AN991 Datasheet, PDF (1/3 Pages) STMicroelectronics – The application has been developed to supply two 18W fluorescent lamps

AN991
®
APPLICATION NOTE
L6569 - L6561 LIGHTING APPLICATION WITH PFC
by I. Dal Santo, U. Moriconi
DESCRIPTION
The application has been developed to supply two 18W fluorescent lamps. It consists of two sections: a
Power Factor Corrected preregulator (PFC), using the L6561, and the lamp ballast stage with the L6569
(see fig.1).
The power factor corrector section is based on the L6561. This is an IC intended to control PFC preregu-
lators by using the transition mode technique. It is especially dedicated for electronic lamp ballast appli-
cations (to better understand the L6561 characteristics refer to AN966).
Referring to the application circuit (see fig.1), the AC mains voltage is rectified by a diodes bridge and
delivered to the boost converter. The converter section boosts the rectified voltage to a DC controlled
value. The section consists of a boost inductor (T1), a controlled power switch (Q1), a boost diode (D1),
an output capacitor (C5) and, obviously, a control circuitry.
Since the input is a time-variable supply voltage (sine wave), to make the input current shaped like the
line voltage, the converter has to produce a boost inductor average current like the rectified input volt-
age. To do so, the L6561 controls the system in transition mode. Transition mode approach consists of a
"zero current turn-on" system, switching at variable frequency and duty cycle. The output voltage value
of the PFC, which can be adjusted by the pair of resistor R7-R8, is set at 400 Volt.
The regulated voltage is delivered to the ballast section. The L6569 is a high voltage half-bridge driver
with a built-in oscillator, similar to a NE555 timer (to better understand the L6569 in lighting applications
refer to AN880). The load consists of a L-C series resonant circuit with the lamps connected across the
capacitors. This topology allows to operate in Zero Voltage Switching, to reduce the transistor switching
losses and the electromagnetic interference generated by the output wiring of the lamp.
The preheating of the lamp cathodes is achieved by a high switching frequency, about 80 kHz (R14,
C14), as current flows in the filaments, without lamp ignition. The preheating time is defined by the time
constant R12 C10. After this time has elapsed, the switching frequency decreases moving towards the
resonance one (L1/L2, C16/C17), thus increasing the voltage across the lamps and causing the ignition. In
steady state the frequency reaches 44 kHz (R14, C13+C14).
At start up, when the PFC is not running, the ballast controller (L6569) is powered by R9. When the bal-
last section is running, the current pump (C12, R13, D3 and DZ2) allows to start up the power factor sec-
tion and supply the overall circuit. This sequence prevents the activation of the PFC over voltage protec-
tion. In fact if the PFC starts first, it is not loaded as long as the ballast is not operating.
The application is provided with a safety circuitry that, in case of open load or ignition failure, shuts down
the supply of the two controllers. This fault condition is latched by the SCR until the mains is removed
and the turn-on sequence is repeated.
June 2002
1/3