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Control Theory
Dimmable Light Ballast with Power Factor Correction, Rev. 1
12 Freescale Semiconductor
Figure 2-1. Typical Low Pressure Fluorescent Tube I/V Characteristic
The value of V
strike
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 V
strike
are in range 500 V to 1200 V. Once the tube is on, the voltage across it drops
to the on-state voltage (V
on)
, the magnitude of this voltage being dependent upon the characteristics of
the tube. Typically, V
on
is in range 40 V to 110 V.
The value of V
on
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 V
strike
to V
on
.