The present invention relates to d.c. - a.c. inverter circuitry connectible at its input to a d.c. power source and at its output to a fluorescent lamp. More particularly, it relates to such circuitry that enables a standard type of fluorescent lamp to be operated from a low voltage battery with a high degree of efficiency, i.e., with minimum electrical power input for a given level of light power output.
There are many applications where it is desirable to have a light source that is capable of operating from a convenient d.c. power source, such as a battery. The incandescent lamp is often used as a means of converting electrical power from a battery into light power (illumination). It is well known in the art, however, that an incandescent lamp is a very inefficient means of converting electrical energy into visible light. Approximately 6% of the electrical energy supplied to an incandescent lamp is converted into visible light, the other 94% being used up in the generation of heat with a small accompanying amount of invisible ultraviolet light.
When the supply of electrical energy is limited, as in the case of a battery-operated lamp, it is desirable to achieve maximum operating efficiency so that the lamp can be operated for the longest period of time before the supply of electrical energy is exhausted. It is also well known in the art that a fluorescent lamp is approximately four to five times more efficient than an incandescent lamp. Because of its higher efficiency, the fluorescent lamp has the potential to out-perform the incandescent lamp in those instances where efficiency ranks as an important operating parameter. In order to take full advantage of this remarkable potential, however, it is essential that transistor circuitry used to supply power to the fluorescent lamp does not itself operate inefficiently. To this end, it has heretofore been recognized that the circuitry should at least take into account that the gas mixture in a fluorescent lamp must first become highly ionized in order to cause the lamp to emit visible light, that the application across the lamp of an a.c. voltage rather than a d.c. voltage makes it possible to use a purely reactive element as a current limiting impedance (ballast) in order to minimize energy loss relative to the loss experienced with the use of a resistive ballast, and that the lamp impedance will remain fairly constant if the frequency of the a.c. voltage applied across the lamp has a period of oscillation shorter than the average recombination period of the positive and negative ions, yet not so short as to unduly reduce the effectiveness of transistor switching. It has moreover been recognized that heating at least one cathode in the lamp just prior to or while the starting voltage is being applied will facilitate the ionization process.