CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from copending U.S. provisional patent application Serial No. 60/262,453, filed Jan. 18, 2001, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power supply to provide electrical power to a xenon bulb. More particularly, but not by way of limitation, the present invention relates to a microprocessor controlled power supply for a xenon bulb which, in one embodiment, provides a constant programmable power to the bulb. 
     2. Background of the Invention 
     Continuous arc xenon bulbs provide bright, stable, daylight balanced light at power levels from a few watts up to tens of thousands of watts. Such bulbs are widely accepted in architectural, entertainment, and medical applications. Typically such bulbs require a moderate DC voltage (on the order of 18 to 150 volts) at a relatively high current for steady-state operation. In addition, a higher voltage is usually provided for starting (usually between 2 and 10 times the operating voltage) along with a very high voltage, short duration ignition pulse (on the order of several kilovolts for a period ranging from a few microseconds to a few milliseconds). This higher start-up voltage and the ignition pulse tend to complicate xenon power supply designs. 
     Presently, xenon power supplies may be logically divided into two distinct groups: a) those that operate at line frequency, otherwise known as magnetic ballasts; and b) those that operate at higher frequencies, commonly referred to as electronic power supplies. It should be noted that the terms “ballast” and “power supply” are often used interchangeably. Magnetic ballasts typically employee a transformer followed by a rectifier and filter capacitors to provide the steady-state electrical power, much like a conventional linear power supply. Magnetic ballasts rely on the inductance of the transformer, or a separate inductor in series with the transformer, to limit the current provided by the ballast. The inductance acts on the line frequency of the AC power supplied to the ballast leading to ballasts which are characteristically large and heavy compared to their electronic counterparts. 
     Electronic power supplies, on the other hand, typically rectify and filter the incoming electrical power. Solid state switches such as transistors, MOSFETs, IGBTs, or the like, are used to “chop” the resulting DC voltage at a relatively high frequency, typically somewhere between 10 kilohertz and 100 kilohertz. A transformer is then used to produce a lower voltage which is again rectified and filtered to provide a steady-state direct current output. The higher frequency provides substantial reductions in the size and weight of the transformer and efficient regulation of the output voltage may be easily achieved by varying the frequency at which switching occurs, the duty cycle provided at the switches, or both. While electronic power supplies are smaller and lighter than their magnetic counterparts, they are also more complex. In addition, electronic power supplies designed to power xenon bulbs above 3600 watts presently stretch the practical limits of the solid state switches employed, resulting in hot components and reduced life of the component parts. Presently, the selection of a particular solid state switch requires balancing switching frequency, and thus the size and weight of the reactive components, against power handling capability. 
     Thus, magnetic ballasts have dominated the high power xenon field. The term “high power” as used in conjunction with the present invention refers to xenon bulbs which are designed to consume more than about 2500 watts of electrical power. Practically speaking, short-arc xenon bulbs may presently be produced up to about 20,000 watts while long-arc xenon bulbs of at least 100,000 watts are presently available. 
     While magnetic ballasts perform satisfactorily in many applications, they are marginal for use in the entertainment industry for a number of reasons. For example, such ballasts often produce “ripple” at the line frequency or, perhaps, at twice the line frequency. In the United States, this results in 60 Hz or 120 Hz flicker. When a filmed scene is lighted with a xenon powered by such a ballast, “beating” between the motion picture frame rate and the flicker can result in flicker at a much lower, perceivable rate in the recorded images. In addition, flicker at any rate will totally preclude the use of frame rates higher than the flicker rate. Furthermore, magnetic ballasts designed for these power levels are often too heavy to be moved manually and therefore require undue time and labor for setup and tear down. 
     While high power electronic power supplies are available, the size and weight of such devices approaches that of magnetic ballasts. Presently, the most palatable solution for the entertainment industry is the ganging of lower power electronic power supplies to supply high power xenon bulbs. “Ganging” involves the parallel connection of two or more power supplies. To date, the ganging of lower power electronic power supplies has proven reasonably effective up to power levels of 10 kilowatts. Unfortunately, not all electronic power supplies are gangable and, of those that are gangable, load sharing among ganged power supplies is less than perfect. Therefore, it is common for one power supply in a ganged configuration to operate at substantially higher temperature than its co-power supplies, resulting in unreliable operation and premature failure of the over-worked supply. In addition, it has been observed that ganging power supplies may produce substantial ripple, and hence flicker, at rates which are much lower than the switching frequency of the power supplies, thus also raising concerns when used to light a motion picture scene. 
     Another problem which arises in the use of high power xenon bulbs is inconsistent bulb voltage. First, bulb operating voltage may vary significantly over the life of the bulb. Second, there are significant variations in bulb voltage from bulbs offered by different bulb manufacturers. Finally, bulb voltage varies significantly with the temperature of an individual bulb and, therefore, varies as the bulb heats during use. Neither magnetic ballasts or electronic power supplies presently handle such variations in bulb voltage appropriately. In virtually all instances, the bulb will be operated above or below rated power depending on whether the bulb operating voltage is above or below the voltage for which the power supply was designed. In many respects, an ignited xenon bulb resembles a zener diode, e.g., large changes in current flowing through the bulb result in relatively small changes in bulb voltage. Thus, proper regulation of bulb brightness requires the operation of the power supply in a “constant power” mode. Typically, presently available electronic power supplies tightly regulate either output voltage or output current, either of which results in inconsistent bulb brightness as the bulb voltage varies. 
     Additionally, prior art electronic power supplies have utilized a transformer to step down the “chopped” input voltage to a voltage closer to the bulb voltage. Thus used, the transformer may serve a number of purposes. For example: the output power to the bulb is isolated from the power line and from earth ground; the transformer may be included in the oscillator design which drives the solid state switches, as with a relaxation oscillator; the inductive nature of the transformer provides an upper limit on the electrical current; and the transformer provides a reduction in voltage, allowing the switches to operate at a higher duty cycle which improves the power supply&#39;s ability to resolve the output voltage. Unfortunately, the transformer is a large, heavy, and costly component of a high power xenon ballast. 
     A final consideration in the design of a high power xenon ballast is the apparent phase angle between the incoming voltage and incoming current, otherwise known as “power factor”. Power factor is defined as the cosine of the phase angle between voltage and current in an AC system. Ideally any system connected to an AC power line will exhibit a power factor of one. Generally speaking, a power factor of less than one poses a problem for the utility company, rather than the user of the electrical power, resulting in increased line losses. However, many jurisdictions require electrical products to carry the mark of a recognized testing laboratory and typically the standards applied by such laboratories set limits on the power factor exhibited by electrical devices connected to AC power. Thus, a xenon power supply aimed at a global market will require power factor correction for compliance with such standards. While some xenon power supplies presently include power factor correction, none of these supplies take advantage of a power factor correction scheme which can reduce the size, weight, and cost of downstream components and actually facilitate a transformerless power supply. 
     It is thus an object of the present invention to provide a transformerless electronic power supply for a xenon bulb. 
     It is still a further object of the present invention to provide a power factor corrected electronic power supply for a xenon bulb. 
     It is still a further object of the present invention to provide a microprocessor controlled xenon power supply wherein performance calculations and safety features may be incorporated in software. 
     It is yet a further object of the present invention to provide a transformerless, power factor corrected high power xenon power supply which weighs substantially less than presently available high power ballasts. 
     SUMMARY OF THE INVENTION 
     The present invention provides a microprocessor controlled, transformerless, high power xenon power supply which is power factor corrected as to the incoming line. The power factor correction provides a first stage of voltage regulation. A second stage, switching regulator, synchronized to the power factor correction, provides power regulation at a predetermined wattage, regardless of bulb voltage as long as bulb voltage remains within a prescribed range. Synchronization of the power factor correction and the second stage regulator allows a reduction in value, and therefore the size, of the filter capacitors required to reduce ripple to a particular level. 
     In a preferred embodiment, a programmable microcontroller monitors the output voltage and output current to derive output power. The microcontroller adjusts the duty cycle of a pulse width modulated output, which drives solid state switches of the second stage regulator, to maintain a substantially constant output power. 
     Preferably, the second stage regulator incorporates one or more current paths depending on the output power desired. Each current path comprises: a solid state switch, i.e. a transistor, MOSFET, IGBT, or the like; an inductor; and a capacitor. The number of current paths employed determines the maximum power output of the power supply. Thus, by way of example and not limitation, if a 4000 watt power supply employed a single current path, a 7000 watt power supply would employ two current paths, and a 10,000 watt power supply would employ three current paths. The individual elements of each current path are therefore no larger than required to attain the maximum output level for a given power supply wattage. 
     The power factor correction circuit employs a controller which monitors the input current and input voltage, and modulates an output to one or more solid state switches to shape the input current to match the input voltage at a phase angle near zero. Similar to the second stage regulator, the power factor correction provides one or more current paths, depending on the desired output power. Preferably each current path comprises: an inductor connected to a solid state switch in a boost configuration and a diode for summing the outputs of the various current paths into one or more capacitors. 
     In one preferred embodiment, the inventive high power xenon power supply includes an input to control dimming of the xenon bulb. In a dimming configuration, a maximum voltage (i.e., five volts DC) applied to the dimming input results in the power supply producing the maximum output power. Zero volts applied to the dimming input results in the power supply producing a minimum output power, typically 40% of the maximum power. A voltage in between the maximum and minimum voltages would result in an intermediate output power proportional to the level of the applied dimming voltage. 
     For starting, the capacitors of the second stage regulator are charged to a starting voltage, typically on the order of 150 volts. An ignition pulse is then triggered by the microcontroller through a conventional ignitor circuit, resulting in a high voltage pulse applied across the xenon bulb. Upon detecting current flow from the second stage regulator, indicating an ignited bulb, the microcontroller begins regulating the output at a predetermined level. 
    
    
     Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 provides a block diagram of the inventive transformerless high power xenon power supply. 
     FIG. 2 provides a block diagram for a preferred power factor correction circuit as incorporated in the inventive xenon power supply. 
     FIG. 3 provides a schematic diagram for a preferred current path of the power factor correction circuit. 
     FIG. 4 provides a schematic diagram depicting three power factor correction current paths as incorporated in a 10,000 watt embodiment of the inventive xenon power supply. 
     FIGS. 5A and 5B provide a schematic diagram for a preferred second stage regulator circuit as incorporated in the inventive xenon power supply. 
     FIG. 6 provides a flow chart of a computer program as used in the inventive high power xenon power supply. 
     FIG. 7 provides a flow chart of additional computer program steps to include a dimming function in the inventive high power xenon power supply. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1 the inventive high power xenon power supply  10  preferably comprises: a power connector  12  for connection to a power source such as conventional alternating current provided by an electric utility company; a current sensor  14  for monitoring the incoming current; a ground fault interrupter  16  for disconnection of the power supply in the event of current path to earth ground; circuit breaker  18  for protection against over current conditions; bridge rectifier  20  for conversion of the incoming AC power to DC power; power factor correction system  22  for sinusoidally shaping the incoming current to match the incoming voltage; second stage regulator  24  for selectively regulating the output at a predetermined voltage, current, or power as discussed herein below; output current sensor  26  for monitoring the electrical current flowing through the xenon bulb  32 ; voltage sensor  28  for monitoring the output voltage applied to the xenon bulb  32 ; microcontroller  30 ; and ignitor  34  for producing a high voltage ignition pulse. In addition, power supply  10  may be provided with a potentiometer  46 , or electronic input means, for providing a dimming input. 
     The term “high power” as used herein refers to xenon bulbs intended to consume 2500 watts or more of electrical power and to power supplies for such bulbs. It should be noted that presently there are no commercially available xenon bulbs designed for continuous use above 10,000 watts. Thus, the description of the preferred embodiment is provided herein with regard to such commercially available bulbs. As will be apparent to those skilled in the art, the present invention could readily be modified to accommodate xenon bulbs far in excess of 100,000 watts, should such bulbs become available, and it is the intention of the inventor that such modifications are within the scope of the present invention. It should also be noted that presently there are 100,000 watt long-arc xenon bulbs produced in small quantities. While the voltage required to operate long-arc xenon bulbs is substantially different from that required for short-arc xenon bulbs, the inventive power supply is, nonetheless, adaptable for use with such bulbs. 
     Turning next to the ignitor  34 , xenon ignitors are well known in the art and the ignitor  34  incorporated in the inventive power supply is a conventional, commercially available xenon ignitor. Such ignitors receive an input (typically on the order of 100 volts, or more) and generate an output pulse of several thousand volts. The ignitor is typically wired in series with the bulb and a power supply such that the voltage across an unignited bulb is the sum of the power supply voltage and the ignitor voltage. Upon the generation of the high voltage pulse from the ignitor, the xenon gas in the bulb ionizes and an electrical arc is started between the internal electrodes in the bulb. After ignition, the voltage produced by the second stage regulator  24  is then sufficient to sustain the arc. 
     Referring next to FIG. 2, preferably power factor correction circuit  22  comprises: one or more current paths  36 ; a power factor correction controller  38 ; bypass diode  40 ; and capacitors  42 . Power factor correction schemes are well known in the art and the power factor correction scheme employed herein is similar to prior art schemes except as discussed hereinbelow. Power factor controllers are likewise well known in the art and typically are provided as a single integrated circuit. One such power factor controller is the UCC3817 BiCMOS power factor preregulator manufactured by Texas Instruments, Inc. of Dallas, Tex. The UCC3817 device is suitable for use in the inventive power factor correction circuit when used with support components as suggested by Texas Instruments, Inc. The use of the UCC3817 device in this manner is within the level of skill of one of ordinary skill in the art. 
     Referring now to FIGS. 2 and 3, power factor controller  38  provides a pulse width modulated output  44  for driving boost switch  48 . Preferably the switching frequency applied to solid state switch  48  is high (typically between 10 kilohertz and 100 kilohertz) relative to the power line frequency (typically 50 or 60 Hertz, depending on the country in which the device is used). Controller  38  varies the duty cycle of the waveform applied to switch  48  to shape the current flowing through current sensing resistor  50  such that the input current waveform matches the sinusoidal shape of the input voltage at approximately a zero degree phase angle between the two waveforms. 
     Bypass diode  40  charges capacitors  42  to substantially the peak of the incoming AC line voltage (minus a small voltage drop across bridge  20  and diode  40 ). As required to shape the current, controller  38  activates switch  48  thereby storing electrical energy in inductor  54 . As appropriate, controller  38  deactivates switch  48 . The energy stored in inductor  54  causes the voltage to rise at node  56  resulting in current flow through diode  52  and increasing the voltage stored in capacitors  42 . The power factor controller  38  includes voltage feedback input  46  through which controller  38  compares the voltage at node  56  to an internal reference to likewise adjust the duty cycle of the output  44  to switch  48  such that the voltage at node  56  is regulated at approximately 350 volts. 
     As shown in FIG. 3, a power factor correction current path  36  preferably involves an inductor  54 , a solid state switch  48  wired in a boost configuration, and a diode  52 . By switching the current through the current path  36 , controller  38  preferably causes capacitors  42  (FIG. 2) to be charged to a voltage greater than that of the incoming AC line. Solid state switch  48  is typically a transistor, a MOSFET, an IGBT, or the like. Presently with known solid state switch types there exists a tradeoff between current handling capability and the switching frequency at which the device may be switched. Thus, while individual devices are available which could switch the electrical current required for a high power xenon power supply above 4000 watts, such devices could only operate in the range of ten to twenty kilohertz. As the operating frequency is reduced, the physical size of the reactive components (i.e., inductors and capacitors) must be increased. Thus, while a single switch could be used, the size and weight of the reactive components becomes prohibitive for ballasts above 4000 watts. On the other hand, switches are available which work well at switching frequencies up to 100 kilohertz and provide adequate current for a 4000 watt power supply. Thus, multiple switches  48  could be employed to achieve higher power outputs while still maintaining a desirable switching frequency. 
     For purposes of this invention, “load sharing” refers to the division of electrical current switched among a group of parallel switches. Unfortunately, if multiple switches  48  were simply wired in parallel, variation between individual switches  48  would normally result in large disparities in the current passing through each of the various switches  48  (uneven load sharing). This results in overheating of the device which takes on more than its fair share of the switched load. To avoid this phenomenon, power factor correction circuit  22  preferably includes a separate current path  36  (as shown in FIG. 4) for each switch  48  employed. In this way, each switch  48  switches only the current associated with temporary storage of energy in its associated inductor  54 . Diodes  52  provide proper summing of the current from each current path  36  into node  56  as each switch  48  is deactivated. Thus, load sharing is primarily dependant on the consistency between inductors  54  rather than between switches  48 . 
     Referring next to FIGS. 5A and 5B, second stage regulator  24  preferably comprises: microcontroller  30 ; one or more current paths  58 ; voltage divider  28  providing feedback of the output voltage in a range readable by the microcontroller  30 ; capacitors  62 ; and current sensor  26 . 
     Second stage regulator  24  is typically a switching regulator, preferably employing a microcontroller  30  such that regulator  24  can be readily programmed to provide a regulated voltage prior to ignition of the bulb and regulated power after ignition of the bulb. In the preferred embodiment, microcontroller  30  includes first analog input  64  for monitoring the voltage from voltage divider  28  and second analog input  66  for monitoring the output of current sensor  26 . Internal to microcontroller  30 , inputs  64  and  66  are connected to an analog to digital converter such that microcontroller  30  can determine the analog level of these inputs. In the preferred microcontroller, for example, a voltage between zero and five volts will be converted to a corresponding number between 0 and 1023. A scale factor may be multiplied by the product of the values read from inputs  64  and  66  to calculate the actual power being delivered to bulb  32  (FIG.  1 ). The duty cycle of the pulse width of modulated output  68  is then adjusted to maintain the desired power level at bulb  32 . 
     In the preferred embodiment, microcontroller  30  is a PIC16F877 manufactured by Microchip Technology, Inc. of Chandler, Ariz. As will be apparent to those skilled in the art, most manufacturers of microcontrollers offer at least one device which would be suitable for use in the present invention. In addition, the terms “microcontroller” and “microprocessor” are used herein interchangeably to denote a programmable computing device, and the terms refer to any such computing device regardless of the level of integration of the computing device. 
     Microcontroller  30  includes a programmable pulse width modulator which provides PWM output  68  (shared with I/O pin RC2 in the PIC 16F877). The timing of the waveform appearing at output  68  is determined by the values written to internal registers within microcontroller  30 . In a regulated voltage mode, i.e. during bulb startup, the microcontroller adjusts the duty cycle of output  68  to maintain the desired voltage at input  64 . During the regulated power mode, i.e., during steady-state operation, the microcontroller adjusts the duty cycle based on the actual power being delivered to the bulb as discussed hereinabove. 
     Continuing with FIGS. 5A and 5B, the pulse width modulator output  68  is connected to one or more solid state switches  72  through a base drive circuit comprising a base drive transformer  70  common to all solid state switches  72  and a resistor  74  connected between the output of transformer  70  and each switch  72 . As with the power factor correction circuit  22  (FIG.  2 ), a solid state switch  72  is preferably a transistor, MOSFET, IGBT, or the like. Unlike the power factor correction circuit, each switch  72  is connected between an inductor  76  and capacitors  62  in a series configuration rather than in a boost configuration as in the power factor correction circuit  22 . With regard to the preferred embodiment, it is intended that the voltage produced by the second stage regulator  24  be a fraction of the voltage at node  56  (the input voltage to the second stage regulator  24 ) rather than producing a voltage greater than the input voltage as does the power factor correction circuit  22 . It should be noted, however, that, if the inventive power supply were adapted for use with a long-arc xenon bulb, it might be more appropriate to wire the second stage regulator in a boost configuration, much like the power factor correction circuit. 
     Again, in reference to solid state switch  72 , there exists a tradeoff between operating current and maximum switching speed of the switch  72 . As in the case of the power factor correction circuit, individual switches  72  are available which work well at the current requirements for a 4000 watt xenon bulb at the desired frequency (preferably on the order of 100 kilohertz), but such switches are not presently available for bulbs of higher wattage. Thus, the second stage regulator  24  also requires multiple current paths  58 . To ensure proper load sharing among the switches  72 , each current path includes an inductor  76  which effectively limits the current in each path  58  in light of the switching frequency produced at output  68 . Thus, the current flowing through each current path  58 , and hence load sharing among the switches  72 , is primarily influenced by the inductors  76 . 
     Referring again to FIG. 1, capacitors  42  and  62  filter the outputs of the power factor correction circuit  22  and second stage regulator  24 , respectively. Preferably, there is one capacitor for each current path  36  or  58 . Since capacitors  36  are connected in parallel and capacitors  58  are connected in parallel, a single capacitor could instead be used on either output. However, by providing a capacitor for each current path, a power supply may be constructed such that, to drive a 4000 watt bulb, a single path  36  and a single path  58  could be employed along with one each of capacitors  42  and  62 . Second current paths  36  and  58 , and second capacitors  42  and  62  could be added for operation up to 7000 watts. Additional current paths  36  and  58  along with capacitors additional corresponding capacitors  42  and  62  could likewise be added to achieve any level of output power desired. In this way, excess capacitance, which would increase the weight of the power supply, is not unnecessarily included in light of the power of the bulb. 
     In order to perform the functions required for proper power regulation, microcontroller  30  requires a suitable computer program. A flowchart for the preferred computer program is shown in FIG.  6 . Referring also to FIG. 1, initially, at step  200 , the program monitors the voltage from voltage divider  80 , indicating that power has been applied to the power supply. Upon the detection of electrical power at step  202 , the microcontroller  30  (FIG. 5B) monitors the output of input current sensor  14  at step  204 . At this point, microcontroller  30  has not yet activated switches  72  (FIG. 5A) and thus, the only input current flowing will be that required for functioning of the power factor correction circuit  22  and to charge capacitors  42 . Thus, as capacitors  42  charge, the input current will decrease until the power factor correction circuit  22  reaches its regulated voltage, at which time, the input current will reach a steady-state value. 
     Upon detecting a steady-state input current indicating that the power factor circuit  22  has achieved regulation at step  206 , the microcontroller then begins operation of the pulse width modulator at step  208  and monitors the output voltage at steps  210  and  212 . 
     Upon charging second stage regulator capacitors  62  to a starting voltage (typically about 150 volts), the microcontroller issues an ignitor pulse at step  214 . After the ignition pulse, if output current is detected at steps  216  and  218 , the bulb has ignited and the program advances to its operational loop at step  220 . If no current is detected at step  218 , the bulb did not ignite and the microcontroller will repeat the ignition pulse at step  214 . 
     At step  220 , the microprocessor reads the output voltage from divider  28  and at step  222  reads the output current from sensor  26 . After multiplying the voltage and current at step  224 , at step  226  the product is multiplied by a scale factor to calculate actual power output to bulb  32 . The desired power is indicated by the selection through jumpers  82  (FIG. 5B) which is read at step  228 . The difference between the desired power and the actual output power is then divided by the desired power to yield a percentage error at step  230 . At step  232 , the duty cycle at output  68  is then adjusted by the same percentage as calculated in step  230 . The process then repeats, returning to step  220  to again read the output voltage. 
     In one preferred embodiment, power supply  10  includes a dimming control  46 . Referring now to FIG. 7, additional steps are added between steps  228  and  230  of FIG. 6 to add dimming capability to the computer program. In step  234 , for the desired power output indicated by jumpers  82 , a minimum power output is determined for dimming. The microcontroller next reads the output of potentiometer  46  at step  236  and at step  238  adjusts the desired output power to a given level between the minimum power of step  234  and the maximum power determined in step  228  depending on the value read at step  236 . As will be apparent to those skilled in the art, the precise method of inputting the dimming level is unimportant. Dimming values could be provided through analog voltages from another source, a series of switches, a digital interface such as RS-232, DMX-512, or the like and the adjustment of the commanded power output (PO) from any such input is well within the skill level of one of ordinary skill in the art. At step  230 , the output power is then adjusted to the result of step  238  rather than the result of step  228 . 
     It should be noted that, if power factor controller  22  includes a synchronizing input (as does the UCC3817), by simply connecting the pulse width modulator output  68  to the synchronizing input (not shown) of power factor controller  38 , controller  38  will automatically synchronize its output  44  to that of output  68 . This results in switch  48  opening at the same time switch  72  closes such that electrical current flowing through current paths  58  will occur contemporaneously with the flow of current through diodes  52 . Managing the electrical current in this fashion reduces the storage requirements of capacitors  42 , allowing the use of capacitors having a smaller physical size than would otherwise be possible. 
     Referring again to FIG. 1, in operation, power applied to connector  12  passes through ground fault interrupter  16  and circuit breaker  18  before rectification by bridge rectifier  20 . The ground fault interrupter  16  and circuit breaker  18  protect the power supply  10 , up-stream equipment, and the operator from various fault conditions. When power is applied to power supply  10 , the power factor correction circuit  22  begins charging capacitors  42  eventually reaching and maintaining a regulated output voltage, preferably around 350 volts DC (most preferably in a range between 150 volts and 550 volts). After the power factor correction circuit has achieved its steady-state voltage, the microcontroller  30  first controls the second stage regulator output  24  in a constant voltage mode at a starting voltage, typically 150 volts. It then produces a high voltage ignition pulses through ignitor  34  until an arc strikes within xenon bulb  32 . Microcontroller  30  then changes to a constant power mode wherein microcontroller  30  monitors the output voltage from divider  28  and output current as sensed by current sensor  26  to monitor the output power and modulate output  68  to regulate the power delivered to the bulb at a substantially constant, predetermined value. As will be apparent to those skilled in the art, a power measurement means is necessary to accurately maintain a constant power output. In the preferred embodiment, the microcontroller  30  acting in concert with the current sensor  26  and voltage divider  28  comprise such a power measurement means. However, many techniques are known in the art for measuring the power output of the power supply (i.e., measuring the light output of the bulb) which are suitable for use in the present invention. 
     As will be apparent to those skilled in the art, while the inventive power supply  10  has been discussed as incorporating a boost regulator  22  for the purposes of power factor correction, followed by a series (or buck) switching regulator  24 , the invention is not so limited. By way of example, and not limitation, a single regulator could be employed, powered by simply rectifying and filtering the AC line to eliminate the power factor correction circuit. However, such a modification would likely preclude use of the inventive device in a jurisdiction which has set limits on the power factor of electrical equipment. In another example, as also mentioned above, the second stage regulator could be wired in a boost configuration for use with higher voltage bulbs such as long-arc xenon bulbs. In yet another example, the power factor correction circuitry could be configured to produce a lower voltage than the incoming line voltage. In such a configuration, bypass diode  40  would be undesirable. 
     It should also be noted that, while all of the switch inputs to current paths  58  are shown wired to a single pulse transformer  70 , the switch inputs could instead be wired to separate pulse transformers  70 , and the operation of the various switches interleaved. This would effectively triple the frequency of operation (assuming three current paths) which would allow a reduction in the size of capacitors  62 . 
     As will also be apparent to those skilled in the art, while the preferred embodiment of the inventive power supply is high-power in nature, the invention is not so limited. While prior art power supplies may be more cost effective for lower wattage xenon bulbs in applications where flicker is not an issue, the inventive power supply is, nonetheless, well suited for use with xenon bulbs of virtually any power rating, particularly where constant power output is a consideration. 
     Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention.
A transformerless, microprocessor controlled xenon power supply including: a DC supply which rectifies and filters the incoming AC electrical power; a plurality of current path connected between the DC supply and a xenon bulb, each current path having an input for receiving power from the DC supply, an inductor and a controllable switch for controlling the flow of electrical current to the bulb; and an output for driving the xenon bulb. Each controllable switch includes a switch input connected to the output of a pulse width modulator for controlling the flow of electrical current through the controllable switch in a binary fashion. Preferably, the output voltage and current are measured and used to control the pulse width modulator such that, prior to ignition of the bulb, the output is regulated at a substantially constant voltage. After ignition of the bulb, the output is regulated at a substantially constant power.