The present invention relates to flash lamps used in reprographic applications such as illumination of original documents or toner image fusing and, more particularly, to a low capacitance power supply for a flash lamp.
With the advent of high speed reprographic copier and duplicators, the use of flash lamps, particularly xenon, has become widespread. These lamps are capable of being rapidly pulsed to provide the high speed exposure of original documents required by these systems. Flash lamps have also been used to fuse toner images which have been transferred to an output sheet from a photoreceptor surface. To enable either type of flash operation, a dc voltage source is required to charge a capacitor, or a series of capacitors, to a desired voltage; the capacitor(s) are then discharged through the lamp creating the flash illumination. The dc source and the associated capacitance must be capable of supplying sufficient energy to accomplish the specific flash function. For document illumination purposes, the energy required to illuminate an 81/2.times.11" document with a xenon flash for exposure on a photoconductor is in the 50-100 joule range. For flash fusing of an image pattern of the same size onto paper output sheet, 500-800 joules is normally required. To store this amount of output energy at typical lamp voltages of approximately 1000 volts, the capacitance requirements for the flash lamp power supply are quite large; on the order of 200 .mu.F for exposure and 1500 .mu.F for fusing. High capacitance in a power supply adds to both the size, weight and cost of the power supply.
The present invention is directed towards a power supply for a flash lamp in which the capacitance requirements are kept to a minimum (far below the values stated above) consistent with the flash energy needed for the particular flash purposes. This is accomplished by utilizing a master dc power supply which has a very high voltage and a very low capacitance. An automatic switching and control circuit associated with the master power supply is activated so as to alternately charge and discharge secondary capacitors of low value, each discharge providing an incremental portion of the total energy to the lamp. The cycling process continues until the lamp has received the total energy required for the specific purpose, e.g. exposure or fusing. More particularly, the invention is directed to a power supply circuit for supplying an output energy E.sub.0 to a flash lamp, said power supply comprising:
a variable output, high voltage dc power supply,
at least a first and second capacitor charging circuit connected to said dc high power supply and said lamp, each said charging circuit including a capacitor for storing an incremental portion of the total energy requirements E.sub.0,
means for cyclically and alternately connecting and disconnecting said charging circuits to and from said power supply and lamp so as to alternately store said incremental energy in each of said capacitors and subsequently to discharge said stored energy into said lamp,
whereby the maximum output energy is delivered to said lamp in incremental portions such that the total energy E.sub.0 is the product of the energy discharged per cycle times to the number of discharges from said charging circuits.
In one embodiment, the charging circuits connected between the lamp and the power supply are connected in a dc resonant charging mode so as to enable switching at zero current crossings, thereby operating the circuit at maximum efficiency.