BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to electric power conversion, and more particularly relates to methods and devices for controlled power conversion through a resonant charge transfer device. 
   2. Description of Related Art 
   Resonant charge transfer devices may be used to formulate a desired output waveform from a given waveform input at the terminals of, for example, a three-phase power device. Generally they operate by transferring a predetermined amount of charge between the various phases of the input terminal and a charge storage device, such as a capacitor, and then, after suitable transfer of charge has occurred, transferring a predetermined amount of charge between the charge storage device and the output terminals. Generally the transfer of charge is mediated through the opening and closing of switches, the operation of which results in a sequential transfer of charge from the various phase inputs of the three-phase input to the charge storage device and then from the charge transfer device to the output. It is known how to predetermine the amount of charge that must be transferred and the precise sequence for doing so to achieve a wide variety of transformations of input power to a desired output power. 
     FIG. 1  illustrates a functional overview of the operation of a prior art resonant charge transfer device  100 . In particular, in converting input AC power  110  to output DC power  170 , for example, the circuit generally operates by passing the input power  110  through a stage of input filtering  120 , after which charge is transferred to the resonant circuit  130 . The amount of charge that is transferred to the resonant circuit  130  is determined by the opening and closing times of input switches  150 . In particular, the input switches  150  open and close at predetermined times in accordance with a desired output power to be achieved by the circuit  100 . Upon the closing of the input switches  150  after charge has been transferred to the resonant circuit  130 , the charge is subsequently permitted to discharge into an output filter stage  140 . The manner in which this discharge occurs is determined by the opening and closing of output switches  160 . In particular, the output switches  160  open and close at predetermined times in accordance with a desired output to be achieved by the circuit  100 . It is known within the prior art how to ascertain the opening and closing times of the input switches  150  and output switches  160  to achieve a desired output power for a given input voltage  110  and output voltage  170 . 
   U.S. Pat. No. 6,118,678, “Charge Transfer Apparatus and Method Therefore,” which is incorporated by reference herein for all purposes, teaches examples of this kind of resonant circuit topology. For example,  FIG. 2 , taken from U.S. Pat. No. 6,118,678, illustrates a resonant charge transfer device within input terminals  11 , an input filter section  10 , input switches  20 , resonant circuit elements  22 , 25 , 26 , output switches  30 , output filter section  40 , and output terminals  12 . As taught therein, turning on the switches at predetermined times and operating them so that they self-commutate leads to a wide variety of circuit applications, including but not limited to AC-to-DC rectifier, AC-to-AC power conversion, and DC-to-AC power conversion. 
   The opening and closing times of the input switches  150  and output switches  160  to achieve a desired output power for a given input power source  110  are a function of the parameters defining the input filter stage  120 , the resonant circuit elements  130 , and the output filter stage  140 . In any particular implementation, however, the parameters of the actual elements comprising the input filter stage  120 , resonant circuit elements  130 , and output filter stage  140  will deviate from their nominally given values as a function of various factors such as, for example, temperature, operating point, etc. Because the actual parameter values in any particular implementation differ from their nominal values, use of predetermined opening/closing times of the input and output switches  150 ,  160  will not lead to the precise desired output power; the actual output power  170  will differ from the desired power in some unknown fashion that will vary as the actual parameters differ with temperature, operating point, etc. Hence, a need arises to develop feedback control strategies that actively monitor the operating values of the various circuit parameters as well as the circuit&#39;s operating point and control the switching times to achieve the desired output. 
   SUMMARY OF THE INVENTION 
   Summary of the Problem 
   Several military as well as commercial applications need high density, lightweight AC and DC power supplies and converters that can provide multiple outputs from a given input or can provide power balancing from multiple sources. These applications include supplying shipboard power as well as supplying commercial grid utility power and power in commercial small production settings. One approach as discussed above is to implement a resonant soft-switching circuit topology. This circuit topology provides benefits such as reduced component count (which results in lower cost), elimination of switching issues (which results in improved efficiency), and enabling of high speed operation at high power levels. However, on-the-fly monitoring of and adjustment to source and load transients is required to permit power sources to re-configure power flows in response to rapidly changing demands. This monitoring and adjustment can be achieved through use of a controlled resonant circuit topology in which a control system as disclosed herein calculates and/or controls voltage and current levels, switching timing, and system parameters within the soft-switching resonant circuit topology. 
   Summary of the Solution 
   Methods and devices for transferring charge to achieve a desired output in a resonant charge transfer device are given. In an exemplary embodiment a resonant charge transfer device comprises a first filter section, a first switch section, a charge storage device, a second switch section, and a second filter section, and a method for controlling this device is given, the method comprising specifying a desired output; specifying a desired charge storage voltage ratio; determining one or more first switch times at which one or more first switches will turn on; turning on the first switches at the first switch times; determining one or more second switch times at which one or more second switches will turn on; turning on the second switches at the second switch times; measuring an actual output of the device; measuring at one or more specific times one or more parameters associated with each of the first filter section, and the charge storage section; computing an actual charge storage voltage ratio from the measured charge storage section parameters; comparing the desired output to the actual output and the desired charge storage voltage ratio to the actual charge storage voltage ratio to formulate an output correction parameter; utilizing the measured first filter parameters, the measured charge storage section parameters, and the output correction parameter to compute one or more corrected first switch times; utilizing the measured first filter parameters, the measured charge storage section parameters, and the output correction parameter to compute one or more corrected second switch times; on a subsequent operation cycle of the resonant charge transfer device, turning on the one or more first switches at the corrected first switch times; and on a subsequent operation cycle of the resonant charge transfer device, turning on the second switches at the corrected second switch times. 
   A further exemplary embodiment describes a charge transfer device comprising: a first filter section, a first switch section, a charge storage device, a second switch section, a second filter section, a feedback control unit configured to operate the first switch section and the second switch so as to achieve a desired output. 
   And a still further exemplary embodiment describes a method for controlling a charge transfer device comprising: (i) inputting a desired output current value to a control system of the charge transfer device; (ii) inputting an actual output current value to the control system; (iii) inputting control parameters for controlling an input filter, resonant circuit element and output filter into the control system; (iv) determining switching parameters for input and output switches based on the control parameters, desired output current value and actual output current value; and (v) switching the input and output switches in accordance with the switching parameters to achieve the desired output value. 


   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     Other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment and certain modifications thereof when taken together with the accompanying drawings in which: 
       FIG. 1  illustrates an overview functional illustration of a prior art resonant charge transfer device without control; 
       FIG. 2  illustrates a prior art resonant circuit structure; 
       FIG. 3  illustrates an overview functional illustration of a controlled resonant charge transfer device in accordance with an exemplary embodiment of the present invention; 
       FIG. 4  illustrates an exemplary embodiment of a resonant charge transfer device that may be controlled in accordance with the present invention; 
       FIG. 5  illustrates an exemplary embodiment of a control system adapted for controlling the operation of a resonant charge transfer device; and 
       FIG. 6  illustrates a more detailed functional overview of a control system illustrated in  FIG. 5 . 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale, and some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. 
   To achieve the precisely desired output power in any particular implementation of a charge transfer device such as that exemplified in  FIGS. 1 and 2 , one may control the circuit using feedback, as illustrated in  FIG. 3 , which shows a functional illustration of a control circuit in accordance with one embodiment of the invention. In this feedback approach, the actual output current  170  along with a desired output current  180  are input into a control system  200 . In addition, various input filter parameters  125 , resonant circuit element parameters  135 , and output filter parameters  145  are also input to the control system  200 , which then ascertains the opening/closing times of the input and output switches  150 ,  160 . 
     FIG. 4  illustrates a resonant charge transfer device circuit  300  with a three phase AC input terminal  301  for receiving a three-phase AC power supply  110  (from  FIGS. 1 and 3 ) and a two phase output terminal  302  for supplying DC power  170  (from  FIGS. 1 and 3 ) as an output. In this particular embodiment, this resonant charge transfer device comprises six input switch assemblies  303 ,  304 ,  305 ,  306 ,  307 ,  308  (which correspond to input switches  150  as shown on  FIGS. 1 and 3 ) and two output switch assemblies  309 ,  310  (which correspond to output switches  160  as shown on  FIGS. 1 and 3 ). The switch assemblies comprise Insulated Gate Bipolar Transistor (“IGBT”) switches coupled to diodes. This arrangement permits the switches to self-commute and eliminates the need to turn the switches off actively. This circuit further comprises three input filter  120  (from  FIGS. 1 and 3 ) capacitors  320 ,  330 ,  340  and one central capacitor  350 . The central capacitor  350  corresponds to the resonant portion  130  of the resonant charge transfer device  100  (from  FIGS. 1 and 3 ). 
   The operation of this circuit may be controlled via a feedback controller, an exemplary embodiment of which is illustrated in  FIG. 5 . As shown in  FIG. 5 , a feedback controller  400  according to one embodiment of the present utilizes a system controller  410  to initiate operation of a digital signal processor  420  (“DSP”) working in conjunction with a programmable logic device  430  (“PLD”) to monitor and control the operation of the circuit  300  illustrated in  FIG. 4 . 
   More particularly as shown in the functional illustration in  FIG. 6 , the system controller  410  comprises the functional operations of startup  501  and current control  510 ; the DSP  420  comprises the functional operations of switch selection  520 , charge ratio control  530 , and current pulse-to-pulse control  540 , whereas the PLD  430  comprises the functional operations of zero crossing detection  550 , t 3  calculation  560 , triggering  570 , trigger switches  580 , and data collection  590 . Each of these functionalities in turn will be discussed in further detail for an exemplary embodiment. 
   Startup controller  410  takes as input a user-supplied power on command. Once the command has been received, an enable command to initiate operation of various processes of the PLD  430  is activated after a suitable delay, where the amount of delay may be supplied as a parameter. An exemplary range of values for this delay is 10-100 milliseconds. Once the enable command is activated, a start control (“StrtCtrl”) command to initiate operation of various processes of the DSP is activated after a suitable delay, which also may be supplied as a parameter. An exemplary range of values for this delay is 10-100 milliseconds. The output of the system controller is the activated enable and StrtCtrl commands. 
   Current control process  510  takes as input a reference current level supplied by the user (“CurRequest”). In this exemplary embodiment CurRequest is the DC output signal desired by the user. It also takes as inputs the output voltage sampled at a particular time t 3  (“VLoadT 3 ”) and the similarly sampled output current (ILoadT 3 ″). This process requires that StrtCtrl be activated. The process operates by first filtering the sample load current according to the following algorithm to obtain an average load current ILoadFlt:
 
ILoadFlt(k)=ccf1*ILoadFlt(k−1)+ccf2*ILoad(k)
 
Where “k” is the value of the present time step, “k−1” is the value of the previous time step, and “ccf 1 ”, “ccf 2 ” are smoothing coefficients the values of which are functions of the DSP sample period t sDSP  which is a parameter of the actual DSP used. The algorithm will be updated at a desired clock pulse rate, for example 20,160 Hz. One of ordinary skill in the art will recognize that other choices for the update rate are possible. In addition, an exemplary choice for ccf 1  and ccf 2  is
 
ccf1=exp(−t sDSP /0.005)
 
ccf2=1−exp(−t sDSP/ 0.005).
 
One of ordinary skill in the art will recognize that other choices are available. The current error may then be computed by subtracting the filtered load current ILoadFLT from the reference load current CurRequest. Applying the error to an integrator with a given loop gain (“ContGain”) (not shown on Figure) then allows the mean trigger time t 0,avg  to be ramped up or down to adjust the output current. One of ordinary skill in the art will recognize that ContGain is a design parameter. For this exemplary embodiment, a suitable value for ContGain is approximately 600 microseconds per ampere. The output of this current control process  510  is t 0,avg , the average time t 0  with respect to the previous t 3  timing event, which will be discussed in further detail below.
 
   Switch selection process  520  determines which input switches  150  (From  FIGS. 1 and 3 ) to fire. In particular, at time t 0  the switches associated with the high and low absolute value phase voltages are fired. At time t 1  the medium phase voltage switch is fired. Accordingly switch selection process  520  takes as its input phase filter voltages  110  (from  FIGS. 1 and 3 ) sampled at time t 3  (“V 1 FltT 3 , V 2 FltT 3 , V 3 FltT 3 ”) and calculates the absolute values of the three input phase voltages. It then sorts these absolute values by magnitude, and computes a corresponding switch number for each of the high, medium, and low voltages. In particular, given the sort order n of the absolute value of the input phase voltages, the switch number is 2*n for a negative phase voltage and 2*n−1 for a positive phase voltage. In addition, this switch selection process  520  also computes a voltage ratio (“R vlt ”) computed by dividing the low input voltage by the medium input voltage. This process  520  then outputs the switch numbers inpHigh, inpMed, and inpLow as well as R vlt . 
   Charge ratio control process  530  adjusts the t 1  trigger time to cause the input charge ratio to track the voltage ratio R vlt . In particular, the charge accumulated by the resonant capacitor  350  is the integral of the charge current, which is proportional to the capacitor voltage. Thus if the capacitor voltage is stored at the trigger times t 0 , t 1 , and t 3 , then the charge ratio can be calculated as (VcT 1 −VcT 0 )/(VcT 3 −VcT 0 ). If this ratio is larger than the charge ratio R vlt  by a sufficient amount, which in an exemplary embodiment is 0.1, then the time t 1  is decreased by a fixed amount, which in one embodiment is 0.05 microseconds. If the ratio is smaller than the charge ratio R vlt  by a sufficient amount, which in an exemplary embodiment is 0.1, then the time t 1  is increased by a fixed amount, which in an exemplary embodiment is 0.05 microseconds. This approach results in fixed increment integral tracking control. One of ordinary skill in the art will recognize that other approaches are available. Thus the charge ratio control process  530  takes as inputs t 3,trig , which is a trigger signal pulse from the PLD, the ratio R vlt , the pulse number, and the central capacitor voltages sampled at times t 0 , t 1 , and t 3 , (“VcT 0 , VcT 1 , VcT 3 ”) and then outputs t 1 , a trigger time measured in counts with respect to the previous t 3  event, as well as the previous voltage ratio input signal (“MR vlt ”) and the observed charge ratio (“MR charge ”), which may be used for diagnostic purposes. (MR vlt  and MR charge  are not shown in  FIG. 5 .) 
   Current pulse-to-pulse control process  540  calculates a t 0  offset for each pulse to minimize the output current distortion caused by the periodic three-phase modulation. This offset t 0,offset  (not shown on Figure) is computed to keep the resonant capacitor  350  voltage constant. It takes as inputs a trigger pulse t 3,Trig , the pulse number, the central capacitor  350  voltage VcT 3  sampled at time t 3 , and the average time t 0,avg  and outputs a trigger time t 0  in counts with respect to the previous t 3  event. In particular, the process computes the average central capacitor voltage V c,mean . Based upon the pulse number, it computes the previous pulse number and from this previous pulse number retrieves the previous t 0,offset . It then computes the error in the central capacitor  350  voltage as the average voltage V c,mean  minus the central capacitor  350  voltage sampled at time t 3 , VcT 3 . If the error in the central capacitor  350  voltage is sufficiently large, then the previous t 0,offset  is decreased by a suitable amount, which in an exemplary embodiment is 0.05 microseconds. If the error is sufficiently small, then the previous t 0,offset  is increased by a suitable amount, which in an exemplary embodiment is 0.05 microseconds. The adjusted value is then stored for future use. This offset is then added to t 0,average  to obtain a new value for t 0 . It may be desirable in some applications to ensure that the value of t 0  is bounded. 
   The zero crossing detection process  550  within the PLD establishes the base timing for the resonant pulse train. Each pulse is triggered based on its time from the start of an input phase voltage sinusoidal period. In particular, this process  550  takes as inputs the input phase filter capacitor  320 ,  330 ,  340  voltages (“V 1 Flt, V 2 Flt, V 3 Flt”) and a trigger trigActive which is an indicator for which half of the energy transfer is active and outputs the occurrence of an input phase  1  filter capacitor voltage zero crossing event (“ZC”). This process  550  works by detecting a positive edge crossing of the phase  1  input voltage signal through zero, debouncing the detected signal to insure a consistent edge, and estimating the actual zero crossing based on the detected and de-bounced zero crossings combined with the expected period of the zero crossing event ZC. 
   The t 3  calculation process  560  takes as input the occurrence of the zero crossing event ZC and an input clock signal and outputs the present t 3  timing value referenced to the previous t 3  value. It also outputs a pulse number, which is an index to the present pulse. It accomplishes this by computing a local time as the time from the previous zero crossing event and, when a zero crossing event is detected, resetting the pulse number counter to 1. Each time the local time then passes a t 3  event time, the pulse number counter is incremented. 
   The triggering process  570  takes as inputs the occurrence of a zero crossing event ZC and trigger times t 0 , t 1 , and t 3  as well as a trigger t inv  and returns various triggers, t 0 Trig , t 1Trig , t 3Trig , t invTrig , and trigActive that are utilized in various other aspects of the controller operation. For an exemplary embodiment, t inv  may be set to a constant of 17 microseconds, although one of ordinary skill in the art will recognize that other choices are possible. Once a zero crossing signal is detected (after the enable signal is applied), the process  570  provides a clock signal to all counters. On each zero crossing it resets the master local counter, which runs for 360 electrical degrees. When the master local counter exceeds the input value for t 3 , the process  570  sets the t 3Trig  output, holds the t 3Trig  output for a given count (the “t 3Trig  persistence period”), and at the end of the t 3Trig  persistence period resets the t 3Trig  output. In an exemplary embodiment, this persistence period along with other persistence periods discussed herein are on the order of 500 microseconds. One of ordinary skill in the art will recognize that other choices are possible. It also resets the t 3  counter. When the t 3  counter exceeds the input value for the process sets the t invTrig  output, holds the t invTrig  output for a given count (the “t invTrig  persistence period”), and at the end of the t invTrig  persistence period resets the t invTrig  output. It also resets the t inv  counter. When the t inv  counter exceeds the input value for t 0 , the process sets the t 0Trig  output, holds the t 0Trig  output for a given count (the “t 0Trig  persistence period”), and at the end of the t 0Trig  persistence period resets the t 0Trig  output. It also resets the t 0  counter. When the t 0  counter exceeds the input value for t 1 , the process sets the t 1Trig  output, holds the t 1Trig  output for a given count (the “t 1Trig  persistence period”), and at the end of the t 1Trig  persistence period resets the t 1Trig  output. It also resets the t 1  counter. When t 3Trig  events are detected, the TriggerActive counter is incremented. 
   Trigger switches process  580  computes the times at which the input switches  150  and output switches  160  fire (from  FIGS. 1 and 3 ). It takes as inputs various triggers t 0Trig , t 1Trig , t 3Trig , t invTrig  and the switch numbers inpHigh, inpMed, and inpLow for the high, medium, and low input switches and outputs the input phase  1  gate trigger signals, Si 1   p , Si 1   n  (positive and negative), the input phase  2  gate trigger signals Si 2   p , Si 2   n , the input phase  3  gate trigger signals Si 3   p , Si 3   n , and the output phase  1  gate trigger signals So 1   p , So 1   n . When a t 3  event is detected, the process  580  sets a t 3TriggerLatch , which is then used to reset the t 1TriggerLatch . When a t 1  event is detected, the process  580  sets a t 1TriggerLatch , which is then used to reset the t 1TriggerLatch . When a t invTrig  event is detected, the process  580  sets a t invTriggerLatch . When either the t 1TriggerLatch  or the t 0TriggerLatch  is set, the high switch trigger vector sHigh is defined, based upon the inpHigh count value. When the t 0TriggerLatch  is set, the low switch trigger vector sLow is defined, based upon the inpLow count value. When the t 1TriggerLatch  is set, the medium switch trigger vector sMed is defined based upon the inpMed count value. When the t invTriggerLatch  is set, the inversion switch trigger vector sInv is defined, based upon the inpHigh value. The input switch command vector Si is then formed as the bitwise OR of sHigh, sLow, sMed, and sInv. 
   Finally data collection process  590  samples the central capacitor  350  voltage and current, the phase  1 ,  2 , and  3  filter capacitor voltages and the output voltage load and currents appropriately at times t 0 , t 1 , and t 3  to supply VcT 0 , VcT 1 , VcT 3 , IcT 3 , V 1 FltT 3 , V 2 FltT 3 , V 3 FltT 3 , VLoadT 3  and ILoadT 3  for use by other processes in this embodiment of the control circuit. 
   In an exemplary embodiment, control system  200  comprises an algorithm, software, and circuitry wherein the algorithm is embedded in software that resides on the circuit board. However, the disclosed methods may readily be implemented partially in software using object or object-oriented software development environments that provide portable source code that can be used on a variety of computer or workstation platforms. Alternatively, the disclosed system may be implemented partially or fully in hardware using standard logic circuits or, for example, a VLSI design. Whether software or hardware is used to implement the systems in accordance with this invention is dependent on the speed and/or efficiency requirements of the system, particular function, and the particular software or hardware systems or microprocessor or microcomputer systems being utilized. The systems and methods illustrated herein can be readily implemented in hardware and/or software using any suitable systems or structures, devices and/or software by those of ordinary skill in the applicable art from the functional description provided herein and with a basic general knowledge of the power conversion and control arts.
Methods and devices for achieving a desired output in a resonant charge transfer device are given. In an exemplary embodiment a controlled resonant charge transfer device comprises first and second filter sections, first and second switch sections, a charge storage device, and a feedback control system. A method for controlling this device is given, the method comprising specifying a desired output and a desired charge storage voltage ratio; turning on first switches at first switch times and second switches at second switch times; measuring an actual output of the device and one or more parameters of the resonant circuit; determining an actual charge storage voltage ratio; computing corrected first switch times and corrected second switch times; on a subsequent operation cycle of the resonant charge transfer device, turning on the first switches at the corrected first switch times and the second switches at the corrected second switch times.