Freescale Semiconductor M68HC08 Chapter Control Theory, Introduction, Fluorescent Lamp Operation

Chapter 2

Control Theory

2.1 Introduction

This chapter covers fluorescent lamp theory and two PFC concepts - discontinuous conduction mode and hysteresis current control mode.

2.1.1 Fluorescent Lamp Control Theory

To light a low-pressure fluorescent lamp, the electronic circuit must perform the following four main functions:

•Provide a startup voltage across the electrodes of the lamp

•Maintain a constant current when the lamp is operating in the steady state

•Ensure that the circuit will remain stable, even under fault conditions

•Comply with the applicable domestic and international regulations (PFC, THD and safety)

Most generally, light ballast topology fairly closely matches target lamps in terms of tube wattage, length, and diameter. The digital electronic lamp ballast includes also additional features like dimming capability, tube end-of-life, startup fault, tube removed indication, and so on. Different tubes require different software settings; also, some hardware components may have to be adapted accordingly.

2.1.2 Fluorescent Lamp Operation

When the lamp is off, no current flows through the tubes, and the apparent impedance is nearly infinite.

When the voltage across the electrodes reaches the Vstrike value, the gas mixture is highly ionized and an arc is generated across the two terminals of the lamp. This behavior is depicted by the typical operating

curve shown in Figure 2-1.

Dimmable Light Ballast with Power Factor Correction, Rev. 1

Freescale Semiconductor 11