Whenever the line or supply voltage is less than the open circuit voltage (OCV) required to operate a gas discharge lamp, the supply voltage magnitude to the lamp must be increased in order to drive the lamp into operation. There must also be some technique to start and restart the lamp, either hot or cold. The required starting voltage is greater than the lamp operating voltage.
Many different systems have been devised to provide this required operating lamp voltage. The conditions described above, wherein the supply voltage is less than the OCV required for lamp operation, are common because the lowest usable voltage is normally employed for reasons of economy and availability at the application site. One normally uses the highest lumen-per-watt output lamp which is often one of the higher voltage lamps. The lighting system must be consistent with the lighting requirements and must be operable on the available line voltage. If a 120 VAC supply is available, lamps of certain types up to some known wattage level and lumen output can be operated; for the newer, more efficient metal halide lamps and higher wattage lamps, one must arrange for a higher lamp supply voltage such as 240-530 VAC, which may not be available.
In these circuits, there are certain basic components, in addition to the lamp itself, which are present, including some form of ballast for voltage transformation and for controlling or limiting the operating current level and lamp power. A semiconductor switching circuit is typically used to step up the source voltage to provide the required lamp ignition and sustaining voltage. A lamp starting circuit is normally present and it is common to switch this starting circuit out of operation, or minimize its influence, after the lamp has entered its normal operation mode.
Stated differently, a lamp operating circuit most often includes a power source, which is normally a low-voltage AC source, some circuit means for controlling the amount of wattage which is delivered to the lamp, and the lamp itself. The circuit usually includes other components for special purposes such as power factor control.
Lamp operating circuits of the prior art have relied upon switching devices such as SCRs, Triacs, transistors or the like to do some of the voltage transformation and control switching, and many of these circuits have included complex and expensive collections of circuits and components. The more components that are used, the more attention that must be paid to the problems associated with heat dissipation and circuit failure rates and life. It is therefore desirable to minimize the number of such components.
It is also very desirable, especially in high wattage lamp circuits, to have a high operating power factor for the lamp and the operating circuit. This is sometimes a problem with circuits using large inductive devices, and many circuits of the prior art include capacitive devices to correct the power factor. Switching circuits that are used in lamp operating circuits most often generate a poor power factor and high line harmonics condition.