Abstract:
A system for charging a capacitor with relatively high energy pulses at a relatively high pulse repetition rate includes a voltage transformer. A resonant circuit is established having an inductor connected to the transformer&#39;s low voltage side and a capacitor connected to the transformer&#39;s high voltage side. A power supply cooperates with a switch assembly to generate a train of pulses, alternating in polarity, in the circuit. With this arrangement, the transformer core is reset after each pulse. A rectifying circuit operates on the alternating polarity pulses to create a train of constant polarity pulses for charging the capacitor. For the system, the maximum charging voltage is regulated by a control circuit having a probe for measuring the voltage across the charging capacitor. This measured voltage is used by the control circuit to selectively operate the switch assembly and regulate the maximum voltage across the capacitor.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains generally to electrical power systems. More particularly, the present invention pertains to high voltage, pulsed power systems. The present invention is particularly, but not exclusively, useful as a power system having a capacitor that is charged with pulses at a relatively high pulse repetition rate.  
       BACKGROUND OF THE INVENTION  
       [0002]     Several types of modern equipment require an electrical power supply that is capable of producing high energy pulses at very high pulse repetition rate. Examples of these include, but are not limited to, certain laser systems, and more modernly, extreme ultraviolet (EAV) sources for advanced photolithography techniques.  
         [0003]     Heretofore, resonant circuits that are configured to charge a capacitor have been employed when power at a high pulse repetition rate has been required. These resonant circuits typically include an inductor in addition to the capacitor. When a power source (e.g. battery) is connected to the resonant circuit and activated, a voltage appears across the capacitor that increases from zero to a maximum value during a time period, t, defined by the LC circuit. After the voltage across the capacitor reaches the maximum value, a load (e.g. UAV laser) can be connected to the resonant circuit to discharge the capacitor and energize the load. This sequence of charging and discharging the capacitor can be repeated, as desired, to drive the load with a train of substantially constant energy pulses.  
         [0004]     One problem associated with the above-described resonant charge circuit is the regulation of the maximum voltage across the capacitor. Specifically, any residual voltage on the capacitor (i.e. voltage remaining between pulses) will affect the maximum voltage on the capacitor at the end of a charge transfer. In attempts to overcome this difficulty, dequeing techniques have been developed and used. In general, these dequeing techniques have employed a voltage probe to monitor the voltage across the capacitor. When the desired voltage is reached, a switch is used to divert current from the resonant circuit, and as a result, stop all charge transfer to the capacitor.  
         [0005]     By way of example,  FIG. 1  shows a typical, prior art resonant circuit (generally designated  10 ) that uses a dequeing technique to charge a capacitor  12  at a high pulse repetition rate. As shown, the resonant circuit  10  includes an inductor  14 , a power source  16 , diodes  18 ,  19 ,  20  and two transistor switches  22 ,  24  (note: the circuit  10  also requires a voltage probe and control circuit that are not shown in  FIG. 1 ). Operation of the resonant circuit  10  begins by closing switch  22  at time t=0. With switch  22  closed, the capacitor  12  is resonantly charged with current that passes through the diode  20  and inductor  14 . Once the voltage probe indicates that a desired voltage across capacitor  12  has been reached, the control circuit quickly closes switch  24 . With switch  24  closed, all remaining current in the circuit  10  is routed through the circuit branch  21  having switch  22 , switch  24  and the inductor  14 . With the circuit  10  in this configuration, all current flow to the capacitor  12  is stopped. Switch  22  is then opened, diverting current from branch  21  through a circuit branch  25  having the power source  16 , diode  18 , inductor  14  and switch  24 . This allows the energy in the inductor  14  to be returned to the source  16  and recovered. The charge across the capacitor  12  can then be maintained until required by a load (not shown). Once the capacitor  12  has been discharged, switch  24  is then opened, configuring the circuit  10  to generate the next pulse.  
         [0006]     In a typical setup of the prior art resonant circuit  10  shown in  FIG. 1 , the desired voltage across the capacitor  12  is selected to be less than the peak voltage generated by the resonant circuit  10 , which in turn, is typically about twice the voltage of the source  16 . For the circuit  10 , the switches  22 ,  24  are preferably constructed of either MOSFET&#39;s or IGBT&#39;s, which unfortunately, have limited voltage ratings. Specifically, operational charging voltages for the circuit  10 , as shown, have been generally limited to a maximum voltage that is below about 2 kV.  
         [0007]     In light of the above, it is an object of the present invention to provide a power supply that is capable of producing pulses at a very high pulse repetition rate and that is operable at relatively high voltages (i.e. greater than about 2 kV). It is yet another object of the present invention to provide a system for charging a capacitor at a high voltage and high pulse repetition rate while accurately regulating the maximum voltage across the capacitor. Yet another object of the present invention is to provide a high pulse rate, pulsed power system which is easy to use, relatively simple to implement, and comparatively cost effective.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0008]     The present invention is directed to a power supply system that charges a capacitor with relatively high energy pulses, at a relatively high pulse repetition rate. The system works in conjunction with a load (e.g. UAV laser lamps) that is switchably connected to the capacitor. At the end of each pulse, the load is connected (i.e. switched) across the capacitor. This then discharges the capacitor and energizes the load.  
         [0009]     For the system, a voltage transformer having a high voltage side, a low voltage side and a transformer core is provided. In addition, a resonant circuit having an inductor and a capacitor is connected to the transformer. Specifically, the inductor is connected to the low voltage side of the transformer and the capacitor is connected to the high voltage side of the transformer. With this cooperation of structure, a power supply and switch assembly are connected to the low voltage side of the transformer and configured to generate a pulse in the resonant circuit. This, in turn, generates a high energy pulse on the high voltage side of the transformer that charges the capacitor. The capacitor is then discharged by the load.  
         [0010]     In greater structural detail, the switch assembly is configured to generate pulses on the low side of the transformer that alternate in polarity. With this arrangement of alternating pulses, the transformer core is reset after each pulse. The system also includes a rectifying circuit that is connected to the high voltage side of the transformer. In functional terms, this circuit rectifies the alternating polarity pulses from the low side of the transformer to produce a train of constant polarity pulses for charging the capacitor.  
         [0011]     To generate the pulses that alternate in polarity on the low voltage side of the transformer, the switch assembly is connected between the power source and the transformer. In one embodiment, a switch assembly is used having four transistors and four diodes that are arranged as an H bridge. Because the transistors operate on the low voltage side of the transformer, standard MOSFET or IGBT type low voltage transistors are typically used. Functionally, with the H bridge arrangement, a first pair of transistors are opened while a second pair of transistors remain closed to produce the first pulse. After the load discharges the capacitor, the first pair of transistors are closed and the second pair of transistors are opened to produce a second pulse having a polarity that is opposite to the first pulse. This process is repeated to generate a continuous train of pulses in the circuit on the low side of the transformer that alternate in polarity.  
         [0012]     In another aspect of the present invention, the system includes a control circuit for regulating the maximum voltage that is applied to charge the capacitor. This control circuit includes a probe for measuring a voltage across the capacitor. This measured voltage, in turn, is used by the control circuit to selectively open and close the transistor pairs in the switch assembly to regulate the maximum voltage across the capacitor. In particular, the transistor pair in the switch assembly, that is initially closed to create a pulse, is subsequently opened when the voltage across the capacitor reaches a pre-selected magnitude.  
         [0013]     In a particular embodiment of the system, the control circuit includes a crowbar switch that is connected to the low voltage side of the transformer. Structurally, the crowbar switch includes a circuit that is connected to the low side of the transformer and includes two switches and two diodes. When activated, the crowbar switch short-circuits the low side of the transformer. The crowbar switch is required to divert the energy stored in the inductor. Energy from the inductor resonantly transfers back to a filter capacitor and is recovered. In use, the crowbar switch is activated in response to the voltage probe and works in concert with the switch assembly to regulate the maximum voltage across the capacitor. Specifically, the crowbar switch closes to short-circuit the low voltage side of the transformer when the voltage across the capacitor reaches a pre-selected magnitude. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0015]      FIG. 1  is a schematic diagram of a prior art, resonant circuit for charging a capacitor;  
         [0016]      FIG. 2  is a simplified schematic diagram of a high pulse rate, pulsed power system in accordance with the present invention;  
         [0017]      FIG. 3  is a schematic diagram showing an arrangement of electrical components for charging a capacitor in accordance with the present invention;  
         [0018]      FIG. 4  is a schematic diagram showing an alternate embodiment of the high voltage side of the transformer shown in  FIG. 3  in accordance with the present invention;  
         [0019]      FIG. 5  is a schematic diagram showing another alternate embodiment of the high voltage side of the transformer shown in  FIG. 3  in accordance with the present invention;  
         [0020]      FIG. 6  is a plot showing waveforms from a SPICE model computer simulation for the circuit shown in  FIG. 3 , with the upper plot showing the voltage across the capacitor as a function of time and the lower plot showing the current through the resonant inductor as a function of time (note: voltage regulation is shown without activation of the crowbar switch); and  
         [0021]      FIG. 7  is a plot as in  FIG. 6  of the voltage across the capacitor as a function of time without the activation of the crowbar switch in comparison with the voltage across the capacitor as a function of time with activation of the crowbar switch. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     Referring to  FIG. 2 , a high pulse rate, pulsed power system is shown and generally designated  26 . As shown in  FIG. 2 , the system  26  includes a pulsed charging circuit  28  for charging a capacitor  30 . The system  26  further includes a voltage probe  32  and a control circuit  34 . In functional overview, the pulsed charging circuit  28  is configured to establish voltage pulses across the capacitor  30  which are monitored by the voltage probe  32 . The control circuit  34  then receives the output of the voltage probe  32  and uses that output to selectively open and close transistor switches in the charging circuit  28 . In this manner, the control circuit  34  cooperates with the charging circuit  28  to regulate a maximum voltage across the capacitor  30 .  
         [0023]      FIG. 3  shows the charging circuit  28  in greater detail. As seen there, the charging circuit  28  includes a voltage transformer  36  having a high voltage side  38 , a low voltage side  40  and a transformer core  42 . In addition, a resonant circuit  44  that includes the charging capacitor  30  and an inductor  46  is connected to the transformer  36 , as shown. Specifically, it can be seen that the inductor  46  is connected to the low voltage side  40  of the transformer  36  and the charging capacitor  30  is connected to the high voltage side  38  of the transformer  36 .  FIG. 3  further shows that the charging circuit  28  includes a power source  48 , a filter capacitor  49  and a switch assembly  50 , all of which are connected to the low voltage side  40  of the transformer  36 . In addition, for the charging circuit  28  shown, a crowbar switch  52  is connected to the low voltage side  40  of the transformer  36 . It can further be seen that the charging circuit  28  includes a rectifying circuit  54  connected to the high voltage side  38  of the transformer  36 .  
         [0024]     Functionally, the switch assembly  50  and power source  48  shown in  FIG. 3  cooperate, under the control of the control circuit  34  (see  FIG. 2 ), to generate a series of pulses in the resonant circuit  44  that alternate in polarity. With this arrangement, the transformer core  42  is reset after each pulse. A rectifying circuit  54  is connected to the high voltage side  38  of the transformer  36  and four high voltage diodes  56   a - d . Functionally, the circuit  54  rectifies the alternating polarity pulses from the transformer  36  to feed the capacitor  30  with a train of constant polarity pulses.  
         [0025]     The switch assembly  50  can now be described with reference to  FIG. 3 . As shown there, the switch assembly  50  is connected between the power source  48  and the transformer  36 . For the embodiment shown in  FIG. 3 , the switch assembly  50  includes four transistor switches  58   a - d  that are arranged as an H bridge. Within the H bridge, each transistor switch  58   a - d  is coupled in parallel with a respective diode and capacitor, as shown. For the crowbar switch  52 , two transistor switches  60   a,b  are arranged with two low voltage diodes  62   a,b , as shown. Because the transistor switches (i.e. switches  58   a - d ,  60   a,b ) are positioned on the low voltage side  40  of the transformer  36 , standard MOSFET or IGBT type low voltage transistors can be used. Alternatively, thyristors may be used instead of transistor switches. Additionally, several transistor switches or thyristors could be used in series in the crowbar switch  52 .  
         [0026]      FIG. 4  shows an alternate embodiment of the high voltage side  38  of the transformer  36 . As seen there, a crowbar switch  52  is repositioned on the high voltage side  38 . For the crowbar switch  52 , two transistor switches  60   a,b  are arranged with two high voltage diodes  62   a,b , as shown. Because these transistor switches  60   a,b  are positioned on the high voltage side  38  of the transformer  36 , instead of on the low voltage side  40 , high voltage transistors must be used. The crowbar switch  52  may be positioned on the high voltage side  38  in addition to, or in lieu of, the crowbar switch  52  provided on the low voltage side  40  in  FIG. 3 . The rectifying circuit  54  remains unchanged from that shown in  FIG. 3 , with high voltage diodes  56   a - d . Functionally, the circuit  54  rectifies the alternating polarity pulses from the transformer  36  to feed the capacitor  30  with a train of constant polarity pulses.  
         [0027]      FIG. 5  shows another alternate embodiment of the high voltage side  38  of the transformer  36 . In  FIG. 5 , two high voltage sides  38   a,b  are shown operating with the transformer core  42 . Each high voltage side  38   a,b  is connected to a rectifying circuit  54  that rectifies the alternating polarity pulses from the transformer  36  to feed the capacitor  30  with a train of constant polarity pulses. The rectifying circuits  54  include high voltage diodes  56   a-d  and  56   e - h  and are connected in series with the capacitor  30 .  
         [0028]     Operation of the charging circuit  28  can perhaps best be understood with cross-reference to  FIGS. 3 and 6 . Initially, all switches  58   a - d ,  60   a,b  are configured in an open state and no current flows through the circuit. Next, at time t=0, switches  58   a  and  58   d  are closed by the control circuit  34  (see  FIG. 2 ).  FIG. 6  shows that after switches  58   a  and  58   d  are closed, a rising voltage is established (plot portion  64 ) across the capacitor  30  and a positive current, which peaks sinusoidally and then falls toward zero (plot portion  66 ), is passed through the inductor  46 . Note:  FIGS. 6 and 7  are SPICE model computer plots for a charging circuit  28  having an 1100V power source  48 , a 240 μH inductor  46  and a 0.44 μF charging capacitor  30 .  
         [0029]      FIG. 6  illustrates that at a time t=50 μS, the desired voltage across capacitor  30 , which in this case is about 3.80KV, is reached. At this point the control circuit  34  (see  FIG. 2 ) opens switches  58   a  and  58   d . Current in the inductor  46  then flows through the two diodes that are connected in parallel with switches  58   b  and  58   c , transferring the energy in the inductor  46  to the power source  48  and the capacitor  30 . As shown in  FIG. 6 , this causes a small overshoot voltage across the capacitor  30  (i.e. the maximum voltage across capacitor  30  reaches about 4.00 KV, 0.20 KV higher than the desired voltage of 3.80 KV). This overshoot can be corrected during calibration, or as detailed below, with the use of the crowbar switch  52 .  
         [0030]      FIG. 6  shows that at about t=56 μS, there is zero current through the inductor  46  and at about t=70 μS, the capacitor  30  is discharged. The charging circuit  28  is now ready to generate another pulse. Continuing with cross reference to  FIGS. 3 and 6 , for the system  26 , the second pulse is generated by simultaneously closing switches  58   b  and  58   c .  FIG. 6  shows the voltage and current plots generated when the switches  58   b  and  58   c  are closed at time t=100 μS by the control circuit  34  (see  FIG. 2 ). Specifically,  FIG. 6  shows that after switches  58   b  and  58   c  are closed, a rising positive voltage is established (plot portion  68 ) across the capacitor  30  and a negative current, which peaks sinusoidally and then returns toward zero (plot portion  70 ), is passed through the inductor  46 . Thus, from  FIG. 6  it can be seen that the polarity of the current pulses through the inductor  46  alternates with each pulse. With this arrangement, the core  42  of the transformer  36  is reset after each pulse. On the other hand,  FIG. 6  shows that the polarity of voltage pulses across the capacitor  30  remains constant due to the rectifying circuit  54 .  
         [0031]     Continuing with  FIGS. 3 and 6 , when the desired voltage across capacitor  30  of about 3.80 KV is reached for the second pulse (i.e. at time t=150 μS), the control circuit  34  (see  FIG. 2 ) opens switches  58   b  and  58   c . Current in the inductor  46  then flows through the diodes that are connected in parallel with switches  58   a  and  58   d , transferring the energy in the inductor  46  to the power source  48  and the capacitor  30 . As shown in  FIG. 6 , this again causes a small overshoot voltage across the capacitor  30  of about 0.20 KV.  FIG. 6  shows that at about t=156 μS, there is zero current through the inductor  46  and at about t=170 μS, the capacitor  30  is discharged. The charging circuit  28  is now ready to generate another pulse having a positive current through the inductor  46 .  
         [0032]      FIG. 7  illustrates the use of the crowbar switch  52  ( FIG. 3 ) to reduce or eliminate the overshoot described above. Specifically, plot portion  72  shows the voltage across the capacitor  30  as a function of time without the activation of the crowbar switch  52 , and plot portion  74  shows the voltage across the capacitor  30  as a function of time with activation of the crowbar switch  52 . In greater detail, the operation of the crowbar switch  52  can perhaps best be understood with cross-reference to  FIGS. 3 and 7 . Initially, all switches  58   a - d ,  60   a,b  are configured in an open state and no current flows through the circuit. Next, at time t=0, switches  58   a  and  58   d  are closed by the control circuit  34  (see  FIG. 2 ).  FIG. 7  shows that after switches  58   a  and  58   d  are closed, a rising voltage is established (plot portion  76 ) across the capacitor  30 .  FIG. 7  illustrates that at a time t=50 μS, the desired voltage across the capacitor  30 , which in this case is about 3.80 KV, is reached. At this point the control circuit  34  (see  FIG. 2 ) opens switches  58   a  and  58   d , and closes switches  60   a  and  60   b . Functionally, the crowbar switch  52  closes to short the low voltage side  40  of the transformer  36  when the voltage across the charging capacitor  30  reaches the desired voltage. With the transformer  36  short-circuited, current in the inductor  46  flows through the diodes that are connected in parallel with switches  58   b  and  58   c , transferring the energy in the inductor  46  to the filter capacitor  49 . As illustrated in  FIG. 7 , this reduces the overshoot voltage across the capacitor  30 .  
         [0033]     Referring back to  FIG. 4 , it can be seen that operation of the charging circuit for this alternate embodiment is similar to the above discussion relating to  FIG. 3 . Initially, all switches  58   a - d  (see  FIG. 3 ) and  60   a,b  are configured in an open state and no current flows through the circuit. Next, at time t=0, switches  58   a  and  58   d  are closed by the control circuit  34  (see  FIGS. 2 and 3 ).  FIG. 7  shows that after switches  58   a  and  58   d  are closed, a rising voltage is established (plot portion  76 ) across the capacitor  30 .  FIG. 7  illustrates that at a time t=50 μS, the desired voltage across the capacitor  30  is reached. At this point the control circuit  34  (see  FIG. 2 ) opens switches  58   a  and  58   d , and closes switches  60   a  and  60   b . Functionally, the crowbar switch  52  closes to short the high voltage side  38  when the voltage across the charging capacitor  30  reaches the desired voltage. With the transformer  36  short-circuited, current in the inductor  46  flows through the diodes that are connected in parallel with switches  58   b  and  58   c , transferring the energy in the inductor  46  to the filter capacitor  49 . As illustrated in  FIG. 7 , this reduces the overshoot voltage across the capacitor  30 .  
         [0034]     While the particular Resonant Charge Power Supply Topology For High Pulse Rate Pulsed Power Systems as herein shown and disclosed in detail are fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that they are merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.