Freescale Semiconductor M68HC08 manual Controlling the Fluorescent Lamp, Freescale Semiconductor

Introduction

Figure 2-2. Typical Fluorescent Tube Equivalent Circuit in Steady State

Up to now, there is no model available to describe the start up sequence of these lamps. However, since most of the phenomena are dependent upon the steady state characteristics of the lamp, one can simplify the analysis by assuming that the passive networks control the electrical behavior of the circuit. This

assumption is wrong during the time elapsed from Vstrike to Von, but since this time interval is very short, the results given by the proposed simple model are accurate enough to design the converter. When a

fluorescent tube is aging, its electrical characteristics degrade from the original values, yielding less light

for the same input power, and different Vstrike and Von voltages. A simple, low-cost electronic lamp ballast cannot optimize the overall efficiency throughout the lifetime of the tube, but the circuit must be designed

to guarantee the operation of the lamp even under worst case “end of life” conditions. As a consequence, the converter will be slightly oversized to make sure that, after 8000 hours of operation, the system will still drive the fluorescent tube.

2.1.3 Controlling the Fluorescent Lamp

As already stated, both the voltage and the current must be accurately controlled to make sure that a given fluorescent lamp operates within its specifications.

The most commonly used network is built around a large inductor, connected in series with the lamp, and associated with a bimetallic switch generally named “the starter”. Figure 2-3gives the typical electrical schematic diagram for the standard, line operated, fluorescent tube control.

Figure 2-3. Standard Ballast Circuit for Fluorescent Tube

Dimmable Light Ballast with Power Factor Correction, Rev. 1

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