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

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

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.

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

®

AN991

APPLICATION NOTE

L6569 - L6561 LIGHTING APPLICATION WITH PFC

by I. Dal Santo, U. Moriconi

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