Abstract:
A resonant switching-type capacitive charging power conditioner circuit includes a trap switch assembly to prevent the energy initially delivered to the circuit by an electrical energy source, from returning to the source. Once trapped, all of the energy is transferred to a capacitive store over a number of cycles. The period for each cycle is a function of the state of charge of the capacitive store, and the period decreases for each successive cycle as the charge on the capacitive store increases to its final value. Switches are turned on and off in response to the absence of certain currents in the circuit, to match the decreasing period of each successive energy transfer cycle throughout the entire energy transfer process. This adaptive clocking prevents energy from returning to the energy source, and eliminates dead time for each cycle.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The conditions under which this invention was made are such as to entitle the Government of the United States under paragraph I(a) of Executive Order 10096, as represented by the Secretary of the Air Force, to the entire right, title and interest therein, including foreign rights. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the transfer of electrical energy from an energy source to a capacitive store and, more particularly, involves energy trapping and adaptive clocking of the energy transfer cycle in conjunction with a resonant circuit. 
     BACKGROUND 
     The charging of a capacitive energy store requires the transfer of energy from an energy source. Energy sources such as generators, batteries, fuel cells, and solar cells are typically voltage sources. The capacitive store initially appears as a short circuit when connected to a voltage source that has a voltage higher than that across the capacitive store. Consequently, the flow of current must be controlled. 
     The simplest control means is a series resistor as shown in FIG.  1 . Voltage source  1  having a voltage V dc  charges capacitor  2 , having a capacitance C, in series with resistor  3 , having a resistance R. This circuit limits the peak current to a value of V dc /R, and results in a relatively long charging time to achieve 99% V dc , i. e., approximately 3RC seconds. The charging efficiency is only 50%; that is, resistor  3  dissipates the same amount of energy that is transferred to capacitor  2 , or C/2V dc   2 . In some low-energy applications, resistive charging is the best engineering choice. However, in high-energy applications, the relatively long charging time or the 50% efficiency is unacceptable. 
     The charging time and efficiency are improved by resonant charging. This accomplished by replacing resistor  3  with an inductor  4 , having an inductance L, as shown in FIG.  2 . The theoretical efficiency of resonant charging approaches 100% and is typically greater than 95% in practice. The charging time is given by π(LC) 1/2  seconds, with the peak current being limited to V/(LC) 1/2 . Diode  5  is used in the circuit because the capacitor  6  charges to almost twice the voltage of d.c. voltage source  7 , V dc , and it is necessary to prevent the charge transferred to capacitor  6  from flowing back into voltage source  7 . 
     The peak energy storage rating of inductor  4  is one-fourth the energy rating of capacitor  6 . The specific energy of a capacitor is on the order of 2000 J/kg, and that of an inductor is typically much less, on the order of 50 J/kg. Therefore, inductor  4  is typically on the order of 40 times larger than capacitor  6 . 
     In moderate low-energy applications, such as pulsed radar transmitters, resonant charging is a good engineering choice. However, when the capacitive stored energy is greater than a few kJ, a better alternative for the charging apparatus is a switching-type capacitive charging power conditioner, or “SCCPC.” The SCCPC operates from a d.c. source and provides fast and efficient charging of the capacitive store. In most applications, it also replaces the large d.c. power supply required for the input power by operating from a directly rectified a.c. power line. The SCCPC can also operate from any other suitable d.c. source, such as a battery. 
     A transformer is an important part of an SCCPC because it accommodates the difference between the voltage source and the load voltages, and isolates the voltage source from the load. Transformers must operate with bipolar voltages that contain no d.c. components. In general, transformers are inversely related in size and cost to the frequency of operation, which is motivation for operating the SCCPC at high frequency. There are two basic configurations of the SCCPC, the center-tapped transformer configuration shown in FIG.  3  and the “H” bridge switch configuration shown in FIG.  4 . 
     The principle of operation is the same for both SCCPC configurations. A small amount of energy is measured out by the primary capacitor, switched through the transformer, then rectified and deposited into the load capacitor. This process is repeated at a high frequency until the load capacitor is fully charged and in a manner such that the transformer is subjected to only a bipolar voltage. 
     More particularly, center-tapped configuration  8  of the prior art is schematically illustrated in FIG.  3 . Center-tapped configuration  8  operates by alternately charging small capacitors  9  and  10  by means of switches  11  and  12 , from voltage source  13 , through the primary winding of transformer  14 . Transformer  14  usually steps up the voltage by a factor of N, i. e., N is typically greater than 1, where N is the turns ratio of a transformers secondary and primary windings; but in some cases the voltage may be stepped down, i. e., N may be less than 1. The secondary current of transformer  14  passes through bridge rectifier  15  and then into load capacitor  16 . This process is repeated at a high frequency such that over a period, load capacitor  16  is charged to the desired voltage. The switches  11  and  12  are operated in an alternating sequence such that the voltage applied to transformer  14  is bipolar and has no d.c. component. 
     H-bridge circuit  17  of the prior art is schematically shown in FIG.  4 . H-bridge circuit  17  has only one small primary capacitor  18 , which is charged through the primary coil of transformer  19 . The H-bridge switches  20 ,  21 ,  24  and  26  are sequentially operated to alternately apply a bipolar voltage through capacitor  18  to transformer  19 . Specifically, in the first energy transfer cycle, the switch pair  20  and  26  are turned on, while switch pair  21  and  24  remain in the off state. This connects the positive side of voltage source  27  through small primary capacitor  18  to the top of the primary coil of transformer  19 . 
     After this energy transfer cycle is completed, the next energy transfer cycle begins with switch pair  21  and  24  being turned on while switch pair  20  and  26  are switched to the off state. This connects the positive side of voltage source  27  through primary capacitor  18  to the bottom side of the primary coil of transformer  19 , thus providing the reverse polarity and ensuring that the bipolar signal received by transformer  19  has no d.c. component. This sequence of operating two the switch pairs is repeated until the required amount of energy is transferred through bridge rectifier  28  to load capacitor  29 . 
     The basic energy transfer process and the functions of the switches during a single switching event of the same polarity can be better explained using simplified equivalent circuit  30  shown in FIG.  5 . Circuit  30  illustrates the operation of both center-tapped circuit  8  of FIG.  3  and H-bridge circuit  17  of FIG.  4 . 
     Transformer  14  in circuit  8  of FIG.  3  and transformer  19  in circuit  17  of FIG. 4, are replaced in FIG. 5 by equivalent leakage inductor  31 . The equivalent inductance of inductor  31  can be obtained by calculation familiar to those skilled in the electrical art, using the transformer turns ratio N. Likewise, load capacitor  16  in circuit  8  and load capacitor  29  in circuit  17  are represented by equivalent load capacitor  32 . The capacitance of capacitor  32  can be calculated using equations and methods well known to those reasonably skilled in the electrical art. The voltage across load capacitor  32  divided by the voltage of source  33  is defined as the charge ratio α. Forward switch  34  is a silicon controlled rectifier, or “SCR,” with parallel back diode  35 . However, any suitable switch may be used, such as an isolated gate bipolar transistor, or “IGBT,” or monolithic oxide silicon field effect transistor, or “MOSFET.” 
     The operation of the switch cycle begins when forward switch  34  closes, i. e., is turned on. A resonant current flows from voltage source  33  through switch  34 , through capacitor  36 , through inductor  31 , through the bridge rectifier formed by diodes  37 ,  38 ,  39  and  40 , and into load capacitor  32 . Being in a resonant circuit, the voltage across capacitor  36  will increase and ultimately exceed the voltage of the voltage source  33 . When this occurs, forward switch  34  is turned off, and the current through capacitor  36  reverses and flows back through back diode  35 , that is, across forward switch  34 . 
     The reverse current deposits energy back into voltage source  33 . This reverse current continues to provide a positive energy transfer to load capacitor  32  because the bridge rectifier allows only a positive flow of current into load capacitor  32 , while at the same time routing the excess energy back to voltage source  33 . As this reverse current continues to flow, it builds up an opposing voltage on capacitor  36  until the opposing voltage is sufficient to reduce the reverse current to zero. When the reverse current reaches zero, the energy transfer cycle is completed. Forward switch  34  is then turned on and the next energy transfer cycle is begun. 
     It can be shown that the energy transferred to load capacitor  32  is a function of the state of charge of load capacitor  32 , and that the fractional transfer is very low when state of charge across load capacitor  32  is low. More particularly, the fractional amount of the energy transferred to capacitor  32  relative to the energy that is delivered to the circuit from voltage source  33 , also known as the energy transfer ratio, is given by the following equation:                J        (     G   ,   α     )       =     8          (     1   -   G     )          [       G        (     1   -     2      α       )       -   1     ]              [       α        (     1   -   G     )       -   2     ]           (     1   +   G     )     4        G                 (   1   )                                
     where G=the ratio of the capacitance of capacitor  32  to the capacitance of capacitor  36 , typically 100 to 10,000. 
     During the initial stages of the charging process begins, the voltage on capacitor  32  is very low, and thus α≅0. Under this condition, the energy transfer ratio J(G, α) simplifies to the following expression:                J        (   G   )       =       4        G   2           (     G   +   1     )     3               (   2   )                                
     Accordingly, the transfer ratio J(G, α) is very low during the initial stages of the charging process, e. g., J(G, α)≅0.004 for G=1000. 
     As the voltage builds up on capacitor  32 , α increases, and thus the transfer ratio J(G, α) also increases. Nonetheless, the average energy transfer ratio taken over the entire charging process is low, and this inefficiency requires a large number of cycles to achieve a useful energy transfer to capacitor  32 . The energy not transferred to capacitor  32  from the energy delivered to circuit  30  during each cycle is returned to voltage source  33  by the reverse current. 
     Each energy transfer cycle is of a short duration and is repeated at a high frequency to accomplish the total energy transfer to load capacitor  32 . The high frequency is a major factor in reducing the size of the transformer and thus the size and cost of the apparatus. However, the high frequency concomitantly imposes a high switching loss because the amount of energy that must be processed is much larger that the amount actually delivered to load capacitor  32 . 
     The period of the energy transfer cycle, T, is also a function of the state of the charge ratio α of load capacitor  32 . The following expressions approximate T for two conditions, T 1  for α≦⅔, i. e., during the initial cycles of the charging process, and T 2  for α&gt;&gt;⅔, i. e., during the latter cycles of the charging process:                      T   1     =     2      π            LC   1          G     1   +   G                       when                 α     ≤     2   3                   (   3   )                       T   2     =     π            LC   1          G     1   +   G                           when                 α     〉     〉          2   3                   (   4   )                                
     where: 
     L=the inductance of inductor  31 ; and 
     C=the capacitance of capacitor  36 . 
     At the present time SCCPC&#39;s are driven at a fixed frequency selected to accommodate the maximum charging cycle period that occurs at the beginning of the charging process, i. e., before the charge on load capacitor  32  has appreciably increased. As a result, the period is much longer than that necessary during the latter stages of the charging process, i. e., when α has significantly increased. Consequently, during a substantial portion of the total time necessary to complete the transfer of energy from voltage source  33  to load capacitor  32 , i. e., during the latter stages of the charging process, approximately 50% of each charging cycle period is comprised of dead time, i. e., the period exceeds that which is necessary to drive the circuit. 
     It follows that there is a need in the art for a charging apparatus capable of transferring all of the energy taken from the voltage source in each switching cycle, while matching the clocking frequency to the period required for energy transfer for each cycle throughout the charging process. 
     SUMMARY 
     A resonant switching-type capacitive charging power conditioner circuit includes a trap switch assembly to prevent the energy initially delivered to the circuit by an electrical energy source from returning to the source. Once trapped, all of the energy is transferred to a capacitive store, such as a load capacitor, over a number of cycles. The period for each cycle is a function of the state of charge of the capacitive store, and the period decreases for each successive cycle as the charge on the capacitive store increases to its final value. Switches are turned on and off in response to the absence of certain currents in the circuit, to match the decreasing periods of the successive charging cycles, respectively, throughout the charging process. This adaptive clocking prevents energy from returning the energy source, and eliminates dead time for each cycle. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, and illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing showing a prior art circuit for resistive charging of a capacitor. 
     FIG. 2 is a schematic drawing illustrating another prior art circuit for resonant charging of a capacitor. 
     FIG. 3 is a schematic drawing of an SCCPC circuit of the prior art having a center-tapped transformer configuration. 
     FIG. 4 is a schematic drawing of a SCCPC circuit of the prior art having an H-bridge configuration. 
     FIG. 5 is a schematic drawing of a simplified equivalent circuit of an SSCPC circuit that is suitable for the analysis of the operation of both the center-tapped transformer and the H-bridge SCCPC configurations respectively shown in FIG.  3  and FIG.  4 . 
     FIG. 6 is a schematic drawing of the circuit of the present invention. 
     FIG. 7 is a graph of the significant voltage and currents in the circuit of the present invention. 
     FIG. 8 is a schematic drawing of the center-tapped transformer configuration of the circuit of the present invention. 
     FIG. 9 is a schematic drawing of the H-bridge switched configuration of the circuit of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The invention is comprised of an energy trapping innovation and adaptive clocking that maximizes the utilization of the trapping process. Referring to FIG. 6, circuit  42  includes start switch  43 , trapping switch  44  and clamping diode  45 . A controlled switch may replace clamping diode  45 . The energy transfer cycle begins when start switch  43  is turned on (closed), with trapping switch  44  being in the off state (open). Current  46  flows as indicated from voltage source  47 , through start switch  43 , capacitor  49 , inductor  50 , the bridge rectifier comprised of diodes  51 ,  52 ,  53  and  54 , and into the positive side of load capacitor  55 . Load capacitor  55  is connected to the load terminals of the bridge rectifier. The configuration of diode  45  in parallel with capacitor  49  comprises clamping circuit  56 . 
     As current  46  flows in the indicated direction, the voltage on capacitor  49  builds up to a level that exceeds the voltage of voltage source  47 , and results in reducing current  46  to zero. The time t 1  required for current  46  to decrease to zero as measured from the moment start switch  43  closed, is given by solving the following equation:                t   1     =     π            LC   1          G     1   +   G                     (   5   )                                
     where: 
     L=the value of the inductance of inductor  50 ; 
     C 1 =the capacitance of capacitor  49 ; and 
     G=the ratio of the capacitance of load capacitor  55  to C 1 . 
     At the end of t 1 , current  46  is zero and start switch  43  is placed in the off (open) state to isolate voltage source  47  from circuit  42 . Trapping switch  44  is simultaneously switched on (closed), and the voltage across capacitor  49  causes current  57  to pass through trapping switch  44  in the direction indicated, to the bridge rectifier. Current  57  flows through the bridge rectifier and deposits charge and energy into the positive terminal of load capacitor  55 , before returning through inductor  50  to complete the circuit back to clamping circuit  56 . The flow of current  57  is resonant and would cause a reversal of the voltage on capacitor  49  except that a voltage reversal is prevented by diode  45  of clamping circuit  56 . 
     The time t 2  required for the voltage on capacitor  49  to decrease from its value at the end of interval t 1 , to zero, is calculated by the following equation:                t   2     =     π     2        ω   2                 (   6   )                                
     where:                ω   2     =     1         LC   1          G     1   +   G                     (   7   )                                
     As diode  45  prevents capacitor  49  from being charged by reverse current  56 , eventually the voltage across capacitor  49  decays to zero, whereupon the only energy remaining in circuit  42  and not yet transferred to load capacitor  55 , is stored in inductor  50 . Current  57  continues to flow, induced by the magnetic field of inductor  50 , until all of the energy stored in inductor  50  is delivered through the bridge rectifier to load capacitor  55 . The time t 3  required to transfer the energy stored in inductor  50  to load capacitor  55 , measured from the time the voltage across capacitor  49  becomes zero, i. e., at the end of t 2 , until current  57  decays to nothing, is given by the following equation:                t   3     =       1     ω   3              sin     -   1            [           G        (     1   -     2      β       )       -   1       G        (     1   +     G                   β   2         )           ]                 (   8   )                                
     where:                ω   3     =     1       LGC   1                 (   9   )                                
     and β=the ratio of the voltage across load capacitor  55  measured at the beginning of the energy transfer cycle, i. e., just prior to the closing of start switch  43 , to the voltage of voltage source  47 . 
     The total time T to accomplish one energy transfer cycle including the trapping is given by the sum: 
     
       
           T=t   1   +t   2   +t   3   (10) 
       
     
     T depends upon the initial state of charge in terms of the charge ratio β at the beginning of each charging cycle, and consequently will change for each cycle during the charging process. More particularly, T is at its maximum at the beginning of the charging process, and decreases as the voltage on the load capacitor  55  increases. Typical graphs of currents  46  and  57  as a function of time are shown in FIG.  7 . Also shown therein is a graph of the voltage, V 49 , across capacitor  49  as a function of time. 
     As previously noted, the time interval t 1 , is the duration of the energy or current flow delivered from voltage source  46  to circuit  42 , and corresponds to the positive part of the trace of current  46  as it rises from zero to a maximum value and then decreases back to zero. During the interval t 1 , the voltage V 49  rises from zero at the start of the cycle, to a peak value at the end of t 1 . 
     The time interval t 2  begins at the end of t 1 , and is defined as the interval from when current  57  is zero until the current reaches its peak negative value. t 2  may also be defined as the interval necessary for the voltage V 49  to decay from its peak value to zero. 
     The time interval t 3  begins at the end of t 2 . The voltage V 49  across capacitor  49  is clamped to zero by clamping diode  45 , while current  57  decays from its peak negative valued at the end of t 2  to zero at the end of t 3 . The intervals may be approximated using equations 5 through 9. 
     To maximize the utilization of the charging circuit and to achieve the shortest over-all charging time, the actual periods of the individual energy transfer cycles should start off long and decrease to match the theoretical period for each particular cycle, as calculated by equations 5 through 10. The adaptive clocking aspect of the invention, as discussed below, accomplishes this. 
     The proper operation of circuit  42  depends upon the operation of the switches  43  and  44  in a precise sequence. The times at which these two switches should operate can be theoretically calculated, using equations 5 through 9, as a function of the intervals t 1 , t 2 , and t 3.  However, this is not practical for real applications because of unpredictable effect of thermal drift, aging and vibration on the various electrical elements. 
     The beginning and end of intervals t 1 , t 2 , and t 3 , can be determined by measuring or sensing currents  46  and  57  as a function of time. More particularly, current measuring means  60  measures current  46 , and current measuring means  61  measures current  57 . Methods and means for the measurement of electrical currents are well known and can be easily implemented by those skilled in the electrical art, e. g., viewing shunts, Hall devices and current transformers. 
     The charging of load capacitor  55  begins by commanding switch  43  to an on state (closed). The end of interval t 1  occurs when value of current  46 , as measured by measuring means  60 , returns to zero after reaching a positive peak. Switch  43  is then commanded off (open) and switch  44  is commanded on (closed), to begin interval t 2 . Methods and means for generating switch commands based on the values of currents are well known to those reasonably skilled in the relevant art. 
     Current  57 , as measured by measuring means  61 , increases in magnitude until it reaches a negative peak value, marking the end of t 2  and the beginning of t 3 . Current  57  then decays to zero, signifying the end of t 3  as well as the end of the charging cycle period T. 
     The beginning of the following energy transfer cycle occurs at the end of t 3  (and T), with commands to turn on (close) switch  43  and turn off (open) switch  44 , as previously explained. The energy transfer process continues with successive cycles until load capacitor  55  is fully charged, i. e., the transfer of energy from voltage source  47  is completed. The sequential turning on and off of switches  43  and  44  in accordance with the measurements of currents  46  and  57  by measuring means  60  and  61 , respectively, i. e., by responding to the signals generated by measuring means  60  and  61 , can be automated using means and methods well know to those reasonably skilled in the art. 
     FIG. 8 is a schematic drawing of the present invention implemented in a center-tapped transformer configuration. FIG. 9 is a schematic drawing of the present invention implemented in a H-bridge switched configuration. Conventional symbols are used to represent the various electrical elements included therein. In each configuration, the transformer provides the equivalent inductance provided by inductor  50  of circuit  42 . The foregoing configurations are examples of the present invention, and their operation is obvious to one skilled in the electrical art in view of the detailed description of the circuit  42 , in addition to the explanations of the operation of circuits  8  and  17  shown in FIGS. 3 and 4, respectively. 
     It is to be understood that the preceding is merely a detailed description of one embodiment of this invention and that numerous changes to the disclosed embodiment can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.