AN993 Datasheet, PDF (18/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

Description of the demonstration application
AN993
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 f_pre,
about 60 kHz, set by R_pre = 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 T_pre is set by the capacitor
C_pre = C13. The choice of this time is strictly dependent on the type of lamp. In the
application T_pre has been set to 1.5 sec.
The ignition sequence begins after T_pre. 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, t_sh, amounts to t_sh = t_pre/10 = 150 ms. At the
end of t_sh 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
(V_PIN2 = 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 R_S2 (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 f_pre.
Once the lamp strikes on (after t_pre and t_sh), the average voltage across RS2 increases and
the feedback can regulate the lamp’s current.
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Doc ID 5656 Rev 10