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july 1999 tinyswitch flyback design methodology application note AN-23 figure 1. typical tinyswitch flyback power supply. introduction this document describes a simple design methodology for flyback power supply design using the tinyswitch family of integrated off-line switchers. the objective of this design methodology is to provide power supply engineers a handy tool that not only eases the design task but also delivers design optimization in cost and efficiency for most applications. basic circuit configuration because of the high level integration of tinyswitch , flyback power supply design is greatly simplified. as a result, the basic circuit configuration of tinyswitch flyback power supplies remains unchanged from application to application. application specific issues outside this basic configuration such as constant current, constant power outputs, etc. are beyond the scope of this document. figure 1 shows the basic circuit configuration in a typical tinyswitch flyback design using tny253. design flow figures 2a, b and c present a design flow chart showing the complete design procedure in 22 steps. the logic behind this design methodology can be summarized as follows: 1. calculate minimum reflected voltage, v or , allowed by a given output diode. 2. design for discontinuous mode operation using this calculated v or . if necessary, increase v or . 3. at v or = 150 v, select bigger tinyswitch to stay in discontinuous mode or go to continuous mode design. 4. design transformer using ee16 core. 5. select feedback circuit and other components to complete the design. tm pi-2336-112098 d s en bp + - c in 0.1 f vac fusible snubber circuit output capacitor v o output post filter l, c bypass pin capacitor feedback sense circuit tny253
AN-23 a 7/99 2 1. system requirements v acmin , v acmax , f l , v o , p o , 2. select output diode based on v o & estimate diode loss 3. select clamp/snubber circuit n y v or < 150 v to step 8 pi-2345-111398 4. estimate efficiency 5. determine v min , v max & c in 6. determine piv calculate v or from v max , v o , v d , & piv 7. choose tinyswitch based on v min ,v max & p o figure 2a. tinyswitch flyback design flowchart steps 1 to 7.
a 7/99 AN-23 3 from step 7 8. calculate d max from v min , p o & i p y 10. fully disc? to step 18 pi-2351-111398 11a. fully disc required? n 12. mostly disc? n 13. continuous ok? n 9. calculate k dp from v min , v or & d max 14. calculate d max from v min , v or 11b. set k dp = (1- d max )/(0.67 - d max ), recalculate v or y n discontinuous 17a. calculate l p y n 13b. set k dp = 1, recalculate v or 15. calculate k rp from v min , p o, i p & d max 16a. k rp 0.6 16b. set k rp = 0.6, recalculate d max 16c. recalculate v or from v min , d max n v or < 150 v n back to step 7 continuous 17b. calculate l p to step 18 y y y v or < 150 v y figure 2b. tinyswitch flyback design flowchart steps 8 to 17a, b.
AN-23 a 7/99 4 figure 2c. tinyswitch flyback design flowchart steps 18-22. n y from step 17a, b 18. design transformer n p , n s , l g discontinuous? 19a. discontinuous calculate currents i rms , i srms 20. determine output short circuit current i os 21. determine output capacitor c out 22. determine feedback circuit and post filter design complete n 19b. continuous calculate currents i rms , i srms pi-2347-112098
a 7/99 AN-23 5 diode type schottky ultrafast-pn fast-pn v d (v) 0.5 1.0 1.0 efficiency loss (0.5/v o ) 100% (1.0/v o ) 100% (1.0/v o ) 100% table 2. diode forward voltage (v d ) and efficiency loss. step by step design procedure symbols and parameters used in this design procedure are defined in application note topswitch flyback design methodology (an-16) . step 1. determine system requirements: v acmin , v acmax , f l , v o , p o , determine input voltage range from table 1. step 2. select output diode. estimate associated efficiency loss. the output diode can be selected based on expected power supply efficiency and cost (see table 2). - use a schottky diode for highest efficiency for output voltages up to 7.5 v. - for output voltages beyond 7.5 v use an ultra fast pn- diode. - if efficiency is not a concern (or cost is paramount), use a fast pn-diode. - the schottky and ultrafast may be used with continuous mode of operation. the fast pn-diode should be used only with discontinuous mode of operation. - choose output diode type. table 2 shows approximate forward voltage (v d ) for types of output diode discussed above. output diode efficiency loss is the power supply efficiency reduction (in percentage) caused by the diode. input (vac) 100/115 230 universal v acmin (vac) 85 195 85 v acmax (vac) 132 265 265 table 1. input voltage range. the estimated efficiency loss due to the output diode is also shown in table 2. table 3 shows some commonly used output diodes. v r is the diode reverse voltage rating. i d is the diode dc current rating. the final diode current rating is to be determined in step 20 to accommodate continuous short circuit current i os . table 3. output diodes. output diode v r (v) i d (a) manufacturer 1n5819 40 1.0 motorola 1n5822 40 3.0 motorola schottky mbr745 45 7.5 motorola mbr1045 45 10.0 motorola mbr1645 45 16.0 motorola uf4002 100 1.0 gi mur110 100 1.0 motorola mur120 200 1.0 motorola uf4003 200 1.0 gi byv27-200 200 2.0 philips, gi uf5401 100 3.0 gi ufr uf5402 200 3.0 gi mur410 100 4.0 motorola mur420 200 4.0 motorola mur810 100 8.0 motorola mur820 200 8.0 motorola byw29-200 200 8.0 philips, gi byv32-200 200 20.0 philips step 3. select clamp/snubber circuit and determine associated efficiency loss. clamp/snubber circuit is required at drain to keep drain voltage below rated bv: - a snubber alone may be used at low power (< 3 w with universal input) and will provide lower video noise and superior emi performance. - an rcd clamp may be used for power levels < 3 w for higher efficiency and is required at power levels > 3 w with universal input. table 4 shows the approximate efficiency loss due to clamp/ snubber circuits. step 4. estimate power supply efficiency . total efficiency loss is the sum of the output diode efficiency loss (from step 2) and the clamp/snubber efficiency loss (from step 3). clamp/snubber rc snubber rcd clamp p o 0 to 3 w > 3 w efficiency loss 20% 15% table 4. clamp/snubber efficiency loss.
AN-23 a 7/99 6 calculate overall power supply efficiency as: = 100% - total efficiency loss. step 5. determine maximum and minimum dc input voltages v max , v min and input storage capacitance c in . (see an-16 for more detail) calculate the maximum v max as: vv max acmax = 2 choose input storage capacitor, c in per table 5. diode type schottky ultrafast-pn fast-pn v r (v) 40-45 100-200 > 200 table 6. diode reverse voltage range. table 7. tinyswitch output power (p o ) capability device tny253 tny254 tny255 p o for single voltage* 5.0 8.0 10 output power capability (w) p o for universal voltage 2.5 5.0 7.5 recommended power range for lowest cost** (w) p o for single voltage* 0-2.5 2.0-5.0 6.0-10 p o for universal voltage 0-1.5 1.0-4.0 3.5-6.5 set bridge rectifier conduction time t c = 3 ms. derive minimum dc input voltage, v min vv p f t c min acmin o l c in = () ? ? ? ? ? ? ? ? 2 2 1 2 2 where c in : input capacitance f l : line frequency t c : diode conduction time step 6. determine output diode peak inverse voltage piv. calculate reflected output voltage v or based on v max , v o , v d and piv. look up output diode reverse voltage v r from diode data sheet or table 3 in step 2. calculate maximum peak inverse voltage piv. the maximum recommended piv is 80% of the reverse voltage rating v r . piv v r = 08 . calculate reflected output voltage v or : v vvv piv v or max o d o = + () ? if v or > 150 v, go back to step 2 and choose a different diode for higher v r . refer to table 6 for approximate v r range for different types of diodes. input voltage 100/115 230 universal c in ( f/watt) 2-3 1 2-3 v min (v) 90 240 90 table 5. c in range . step 7. choose tinyswitch based on input voltage range and output power p o . select appropriate tinyswitch according to table 7 based on output power p o and input voltage range (from step 1). * single voltage 100/115 vac with voltage doubler, or single voltage 230 vac without doubler ** based on ee16 core transformer for universal input voltage and an output power range of 1w to 1.5 w, tny254 is usually a better choice than tny253 except for applications requiring low video noise. for universal input voltage and an output power range of 3.5w to 4 w, tny255 usually results in smaller transformer size and higher efficiency than tny254. step 8. determine primary peak current i p . calculate maximum duty cycle d max for discontinuous mode of operation based on v min , p o and i p . primary peak current is 90% of minimum i limit from the data sheet of the selected tinyswitch . 0.9 is the over temperature derating factor for i limit :
a 7/99 AN-23 7 ii p limit = 09 . minimum calculate maximum duty cycle d max for discontinuous mode of operation as: d p vi max o min p = 2 step 9. calculate k dp from v min , v or and d max . kdp is the ratio between the off-time of the switch and the reset time of the core: k vd vd dp or max min max = ? () 1 step 10, 11a, 11b, 12, 13a, 13b. check k dp to ensure discontinuous mode of operation. raise v or if necessary. the mode of operation can vary depending on power supply requirements. however, discontinuous mode of operation is always recommended wherever it is possible. with discontinuous mode of operation, generally, the output filter is smaller, output rectifier is cheaper with pn junction diode, emi and video noise are lower. fully discontinuous mode of operation (discontinuous under all conditions) may be necessary in some applications to meet specific requirements such as very low video noise, very low output ripple voltage. use of rc snubber, and/or pn junction diode as output rectifier also demand fully discontinuous mode of operation. this can be accomplished by raising v or higher if necessary until k dp (1- d max )/(0.67 - d max ). to keep the worst case drain voltage below the recommended level of 650 v, v or should be kept below 150v. mostly discontinuous mode of operation (k dp 1 ) refers to a design operating in discontinuous mode under most situations, but do have the possibility of operating in continuous mode occasionally. continuous mode operation (k dp < 1 ) provides higher output power. in this mode a schottky output diode should be used to prevent long diode reverse recovery times that could exceed leading edge blanking period (t leb ). step 10. check for fully discontinuous operation. k dp (1- d max )/(0.67 - d max ): fully discontinuous. go to step 17a. k dp < (1- d max )/(0.67 - d max ): go to step 11. 0.67 is the reciprocal of the percentage of duty cycle relaxation caused by various parameters such as the tolerance in tinyswitch current limit and frequency. step 11a, b. determine if fully discontinuous is necessary. if yes, set k dp = (1- d max )/(0.67 - d max ). recalculate v or as v kv d d or dp min max max = ? 1 - if v or < 150 v, go to step 17a. - if v or > 150 v, go back to step 7 and select higher current tinyswitch . if not, go to step 12. step 12. check for mostly discontinuous. k dp 1. operation is mostly discontinuous. go to step 17a. k dp < 1. go to step 13. step 13a, b. determine if continuous is acceptable for the application. if yes, go to step 14. if not, set k dp = 1. recalculate v or as: v kv d d or dp min max max = ? 1 - if v or < 150 v, go to step 17a. - if v or > 150 v, go back to step 7 and select higher current tinyswitch .
AN-23 a 7/99 8 step 14. recalculate d max for continuous mode of operation from v min and v or . start continuous mode design. recalculate d max as: d v vv max or or min = + step 15. calculate k rp from v min , p o , , i p , and d max . k rp is the ratio between the primary ripple current i r and primary peak current i p . and i p is 90% of minimum i limit . from an-16, i i k d p avg rp max = ? () 1 2 and i p v avg o min = by combining the above equations, k rp can be expressed as: k id v p id v rp p max min o p max min = ? 2( ) step 16a, b, c. check k rp against 0.6. k rp 0.6, go to step 17b. k rp < 0.6, set k rp = 0.6. - recalculate d max using step 15 equation. - recalculate v or using step 14 equation. - if v or < 150 v, go to step 17b. - if v or > 150 v, go back to step 7and select higher current tinyswitch . step 17a, b. calculate primary inductance l p . discontinuous mode: l p if z p o ps = ? + 10 1 2 1 09 1 6 2 . () ? continuous mode: l p k k if z p o rp rp ps = ? ? + 10 1 2 1 09 1 6 2 () . () ? i p is 90% of minimum i limit from tinyswitch data sheet as previously defined in step 8. f s is minimum switching frequency from tinyswitch data sheet. please note the cancellation effect between the over temperature variations of i p and f s resulting in the additional 1/0.9 term. z is loss allocation factor. if z = 0, all losses are on the primary side. if z = 1, all losses are on the secondary side. since output diode loss and clamp/snubber loss are both secondary losses, z = 1 is a reasonable starting point. step 18. design transformer. calculate turns ratio n p /n s: n n v vv p s or od = + selecting core and bobbin - with triple insulated secondary wire and no margin winding, ee16 core is suitable for most tinyswitch applications. - to accommodate margin winding, eel16 core must be used. - in below 2 w and/or space constrained applications, ee13 or ef13 cores with special bobbin meeting safety requirements may be used. calculate primary and secondary number of turns for peak flux density (b p ) not to exceed 3000 gauss. limit b p to 2500 gauss for low audio noise designs. use the lowest practical value of b p for the greatest reduction in auido noise. see an-24 for additional information. calculate primary number of turns (n p ) ni l ba pp p pe = 100 where i p equals to maximum i limit
a 7/99 AN-23 9 calculate secondary number of turns n s : n nvv v s pod or = + () calculate gap length (l g ). gap length should be larger than 0.1 mm to ensure manufacturability. la n la ge p pl = ? ? ? ? ? ? ? 40 1000 1 2 please refer to power integrations web site www.powerint.com for audio noise suppression techniques applicable to transformer design. step 19a, b. calculate primary rms current i rms and secondary rms current i srms . discontinuous mode: - calculate primary rms current i rms id i rms max p = 2 3 where i p equals to maximum i limit - calculate secondary rms current i srms ii d k srms sp max dp = ? 1 3 where ii n n sp p p s = and i p equals to maximum i limit continuous mode: - calculate primary rms current i rms iid k k rms p max rp rp = ?+ ? ? ? ? ? ? 2 3 1 where i p equals to maximum i limit - calculate secondary rms current i srms ii d k k srms sp max rp rp = ? () ?+ ? ? ? ? ? ? 1 3 1 2 where ii n n sp p p s = p equals to maximum i limit choose wire gauge for primary and secondary windings based on i rms and i srms . in some designs, a lower guage (larger diameter) wire may be necessary to maintain transformer temperature within acceptable limits during continuous short circuit conditions. do not use wire thinner than 36 awg to prevent excessive winding capacitance and to improve manufacturability. step 20. determine output short circuit current i os . calculate maximum output short circuit current i os from i p and n p /n s , where i p is the maximum i limit from tinyswitch data sheet and n p /n s is the turns ratio from step 18: ii n n k os p p s = where k is the peak to rms current conversion factor the value of k is determined based on empirical measurements: k = 0.9 for schottky diode and k = 0.8 for pn junction diode. check i os against diode dc current rating i d. if necessary, choose higher current diode (see table 3). step 21. determine output capacitor c out . calculate output ripple current: iii ripple srms o =? 22 choose output capacitor with rms current rating equal to or larger than output ripple current. use low esr electrolytic capacitor rated for switching power supply use. examples are lxf series from ucc, pl series from nichicon, and hfq series from panasonic.
AN-23 a 7/99 10 step 22. determine feedback circuit and output post filter. the output voltage of the tinyswitch flyback power supply should be sensed at the first output capacitor, which is before the output post lc filer. this way the output post lc filter is outside the feedback control loop and the resonant frequency of the output post lc filter can be as low as required to meet the output ripple specification requirement. use zener diode in series with the optocoupler led. output voltage v o is determined by vvv o z led =+ where v led 1v replace the zener with a tl431 for better output accuracy. in non-isolated design, use a bipolar npn transistor in place of the optocoupler. replace the led with the base emitter junction and connect the collector to the enable pin of the tinyswitch .
a 7/99 AN-23 11
AN-23 a 7/99 12 korea power integrations international holdings, inc. rm# 402, handuk building 649-4 yeoksam-dong, kangnam-gu, seoul, korea phone: +82-2-568-7520 fax: +82-2-568-7474 e-mail: koreasales@powerint.com world headquarters americas power integrations, inc. 5245 hellyer avenue san jose, ca 95138 usa main: +1 408-414-9200 customer service: phone: +1 408-414-9665 fax: +1 408-414-9765 e-mail: usasales@powerint.com for the latest updates, visit our web site: www.powerint.com power integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. power integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any license under its patent rights or the rights of others. the pi logo, topswitch , tinyswitch and ecosmart are registered trademarks of power integrations, inc. ?copyright 2001, power integrations, inc. japan power integrations, k.k. keihin-tatemono 1st bldg. 12-20 shin-yokohama 2-chome kohoku-ku, yokohama-shi kanagawa 222-0033, japan phone: +81-45-471-1021 fax: +81-45-471-3717 e-mail: japansales@powerint.com taiwan power integrations international holdings, inc. 17f-3, no. 510 chung hsiao e. rd., sec. 5, taipei, taiwan 110, r.o.c. phone: +886-2-2727-1221 fax: +886-2-2727-1223 e-mail: taiwansales@powerint.com europe & africa power integrations (europe) ltd. centennial court easthampstead road bracknell berkshire, rg12 1yq united kingdom phone: +44-1344-462-300 fax: +44-1344-311-732 e-mail: eurosales@powerint.com china power integrations international holdings, inc. rm# 1705, bao hua bldg. 1016 hua qiang bei lu shenzhen, guangdong 518031 china phone: +86-755-367-5143 fax: +86-755-377-9610 e-mail: chinasales@powerint.com india (technical support) innovatech #1, 8th main road vasanthnagar bangalore, india 560052 phone: +91-80-226-6023 fax: +91-80-228-9727 e-mail: indiasales@powerint.com applications hotline world wide +1-408-414-9660 applications fax world wide +1-408-414-9760

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