AN4213 datasheet(25/42 Pages) STMICROELECTRONICS

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AN4213

DC/AC converter and lamp

At startup when the PTC is cold, it can be considered as a short-circuit, the circuit operating

frequency is determined by C’res and Lres (we can neglect Cres). During the preheating the

current flows through the PTC and C’res heats both the cathodes and the PTC at the same

time. The value of C’res must be chosen in order to avoid high voltage on the lamp and

consequent switch-on. When the PTC is hot (end of preheating) its resistance increases

until it can be considered as an open circuit. In this case the current flows through the series

formed by C’’res and C’res. Therefore the equivalent capacitance across the lamp becomes

lower than the initial value, increasing the capacitive reactance and allowing the tube

ignition. We can summarize the turn-on sequence in three phases: preheating, ignition and

normal lamp operation. The preheating of the lamp filaments is achieved by a high switching

frequency f

PRE

, to ensure that a current flows in the filaments without lamp ignition. In fact

the initial voltage applied across the lamp is below the strike potential. The duration of the

preheating period t

PRE

is set by the capacitor C’res together with Lres. The choice of this

time is strictly dependent on the lamp type. The ignition sequence begins after t

PRE

. During

the ignition phase, the frequency shifts from f

MAX

to f

MIN

(that is the normal operation

frequency) in a period TSH. The voltage across the lamp increases causing the ignition. At

the end of the ignition time the frequency reaches the normal operation value, f

MIN

, and the

lamp is in run mode (as shown in

Figure 14).

Figure 14. Lamp output voltage timing diagram

The following figures indicate the preheating, ignition and run mode phases, respectively.

AM17328v1

time

Vlamp

Vph

Vign

Ignition Ramp

Vrun

Preheat

fmax

Frequency

Ignition Phase

Run

Ignition

fmin

Tpre

Tsh

Trun

fmin

Normal Operation

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