AN993 Datasheet, PDF (22/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

Design tips
AN993
When the lamp is not yet ignited, C has to allow a sufficient current to flow into the lamp
filament in order to preheat correctly. The power to be delivered to the lamp filaments (Pfil),
the preheating frequency, the lamp filament resistance r and the maximum voltage to be
applied across C without causing lamp ignition are constraints that help to evaluate the size
of the capacitor. By setting the current through the lamp filament and the maximum voltage
across the capacitor, one obtains a range of capacitor values.
Equation 17
These L, C_b and C values have to be corrected to obtain standard and commercial
component values.
Using these values and lamp equivalent resistances, the transfer functions during the
preheating and operating conditions can be calculated.
The preheating transfer function allows you to see if, when moving towards the resonant
frequency (L-C), there is a frequency at which the voltage across the lamp enables its
ignition. This frequency has to be between f_max and f_min. The gain of the transfer function
depends on r also, but r changes greatly during the preheating phase (also 3 to 4 times) and
this must be taken into consideration (see Figure 20).
Figure 20. Preheating transfer function
V_lamp
V_in
F=F(L,C,r@25C)
F=F(L,C,r@Tpre)
freq
1/2∗π∗sqrt(L∗C)
fpre
fmin
frequency shifting during Tsh
AM01328v1
The operating transfer function can be used to check if the operating voltage across the
lamp (at f = f_min) is similar to the one used to evaluate L (Figure 21).
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Doc ID 5656 Rev 10