Dimming fluorescent lamps is an attractive feature that provides mood and brightness effects as well as having tremendous energy savings potential. Designing a dimmable electronic ballast, however, is a complicated and challenging task. This paper presents an overview for designing a dimming electronic ballast for a T5 35W lamp. Lamp requirements, dimming control methods, and resonant output stage design are all discussed in this paper. A new ballast design software program is also demonstrated which greatly aids with the design of the dimming ballast and generates the necessary schematic and bill of materials for prototyping. Finally, a fully-functional prototype is built, tested and compared against original design results.
A fluorescent lamp requires preheating of the filaments, a high ignition voltage to strike, current or power control for dimming, and additional filament heating during low dimming levels. To understand how a dimming fluorescent lamp works, a simple resistive model for the lamp is used (Figure 1). The lamp resistance (Rlamp) is connected between each filament resistor pair (R1, R2, R3 and R4).
Figure 1, Resistive lamp model
During dimming, the current flowing through the lamp resistance determines the lamp power and brightness. At low brightness levels (typically below 20%), the lamp requires additional heating current through the filament resistors for maintaining the arc. To calculate the equivalent lamp resistance some basic electrical lamp parameters must first be known (Table I).
|| Filament preheat time
|| Ignition voltage amplitude to strike the lamp
|| Lamp power at 100% brightness
|| Lamp voltage amplitude at 100% brightness
|| Lamp power at 5% brightness
|| Lamp voltage amplitude at 5% brightness
|| Equivalent lamp resistance at 100% brightness
|| Equivalent lamp resistance at 5% brightness
|| Cold filament resistances
Table I, Typical 35W/T5 lamp requirements
The lamp resistance [Ohms] at the corresponding % dimming level is determined from the running lamp power and voltage:
P%= Lamp power at % dimming level [Watts]
V%= Lamp voltage amplitude at % dimming level [Volts]
The lamp manufacturer does not typically specify the filament resistor values so they must be measured directly on the bench across one end of the lamp. The exact location where the arc leaves the filament (hot spot) is not known. For purposes of the model, the hot spot is assumed to be exactly in the middle of each filament, therefore, R1=R2=R3=R4= 1/2 x (total resistance measured across one filament). Also, the filament resistance is a strong function of temperature so the values given are measured at cold.