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 (t 1 ), a controlled power switch (q 1 ), a boost diode (d 1 ), an output capacitor (c 5 ) 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 r 7 -r 8 , 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 (r 14 , c 14 ), as current flows in the filaments, without lamp ignition. the preheating time is defined by the time constant r 12 c 10 . after this time has elapsed, the switching frequency decreases moving towards the resonance one (l 1 /l 2 , c 16 /c 17 ), thus increasing the voltage across the lamps and causing the ignition. in steady state the frequency reaches 44 khz (r 14 , c 13 +c 14 ). at start up, when the pfc is not running, the ballast controller (l6569) is powered by r 9 . when the bal- last section is running, the current pump (c 12 , r 13 , d 3 and d z2 ) 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 ? AN991 application note l6569 - l6561 lighting application with pfc by i. dal santo, u. moriconi 1/3
176 to 265 vac t1 l6561 fuse c1 220nf 400v c2 10nf r1 750k r1a 750k r2 8.2k c3 680nf l6569 r7a 750k r7 750k r6 1 d1 byt11600 r11 22 r31 1.5m r4 10 r12 2.2m r3 68k r14 16k dz1 4.7v d2 1n4148 d3 1n4148 dz2 18v d4 1n4148 dz3 5.6v dz4 20v d5 1n4148 r16 10 r18 10k r20 180k r10 56k r26 2.2m r30 2.2m r27 220 r25 82k r22 1.5m r24 1.5m r21 820k r23 820k r19 330k r15 10 r17 10k r9 220k 1w r13 47 c8 10 m f c6 10 m f c10 470nf c13 470pf c14 560pf c15 100nf 250v c15a 100nf 250v c17 6.8nf 630v c18 1 m f c16 6.8nf 630v c12 820pf 600v c11 100nf c5 22 m f 450v ntc 100 bridge q1 stp5nk50z scr p0102aa q4 bs170 q2 stp6nb50 q3 stp6nb50 lp2 lp1 l2 2.3mh l1 2.3mh r29 5.6k c9 100nf 1 5 3 48 7 1 4 23 5 6 7 8 r32 6 2 r8 9.53k r28 56k d98in822a figure 1. schematic circuit AN991 application note 2/3
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