timing of selected individual drive signals of the plurality of drive signals; and (b) a measuring means for measuring, or observing, the duty cycle. The measuring or observing means is connected with the drive varying means. In its most preferred embodiment, the apparatus of the present invention further comprises: (c) a control means for controlling the drive varying means. The control means is connected with the measuring or observing means and with the drive varying means, and effects the controlling to operate the converter device proximate an extremum for the duty cycle. 
     Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical schematic diagram of the preferred embodiment of the apparatus of the present invention incorporated in a representative conversion device. 
     FIG. 2 is a schematic graphic representation of a relationship between duty cycle and a parameter in a conversion device. 
     FIG. 3 is a schematic graphic representation of a relationship between duty cycle and conversion efficiency in a conversion device. 
     FIG. 4 is a schematic diagram illustrating relationships among various drive signals in a conversion device, such as the apparatus illustrated in FIG.  1 . 
     FIG. 5 block diagram illustrating the preferred embodiment of the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is an electrical schematic diagram of the preferred embodiment of the apparatus of the present invention incorporated in a representative conversion device. In FIG. 1, a converter apparatus  10  includes an isolation transformer  12  having a primary winding  14  and a secondary winding  16 . Primary winding  14  is included in a primary-side circuit  18  , and secondary winding  16  is included in a secondary-side circuit  20 . Isolation transformer  12  has a turns ratio of its primary winding  14  to its secondary winding  14  in the ratio:                N                 p     Ns           [   1   ]                                
     Primary-side circuit  18  has a power source, schematically represented by a battery  22  providing an input voltage V IN  in FIG. 1. A second battery  24  is in opposing orientation with respect to battery  22  to schematically represent all the losses in converter apparatus  10 . Battery  24  provides a voltage V η in an orientation opposing battery  22 . Thus, battery  24  absorbs a portion of the energy supplied by battery  22 . Expressing efficiency η of converter apparatus  10  in terms of power in (Pin) and power out (Pout) as:              η   =     Pout   Pin             [   2   ]                                
     Opposing voltage V η provided by battery  24  may be expressed as: 
     
       
         V η =(1−η)·V IN   [3] 
       
     
     As a result, the voltage applied to primary winding  14  is (V IN ·η), the difference between V IN  and expression [3 ]. 
     Primary-side circuit  18  also includes a core reset circuit  26  connected with primary winding  14 , and a field effect transistor (FET) Q 1  connected with primary winding  14 . FET Q 1  operates in response to a drive signal D 0  to connect primary winding  14  with ground at a point  28  at one value of drive signal D 0 , and operates to disconnect primary winding  14  from ground at point  28  at another value of drive signal Do. Drive signal Do establishes the duty cycle D for converter  10 . Thus, FET Q 1  operates as an inverter in alternately connecting and disconnecting one end of primary winding  14  with ground point  28 . 
     Secondary circuit  20  includes an output filter inductor  30  and a resistor  32  representing a load connected in series with secondary winding  16 . An output filter capacitor  34  is connected in parallel with load resistor  32 . A first synchronous rectifier SR 1  is connected in series with load resistor  32 , and a second synchronous rectifier SR 2  is connected in parallel with load resistor  32  and filter capacitor  34 . First synchronous rectifier SR 1  is driven by a drive signal D 1 ; second synchronous rectifier SR 2  is driven by a drive signal D 2 . 
     A sense line  36  senses a parameter at output  38  of converter  10 . The parameter sensed by sense line  36  is commonly output voltage V OUT  of converter  10 , but may be another parameter, if desired. It is common that output voltage V OUT  is regulated to a fixed voltage in employing converters in circuits. It is for their regulated output voltage characteristics that such converter apparatuses are employed. 
     In converter apparatus  10  of FIG. 1,                V   OUT     =       V   IN     ·     Ns     N                 p       ·   D   ·   η             [   4   ]                                
     Sense line  36  provides the sensed parameter from output  38  to a controller  40 . In the preferred embodiment of converter  10 , controller  40  is a pulse width modulator, but other control arrangements may be employed with success. Controller  40  preferably includes a control/adjust section  42  that enables controller  40  to vary drive signals D 1  and D 2 . 
     The apparatus and method of the present invention are particularly advantageously employed with a converter apparatus in which controller  40  is embodied in a digital control device. Such a digital control device may be manifested in control circuitry involving microprocessors or digital signal processors or a combinatoin of such devices. The teachings of the present invention are not limited to digitally controlled converter apparatuses. 
     In the preferred embodiment of converter apparatus  10 , controller  40  varies the timing of drive signals D 1  and D 2 , as will be described later herein in connection with FIG.  4 . Drive signal D 1  controls operation of synchronous rectifier SR 1 ; drive signal D 2  controls operation of synchronous rectifier SR 2 . An output line  44  from controller  40  provides a signal to a measuring unit  46 . The signal provided via output line  44  is affected by the parameter sensed at output  38  and delivered to controller  40  via sense line  36 . Measuring unit  46  provides an indication of the signal delivered via output line  44  to control/adjust section  42  of controller  44  via a signal line  48 . Preferably the signal provided via output line  44  to measuring unit  46  is applied to a drive line  50  substantially without modification. The signal provided on drive line  50  is drive signal D 0 . Drive signal D 0  is employed to drive FET Q 1  and establishes the duty cycle D of converter  10 . Thus, converter apparatus  10  is configured to measure an output parameter at an output  38 , employ the measured output parameter in establishing a drive signal D 0  to fix the measured output parameter at a desired value, and in deriving additional drive signals D 1 , D 2  for operating synchronous rectifiers SR 1 , SR 2 . Further, duty cycle D (duty cycle D is established by drive signal D 0 ) is measured by measuring unit  46 , and information relating to duty cycle D is provided to control/adjust section  42  of controller  40  to adjust drive signals D 1 , D 2  to appropriately to urge converter apparatus  10  toward high power efficiency operation. Operation of converter apparatus  10  to achieve high power conversion efficiency will be discussed in greater detail in connection with FIGS. 2-5. 
     FIG. 2 is a schematic graphic representation of a relationship between duty cycle and a parameter in a conversion device. In FIG. 2, an adjustable parameter P is plotted on a horizontal axis  62  of a graph  60 . Duty cycle D of a converter apparatus, such as converter apparatus  10  (FIG. 1) is plotted on a vertical axis  64  of graph  60 . A curve  66  graphically depicts the relationship between duty cycle D and adjustable parameter P of a converter apparatus. Curve  66  is representative of duty cycle-parameter relationships for several types of converter apparatuses. Other converter apparatuses may exhibit a somewhat different duty cycleparameter relationship, with the curved relationship bowing upwards. The important aspect common to such curves investigated by the inventor relating to duty cycle-to-parameter relationships is that each curve exhibits an observable and detectable extremum—a maximum or a minimum. It is the extremum, as at point  68  of curve  66 , that is sought in practicing the method and operating the apparatus of the present invention. The precise shape of curve  66  is generally not known a priori for a particular converter apparatus; its values may be determined by trial and error. 
     By way of example, one may select an initial value P INIT  for measured parameter P. As indicated in FIG. 2, a value of P INIT  for parameter P gives one an initial value of duty cycle D, D INIT . Next one may select a value of P B  for parameter P. A value P B  for parameter P yields a duty cycle D having a value D B . Value D B  for duty cycle D is higher than value D INIT , associated with initial value P INIT  of parameter P, for the example shown. An operator skilled in the pertinent art knows whether duty cycle D exhibits an extremum at a minimum value (as in FIG. 2, for example) or exhibits an extremum at a maximum value(not shown). Thus, in the case illustrated in FIG. 2, where duty cycle D is known to exhibit an extremum at a minimum value, on observing that duty cycle value D B  is greater than duty cycle value D INIT , where the value of D B  is close to the value of D INIT , an operator may conclude that parameter P should be adjusted in a direction opposite to the first adjustment in order to more closely approach maximizing duty cycle D. Accordingly, a new trial value P A  is selected for parameter P. Parameter value P A  yields a value of D A  for duty cycle D. Value D A  is lower than both previous values selected for parameter P (i.e., values P INIT , P B ). A knowledgeable operator thus may confidently conclude that the optimum value D OPT  for duty cycle D is related with a value for parameter P somewhere between values P A  and P INIT  . Optimum value D OPT  for duty cycle D may be stepwise approached in a manner well known for seeking to maximize a resulting value (such as duty cycle D) by searchingly varying an input value or values (such as adjustable parameter P), for example by fitting a parabolic curve to three trial points. In an operational environment, a converter apparatus may never reach and settle on an optimal value for duty cycle, but may iteratively approach such an optimal value throughout operations. Such continued operation at or about an optimal value for duty cycle results in overall improvement in conversion efficiency compared with operating with no dynamic adjustment relating to optimization. 
     Achieving (or operating in the vicinity of) an extremum for duty cycle is not readily apparent as a desirable end simply based upon the relationship illustrated in FIG.  2 . However, if one observes the representative relationship illustrated in FIG. 3 in light of the relationship illustrated in FIG. 2, the benefits of operating at an extremum for duty cycle become clear. 
     FIG. 3 is a schematic graphic representation of a relationship between duty cycle and conversion efficiency in a conversion device. In FIG. 3, duty cycle D is plotted on a horizontal axis  72  of a graph  70 . Efficiency η of a converter apparatus, such as converter apparatus  10  (FIG. 1) is plotted on a vertical axis  74  of graph  70 . A curve  76  graphically depicts the relationship between efficiency η and duty cycle D of the converter apparatus addressed by FIG.  3 . Curve  76  is representative of several efficiency-duty cycle relationships for several types of converter apparatuses. Curve  76  appears to be nearly linear, and may be linear over a range of values of duty cycle D. The important representative relationship illustrated by curve  76  is a monotonic relationship between efficiency η and duty cycle D. For the converter apparatus represented by exemplary graph  70 , the efficiency-duty cycle relationship is negatively monotonic. That is, as duty cycle increases, efficiency decreases. Conversely, as duty cycle decreases, efficiency increases. Thus, it is clearly beneficial to minimize duty cycle in order to maximize efficiency as much as possible for this example. Graph  60  (FIG. 2) yielded a value for optimal duty cycle D OPT  (a negative, or minimum extremum). Plotting the optimum duty cycle value D OPT  on Graph  70  (FIG. 3) yields an optimal value for efficiency η OPT . The shape of the relationships, such as the relationships illustrated in graphs  60 ,  70  (FIGS. 2-3 ), may be determined beforehand for a respective converter product and employed for “on the fly” adjustments to maximize efficiency. No special measurements of output power or other output parameters other than the normal parametric monitoring to assure constant output voltage (a common requirement for converters) is necessary. Other converter apparatuses may exhibit a somewhat different efficiency-duty cycle relationship than the relationship illustrated in graph  70  (FIG.  3 ). The important aspect common to such curves investigated by the inventor relating to efficiency-to-duty cycle relationships is that each curve exhibited a monotonic relationship. That is, as a first parameter (e.g., duty cycle) varies in a first direction, the second parameter (e.g., efficiency) varies in one direction. Conversely, as the first parameter (e.g., duty cycle) varies in a second direction opposite to the first direction, the second parameter varies in a direction opposite to the one direction first observed. 
     Duty cycle-to-parameter relationships exhibiting an extremum at a maximum value are associated with positively monotonic efficiency-to-duty cycle relationships. Duty cycle-to-parameter relationships exhibiting an extremum at a minimum value are associated with negatively monotonic efficiency-to-duty cycle relationships. Thus, seeking to optimize duty cycle to a maximum extremum will yield a maximum efficiency value using a positively monotonic efficiency-to-duty cycle relationship. Likewise, seeking to optimize duty cycle to a minimum extremum will yield a maximum efficiency value using a negatively monotonic efficiency-to-duty cycle relationship. It is not necessary to know the specific numerical values of the curves a priori, since only an efficiency extremum is sought by the process. 
     The apparatus and method of the present invention are valuable, but their value is offset by increased complexity and expense if additional parts are needed to effect measurements necessary to practice the inventions. Accordingly, it is preferred that a parameter used to seek to optimize duty cycle (e.g., as in connection with FIG.  2 ), is easily measured and easily controlled. FIG. 4 illustrates a selection of representative such parameters. However, the invention is not intended to be limited to using the parameters illustratively discussed in connection with FIG. 4 as the only parameters that can be used in practicing the invention. Any parameter that can be measured in its effect upon duty cycle is a candidate parameter for use with the apparatus and method of the present invention. For example, another useful parameter might be the specific gate voltages for driving a particular synchronous rectifier switch on or off. 
     FIG. 4 is a schematic diagram illustrating relationships among various drive signals in a conversion device, such as the converter apparatus illustrated in FIG.  1 . In FIG. 4, three parameters are plotted on a common horizontal scale indicating time. Thus, in FIG.  4 ( a ) gate drive signal D o  driving FET Q 1  (FIG. 1) is illustrated. Drive signal D o  commences as a positive pulse at time t 01  and has a duration of a time interval t 01 -t 02 . Drive signal Do remains at a zero value until producing another pulse in the next subsequent period of the drive signal scheme during a time interval t 03 -t 04 . Thus, drive signal Do has a period with a duration measured by time interval t 01-t   03 . The “ON” time for drive signal D o , during time interval t 01 -t 02 , compared with the off time for drive signal D 0 , during the time interval t 02 -t 03 , establishes a duty cycle D for drive signal D 0 . 
     In FIG.  4 ( b ), gate drive signal D 1  driving synchronous rectifier SR 1  (FIG. 1) is illustrated. Drive signal D 1  is manifested as a pulse having a duration of a time interval t 11 -t 22 . Drive signal D 2  is repeated in a next subsequent period of the drive signal scheme during a time interval t 13 -t 14 . 
     In FIG.  4 ( c ), gate drive signal D 2  driving synchronous rectifier SR 2  (FIG. 1) is illustrated. Drive signal D 2  is manifested as a pulse having a duration of a time interval t 21 -t 22 . Drive signal D 2  is repeated in a next subsequent period of the drive signal scheme (not shown in FIG.  4 ). 
     Of particular interest in comparing drive signals D 0 , D 1 , D 2  are the adjustable delay intervals among the signals. Specifically, first delay interval Δ 1  is defined by a time interval t 01  -t 11 . First delay interval Δ 1  is the delay between the commencement of drive signal D 0  and the commencement of drive signal D 1 . A second delay interval Δ 2  is defined by a time interval t 02 -t 12 . Second delay interval Δ 2  is the delay between ending the drive signal D 0  pulse and ending the drive signal D 1  pulse. A third delay interval Δ 3  is defined by a time interval t 02 -t 21 . Third delay interval Δ 3  is the delay between ending the drive signal D 0  pulse and beginning the drive signal D 2  pulse. A fourth delay interval Δ 4  is defined by a time interval t 22 -t 13 . Fourth delay interval Δ 4  is the delay between ending the drive signal D 2  pulse and beginning the drive signal D 1  pulse in the next subsequent period of the drive signal pattern. 
     Delay intervals Δ 1 , Δ 2 , Δ 3 , Δ 4  are controllable, as by control/adjust section  42  of controller  40  (FIG.  1 ). Thus, delay intervals Δ 1 , Δ 2 , Δ 3 , Δ 4  may be easily employed as parameters for practicing the method and operating the apparatus of the present invention. Employing adjustable time delays such as delay intervals Δ 1 , Δ 2 , Δ 3 , Δ 4  to ascertain an extremum for duty cycle (FIG. 2) is easily accomplished without any need for complex or expensive additional circuitry to make the required measurements. Any one of the delay intervals Δ 1 , Δ 2 , Δ 3 , Δ 4  may be employed to practice the method and operate the apparatus of the present invention. The preferred embodiment of the apparatus and method of the present invention is to employ more than one of the delay intervals Δ 1 , Δ 2 , Δ 3 , Δ 4  in a cyclical application of the adjustment to ascertain an extremum for duty cycle D (FIG. 2) and adjusting each respective delay interval under consideration in turn to achieve maximum efficiency (FIG.  3 ). A continued cyclical, repetitive application of the measurement and adjustment to ascertain an extremum for duty cycle D will provide the greatest likelihood of operating near the highest level of efficiency during the operation of the converter apparatus. As mentioned earlier, the parameters identified in FIG. 4 are representative only. Any parameter that impacts duty cycle of a converter may be employed in practicing the present invention 
     FIG. 5 is a block diagram illustrating the preferred embodiment of the method of the present invention. In FIG. 5, the method begins with having selected a parameter for use in varying duty cycle of a converter apparatus and varying the selected parameter, as indicated by a block  80 . As the selected parameter is varied, one observes behavior of the duty cycle of the converter apparatus, as indicated by a block  82 . Having noted any change in duty cycle (block  82 ) resulting from having varied the selected parameter (block  80 ), a query is posed inquiring whether it is desirable to repeat the steps represented by blocks  80 ,  82 , as indicated by a query block  84 . If it is determined that it is desirable to repeat the method steps represented by blocks  80 ,  82 , the method proceeds via “YES” response line  86  to return the method execution to block  80  for the desired repetition. Such a decision to repeat the method steps represented by blocks  80 ,  82  may be made, for example, when it is determined that the converter apparatus is not yet operating satisfactorily close enough to an extremum for duty cycle, or that a series of adjustments is no longer producing an appreciable increment of efficiency. 
     If it is determined not desirable to repeat the method steps represented by blocks  80 ,  82 , the method proceeds via “NO” response line  88 . Such a decision to not repeat the method steps represented by blocks  80 ,  82  may be made, for example, when it is determined that the converter apparatus is operating satisfactorily close enough to an extremum for duty cycle. This process of choosing whether to repeat method steps represented by blocks  80 ,  82  depending upon responsive changes in duty cycle (block  82  ) to changes in the selected parameter (block  80  ) and whether the converter apparatus is operating satisfactorily close enough to an extremum for duty cycle is the process described briefly in connection with FIG.  2 . In that process, one looks to a resulting value for duty cycle associated with a selected value for a parameter in deciding whether to vary the parameter again to seek a value for duty cycle closer to an extremum for duty cycle (DOPT in FIG.  2 ). 
     The method continues in FIG. 5 by posing another query whether another parameter is desired to carry the method further, as represented by a query block  90 . This query represents the preferred embodiment of the present invention discussed briefly in connection with FIG. 4 wherein it was established that practicing the method cyclically for a plurality of parameters likely yields a higher sustained level of conversion efficiency than would be attainable using only a single variable parameter in practicing the invention. If it is desired that another parameter be employed in practicing the invention, the method proceeds via “YES” response line  92  and another parameter is selected, as represented by a block  94 . The method then begins anew using the parameter selected according to block  94 , as indicated by a return line  98  to block  80 . If it is not desired that another parameter be employed in practicing the invention, the method proceeds via “NO” response line  96  and the first parameter is employed to begin the method anew, as indicated by return line  98  to block  80 . 
     The method illustrated in FIG. 5 contemplates continuous adjustment operations. The method could incorporate a decision block (not shown) to suspend adjustment operations when a predetermined condition is achieved. Recommencement of adjustment operations may be effected when certain conditions are met, after a predetermined time delay, or based upon some other parameter or parameters. 
     It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims: