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www.fa irchildsemi.com ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 AN-9745 design guide for triac dimmable led driver using fl7730 introduction an led has become a promising light source for replacing conventional lighting systems, such as fluorescent and incandescent lights. especially in the conventional triac dimmer infrastructure, there ha s been much research into development of an led bulb compatible with triac dimmers. because the incandescent light source consumes a hundred watt with short life time, an led bulb can be the excellent substitute with considerably less power dissipation and longer life. the biggest recent issue of triac dimmable led bulb is dimmer compatibility. the conventional triac dimmer was originally designed to handle hundreds of watts induced by incandescent bulbs. an led bulb consuming less than 20 w should interact with those dimmers composed of high-power devices. if the interaction between dimmer and led bulb is not stabilized, visible flicker is perceptible. to manage the interaction without flicker, some requirements for dimmer operation need to be considered. triac dimmer needs latching current at firing and holding current during triac turn-on after firing. if those two currents are not met, triac dimmer misfires and led light flickers. figure 1 shows the connection of triac dimmer and led bulb. as shown in figure 2, the triac dimmer blocks input line in the beginning of line cycle, then connects input line and led bulb after firing. the triac dimmer turns off if latching or holding current flowing through the dimmer is inadequate, as shown in figure 3. the latching and holding currents are different from dimmer models. the typical range of latching and holding currents is around 5 ~ 50 ma. those operating requirement do not cause problems using incandescent bulbs due to high power consumption. an led bulb with less than 20 w output power cannot maintain this amount of current over the whole line cycle. this application note provides a practical guideline of triac dimmable led bulb board design. passive and active bleeder design guides detail how to maintain latching and holding current without visible flicker. active damper design improves efficiency by minimizing the count of external components. the input filter design section covers the effect of filter components on pf, thd, and emi. figure 1. triac dimmer and led bulb figure 2. dimmer operation with adequate latching / holding current figure 3. dimmer operation with inadequate latching / holding current
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 2 1. passive bleeder design the passive bleeder is designed to supply latching and holding current to eliminate misfire and flicker. figure 4 shows a board schematic using a passive bleeder. figure 4. led driver schematic with passive bleeder a passive bleeder is composed of a resistor (r b ) and a capacitor (c b ). l f1 and l f2 are input filter inductors. c in is input filter capacitor and r d is damper resistor. in dimmable board design, a resistor (ex. r b , r d ) needs to be connected in series with a capacitor (ex. c b , c in ) in case that the capacitor is located in between input lines. without the series resistor, a large vo ltage and current spike occurs due to the quickly charged energy in the capacitor at dimmer firing. the current spike can damage the triac dimmer, especially when led bulbs are connected in parallel with the dimmer becau se the sum of the current spike from each led bulb can be over the rated current of the triac dimmer. current ri nging after the current spike can also cause the triac dimmer to misfire due to negative current of less than the holding current in the oscillation. the voltage spike can destroy external components if it is over the rated breakdown voltage. the passive bleeder includes a hundreds-of-nf capacitor (c b ) to provide latching and holding current. to remove the voltage and current spike descri bed above, a bleeder resistor (r b ) is necessary to dampen the spike. 1.1 passive bleeder capacitor (c b ) selection the capacity of c b determines the bleeder current to retain triac turn-on. in terms of triac dimming, bigger c b has better stability in dimming control due to large bleeder current. figure 5 and figure 6 show the line current of small and large bleeder capacitors. the input current (i in ) is the current from the flyback converter behind the bridge diode. i in is in-phase with line voltage by power factor correction controlled by fl7730. i b is bleeder current and line current (i line ) is the sum of i in and i b . figure 5. line current, small bleeder capacitor (c b ) figure 6. line current, large bleeder capacitor (c b ) i line should be higher than latching and holding current because i line directly flows through the triac dimmer. in figure 5, i line at firing is not large enough due to the small c b . the triac dimmer can misfire right after firing, as shown in figure 3. in figure 6, i line is higher at dimmer firing with the large c b , which can maintain normal turn-on state of triac, as shown in figure 2. therefore, a large c b maintains dimmer firing better than a small c b by supplying higher i b . however, a large c b has a drawback in pf, thd, and efficiency. table 1 show s the system performance comparison between 100 nf and 220 nf c b . c b has a significant influence on pf and power dissipation in r b . compared to 100 nf c b , the 220 nf c b seriously drops pf and increases power dissipation of r b due to the larger charging and discharging current of c b . table 1. c b effect on system performance test condition: v in = 230 v ac , p out = 8 w, r b = 2 k ? pf thd p d in r b c b [100 nf] 0.93 13% 162 mw c b [220 nf] 0.85 11% 684 mw therefore, triac dimming control and pf require balanced trade-off when selecting c b in the passive bleeder. especially in high-line bulb with high pf requirements; these two factors can make finding the proper c b a challenge. in the c b selection, the first step is to see i b during dimmer firing by changing c b to check if there is any misfire at dimmer firing due to inadequate i b . in the range of c b without abnormal operation in dimmer firing, choose the minimum c b for higher pf and efficiency. the emi is not affected by c b because r b is connected in series and interrupts noise filtering by c b .
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 3 1.2 passive bleeder resistor (r b ) selection r b is the damper for reducing the spike current caused by quick charging of c b at firing. figure 7 shows line current with excessively large r b . too large r b dampens i b too much and limits i b less than latching current at firing. then, the triac dimmer can misfire right after firing so that visible flicker is appears. figure 7. line current with excessively large r b figure 8 shows i line with excessively small r b . if r b is too small, r b doesn?t fully dampen the spike current and ringing current occurs. the ringing current fluctuates under the negative i b , which causes misfire of the triac dimmer and visible flicker. figure 8. line current with excessively small r b another consideration in r b selection is power loss. table 2 compares system performance using two different bleeder resistors. in the system specification, r b doesn?t affect pf and thd; however, large r b makes increases power dissipation in r b . table 2. r b effect on system performance test condition: v in = 230 v ac , p out = 8 w, c b = 100 nf pf thd p d in r b r b [1 k ? ] 0.93 13% 100 mw r b [2 k ? ] 0.93 13% 162 mw in r b selection, the excessively large and small r b values should be found first. then, the minimum r b can be selected in the proper range of r b for better efficiency. 2. active bleeder design another method to maintain triac holding current is active bleeding technique. the active bleeder can cover a wider range of triac turn-on in a line input cycle compared to passive bleeder. the proposed active bleeder retains triac holding current by regulating input current, which minimizes power loss in the bleeder circuit. figure 9. active bleeder schematic in figure 9, i line is the sum of i b (active bleeder current) and i in (flyback input current). r sense is sensing resistor detecting line current, i line . c filter is the filter capacitor to filter switching noise at r sense voltage. q reg is a shunt regulator, such as ka431. at dimmer firing, a large current spike causes a large voltage drop at r sense . zd lim limits r sense voltage to protect reference block of q reg . biasing current to drive q bleed (bleeder mosfet) as a linear regulator is supplied by auxiliary winding. the biasing circuit consists of d bias and c bias . the gate of q bleed is controlled by the c bias biasing voltage and cathode of q reg . the amount of driving current is limited by r source and r sink . c comp reduces response of the regulation loop. r comp compensates control loop as a negative feedback resistor. i b i line (i in +i b ) set holding current = v ref (q reg ) / r sense i line regulation i in figure 10. line current using active bleeder
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 4 the functional operation is shown in figure 10. in this active bleeder, v gs (gate-source voltage) of q bleed is increased and i b becomes higher when r sense voltage is less than v ref of q reg . the holding current is given as: sense reg ref hold r q v i ) ( ? (1) in the selection of the i hold , there is a trade-off between dimmer compatibility and system efficiency. if i hold is set high, the active bleeder is more compatible with more dimmers; but the amount of i b increases with more power dissipation in the active bleeder. r source , r sink , c comp , r comp , and c filter have a close relationship with the feedback response of the active bleeder. smaller resistance (r source , r sink , r comp ) and capacitance (c comp , c filter ) increase the speed of the feedback loop. if feedback loop is too fast, i b oscillates with a large current ripple. the operation of the active bleeder should be synchronized with the normal ic operation period. when the ic is in an abnormal condition, such as an led short and open, there is no i in due to shutdown gate signal. if active bleeder is still activated in this abnormal condition, the active bleeder should maintain holding current without i in and the power dissipation in the active bleeder is very high and q bleed is thermally destroyed. therefore, the biasing current should come from the auxiliary winding. then, the active bleeder can be disabled when switching is shut down. figure 11 is a design example of an active bleeder. probe ground is connected to v ref of the shunt regulator (ka431). c1 is the r sense voltage and c2 is the input voltage. c3 is the bleeder mosfet source voltage, which is proportional to bleeder current. c4 is current probed line current. 100/0.5w ka431 100n 1k 100/0.5w fqpf2n50 3k 100n 680n c1(v_r sense ) c2(v in ) c3(q bleed source) 3v 1n4003 aux. winding c4(i line ) probe gnd figure 11. example of active bleeder in 8 w bulb figure 12. measured waveform at high dimming angle figure 13. measured waveform at low dimming angle figure 12 and figure 13 show the waveforms of the active bleeder at high and low dimming angle. at low dimming angle, output current is reduced by the dimming function in fl7730. the active bleeder should compensate more i b current due to the reduced i in (c3). that is why the power dissipation in the active bleeder is in the middle dimming angle range. to check the maximum bleeder temperature, the test condition should be a middle dimming angle and maximum line input voltage. 3. active damper design a resistive damper is necessary in series with input filter capacitor (c in ) when triac dimmer is fired. at dimmer firing, a large current spike is induced through input line to quickly charge c in . without the resistive damper, the large spike creates line current oscillation, causing dimmer misfire and damage to the triac dimmer with the excessive current. while the damper resistor suppresses the spike current, the power loss in the damper resistor is very high. the damper resistor not only dampens the spike current, but also handles the input current from the flyback. therefore, fairchild?s proprietary active damper is proposed to reduce the power loss w ith minimized external components. in figure 14, r ad is the active damper resistor and q ad is damper mosfet to reduce power loss of r ad . r d and c d are delay circuit components and d d is reset diode to discharge c d .
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 5 c in r ad d d r d c d q ad v ad v gate v in i in single-stage flyback figure 14. active damper schematic figure 15. active damper waveforms figure 15 shows the operational waveforms of the active damper. mode analysis is as according to the sequence: m1: dimmer turn-off period; q ad turns off. m2: dimmer is fired and spike current occurs. v gate is gradually increased by the delay circuit (r d and c d ) m3: q ad turns on by the charged v gate . v ad is regulated as v th of q ad . m4: c d is discharged by d d and v gate is reset for the next line cycle. the discharging current path is d d - r ad - c d . during m3 period, q ad can considerably reduce power loss in r ad by regulating v ad as its threshold voltage (v th ). table 3 shows power dissipation of passive and active dampers. the power loss of ac tive damper is much lower than passive damper resistor. at low line (110 v ac ), input current is high and the damper resistor handles the large current. therefore, the active damper is strongly recommended at low line model. table 3. passive vs. active damper power loss p out = 8w damper power dissipation [mw] v in : 110 v ac v in : 220 v ac passive damper, 200 ? 1200 290 active damper, 200 ? + fqn1n50c (v th : 2~4 v) 278 161 active damper, 200 ? + fdd10n20lz (v th : 1~2.5 v) 171 113 3.1 active damper resistor (r ad ) selection a voltage and current spike should be checked first when selecting r ad . voltage spikes can damage the mosfet and filter capacitor over the rated voltage. current spikes create current ringing at dimmer firing. as shown in figure 16, i in ringing occurs at firing with small r ad . this ringing current drops i in and the lowered i in can lead to misfire of the dimmer and visible flicker. also, a large peak current spike by using small r ad might damage the triac dimmer, especially when the dimming led bulbs are connected in parallel. therefore, check points when selecting r ad are: ? voltage spike (should be less than the part?s breakdown voltage.) ? current spike (should be less than the triac dimmer?s rated current. if considering connecting bulbs in parallel, the current spike should be lower inversely proportional to the number of led bulbs.) ? current ringing (check the dropped i in at firing if it is enough higher than triac holding current.) after checking the above considerations, choose the minimum r ad to maximize efficiency. figure 16. v in and i in with small damper resistor (r ad ) 3.2 active damper mosfet (q ad ) selection the maximum v ad should be less than the breakdown voltage of q ad . after selecting r ad , maximum v ad can be checked at 90 o dimming angle and the highest input line
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 6 voltage. then, choose proper q ad with breakdown voltage margin. 1~2 a current rating is enough in the 8w led bulb. as shown in table 3, logic-level mosfet with low threshold voltage can additionally reduce power loss because the regulated v ad is q ad threshold voltage. 3.3 active damper diode (d d ) selection the active damper diode discharges cd to reset vgate. diode with 1a rated forward current is enough to discharge cd. same as the qad selection, maximum vad at 90 dimming angle and the highest input line voltage should be checked first to select dd re verse voltage specification. 3.4 active damper delay circuit (r d , c d ) selection the delay circuit (r d , c d ) should create a long enough delay time before q ad turns on to let r ad dampen the current spike. the worst case for the spike current is 90 dimming angle. spike current ringing needs to be checked first at 90 dimming angle to determine how long the spike current is dampened. then, adjust r d and c d to guarantee the dampened period. the recommended c d and r d values are hundreds of nf and tens of k ? . if c d is too large and r d is very small, d d cannot fully discharge c d in m4, as shown in figure 15. design example figure 17 shows the design example of the active damper in an 8w led bulb system. as shown in figure 18 and figure 19, the delay by 80 k ? r d and 100 nf c d is around 1ms. during the delay, 220 ? r ad dampens voltage and current spike without current ringing or dimmer misfire. c in 220/1w es1j 80k 100nf fqn1n50c v ad v gate v in i in figure 17. design example: active damper in 8w bulb figure 18. measured waveform at high dimming angle figure 19. measured waveform at low dimming angle
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 7 4. features of fl7730 the fl7730 is an active power factor correction (pfc) controller using single-stage flyback topology. dimming control with no flicker is implemented by the analog sensing method. primary-side regulation and single-stage topology reduce external components, such as input bulk capacitor and feedback circu itry to minimize cost. to improve power factor and thd, constant on-time control is utilized with an internal error amplifier and low bandwidth compensator. precise constant-current control regulates accurate output current, indepe ndent of input voltage and output voltage. operating frequency is proportionally adjusted by output voltage to guarantee dcm operation with higher efficiency and simpler design. fl7730 provides protections such as open-led, short-led, and over- temperature protection. figure 20. package diagram table 4. pin definitions pin # name description 1 cs current sense . this pin connects a current-sense resistor to detect the mosfet current for the output-current regulation in c onstant-current regulation. 2 gate pwm signal output . this pin uses the internal totem-pole out put driver to drive the power mosfet. 3 gnd ground 4 vdd power supply . ic operating current and mosfet driving current are supplied using this pin. 5 dim dimming . this pin controls the dimming operation of the led lighting. 6 vs voltage sense . this pin detects the output voltage info rmation and discharge time for linear frequency control and constant-current regulation. this pin connects divider resistors from the auxiliary winding. 7 comi constant-current loop compensation . this pin is the output of the transconductance error amplifier. 8 gnd ground figure 21. functional block diagram
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 8 design summary figure 22 shows the schematic of the triac dimmable led driver using fl7730. this schematic is dedicated to low-line voltage (90~140 v ac ). n1 n3 r 12 510k c 8 10n d 4 rs1m d 5 es3d c 10 35v/330uf 1 n2 v o cs gate vdd dim comi n.c gnd vs 7 8 3 6 2 4 5 r 8 150k r 9 20k r 13 10 ? r 14 1.2 ? d 3 1n4003 c 5 10p c 9 4.7nf c 4 3.3u l 1 4.7mh r 15 1.0 ? c 6 2.2u r 17 51k d 2 11v r 5 75k r 6 62k r 4 1m q1 mb8s r 1 560/0.5w c 1 330n c 2 330n q3 fl7730 q4 fqu5n60c r 10 100k ? 0.5w l 2 4.7mh c 7 47u f 1 1a/250v r 11 510k r 2 100/0.5w d 1 es1j r 3 20k c 3 100nf q2 fqn1n50c r 16 200 ? r 7 0 c 11 35v/1000uf figure 22. schematic of triac dimmable led driver using fl7730 (low line: 90~140 v ac ) n p1 (5 3) n s (ns- ns+) n a (2 6) n p2 (3 4) figure 23. transformer structure table 5. winding specifications no winding pin (s f) wire turns winding method 1 np1 5 ? 3 0.13 38 ts solenoid winding 2 insulation: polyester tape t = 0.025 mm, 2-layer 3 ns ns- ? ns+ 0.3 (tiw) 24 ts solenoid winding 4 insulation: polyester tape t = 0.025 mm, 2-layer 5 na 2 ? 6 0.13 18 ts solenoid winding 6 insulation: polyester tape t = 0.025 mm, 2-layer 7 np2 3 ? 4 0.13 38 ts solenoid winding 8 insulation: polyester tape t = 0.025 mm, 6-layer table 6. electrical characteristics pin specification remark inductance 1 ? 2 1 mh 10% 50 khz, 1 v leakage 1 ? 2 8 h 50khz, 1 v short all output pins
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 9 experimental verification the design example with passive bleeder and active damper was experimentally verified in an 8 w led lighting system. figure 24 shows constant current regulation at input voltage and output voltage change. cons tant-current deviation in the wide output voltage range from 10 v to 28 v is less than 2.1% at each line input voltage. line regulation at the rated output voltage (22 v) is less than 3.9%. operation waveforms are shown in figure 25, figure 26, and figure 27. in this dimmable board, triac dimmer firing is stabilized without any misfire. fl7730 keeps constant t on so v cs is in phase with v in . the maximum spike current of i in is 1.2 a. figure 28 shows the dimming curve. rms input voltage indicates triac dimming angle. led current is smoothly controlled by the fl7730 dimming function and external circuits, such as the passive bleeder and active damper. table 7 provides compatibility with common dimmers for a design without visible flicker. maximum and minimum current vary because each dimmer?s maximum and minimum angles are different. system efficiency is from 80.7% to 82.9% at low line input voltage (90 ~ 140 v ac ). the active damper helps improve the efficiency with a comp act and inexpensive design solution. table 8 shows pf and thd in a low line input voltage range of 90~140 v ac . pf is over 0.9 and thd is much less than 30% by constant t on and linear frequency control in the fl7730. the performances obtained in the design example show a powerful led lighting solution with accurate constant current regulation, stable dimming control, high efficiency, high pf, and low thd with low bom cost. i out [ma] ovp figure 24. cc regulation, measured by cr-load figure 25. waveforms at maximum dimming angle v cs v in i in figure 26. waveforms at half dimming angle v cs v in i in figure 27. waveforms at minimum dimming angle
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 10 table 7. dimmer compatibility manufacturer dimmer maximum current minimum current flicker lutron s-600p-wh 330 ma 40 ma (12%) no lutron cn-600p-wh 328 ma 11 ma (3.4%) no lutron gl-600h 365 ma 8 ma (2.2%) no lutron tg-603pgh-wh 252 ma 12 ma (4.8%) no lutron tg-600ph-wh 333 ma 14 ma (4.2%) no lutron lg-600p 327 ma 3 ma (0.9%) no lutron ctcl-153pd 320 ma 58 ma (18%) no leviton ip106 380 ma 36 ma (9.5%) no leviton 1c4005 344 ma 0 ma (0%) no leviton 6631-lw 340 ma 0 ma (0%) no legrand f 165h 344 ma 3 ma (0.9%) no figure 28. dimming curve (input voltage vs. led current) figure 29. efficiency table 8. power factor (pf) a nd total harmonic distortion (thd) input voltage output current output voltage pf thd 90 v ac 360 ma 21.70 v 0.98 7.4% 110 v ac 376 ma 21.77 v 0.96 9.5% 120 v ac 380 ma 21.77 v 0.95 10.4% 140 v ac 386 ma 21.79 v 0.91 12.4%
AN-9745 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.2 ? 10/11/12 11 related datasheets fl7730my ? single-stage primary-side-regulation pwm controller for pfc and led dimmable driving ka431 ? programmable shunt regulator disclaimer fairchild semiconductor reserves the right to make changes without further notice to any products herein to improve reliability, function, or design. fa irchild does not assume any liability arising out of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights, nor the rights of others. life support policy fairchild?s products are not authorized for use as criti cal components in life support devices or systems without the express written approval of the preside nt of fairchild semiconductor corporation. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. a critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.

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