AN1902 Datasheet, PDF (10/32 Pages) STMicroelectronics – DEVICES TO DRIVE 58W TL TUBES

AN1902 - APPLICATION NOTE
Figure 12: Load typical resonance curves
Resonance frequency
after the tube ignition (f2).
Resonance frequency
before the tube ignition(f1).
2
3
1
fsteady
fign ^fpreh
f
duct(s) ^{Where the two frequencies are:}
f1
=
-----------------------¹------------------------
2π LP(C1seriesC2)
(before the tube ignition)
Pro ^f2
=
-----------¹-----------
2π LPC2
(after the tube ignition)
te _{In good ballast design the cathode preheating is requested in order to increase the tube lifetime. It is}
le _{obtained making high current flow through the cathodes for a fixed time. A simple rule for the preheating}
o _{efficiency check is reported:}
Obs _{1) measure the cathode resistance at the beginning of the preheating;}
- _{2) measure this resistance at the end of the preheating;}
t(s) ^{3) if its value is increased 3-4 times, the cathodes will work at the right temperature during ignition.}
uc During the preheating the current level has to be able to heat the cathodes without generating the ignition
d _{voltage on the start-up capacitor C1. Still referring to figure 12, the converter will operate as follows: it will}
ro start working at the preheating frequency (dot 1) that must be higher than the resonance frequency f1. It
P will remain in this condition for the time fixed by the preheating capacitor. After the preheating the device
frequency control is taken by the two secondary windings moving the working frequency up to the dot 2
te _{where the tube is supposed to ignite. Once the tube is ignited the converter resonance frequency is}
le _{lowered to f2 and the converter can work at the steady state frequency (dot 3) fixed by the oscillator}
o _capacitors.
Obs^{In figure 13 the complete start-up sequence is shown.}
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