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www.fairchildsemi.com ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 an9737 design guideline for singlestage flyback acdc converter using fl6961 for led lighting summary this application note presents singlestage power f actor correction (pfc) and focuses on how to select and d esign the flyback transformer for 16.8w (24v/0.7a) soluti on for universal input for led lighting applications using fl6961. the flyback converter using fl6961 operates in crit ical conduction mode (crm) and has functions such as cc/ cv feedback circuit, softstarting, and the cyclebyc ycle current limit for led lighting applications. introduction these days, engineers use various types of leds for general lighting systems because of their long life, excell ent efficacy, price, environmental benefits, and requir ements from end users. at the same time, high power factor (pf), isolation for safety, and constant current control (cc) for constant led color are becoming requirements. conventional regulation is the minimum power factor correction for input power base above 25w, but many want to reduce power ratings and the new energystar dir ective for solidstate lighting requires a power factor gr eater than 0.9 for commercial applications. expect pf regulati ons to become more stringent. basic operation: high power factor flyback converter the basic idea of achieving high power factor (pf) flyback converter is to use a critical conduction mode (crm ) pfc controller. the conventional pfc ic, such as fl6961 , has constant ontime and variable offtime control meth od, which means the input average current always follow s the input voltage shape. figure 1 shows the typical application schematic of single stage pfc topology. the main difference of normal c rm boost converter is that singlestage pfc doesnt us e a large electrolytic capacitor after the full rectification diode. normally, the singlestage pfc method uses a small capacitor (c1 in figure 1) to act as a noise filter to attenuate highfrequency components and doesnt use the inv p in for output voltage regulation. figure 1. simplified schematic of highpower factor flyback converter with fls6961 fuse br d2 c4 r8 d1 d3 c5 fl6961 12 3 4 87 6 5 c1 c2 r8 r7 r6 q1 t1 inv comp mot cs zcd gnd out vcc u101 emi filter r1 r2 r3 r4 c3 r5 feedback
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 2 figure 2 shows typical waveforms of the simplified circuit of a flyback converter with crm. when the mosfet (q 1) turns on, the primary current in primary side linea rly increases and is clamped at a certain internal leve l because the fl6961 doesnt have cyclebycycle current limi t like a conventional current mode control ic (such as fan75 27b). its peak level is determined by the primary magneti zing inductance value and the fixed ontime. instead of the cycle bycycle primary current limit, the fl6961 has an o ver current protection (ocp) function. if the current s ensing signal is larger than internal detection level, the fl6961 doesnt get output signal for operating the mosfet (q1). figure 2. key waveforms of flyback converter on crm the fl6961 has a constant ontime across the whole range. the input average current always follows the peak i nput current, as shown in the equation: on pk mosfet avg t i i 2 1 ) ( = (1) this is also proportional to the instantaneous inpu t voltage. this means the input current shape is always the sa me as the input voltage shape. the reverse diode voltage is l inearly increased and is equal to: p s in o diode pk n n v v v + = ) ( (2) during the mosfet offtime, which is also the diode on time; the input current instantly drops to zero, th e diode in the secondary side conducts, and the diode current linearly decreases. the peak current of the secondary side i s the same as the multiplication of the primary peak curr ent and turns ratio between the primary side (n p ) and secondary side (n s ) and naturally decreases to zero. the average curr ent of the secondary side is: off pk s p diode avg t i n n i 2 1 ) ( = (3) since the diode forwardvoltage drop decreases as c urrent decreases, the output voltage reflects the primary winding and adds additional voltage due to overshoot made b y resonance between the leakage inductance on primary side winding and parasitic capacitance on the mosfet (q1 ). as a result, a superimposed voltage occurs on the mosf et during offtime as: os r in off mosfet v v v v + + = ) ( (4) where v r is the reflected voltage and v os is the voltage overshoot term. the reflected voltage, v r , is affected by the turns ratio between the primary and secondary side of the trans former and the output voltage, calculated as: o s p r v n n v = (5) figure 3 shows the ideal waveforms of the primarys ide current at mosfet (q1) and the secondaryside curre nt at the diode. the input peak and average current on th e primary side follows input voltage instantaneously. normally, secondaryside current on the diode is la rger than the primary side because of the turns ratio. figure 3. ideal waveforms time time time i pk ( mosfet ) i ds ( mosfet draintosource current) ) i d (diode current ) v ds ( mosfet voltage) t on t off t s i avg (mosfet ) i pk ( diode ) i avg (diode ) v in v r v os
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 3 as a result, designers should consider two conditio ns before component selection: voltage and current capacity o n primaryside mosfet(q1) and secondaryside diode (d 3) to make a stable system with margin. figure 4 shows a guide to deciding two components o n the boundary condition of flyback converter topology. figure 4. boundary conditions of flyback converter topology ( refer to an-8025 ) design example a. transformer design a design guideline of 16.8w singlestage flyback ac dc converter using fl6961 is presented. the applied sy stem parameters are shown in table 1. table 1. system parameters parameter value main input voltage range, v ac(main) 90v~265v output voltage, v out 24v output current, i out 0.7a minimum switching frequency at v ac(min)_pk 50khz diode voltage drop, v d 1v mosfet on resistance, r mos 1 window utilization 0.4 target system efficiency 0.82 maximum duty at v ac(min)_pk 0.35 operating maximum flux density 0.35 regulation, 0.5% note: 1. regulation is strongly related with the copper l oss and 0.5% regulation means 0.084w loss on transformer. there are many ways to decide core and coil size an d turns, such as using al value and following common practic es. in this note, use the k g value related with the core geometry to find optimum core and coil information. step 1. calculate the total period, t: 20 1 = = f t [ s] step 2. calculate the maximum ontime at mosfet in primary side. 7 ) 35 .0 )( 10 20 ( 6 max = = = ? td t on [ s] step 3. calculate the output power: 5. 17 )1 24 (7.0 ) ( = + = + = d o o v v i p [ w ] step 4. calculate the maximum input current, i max : 168 .0 ) 82 .0 )( 90 2 ( 5. 17 min (max) = = = v p i o in [ a ] step 5. calculate the mosfet voltage drop, v vd : 168 .0 (max) = = mos in vd r i v [ v ] step 6. calculate the primary voltage on transforme r, v p : 127 168 .0 127 min ? = ? = vd p v v v [v] v p =126.83 use 127 step 7. calculate the primary peak current, i ppk : 96 .0 ) 10 7 )( 127 ( 82 .0 )5. 17 )( 10 20 (2 2 6 6 (max) = = = ? ? on p ppk t v tp i [ a ] step 8. calculate the primary rms current, i prms : 32 .0 ) 10 20 (3 ) 10 7( 96 .0 3 6 6 = = = ? ? t t i i on ppk prms [ a ] step 9. calculate the required minimum inductance, l: 926 .0 96 .0 ) 10 7( 127 6 (max) = = = ? ppk on p i t v l [ m h] l=0.926[ m h] use 1[mh] step 10. calculate the energyhanding capability in watt seconds, ws: 0004608 .0 2 ) 96 .0 )( 10 1( 2 2 3 2 = = = ? ppk li eng [ws] step11. calculate the electrical conditions, k e : 00003108 .0 10 ) 35 .0 )( 5. 17 ( 145 .0 10 145 .0 4 2 4 2 = = = ? ? m e pb k step 12. calculate the core geometry, k g :
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 4 0136 .0 )5.0( 00003108 .0 ) 0004608 .0( ) ( 2 2 = = = e g k eng k [cm 5 ] step 13. see table 2 for core size. to prevent core saturation, select a little big cor e after comparing two k g values: calculate value at step 12 vs. the existing value in table 2. the pq42016 has a little bit big k g value (0.01327) in table 2 with 2500 permeability (i). step 14. calculate the current density, j.: 265 )4.0 )( 2484 .0( 35 .0 10 ) 0004608 .0(2 10 ) (2 4 4 = = = u p m k a b eng j [a/cm 2 ] step 15. calculate the required wire area. a w(b) : 001207 .0 265 32 .0 ) ( = = = j i a rms b w [cm 2 ] step 16. calculate the number of turns, n: 93 . 141 001207 .0 4.0 4283 .0 ) ( = = = b w u a a k w n [t] n=141.93; use 142 turns. step 17. calculate the required gap, l g : 0489 .0 35 .0 10 ) 96 .0 )( 142 ( 4.0 10 ) )( ( 4.0 4 4 = = = ? ? m g b i n l [cm] step 18. calculate the new turns using a gap from s tep 15. 153 . 83 ) 58 .0( 4.0 ) 10 )( 2500 74 .3 0489 .0( 10 1 ) ( 4.0 ) ( 8 3 = + = + = ? c i g a mpl l l n [t] n=83.153; use 83[t]. where i is permeability of selected core material and mpl is magnetic path length of selected core. step 19. calculate the fringing flux, f: 238 .1 ) 0489 .0 ) 001 .1(2 ln 58 .0 0489 .0 1( ) 2 ln 1( = + = + = g c g l g a l f where g is window height of selected core. step 20. calculate the new turns, n new : 6. 73 ) 238 .1 )( 58 .0 )( 4.0( 10 1 0489 .0 ) 10 ( ) )( 4.0( 5 8 = = = ? f a l l n c g [t] n new =73.6; use 74. step 21. calculate the ac flux density in tesla, b ac : 113 .0 0489 .0 ) 10 )( 238 .1 )( 2 96 .0 )( 74 )( 4.0( ) 10 ( ) 2 ( ) 4.0( 4 4 = = = ? ? g pk ac l f i n b [t] step 22. calculate the new wire size, a w(b) : 002315 .0 74 4.0 4283 .0 ) ( = = = new u a b w n k w a [a/cm 2 ] step 23. calculate the skin depth at expected opera ting frequency at low input voltage. the skin depth is t he radius of the wire. 02960 .0 10 50 62 .6 62 .6 3 = = = f [cm] step 24.calculate the required wire area under cons idering skin depth : 0027535 .0 ) ( 2 = = r wire a [cm 2 ] step 25. select a wire size with the required area from table 4. if the area is not within 10% of the required ar ea, then go to the next smallest size. awg=#23 a w(b) =0.00259[cm 2 ] /cm=666 step 26. calculate the required number of primary s trands, s np : 8938 .0 00259 .0 002315 .0 ) ( = = = a b w np wire a s this means that the selected wire from the step 25, awg23, is enough or has enough margins for supplying the p rimary side current on the flyback converter. step 27. calculate the secondary and auxiliary turn s, n s n aux : 05 . 27 ) 35 .0 )( 90 2 ( ) 35 .0 1 )( 1 24 ( 74 ) ( ) 1 )( ( max max = ? + = ? + = d v d v v n n p d o p s n s =27.05; use 27. 31 . 17 ) 35 .0 )( 90 2 ( ) 35 .0 1 )( 1 15 ( 74 ) ( ) 1 )( ( max max = ? + = ? + = d v d v v n n p d o p aux n aux =17.31; use 17. step 28. calculate the secondary peak current, i spk : 153 .2 35 .0 1 )7.0(2 ) 1( 2 max = ? = ? = d i i o spk [a] step 29. calculate the secondary rms current, i srms : 0021 .1 3 ) 35 .0 1( 153 .2 3 ) 1( max = ? = ? = d i i spk srms [a] step 30. calculate the secondary wire area, a sw(b) : 003781 .0 265 0021 .1 ) ( = = = j i a rms b sw [cm 2 ] step 31. select a wire size with the required area from table 4. if the area is not within 10% of the required ar ea, go to the next smallest size.
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 5 awg=#22 a w(b) =0.003243[cm2] /cm=531.4 step 32. calculate the required number of primary s trands, s np : 2521 .1 00259 .0 003243 .0 ) ( = = = a b sw np wire a s this requires the awg21 wire with two strands for secondaryside winding on the flyback converter. adapted core size pq42614 awg turns primary 74 23 secondary 27 22/ 2 strands auxiliary 17 estimated gap[mm] 0.489 b. mosfet and diode selection step 33. calculate the maximum voltage of mosfet dr ain voltage at primary side: 54 . 490 ) ( = + + = + + = os o s p in os r in off mosfet v v n n v v v v v [v] where v os is assumed ~50v and its peak can degrade external snubber circuit performance. this means a 600v mosfet can be used with some margin. minimum requirements of the mosfet are summarized below. current rating [a] voltage rating [v] calculation +20% margin calculation +20% margin 0.96 1.152 490.54 588.65 step 34. calculate the maximum voltage of diode at secondary side: 74 . 160 74 27 2 265 24 ) ( = + = + = p s in o diode pk n n v v v [v] this means a 200v diode can be used with some margin. the minimum requirement of the secondary diode as summarized below. current rating [a] voltage rating [v] calculation +20% margin calculation +20% margin 2.153 2.584 160.74 192.88 c. sensing resistor the cs pin of fl6961 has overcurrent protection (ocp) over the whole operating period and its internal clamping level, v limit , is 0.8v. figure 5. switching current limit normally, it is reasonable to set the ocp level to 1.5 times higher than the peak current at primary side. 44 .1 3 5.1 (max) = = = on p ppk limit t v tp i i calculate the sensing resistor as: 55 .0 8.0 sin = limit g sen i r [ ] d. voltage and current feedback for cc/cv function the constant voltage and current output is adapted by measuring the actual output voltage and current wit h external passive components and an op amp in the evaluation board. because the output loads, the hig h bright led (hb led) and passive components are effected by ambient temperature. use the feedback p ath for stable operation. figure 6. feedback circuit for cc/cv operation
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 6 normally, the cc block is dominate over the cv block in steady state and the cv block acts as the overvoltage protection (ovp) at transient or abnormal mode, such as no load condition. the output signal of cc block is determined as: ? + ? = dt r v r v c r v r v r v ref cc g sen ref cc g sen cc o ) ( 1 ) ( 3 2 _ sin 1 3 2 _ sin 4 _ where the v sensing_cc means the sensing voltage from the sensing resistor (r1) and its values is as: 1 _ sin r i v o cc g sen = the output signal of cv block is determined as: dt v v r r r r c v v r r r r r v r r r v ref cv g sen ref cv g sen cv g sen cv o ) ( 1 1 ] ) [( ) ( _ sin 6 5 6 7 2 _ sin 6 5 6 7 8 _ sin 6 5 6 _ ? + + ? + + + = where the v sensing_cv means the output voltage on this circuit and this voltage is divided by two resistors, r5 and r6, and connected to noninverted pin at the op amp. normally, set this divided voltage, cv g sen v r r r _ sin 6 5 6 ) ( + , to ref v or a little bit smaller value in steady state condition because the main role of this block is overvoltage protection. there are more highvoltage transfers to the output stage at transient or an abnormal case such as over voltage output condition than in the steady state. e. softstart / overshoot prevention function normally, the high bright (hb) led has a forwardcurrent limitation to prevent the led burnout due to overpower dissipation. thererfore, the output overshoot function is needed through the whole operating period. though there are cc/cv blocks for output regulation, those blocks do not operate in transient modes, because they block have a long response time and cannot act instantly. figure 7 shows the output voltage overshoot compression method using diode and resistor. the current flows through resistor, r9, and diode, d204, at startup, which is the period before activating the cc/cv block, and then decrease at steady state. the quantity of bypassing current goes into the feedback block on the control ic, fl6961, and controls the output power gradually. figure 7. softstart / overshoot prevention method
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 7 table 2. various core types and size part # mlt [cm] mpl [cm] g[cm] a c [cm] w a [cm 2 ] a p [cm 4 ] k g [cm 5 ] perm al manufacturer rm42316 4.17 3.80 1.074 0.640 0.454 0.2900 0.01782 0 2500 2200 magnetics pq42610 5.54 2.94 0.239 1.05 0.1177 0.1235 0.00937 2500 6310 magnetics pq42614 5.54 3.33 0.671 0.709 0.3304 0.2343 0.0120 0 2500 4585 magnetics pq42016 4.34 3.74 1.001 0.580 0.4283 0.2484 0.0132 7 2500 2930 magnetics epc25 4.930 5.92 1.800 0.4640 0.8235 0.3810 0.0143 8 2300 1560 magnetics ei44008 7.77 5.19 0.356 0.9950 0.3613 0.3595 0.018 416 2500 4103 magnetics efd25 4.78 5.69 1.86 0.5810 0.6789 0.3944 0.01917 1800 1800 philips table 3. pq42016 core dimensions (magnetics: http://www.maginc.com/home/advancedsearchresults?pn=42016 table 4. wire table awg bare wire area /cm heavy insulation cm2 cirmil cm2 turns/cm turns/cm2 20 0.005188 1024.0 332.3 0.006065 11.37 98.93 21 0.004116 812.30 418.9 0.004837 12.75 124.0 22 0.003243 640.10 531.4 0.003857 14.25 155.5 23 0.002588 510.80 666.0 0.003135 15.82 191.3 24 0.002047 404.0 842.1 0.002514 17.63 238.6 25 0.001623 320.40 1062.0 0.002002 19.8 299.7 26 0.001280 252.80 1345.0 0.001603 22.12 374.2 27 0.001021 201.60 1687.6 0.001313 24.44 456.9 28 0.008048 158.80 2142.7 0.0010515 27.32 570.6 29 0.0006470 127.70 2664.3 0.0008548 30.27 701.9
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 8 schematic fl6961 figure 8. schematic
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 9 bill of materials item number part reference value quantity description (manufacturer) 1 u101 fl6961 1 crm pfc controller (fairchild semic onductor) 2 u102 fod817 1 optocoupler (fairchild semiconduct or) 3 u201 ka431 1 shunt regulator (fairchild semicondu ctor) 4 u202 ka358a(lm2904) 1 dual op amp (fairchild semiconductor) 5 q101 fqpf3n80c 1 800v/3a mosfet (fairchild semico nductor) 6 d101 df04 1 1.5a smd bridgediode (fairchild semi conductor) 7 d102 rs1m 1 1000v/1a ultrafast recovery diode (f airchild semiconductor) 8 d103 rs1g 1 400v/1a fast recovery diode (fairchil d semiconductor) 9 d201,d204 egp30d 2 200v/3a ultrafast recovery di ode (fairchild semiconductor) 10 d202,d203, d205,d206 ll4148 3 generalpurpose diode (fairchild semicondu ctor) 11 r101,r102, r103 82k 3 smd resistor1206 12 r104 120k 1 smd resistor1206 13 r105 10k 1 smd resistor1206 14 r106 20k 1 smd resistor1206 15 r107 9.1k 1 smd resistor1206 16 r108 47 1 smd resistor 1206 17 r109 10 1 smd resistor 1206 18 r110 220k 1 2w 19 r111 30k 1 smd resistor 1206 20 r112,r113 1 2 smd resistor 1206 21 r201,r202, r203 1 3 smd resistor 1206 22 r204 2.2 1 smd resistor 0806 23 r205 4.3k 1 smd resistor 0806 24 r206 1.5k 1 smd resistor 0806 25 r207 30k 1 smd resistor 0806 26 r208 51k 1 smd resistor 0806 27 r209 33k 1 smd resistor 0806 28 r210 3.9k 1 smd resistor 0806 29 r211 120k 1 smd resistor 0806 30 r212 47k 1 smd resistor 0806 31 r213 4.7k 1 smd resistor 0806 32 r214 47k 1 smd resistor 0806
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 10 bill of materials (continued) item number part reference value quantity description (manufacturer) 33 c101 100nf/250v 1 x C capacitor 34 c102 47nf/250v 1 x C capacitor 35 c103 100nf/630v 1 film capacitor 36 c104 33f/35v 1 electrolytic capacitor 37 c105 2.2nf/1kv 1 ycapacitor 38 c106 2.2f 1 smd capacitor 0805 39 c107 30pf 1 smd capacitor 0805 40 c108 100nf 1 smd capacitor 0805 41 c201,c202 470f/35v 2 electrolytic capacitor 42 c203 1f 1 smd capacitor 0805 43 c204 470nf 1 smd capacitor 0805 44 c205 10f/35v 1 electrolytic capacitor 45 lf101,lf102 80mh 2 line filter 46 l101 27h 1 line filter 47 l102 6.8h 1 line filter 48 l201 5h 1 output inductor 49 f101 1a/250v 1 fuse 50 t1 pq42016 1 1mh
an9737 application note ? 2011 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 4/13/11 11 related datasheets fl6961 singlestage flyback and boundary mode pfc con troller for lighting an8025 design guideline of singlestage flyback acdc converter using fan7530 for led lighting disclaimer fairchild semiconductor reserves the right to make changes without further notice to any products herein to improve reliability, function, or design. fairchild does not assume any liability arising ou t of the application or use of any product or circuit descri bed herein; neither does it convey any license unde r its patent rights, nor the rights of others. life support policy fairchilds products are not authorized for use as critical components in life support devices or syst ems 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 s ystems which, (a) are intended for surgical implant into the body , or (b) support or sustain life, or (c) whose failure to pe rform when properly used in accordance with instructions for u se provided in the labeling, can be reasonably expected to resu lt 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 reasonabl y expected to cause the failure of the life support d evice or system, or to affect its safety or effectiveness.

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