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the l6569: a new high voltage ic driver for electronic lamp ballast by g. calabrese and t. castagnet introduction electronic lamp ballasts are now popular in both consumer and industrial lighting. they offer power saving, flicker free operation and reduced sizes. improvements to the light control and cost reduc- tion of the ballast will broaden their market accep- tance. today designers focus on reducing the cost of the ballast, but also work to add features to the bal- last like saving energy by dimming the light, or in- creasing the life time with better preheat and pro- tections. such requirements have contributed to the development of dedicated high voltage con- trollers like the l6569, which are able to drive the floating transistor of a symmetric half bridge in- verter. this device is a simple, monolithic oscilla- tor-half bridge driver that allows quick design of the ballast. high voltage ic drivers in ballast ap- plications the voltage fed half bridge voltage fed series resonant half bridge inverters are currently used for compact fluorescent lamp ballasts (cfl), for halogen lamp transformers, and for many european tube lamp (tl) ballasts. this simple converter is preferred for new de- signs, because it minimizes the off state voltage of the power transistors to the peak line voltage, and requires only one resonant choke. in addition this choke protects the half bridge against short circuits across lamp terminals. however overheat- ing and overcurrent occur during open load op- eration. the inverter robustness must be im- proved, or some protections are required. the half bridge inverter operates in zero voltage switching (zvs) resonant mode [1], to reduce the transistor switching losses and the electromag- netic interference generated by the output wiring and the lamp. fully integrated ballast controllers by varying the switching frequency, the half bridge inverter is able to modulate the lamp power. however most current designs use a sin- gle frequency with a saturable pulse transformer (see fig. 1) to drive the transistors. this type of design has a higher component count, a higher tolerance on the switching frequency, and it can- not adjust the lamp power. the only way to design a cost effective, compact and smart control of the lamp is to use a dedi- cated i.c. that is able to drive the upper transistor of an symmetric half bridge inverter. such control- lers require a high voltage capability for the float- ing transistor driver [2]. mosfets are preferred over bipolar transistors as power switches be- cause their gate driver requires a lower supply current and a smaller silicon size [3]. june 2002 ? figure 1: cfl series resonant half bridge inverter. 2 m s/dv ; 50 v/dv ; 0.1 a/dv i d v ds rf lvg gnd gnd gnd figure 2: current and voltage of the std3na50 mosfets when driven in zvs with the l6569. AN880 application note 1/14
the l6569 and its applications the l6569 the l6569 is able to directly control a symmetric half bridge inverter of a fluorescent lamp ballast, or a low voltage halogen lamp transformer.two 270ma buffers drive the inverter mosfets in complementary fashion with a 1.25 m s built-in dead time to prevent cross conduction. the buffer for the upper mosfet is driven through a 600v level shifter realized in bcd off line technology. the oscillator, similar to a cmos 555 timer, oper- ates from 25 to 150 khz with a +/-5% maximum tolerance. the internal 15v shunt regulator has a 9v under voltage lock out with an 1v hysteresis, and the circuit requires only 150 m a at power up. the l6569 integrates a high voltage lateral dmos transistor in place of the usual external di- ode [2] to charge the bootstrap capacitor for the upper buffer. figure 5 shows dmos operating as a synchronous rectifier. the applications the primary application for the l6569 is the com- pact fluorescent lamp. with the oscillator, the supply and the mosfet drivers it is the core of the application, and designers can customize the cir- cuit to their requirements. rf cf hvg out lvg boot vs gnd level shifter logic control with dead time low side driver high side driver charge pump uvlo figure 3: block diagram of the l6569. ac line 180k w l6569 10k w 22 w 22 w 10 m f 10 m f std4nk50z lamp 1nf 100nf d02in1385 figure 4: basic application diagram using the l6569 and two std4nk50z mosfets. AN880 application note 2/14
typical industrial tl ballasts requires complex control with dimming or automation interface. here the l6569 is a driver between the power and control blocks. to use it with an external os- cillator, pin cf is used as an 0-12v logic input, and the l6569 becomes a high voltage buffer. applications with power above 150w require a full bridge inverter. figure 6 shows how two l6569 drive such a mosfet bridge. if no external con- trol is required, the first l6569 master can control the switching with its oscillator, and synchronizes the other driver as (slave). the l6569 start up two versions of the l6569 are available with dif- ferent start up characteristics. the l6569 drives the lower mosfet on at power-up until the sup- ply voltage reaches the under voltage lock out. the bootstrap capacitor is precharged to 4.6v and both the lower and the upper mosfets will switch immediately with the oscillator. this is in- tended for inverters which use only one dc block- ing capacitor connected to the power ground, as shown on figure 4 for cfl ballast. charge pump circuit logic l6569 600v 120 w on 15.6 v on figure 5: bootstrap capacitor charge. l6569 47 47 external oscillator stb9nk50z 100nf v s v s hv rf cf gnd d02in1386 l6569 47 47 100nf v s boot hvg out lvg boot hvg out lvg rf cf gnd figure 6: basic diagram for 2x105 w lamp ballast in full bridge configuration. AN880 application note 3/14
the l6569a holds both mosfets off until the under voltage lock out is reached. this is in- tended for inverters using 2 decoupling capacitors in half bridge as shown on figure 12. the inverter is totally off, so that the voltage at the capacitors center node is not unbalanced by the leakage path during power on. considerations on the l6569 environ- ment to illustrate the benefits of the l6569 in the cfl applications, a demonstration board was devel- oped to supply sylvania 18w dulux lamp (ref: cf18dt/e). the following chapters summarize the application considerations applied in this de- sign. the schematic, lay out and components list are shown in appendix a. symmetric half bridge operation to supply a fluorescent lamp, the ballast has to achieve 3 functions: pre heat, ignition, and normal lamp operation. the serial resonance occurs be- tween the choke and the capacitor in parallel with the lamp. the choice of these components deter- mines the lamp ignition voltage and the nominal lamp current. since the inverter using the l6569 and mosfets can operate at a higher frequency than conven- tional solutions, the size of the passive compo- nents will be reduced. such inverter can operate up to 150 khz in zvs mode, and the switching losses of the power transistors only limits the fre- quency. in new design this frequency should be set between 50 and 100 khz. for instance with an 18w lamp, a frequency increase from 33 to 50 khz will lead to a 40% reduction of the choke size. to operate in zero voltage switching (zvs), the switching frequency is higher than the resonant frequency. all operation phases of the ballast are secure in this mode. when the bootstrap transis- tor is conducting, no pulse current will flow from pin boot to pin v s , as it might happen in zero current switching. the bootstrap transistor re- mains in its safe operating area, and its dissipa- tion is negligible. the mosfet drive the zvs drive technique requires only a fast turn off capability as shown on figure 2, and the tran- sistor buffers are designed with a stronger sink current. the two mosfet buffers of the l6569 can sink a 400 ma peak current on capacitive load. typically these buffers can drive any mos- fets in to220 package. figure 7 shows an example with the stp8na50 that has an 0.85 w resistance r ds-on . the built-in dead time circuit acts when a mos- fet turns off, delaying the turn on of the opposite transistor for 1.25 m s. the voltage v out between the 2 mosfets must switch within the minimum dead time (0.85 m s), as shown on figure 8, to avoid bridge cross conductions and transistors overheat. the mosfet voltage selection since the ballast is connected to the ac mains, it must handle any spurious voltage spikes. when the front end rfi filter and the clamping device, such as a varistor, absorbes totally the spike en- ergy, mosfets can have the same 600v mini- mum breakdown voltage bv dss as the l6569. otherwise when the upper mosfet is on, the re- sidual default may be applied to the l6569. al- though the pin out breakdown voltage is higher than 600v, it has a poor avalanche robustness. therefore the lower mosfet protects the driver by having a lower bv dss . a mosfet with a mini- mum bv dss up to 500v will achieve safely this task. 50 ns/dv ; 1 a/dv ; 5 v/dv ; 50v/dv v d v gs i d gnd gnd gnd t gd tc figure 7: current and voltage of the stp8na50 mosfet at turn off with the l6569. t gd = 245 ns ,tc = 95 ns, e = 93 m j @tj=50 c, rg = 22 w . 200 ns/dv ; 50 v/dv ; 0.1 a/dv rf lvg i d v ds t gd t c gnd gnd gnd t d figure 8: std3na50 mosfet turn off when driven by the l6569. t c +t gd AN880 application note 4/14
the auxiliary supply of the converter the circuit consumption is defined by the mos- fets gate charge, the i.c. consumption, the os- cillator, and the shunt regulator. several circuits are possible. in many applications a snubber is used to reduce the dissipation in the mosfets. when this snub- ber is used in conjunction with a start up resistor (r s in figure 10), a non dissipative supply is achieved almost for free. at start up the i.c. is consuming 150 m a, and there- fore only a small supply resistor is required. during operation the capacitor provides the supply current. to avoid cross conduction, the capacitance is lim- ited by the driver dead time t d . hence the capaci- tive supply current i c is also limited.for a cfl bal- last this circuit easily supplies the required operating current. using a cf18dt lamp ( i l > 230 ma) the required capacitance is 470 pf on 230 vac line. at 50 khz the average capacitive current is 6 ma, as described in appendixb. when the required driver current is higher than 10 ma, a secondary winding on the resonant choke is an easy supply alternative. the ballast shutdown the l6569 allows several ways (see figg. 11, 12 and 13) to shutdown the ballast [4]: by acting on the c f input oscillator pin to turn off the upper mosfet or by acting on the v s supply pin with the under voltage lock out. acting on c f (fig. 11) a limiting resistor r l has to be used, and it has to be: r l ? c f >1 m s. when the shutdown is realized acting on vs pin, (see fig. 12) a limiting resistor rs must be used to slow down the discharge of the supply filter cs. the constant time of the discharge must be greater than 10 periods of the switching fre- quency: r s 10 c s ? f sw connecting the c f pin to ground gnd stops the oscillator, and the lower mosfet will remain on. therefore the bootstrap capacitor remains l6569 v out i 15 v h.v.+ bv out > 600v on off figure 9. l6569 driver protection against voltage spikes. bootstrap circuit l6569 6 ma when 50 khz switching 1ma when starting 220k w 470 pf 310 v rs cs c figure 10: non dissipative auxiliary supply using the transistor snubber. AN880 application note 5/14
charged and the circuit can restart immediately. this method is suitable when the inverter uses only one dc blocking capacitor connected to the power ground, as used on figure 11 for compact fluorescent lamp. pulling the v s voltage below the uvlo turns off the oscillator and gives the same bridge configuration. for the l6569a, discharging the v s supply below the uvlo turns off both mosfets. an scr like the x0202ma may be used for the reset function. if the current flowing through the supply resistor is higher than the scr holding current (see figure 12), the scr will remain on and the two mos- fets off. removing power or commutating the scr allows a new start up [4]. otherwise a disable circuitry that turns off the two mosfets (see figure13), can achieve the shut- down function. compared to the scr solution, the shutdown is immediate and the inverter can restart on the disable order. h.v. l6569 vs 200 200 100nf 5.6k vs boot hvg out lvg gnd cf rf v in disable hcf4011 22 4.7 k w bc327 figure 13: l6569 disable circuitry with both mosfets off. l 6569a rs r r c shutdown off off figure 12: shutdownwith a thyristor & a serial resistor to slow down the supply voltage decay. l6569 on r f r l c f figure 11: l6569 shutdown through the c f oscillator pin. AN880 application note 6/14
the lamp seen by the electronic de- signer the lamp equivalent impedance the compact fluorescent lamps are specified at 25 khz (iec 929). the mosfets and the l6569 allow to increase the switching frequency, but the sensitivity of the lamp to the frequency needs to be analyzed. a few samples of the cf18dt/e lamp were tested by varying the frequency and the current of the lamp. the figure 14 shows the lamp imped- ance versus its current as it varies from 0.1a to 0.23a with 5 frequencies from 25 to 150 khz (t amb =25 c). from the tests the impedance appears insensitive to the frequency for such lamps. the specified im- pedance might be valid for higher frequency op- eration. the relative lamp light output was meas- ured as proposed in reference [5]. the light flux increases slightly in that frequency range, but can be considered constant. obviously the impedance is sensitive to the cur- rent with a negative coefficient, and the ballast operates with a non linear impedance [6]. when current is half the nominal one, the impedance is 2.6 times higher, and the voltage is only 25% higher (see figure 15). the preheat preheat techniques are used in cfl ballasts to reduce the ignition lamp voltage. during this phase the lamp is characterized by a high imped- ance that forces the electrical conduction through the preheat filaments. these filaments initially have a low resistance that will increase by 5 times during the preheat. the preheat typically lasts from 400 ms to 1 s, and is achieved by controlling either the current or the voltage of the filaments. for a current control the filaments are in series with the resonant network as shown on figure 16a. when the inverter frequency is constant, a positive temperature coefficient thermistor (ptc) in parallel with the lamp achieves the task by ad- justing both the filament current and the preheat duration. the board uses a 150 w ptc with two 8.2 nf capacitors. the preheat lasts 0.8s and the filament current is 0.45 arms. the ptc is a cheap device, but it is dissipative and works only once at power-up. the preheat can be achieved with a filament volt- age control. the filaments are supplied by two auxiliary windings of the resonant choke as shown figure 17a. during the preheat the l6569 frequency is increased, and the choke operates 0.05 0.1 0.15 0.2 0.25 0.3 0 200 400 600 800 1000 1200 i lamp (a) r lamp (ohms) 25 khz 50 khz 100 khz 150 khz figure 14: variation of the lamp impedance versus its current for several switching frequencies. 0.05 0.1 0.15 0.2 0.25 0 300 600 900 1200 0 50 100 150 200 i (a) r (ohms) u (v) rlamp vlamp figure 15: variation of the average impedance and voltage of the lamp e if i ctl t e=r.i (a) (b) i ctl lamp figure 16: basic preheat current control diagram (a); preheat filament energy curve (b) AN880 application note 7/14
as a transformer supplying the voltage to the fila- ments. only few components are added around the l6569 (see figure 18), and the control of the preheat energy is less sensitive to the preheat du- ration and the inverter frequency (see figure 17b). the start up initialization the initial conditions of the power switching start up requires care; especially if the resonant and switching frequencies are close to each other. the resonant network is not loaded and the full e vf v ctl t e=v /r (a) (b) v ctl lamp figure 17: basic preheat voltage control diagram (a); preheat filament energy curve (b) 2_rf 3_cf 1_vs c f_st c f r f c r l6569 figure 18: double frequency control for the l6569 with programmed frequency and duration. AN880 application note 8/14
line voltage v dc is applied when the oscillator starts. the ballast has to start directly with its nominal conditions to remove any transient oscil- lation. hence the operation runs in zvs mode with no spurious lamp ignition. this situation does not occur with the saturable transformer drive, be- cause the saturation limits naturally the current by increasing the frequency. in the example the resonant capacitors are pre- set to be compatible with the choke current rise (see figure 19). the blocking capacitor is pre- charged to approximately half v dc by 2 biasing resistors, and the lower mosfet also discharges the resonant capacitor to ground (see figure 20). therefore the blocking capacitor never goes above 2/3 of the line voltage v dc (250v rating), the operation is safe in zvs mode. the l6569 is here preferred to the l6569a, because the lower mosfet is on at power-up. the lamp removal protection used in tl ballast, the lamp removal protection is frequently also requested in the oplug-ino cfl bal- last . depending of the topology and the preheat mode, the lamp removal behaves as: - a noload resonant mode when the choke and the capacitor are still connected to the in- verter ; a required overcurrent protection in- creases the frequency to reduce the current; - an open circuit mode when the lamp filaments are inserted in the resonant circuit. when the circuit is open, the choke is not sup- plied. the mosfets turn off slowly generating bridge cross conduction, and undesirable dissipa- tion losses (see figure 21). the detection stops the switching to eliminate the cross conduction. 5 m s/dv ; 50 v/dv ; 0.5 a/dv gnd gnd i i v b v bi ideal initial time figure 19: waveformsof the choke current and the capacitor voltages in steady state preheat. i i v b v bi 100 nf 4nf on l6569 vsAN880 application note 9/14
several ways can achieve the protection task. first it can be done by sensing the resonant cur- rent through a mosfet source resistor or a sec- ondary winding on the choke. the switching is stopped when a large current reduction is de- tected by analog means. a logic circuit can also detect the presence of the lamp filaments. one end of a filament is always connected to a fixed voltage. if the other end of the filament is connected through a high imped- ance resistor to another voltage, the absence of the filament can be easily detected by monitoring the resistor voltage change as shown on figure 22. conclusion the foregoing note shows how high voltage driv- ers, like the l6569, simplify the design of the lamp ballast. these devices includes all the cir- cuitry to drive mosfets in half bridge inverter. since the optimized switching frequency in- creases above 50 khz with a low tolerance, the size of the passive resonant components is re- duced, and the ballast becomes cheaper.with its supply and its oscillator the l6569 is versatile, and its flexibility permits to design any improved power control. bibliography [1]: an 527 oelectronic fluorescent lamp ballasto a.vitanza, r.scollo, sgs-thomson [2]: smart power ics, chapter 8, high voltage in- tegrated circuits for off-line power applications. c.diazzi, sgs-thomson [3]: an 512 ocharacteristics of power semicon- ductorso jm peter, sgs-thomson [4] : othe l6569 half bridge driver: the shutdown functiono [5] : ocompatibility test of dimming electronic bal- lasts used in daylighting and environment con- trolso, a.buddenberg, rensselaer polytechnic in- stitute [6]: opspice high frequency dynamic fluores- cent lamp modelo, bryce hesterman, apec'96, p.641 4_ gnd 3_cf 18v 10.r 100.r 10.r lamp l6569 11.r figure 22: open load detection example. AN880 application note 10/14
appendix a: cfl demonstration board with the l6569 a demonstration board was developed as an ex- ample for compact fluorescent lamp ballast. it is optimised for a cf18dt/e/830 18w lamp from osram-sylvania. using the l6569 the circuit achieves preheat, ignition and normal lamp op- eration. the power transistors are two std3na50 500v-3 w mosfets in i-pack package. board description the three sections of the board are an ac input rectifier, the half bridge inverter, and the resonant ballast. by changing the connection on the input mains, the ballast can operate either on 120 vac mains with a voltage doubler rectification, or on 230 vac mains with a full wave rectification. the input resistor r 1 limits the initial inrush current charging the bulk capacitors. the l6569 operates with a single 50 khz switching frequency pro- grammed by r 4 and c 1 . two fast diodes d 2 &d 3 synchronize the oscillator to keep the switching in zvs mode. the control circuit requires 4.5 ma to supply the i.c., the mos gate drives, and the os- cillator. its supply delivers at least 6.5 ma as de- scribed in appendix b. the start up resistor also balances the voltage across the two bulk capaci- tors. figure 23. cfl ballast diagram for a 18w cfd18t/e lamp with 120/230 vac inputs. the resonant ballast the value of the choke (l 1 ) and the two capaci- tors (c 7 &c 8 ) in parallel with the lamp determine the lamp ignition voltage and the nominal lamp current. during the ignition the lamp impedance is essentially infinite, and the filaments resistance is only the serial load. to generate the ignition volt- age, the switching frequency is set close to the resonance frequency. in normal operation the choke resonates with the capacitors c 7 &c 8 (parallel loading), but also with the decoupling ca- pacitor c 6 (serial loading). the current mode pre- heat uses a 150 w positive temperature coeffi- cient thermistor. inserted in the capacitive series (c 7 &c 8 ), the ptc produces a 0.45 arms fila- ment current during the initial 0.8 s (reference: 307c1253bheab from cera-mite). basic ballast electrical characteristics input voltage: 120 or 230 vac by input connec- tions change switching frequency: 50khz average dc line voltage range: vdc from 260 to 355 nominal supply current: 0.17a rms @ 310 vdc nominal output power: 17w minimum ignition voltage: 700v peak @ 260 vdc nominal preheat current: 0.45 arms during 0.8s @ 310 vdc l1=2.4mh l1=2.4mh core th lcc e2006-b4 ref also vogh 575 0409200 2.4mh c7-c8=ps8n2j h3 630-2a th d7 d4 d5 d6 r1 15 1w c2 47 m f 250v r8 120k 1/2w r5 100k c5 47 m f 250v c3 4.7 m f 25v d8 1n4148 d1 zpd 18v r4 27k 1/4w c1 560pf 50v 220v n 110v r6 47 1/4w r2 22 1/4w 1/4w r3 22 1/4w c4100nf 50v d2 d3 byw100-100 c9 470pf 630v q1 std3na50 q2 std3na50 r7 180k 1/4w c6 100nf 250v c8 8.2nf 630v c7 8.2nf 630v cfl lamp sylvania delux t/e 18w r9 180k 1/4w l6569 d96in419a v s rf cf gnd lvg out hvg boot 4 x 1n4006 1/2w byw100-100 r10 10k rv1 ptc 150 350v figure 23. AN880 application note 11/14
the resonant choke the inductance of the choke is 2.4 mh with a minimum saturation current of 0.65 a. in the prac- tical example it has been done with: core: thomson lcc e2006 material b4; air gap: 2 spacers of 0.4 mm each (total 0.8 mm); al = 75 nh; bobbin: hc2006ba-06; number of turns: 175; measured saturation current: 1 a peak @ 25 c; customization of the board some flexibility is added to the board to extend its evaluation. the mosfets have two foot prints to mount either i-pak or to220 packages. and two choke footprints are also avalaible for e1905a and e2006a magnetic cores. fi gure 24: pcb layout of the board. figure 25: pcb component placement diagram. comp. side copper side AN880 application note 12/14
appendix b: rating of the capacitive supply with the l6569 driver the supply is made with the snubber and a start up resistor r s . a snubber circuit is used to minimize the mos- fets dissipation. it also achieves a non dissipa- tive supply as shown on figure 10. the mosfets gate charge, the driver consump- tion, the oscillator, and the shunt regulator, define the circuit consumption. we can estimate this current is i sav : i sav >2 ? i g +i qs +i osc +i reg = =2 ? q g ? fsw + i qs + v s r f ? 2 +i reg where q g the mosfet gate charge i qs the driver supply current v s the supply voltage r f the oscillator resistor and v s the driver supply voltage i reg the shunt regulator current. when v s is lower than the uvlo threshold u uvlo , the driver is only consuming. its current must be mini- mal to reduce the dissipation of the resistor r s . the l6569 has a 150 m a start up current, and the maximum resistance is 2m w for a 230vac line ap- plication. we can also reduce the resistor value to get a faster start up time t s . t s = r s ? c s ? u uvlo v dc where c s is the supply capacitor, and v dc the line voltage. when the timer oscillates, the capacitor c sup- plies the lamp current during the lower mos turn off. to avoid any cross conduction its capacitance is limited by the driver dead time t d (see figure 26). hence the capacitive supply current i c is also limited. c < t d ? i l v dc i cav =c ? v dc ? f sw 230 ma) the capacitor is 1nf on 120vac line, 470 pf on 230 vac line. at 50 khz the average capacitive current is 6 ma in both cases. t d v hvg +v out gnd gnd gnd rf i c 200 ns/dv ; 50 v/dv ; 0.1 a/dv figure 26: cross conduction of the snubber capacitor with the upper mosfet: capacitor current and voltage waveforms. AN880 application note 13/14
information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specification mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. the st logo is a registered trademark of stmicroelectronics ? 2002 stmicroelectronics printed in italy all rights reserved stmicroelectronics group of companies australia - brazil - canada - china - finland - france - germany - hong kong - india - israel - italy - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states. http://www.st.com AN880 application note 14/14

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