AN993 datasheet(18/27 Pages) STMICROELECTRONICS

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Description of the demonstration application

AN993

18/27

Doc ID 5656 Rev 10

The load consists of a series resonant circuit (L2-C18) with the lamp connected across the

capacitor (C18). This topology allows operation in zero voltage switching mode, to reduce

the transistor switching losses and the electromagnetic interference generated by the output

wiring of the lamp.

The blocking capacitor (C17) allows a zero average lamp current. In steady state the voltage

across these capacitors is as high as half the high voltage bus, approximately 200 V.

4.3

Preheating and ignition sequence

The turn-on sequence can be divided in three phases: preheating, ignition and normal lamp

burning. The preheating of the lamp filaments is achieved by a high switching frequency fpre,

about 60 kHz, set by Rpre = R12+ P1 + R13 and CF = C12, 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 Tpre is set by the capacitor

Cpre = C13. The choice of this time is strictly dependent on the type of lamp. In the

application Tpre has been set to 1.5 sec.

The ignition sequence begins after Tpre. The switching frequency decreases towards the

resonance point (L2-C18), increasing the voltage across the lamp and causing the ignition.

The time interval in which the frequency shifts, tsh, amounts to tsh = tpre/10 = 150 ms. At the

end of tsh the frequency reaches 31 kHz (R18-C12), and the current feedback loop is then

activated.

4.4

Current feedback loop

The current control is achieved by varying the switching frequency of the VCO. Since

controlling the average current in the lamp means controlling the output power, it is quite

easy to perform the control function. The operational amplifier compares the low-pass

filtered half-bridge current with a reference, achieved by a portion of the voltage at pin 2

(VPIN2 = 2 V). This set-point could be changed by the trimmer P1 to perform the dimming

function. The amplifier’s output is connected to the RING pin by D4 and R16. The diode D4 is

necessary to prevent the switching frequency from decreasing to below the value set by

R18.

At start-up the voltage across RS2 (Figure 15) remains low until the lamp ignites. As such,

the inverting input of the amplifier (pin 6) also stays low, while the non-inverting input (pin 7)

is set to a constant voltage (set-point) by the divider R12, P1 and R13.

Therefore, the amplifier’s output (pin 5) remains high (5 V) until the lamp ignites, and D4 is

off. In this condition, the L6574 oscillates at fpre.

Once the lamp strikes on (after tpre and tsh), the average voltage across RS2 increases and

the feedback can regulate the lamp’s current.

AN993 Datasheet(PDF) 18 Page - STMicroelectronics