Dynamic Interaction of High Frequency Electronic Ballasts and Fluorescent Lamps

Fluorescent lamps are non-linear devices that exhibit negative resistance over the useful range of operation. At low frequency, this negative resistance peculiarity is passed through during each period of the AC driving current [1]. When the fluorescent lamp operates at high frequency, the negative resistance is associated with the average power rather than with the instantaneous, cycle by cycle, power [2]. To avoid problems of instability in open loop operation due to the negative resistance characteristics of the lamp, the driver needs to have a high output impedance [3]. The classical treatment of the problem is normally carried out by examining static V-I plots similar to Fig. 1. Open loop operation calls for a "current source" driver since a "voltage source" driver will result in unstable operation. At present, commercial electronic ballasts are designed to comply with the "current source" behavior by placing a high impedance element (the ballast) in series with the lamp. The system may include some feedback to stabilize the operating point or to facilitate controlled dimming. This low frequency feedback network is normally designed under the assumption that the lamp can be represented by a resistor when operated at high frequency [2]. This assumption is a good approximation as long as the open loop system (e.g. ballast and lamp) is stable. An alternative approach to stabilizing the ballast-lamp system is to obtain the current source behavior by incorporating a wide bandwidth feedback to emulate, in closed loop, a “current source” output characteristic. The advantage of such a solution would be the smaller size of the physical ballasting element (e.g. the series inductor). As the open loop output impedance of the ballast is decreased, the effect of the incremental impedance of the lamp must be considered [3-5]. To handle such cases one would need design tools that can deal with the full dynamic behavior of the ballast-lamp system. At present, such tools are unavailable. The objective of this study was to develop a simulation procedure that will enable the examination of ballast-lamp systems over a wide frequency range. Such a tool may conceivably be used to study and design close loop ballast-lamp systems.