Design tips

AN993

22/27

Doc ID 5656 Rev 10

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

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

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).

The operating transfer function can be used to check if the operating voltage across the

Figure 20.

Preheating transfer function

AM01328v1

F=F(L,C,r@25C)

freq

F=F(L,C,r@Tpre)

1/2