How to design a dimming fluorescent electronic ballast

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.

Lamp Requirements

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).

Parameter   Value   Units   Description   tph  1.0  sec  Filament preheat time   Vign  900  Volts   Ignition voltage amplitude to strike the lamp   P100%  35  Watts   Lamp power at 100% brightness   V100%>  310  Volts   Lamp voltage amplitude at 100% brightness   P5%>  0.7  Watts   Lamp power at 5% brightness   V5%  425  Volts   Lamp voltage amplitude at 5% brightness   RLamp100%  1.4  kΩ  Equivalent lamp resistance at 100% brightness   RLamp5%  129  kΩ  Equivalent lamp resistance at 5% brightness   R1,2,3,4  10  &Omega:  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:

(1)

where,

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.