Abstract:
An apparatus for providing power to a gas discharge lamp comprises a storage capacitor and an ignitron switch coupled through a primarily parasitic first inductor to a parallel combination of a diode assembly and a second inductor in series with a gas discharge lamp. The second inductor is selected to optimize the energy transfer from the capacitor to the gas discharge lamp. During a first interval determined by the time constant of the series combination of a storage capacitor, a first inductor, and a second inductor, the diode assembly is not conducting and a forward sense current builds in the first and second inductors. During a second interval determined by the interaction of the two parallel circuits driving the gas discharge lamp, during which the diode array is conducting, the smaller reversed sense current flowing in the first inductor and a larger forward sense current flowing in the second inductor add, thereby generating a unipolar, forward sense, single pulse current output for the generation of optical energy by a gas discharge lamp.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to the class of power supplies used to deliver a shaped current pulse to a gas discharge lamp or tube for the generation of a maximum intensity, single pulse, optical output.  
       BACKGROUND OF THE INVENTION  
       [0002]      FIG. 1   a  shows a prior art gas discharge power supply  10  including a capacitor  12  which is charged by voltage source  15  in series with current limiting resistor  16 . When the voltage level of capacitor  12  reaches a desired level, an ignitron  14  is triggered, which acts as a switch device delivering charge from the storage capacitor  12  to a series combination of lead inductance  18 , and a lamp assembly  21  which is electrically modeled as a gas discharge lamp  22 , which acts as a constant voltage drop, in series with an arc resistance  20 , which has a current-dependant voltage drop. Typically, the arc resistance  20  is very small compared to either the inductive impedance of lead inductance  18  or the capacitive reactance of storage capacitor  12 , thereby producing an under-damped series RLC circuit.  FIG. 2  shows the waveforms of operation of  FIG. 1   a.  At a time t=0us, ignitron  14  is triggered and operates as a closed circuit, resulting in the transfer of energy from storage capacitor  12  to the series circuit of lamp assembly  21  including resistance  20 , and lead inductance  18 . The current which results from the ignitron  14  switch closing is an oscillatory LRC decay I 1   32  shown in  FIG. 2 , where frequency and decay are determined by L R and C according to the well-known formula:  
         I   ⁡     (   t   )       =       I   max     ⁢     ⅇ         -   R       2   ⁢   L       ⁢   t       ⁢     sin   ⁡     (           1   LC     -       (     R     2   ⁢   L       )     2         ⁢   t     )               
         [0003]     When R=0.01 ohms, C=0.5 uF and L=50 nH in  FIG. 1   a,  the current waveform I 1   32  is oscillatory as shown in  FIG. 2 , and lamp  22  generates multiple bursts of optical energy  28 , shown as waveform E 1   30 . Each burst of optical energy  28  is approximately 1 μs in duration, and multiple bursts are emitted until the oscillatory voltage which appears across the gas discharge lamp  22  falls to below the actuation level of the lamp  22 . This results in a plurality of optical bursts at the rate of oscillatory decay, with each subsequent optical pulse of reduced magnitude compared to the previous burst.  
         [0004]     In applications where the lamp  22  is generating an optical burst  28  for use as control energy for an UV/optical switch such as a diamond switch, or some other photo-conducting device using UV/optical control, and the optical energy level is often required to be large in magnitude and short in duration, a problem arises whereby the size of the capacitor C  12  (due to limits on the applied voltage V  15 ) becomes too large to support the burst energy requirement. This increased capacitance  12  causes the resonant frequency to be reduced, which increases the time duration and reduces the rise time of the optical control signal produced by the gas discharge lamp  22 .  
         [0005]     It is desired to reduce the duration of the oscillatory decay, and further to capture the energy associated with the oscillatory decay and redirect it to the optical lamp, thereby producing a single, uni-polar pulse of current, which translates into a single burst or pulse of emitted optical energy  28 .  
         [0006]     An alternative embodiment  21  of prior art  FIG. 1   a,  shown in  FIG. 1   b,  places a second closing switch  15  directly in parallel with both the capacitor  12  and switch  14 , and the flash lamp assembly  21 . The first switch  14  is closed at an initial time t 1 , followed at time t 2  by second closing switch  15 , where the first switch  14  closing time and second switch  15  closing time is controlled by controller  17 , and the second switch  15  is triggered to close at the time of the first quarter period following the first switch  14  closure. This method also has the disadvantage that for some circuit parameters, the current through the gas discharge lamp can reverse direction, thereby allowing the current to pass through zero and allowing the lamp discharge gas to begin cooling, which results in reduced optical emission from the lamp.  
         [0007]     U.S. Pat. No. 3,465,203 by Galster et al describes a circuit for discharging stored charge into a flashlamp using inductors, capacitors, and diodes. Resonant current from the inductor/capacitor combination is redirected through clamping diodes to extend the capacitor discharge time.  
         [0008]     U.S. Pat. No. 4,194,143 by Farkas et al describes the use of a resonant LC circuit to generate multiple flash lamp discharges.  
         [0009]     U.S. Pat. No. 4,524,289 by Hammond et al describes a flash lamp using inductors, capacitors, and switches to transfer current from two resonant LC circuits to a flash lamp load.  
         [0010]     A flash lamp control circuit is desired which generates a single pulse of current which can be optimized for power output and minimized for time duration.  
       OBJECTS OF THE INVENTION  
       [0011]     A first object of the invention is a power source for a gas discharge lamp which generates an optimized pulse of current for use by the gas discharge lamp.  
         [0012]     A second object of the invention is a power source for a gas discharge lamp which allows redirection of the majority of the energy stored in a secondary inductor, to the gas discharge lamp, through a circuit bypassing the initial energy storage capacitor, thereby maintaining a unipolar current drive to the gas discharge lamp.  
       SUMMARY OF THE INVENTION  
       [0013]     A power supply  40  for a gas discharge lamp comprises a switch  44 , an energy storage capacitor  42 , a first inductor  54 , primarily associated with the parasitic inductance of the switch  44 , capacitor  42 , and their connections to the remaining circuit, a diode assembly  49  having a series inductance Ld  60  and resistance Rd  47 , where the diode assembly  49  is also in parallel with the series combination of a gas discharge lamp  51  and a secondary, inductor L 2   58 , which includes the inductance associated with the gas discharge lamp  52 . The secondary inductor  58  is chosen for a level of inductance such that at peak current the energy inductively associated with the secondary inductor  58  is preferably much larger than that of the first inductor  54 , and such that the sum of the first inductor  54  and second inductor  58 , when combined with the capacitance of the initial storage capacitor  42 , results in an initial oscillatory period on the order of the time scale desired for the optical pulse width. Following the first quarter period of this oscillatory period, and then subsequently following with each further same-sense reversal of the time-derivative of the current I 2  (dI 2 /dt) through the secondary inductor  58 , the polarity of the reactive L 2 * dI 2 /dt voltage drop across the secondary inductor  58  reverses. Each time this same-sense polarity reversal occurs and as the L 2  times dI 2 /dt voltage exceeds that of the voltage drop across the gas discharge lamp, V 3 , which has a voltage drop of the opposite polarity sense at that time, the polarity of the net voltage drop across the combined secondary inductor and gas discharge lamp puts the diode  53  in forward bias, allowing a substantial portion of the current I 2  flowing through the secondary inductor  58  to be redirected to the gas discharge lamp  52  through the diode  53 , a circuit independent of the initial storage capacitor C 0  and inductor L 1 , thereby changing the discharge circuit associated with inductor L 2   58  to include the diode  49 , inductor  58 , and flashlamp  52 , and resulting in a continuous unipolar flow of current through the flashlamp, thereby increasing the peak output of the initial optical burst from the lamp and reducing the number of cycles of lamp reignition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1   a  shows a schematic diagram for a prior art power source for a gas discharge tube.  
         [0015]      FIG. 1   b  shows a schematic diagram for an alternate prior art power source for a gas discharge tube.  
         [0016]      FIG. 2  shows the waveforms of operation for  FIG. 1   a.    
         [0017]      FIG. 3  shows a schematic diagram for a power source for a gas discharge tube.  
         [0018]      FIG. 4  shows two cycles of waveforms of operation for the circuit of  FIG. 3 .  
         [0019]      FIG. 5  shows several cycles of waveforms of operation for the circuit of  FIG. 3 .  
         [0020]      FIG. 6  shows the schematic diagram for a diode array. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]      FIG. 3  shows a gas discharge lamp power supply  40  comprising an energy storage capacitor  42  which is charged by a voltage source  45  and bleed resistor  46 . An ignitron  44  is used to instantaneously apply the capacitor  42  charge to a first, primarily parasitic inductor  54  which is coupled to a diode assembly  49  in parallel with a second, energy storage inductor  58  which is in series with a gas discharge lamp assembly  51 . The diode assembly  49  includes an array of diodes  53 , and also has a characteristic resistance Rd  47  and inductance Ld  60 . The gas discharge lamp assembly  51  includes a series resistance R f1    50  and the gas discharge lamp  52  which emits an optical output E 2   48 . The capacitor  42  is first charged to a high potential on the order of kilovolts by voltage source  45 , and trigger circuit  43  causes ignitron  44  to trigger, where after it becomes conductive with a very low series resistance. For an initial duration of time, current builds in both inductors L 1  and L 2 , in accordance with the time constant of C 0   42  and series inductors L 1   54  and L 2   58 , modified slightly by the gas discharge lamp resistance R f1 . After a quarter period of ringing as determined by the LC circuit comprising C 0   42 , L 1   54  and L 2   58 , dI 2 /dt, the derivative of the current through inductor L 2   58 , changes sign resulting in the voltage V 2  at the diode assembly  49  reversing polarity, once the L 2  times (dI 2 /dt) voltage exceeds that of the opposite signed voltage drop, V 3 , across the gas discharge lamp, and diode assembly  49  begins to conduct. After this point in time, a substantial portion of the current which was carried through L 2  and the flashlamp begins to flow through the diode assembly  49 , thereby changing the characteristic time for discharge of the energy stored in inductor L 2  to be dominated by (L 2 +Ld)/(R f1 +Rd), until the voltage V 2  becomes positive again due to the loss of energy into the parallel capacitor circuit, now parasitic, which recharges the capacitor  42  and begins its second discharge cycle, where after the current in inductor L 1  changes direction, the diode assembly  49  stops conducting, and the current of L 1  is once again flowing in the same direction as the current of L 2 . Optimization involves, among other considerations, minimizing the energy put back into the capacitor following the first quarter period and the L/R decay time of the diode  49 , inductors L 2  and Ld, and the gas discharge lamp  22  circuit. In addition, minimization of L 1  and Ld is preferred. A condition for optimization is reached when the following equation is satisfied in the case where Ld is small compared with L 2 , which may be used for the selection of L 2 :  
                 I   mFL     ·     exp   ⁡     (       -     (         T   0     4     +       T   1     2       )       ·       (       R   d     +     R   fl       )       L   2         )         &gt;       I   mC     ·       R   d             (       R   d     +     R   fl       )     2     +       L   2   2         (       L   1     +     L   2       )     ·     C   0                                         
 where: 
 
         [0022]     L 1  and L 2  are the inductances of the associated inductors of  FIG. 3 ;  
         [0023]     C 0  is the capacitance of capacitor  42  of  FIG. 3 ; 
        T 0 =2·π·((L 1 +L 2 )·C 0 ) 0.5 ;     T 1 =2·π·[(L 1 )·C 0 ] 0.5 ,     I mFL  is peak current through the gas discharge lamp,     I mC  is peak current of the storage capacitor during the time period T 0 /4&lt;t&lt;T 0 /4+T 1 /2,     R d  is the average resistance of a diode during the time T 0 /4&lt;t&lt;T 0 /4+T 1 /2;     R f1  is the average resistance of gas discharge lamp during the time T 0 /4&lt;t&lt;T 0 /4+T 1 /2.        
 
         [0030]     Additionally, R f1 &lt;&lt;2·(C 0 /(L 0 +L 1 )) 0.5    
         [0031]      FIG. 4  shows an example of waveforms for operation of the lamp power supply of  FIG. 3  at various voltage and current nodes. The operation of the invention involves the interaction of two coupled circuits; the first involving the ignitron switch  44 , storage capacitor C 0   42 , and the primarily parasitic inductance L 1   54 ; the second involving the diode assembly  49  and the inductance Ld  60  associated with the diode assembly  49  and their connection with series L 2   58  and gas discharge lamp  52 . These two circuits are coupled across the common elements of inductor L 2   58  and gas discharge lamp  52 . For the purposes of discussion, forward current flow will be adopted as that shown in the sense of I 1  and I 2   56  as shown in  FIG. 3 , through L 1   54  and L 2   58 , respectively. Reverse current flow will be taken as opposite to the respective forward current flows.  FIG. 4  shows only two cycles of operation: a first interval  63  and a second interval  65 .  
         [0032]     Time t=0  74  is the instant the ignitron  44  fires, completing the RLC circuit. At this instant, diode  49  is reversed biased and not conducting, so the RLC circuit has a resonant frequency determined by L=L 1 +L 2 , C=C 0 , and Rf 1  and the capacitor voltage V 1  of C 0   42  is shown as waveform  64 . During the first quarter cycle from firing time  74  to T 0 /4  76 , the capacitor voltage waveform V 1   64  varies sinusoidally, as does the current I 1   66  which flows through inductor L 1   54 . When diode  49  is not conducting, waveform V 2   68  varies roughly proportionally to V 1   64  as shown, and current I 2   70  is identical to that of I 1   66 .  
         [0033]     Following peak current at time  76 , and through to time  80  when difference between the relative polarity of the reactive voltage drop of L 2 , L 2 (dI 2 /dt), reverses and exceeds that of the then oppositely signed gas discharge lamp voltage drop, V 3 , and the diode  53  becomes forward biased and begins to conduct. The diode  49  causes the voltage V 2   68  to clamp near 0V as shown, and a majority of the current I 2  flowing through L 2   58  now flows through diode  49  as Id  72 . During this period of diode conduction, from  76  to  80 , the finite remaining voltage V 2  allows the storage capacitor to recharge in the reverse polarity. Also during this interval, the diode circuit  49  allows significantly higher Id currents associated with a faster discharge period of the energy in L 2  through the diode, which contributes to maintaining the current through the gas discharge lamp in the forward direction during the subsequent capacitor charging and discharge cycle which would normally have resulted in a reversal of current flow through the gas discharge lamp due to I 1 . To achieve a unipolar current drive in the flashlamp, the level of forward going current circulation in the diode must always dominate over the reverse current, −I 1 , flowing through L 2  associated with reverse polarity, relative to the initial capacitor charge polarity, of the cycles of the reverse current discharge-recharge of the storage capacitor. At the time  80 , the above described cycle shown as interval  63  begins to repeat as shown in interval  65  with the capacitor recharged in the original polarity from  80  to  82  and with the subsequent change in V 2  polarity due to the positive L 2 (dI 2 /dt) reactive voltage drop. As illustrated in  FIG. 4 , the gas discharge lamp current I 2   70  is initially supplied solely by the capacitor through the period  74 - 76 , waveform  66 , ending shortly after the first quarter period. At time  76 , the reactive voltage drop across L 2 , waveform  64 , is reversed and exceeds the opposite polarity gas discharge lamp resistive voltage drop, V 3 , causing the diode to be forward biased, allowing the voltage across inductor L 2  to drive current through the gas discharge lamp and the diode circuit during the period  76 - 80 . During this interval the current I 2  through the gas discharge lamp is the sum of the capacitor discharge current I 1 , waveform  66  and the diode circuit current Id, waveform  72 . At time  80  the voltage across the diode circuit, V 2 , drops to zero and again changes polarity, putting the diode in reverse bias, thereby decoupling the diode circuit from the flash lamp. Following time  80  the above described cycle of operation repeats. The actual pulse formed by the power supply over a multiple such cycles of  FIG. 3  is shown in  FIG. 5 . Waveform I 2   88  shows the actual current I 2  waveform produced, while the optical output power E 2  is shown in waveform  86 .  
         [0034]     The diode assembly  49  is typically not a single diode, as semiconductor diodes have reverse breakdown characteristics which cause avalanche breakdown, as known in the art of high voltage rectification. Also known as a solution to this problem in the prior art is the diode array  90  of  FIG. 6 , which comprises parallel strings of series diodes and voltage compensating components, one such string shown as a single string  106 . The series diodes  94 ,  98 ,  102  may be any number of matched diodes, but three are shown. Resistor  92  ensures current sharing between the strings of series diodes, while capacitors  96 ,  100 ,  104  are used to divide the reverse voltage present across the diode string equally across each diode, thereby preventing a single diode from receiving all of the reverse voltage and suffering avalanche breakdown. The equal-value capacitors  96 ,  100 ,  104  could also be replaced by equal value resistors without loss of generality.  
         [0035]     While the circuit of  FIG. 3  is set forward as best mode of the invention, variations in the circuit and components are possible. Ignitron  44  acts as a switch, and any switch element suitable for high voltage switching may be used as ignitron  44 . Also, while ignitron  44  is shown as a switch element with a control trigger, it is possible to use a two terminal breakdown-mode switch which triggers simply when a threshold voltage across the terminals exceeds a particular level. The voltage source  45  and bleed resistor  46  may be replaced by any mechanism that delivers charge to capacitor  42 , including a current source, or any device capable of delivering charge. Clamp diode assembly  49  may include series inductance and resistance, or any other source of loss and energy storage including but not limited to shunt and series capacitance across any nodes shown. Inductances L 1   54  and L 2   58  may be intentionally designed inductances, or they may be formed from component leads, or intrinsic circuit values associated with the topology of the physical elements used to realize the circuit. Flashlamp  51  may be a gas discharge lamp, or any type of optical source suitable for converting a flowing current into an optical output. It should be noted that the waveforms of  FIG. 3  are approximations given to suggest the operation of the circuit over some particular time boundaries. It is clear to one skilled in the art of non-linear circuits and higher harmonic frequency current flow that the effect of currents flowing in the three mesh loops of the circuit of  FIG. 3  will effect the T 0  and T 1  time constants, and for this reason, approximations are given for the durations of these periods, and the time references to T 0  and T 1  are not intended to be exact time periods. A reasonable range for T 0  and T 1  to vary from the values shown in the equations of the present letters patent because of inter-mesh loop coupling is from +100% to −50% of the computed value, although larger transient variations are possible during 10% of the duration T 0  or T 1 , particularly when a current or voltage discontinuity occurs.  
         [0036]     In this manner, an improved power supply for a gas discharge lamp is described.