Freescale Semiconductor M68HC08 manual Gas filling mixture

Control Theory

Figure 2-1. Typical Low Pressure Fluorescent Tube I/V Characteristic

The value of Vstrike is a function of several parameters:

•Gas filling mixture

•Gas pressure and temperature

•Tube length

•Tube diameter

•Temperature of electrodes: cold or hot

Typically, values of Vstrike are in range 500 V to 1200 V. Once the tube is on, the voltage across it drops to the on-state voltage (Von), the magnitude of this voltage being dependent upon the characteristics of

the tube. Typically, Von is in range 40 V to 110 V.

The value of Von will vary during the operation of the lamp but, in order to simplify the analysis, we will assume, as a first approximation, that the on-state voltage is constant when the tube is running in steady state.

Consequently, the equivalent steady state circuit can be described by two back-to-back Zener diodes as shown in Figure 2-2. The startup network is far more complex, particularly during ionization of the gas. This is a consequence of the negative impedance exhibited by the lamp when the voltage across its electrodes collapses from Vstrike to Von.

Dimmable Light Ballast with Power Factor Correction, Rev. 1

12 Freescale Semiconductor