This invention relates to a power source for gas discharge lamps and other loads operated at high frequency, and in particular is directed to a new and improved power source suitable for energizing fluorescent lamps with improved protection for components during starting, open circuit and no-load conditions, and with lamp output dimming capability. The present circuit is an improvement on that shown in U.S. patent to Campbell, U.S. Pat. No. 3,396,307 and that shown in copending application, Ser. No. 402,317, now Pat. No. 3,889,153, filed Oct. 1, 1973, and Ser. No. 530,384, filed Dec. 6, 1974 and assigned to the same assignee as the present application.
The power source of the present invention has an inverter and a ballast circuit generally similar to that of the prior art devices. In these devices, a transistor oscillator generates starting pulses and then generates the operating wave form when a load such as a fluorescent lamp, is connected.
In the use of solid state circuitry in inverters powered from a 115 volt or higher ac line with direct rectification to provide the dc input, it is readily possible for the emmitter-to-collector voltage of the transistor oscillator to build up under open circuit or no-load conditions beyond the breakdown value, with rapid and destructive results. The prior art circuitry has utilized various protective techniques including fast blow fusing in the transistor base circuit, thermal switching in the base circuit, capacitive output loading to reduce open circuit pulse amplitude, and parallel transistor operation to lower the impedance level.
None of these approaches has proven entirely satisfactory. In order to operate at sufficient base current to blow a fuse rapidly or switch thermally to a lower level, it is necessary to sacrifice the capability of an efficient low base current mode of operation. When capacitive loading is used to reduce the open circuit pulses, it also reduces the starting capability, and fast starts at low temperatures are more difficult to achieve. Parallel transistor operation represents an over design for achieving a modest reduction in voltage swing.
It is an object of the present invention to provide a new and improved inverter circuit which provides substantially instantaneous open circuit and starting protection, virtually zero power consumption under no-load, flexible and efficient dimming, early starting ability, and closed loop output controllability, with reduced needs for design compromise.