Application Notes for AP3772 System Solution

Transcription

1 lcaon oe 08 lcaon oes for 377 ysem oluon reared by Zhao Jng Jng ysem Engneerng De.. nroducon The 377 uses ulse Frequency Modulaon (FM) mehod o realze Dsconnuous Conducon Mode (DCM) oeraon for Flyback ower sules. The oerang rncle of FM s dfferen wh ulse Wdh Modulaon (WM), so he desgn of ransformer s also dfferen. The 377 can rovde accurae consan volage (C), consan curren (CC) regulaon wh rmary de egulaon () srucure. uses nernal lne comensaon and cable comensaon o reduce he F number of exernal sysem comonens. Fxed cable comensaon s used n dfferen C versons o ada he dfferen volage dro on ouu cable and good C regulaon s acheved. Besdes, audo nose s reduced by he creave audo suresson echnque. The 377 s desgned for drvng bolar ranssor n Flyback converer, wh more drvng curren of abou 40m. Wh sysem arameers roerly desgned, 377 can acheve sandby ower less han 50mW. T D C BG T C s C C T D Da a C UT C UT DUMMY C 5/. C CC Q a U 377 CC CC UT C CC GD C E C Fgure. Tycal lcaon Crcu of 377 Fgure s he ycal alcaon crcu of 377, whch s a convenonal Flyback converer wh a 3-wndng ransformer---rmary wndng ( ), secondary wndng ( ) and auxlary wndng ( ). The auxlary wndng s used for rovdng CC suly volage for C and sensng he ouu volage feedback sgnal o n. Fgure shows he ycal waveforms whch demonsrae he basc oerang rncle of 377 alcaon. nd r. 0 ev.. 0 he arameers are defned as followng. dr ---The drvng sgnal of rmary ower swch ---The rmary sde curren s ---The secondary sde curren K ---eak value of rmary sde curren K ---eak value of secondary sde curren EC ---The ransen volage a secondary wndng ---The sable volage a secondary wndng when BCD emconducor Manufacurng med

2 lcaon oe 08 recfcaon dode s n conducng saus, whch equals he sum of ouu volage UT and he forward volage dro of dode UX ---The ransen volage a auxlary wndng --- The sable volage a auxlary wndng when recfcaon dode s n conducng saus, whch equals he sum of volage CC and he forward volage dro of auxlary dode W ---The erod of swchng frequency ---The conducon me when rmary sde swch s ---The conducon me when secondary sde dode s FF ---The dead me when neher rmary sde swch nor secondary sde dode s FF --- The me when secondary sde dode s FF W dr K K FF UX EC FF Fgure. eraon Waveforms of Flyback Conrol ysem. Gudelne of ysem Desgn. ow andby ower Desgn. wchng Frequency Desgn 3. Transformer and ower Devces Desgn 4. Feedback essors Desgn 5. ne Comensaon Desgn 6. Cable Comensaon Desgn. ow andby ower Desgn n order o acheve low sandby ower, 377 decreases he mnmum oerang volage. nd he saru ressors T T should be hgh enough o furher lower he ower loss. However, here s a radeoff beween low sandby ower T and small saru me TT, whch s r. 0 ev.. 0 TT ( ) C / () T T vcc TH _ T DC _ M Where TH_T s he aru Threshold of CC, and DC_M s he recfed DC volage from he lowes C nu. Besdes, he selecon of dummy load ressor s a radeoff beween sandby ower and - curve. The recommended value of dummy load ressor DUMMY s 4.7kΩ o 0kΩ for an alcaon wh 5 ouu volage.. wchng Frequency Desgn BCD emconducor Manufacurng med

3 lcaon oe 08 s we know, n DCM Flyback converer, he sored energy of rmary sde wll be ransferred o secondary sde a he me when he rmary swch s urned off. nd assume he curren ransfer effcency from rmary o secondary sη, hen s η () Here, s he urn rao of rmary wndng o secondary wndng. s obvous n Fgure ha he ouu curren s he average curren of secondary sde, o s (3) W Then, (4) W o η lways volage of CC n ( CC ) s deermned by, cc DD (5) W Here DD s a consan volage generaed by C. Then, cc DD (6) η K f η s effcency of ower ransmsson from ransformer T rmary o he ouu, hen f W η (7) T Where, f W s he swchng frequency. o, f W (8) η T When volage a he sense ressor reaches he reference volage se by 377, he swch wll be urned off and rmary curren reaches s maxmum value, cs _ ref K (9) cs When he consan reference C_EF s used, he eak curren K s consan. From formula (6) and (8), s obvous ha CC and f W ncreases lnearly wh he ouu curren. H 0.5 C_EF.4 CC f W 47.6kHz f W 0kHz 4% Fgure 3. elaonsh Beween CC, f W and a Consan eak Curren Mode r. 0 ev.. 0 BCD emconducor Manufacurng med 3

4 lcaon oe 08 n 377, n order o realze audo nose suresson, wo-segmened of curren reference volage C_EF s used. The reference s abou 0.5 when >4%* _MX and s decreased o 0.5/.5 when <4%* _MX, as follows n Fgure 4. D CC H /.5 C_EF 0.4х _MX _MX f W 55kHz 5kHz f W 3.kHz 8.89kHz 3.95kHz 0kHz 0.4х _MX _MX UCE _MX Fgure 4. elaonsh Beween CC, f W and a arable eak Curren Mode Then from formula (6) and (8), we can see he CC and f W boh has a lea a abou 4% of maxmum load. he lea on, f he eak curren s decreased by.5 mes, he volage of CC n a low K wll be ncreased o.5 mes, and he swchng frequency f W a low K wll be ncreased o.5 mes. o he load range n audo s largely narrowed. H 0.5 C_EF 0.5/.5 39% 4% Fgure 5. Hyseress a Converson Beween ow K and Hgh K n order o avod unsable oeraon, a hyseress s added a he converson beween low K and hgh K. Consderng he relaonsh beween audo nose and flux densy of ransformer, delab 500 gauss s beer for audo nose suresson. he 377 can be u o 0kHz. Bu hs s only he lm of he C; he fnally desgned maxmum swchng frequency s deermned by he radeoff beween he effcency, mechancal dmensons and hermal erformance. The low lmaon of maxmum swchng frequency s gven by audo nose suresson. nd he uer lm of r. 0 ev.. 0 BCD emconducor Manufacurng med 4

5 lcaon oe 08.3 Transformer and ower Devces Desgn n he desgn of 377, consan curren conrol funcon wll kee a fxed rooron beween on-me and off-me FF of recfer D (n Fgure ) by dschargng or chargng a caacor embedded n he C. The fxed rooron s W s assumed k W 4 Then he ouu consan-curren value s r. 0 ev.. 0 (0) () K η () K k k.3. Calculae Turn ao of Transformer ( ) The urn rao of ransformer should be desgned frs, whch ensures he ower converer oerang n DCM whn he whole condons, W (3) s we know, f equaon (3) s me a mnmum nu volage and full load, can ensure ha he ower converer oeraes n DCM n all condons. For he rmary sde curren, ndc (4) Where s he nducance of rmary wndng. ndc s he recfed DC volage of nu. When ndc s he mnmum value, he maxmum can be obaned. o, _MX ndc _ mn (5) For he secondary sde curren, s he nducance of secondary wndng, d s he forward volage of secondary dode. There s an oscllang sgnal on waveform afer secondary choky dode curren decrease o zero, whch s caused by rmary nducance and equvalen ouu caacance of rmary swch. Then some margn s added o as s. (6) d From formula (4) and formula (6), we can ge s o s s Then, η W fsw ( ) fsw (7) fsw η η BCD emconducor Manufacurng med (8) (9), and W n (3) are relaced wh (5), (6) and (9), hen η s s. (0) s ndc_mn elaonsh beween nducance of rmary sde and secondary sde s, s () full load, he sysem wll work n he boundary of CC regulaon. can be gven by formula (),he followng can be obaned, _ MX ndc_mn η k (.) () Then desgned urns rao should be no more han _MX defned n formula ()..3. Check ress olage of rmary de wch and everse olage of econdary Dode f s fxed by cusomer accordng o desgn se.3., real sress volage of rmary sde swch and reverse volage of secondary dode can be calculaed. 5

6 lcaon oe 08 The maxmum sress volage of rmary sde swch s, ds _ swch dc_ske ndc_max (3) Be careful ha he value of dc_ske s deermned by he snubber crcu desgn. Maxmum reverse volage of secondary sde, Where, f W was se by he user based on defne requremen. Then, can be goen by, f K W η (7).3.5 Calculae he Turns of rmary, econdary and uxlary (,, ) The urns of rmary wndng, ndc_max dr (4) K K e B e B max (8) For Flyback converer desgn, hgher urns rao brngs hgher sress volage of rmary sde swch, hgher ransformng effcency, and he lower reverse volage of secondary dode. Fnally, n desgn of urns rao, formula (), (3) and (4) should be oally consdered. s and are fxed, we can ge by (9).3.3 Calculae he eak Curren of rmary de and Curren ensed essor ( K & C ) K can be calculaed by he ouu curren. Turns of auxlary wndng s, (30) k (5) η n 377, 0.5 s an nernal reference volage. f he sensed volage C_EF reaches 0.5, he ower swch wll shu down and wll be ended. o C can be obaned by formula (9) and seleced wh a real value from he sandard ressor seres. We recommended usng % olerance ressors for C. fer C s seleced, K should be modfed based on he seleced C..3.4 Calculae he nducance of rmary de--- The rmary sde nducance s relave wh he sored energy. should be bg enough o sore enough energy, so ha _MX can be obaned from hs sysem..3.6 Check he Maxmum Duy Cycle of rmary de fer urn rao of rmary sde and secondary sde s desgned, he maxmum duy cycle of rmary sde a low lne volage can be calculaed agan. Consderng he ol-second balance beween magnezng and de-magnezng, he formula of duy cycle s D ( ) d ons max (3) ndc η sw.3.7 Check everse olage of uxlary Dode f and s fxed accordng o desgn se.3.5, real reverse volage of auxlary dode can be calculaed by formula (3). ccordng o formula (8), he ouu ower can be gven by, f W η (6) dar ndc_max (3) r. 0 ev.. 0 BCD emconducor Manufacurng med 6

7 lcaon oe 08.4 Feedback essors Desgn Fgure 6. Feedback essors Crcu From above Fgure 6, o ( ) (33) D o D (34) Through adjusng and, a suable ouu volage can be acheved. The recommended values of and are whn 5kΩ o 00kΩ..5 ne Comensaon Desgn The nernal lne comensaon funcon n 377 s shown n Fgure 7. s closed when he rmary swch s. The lne volage can be deeced from he n. The deeced volage nernally comensaes he eak curren. o he lne comensaon s deermned by E. n dfferen alcaons, he value of E s dfferen. Fgure 7. ne Comensaon Crcu r. 0 ev.. 0 BCD emconducor Manufacurng med 7

8 lcaon oe 08 Fgure 8. Waveform of n The negave volage of n (n Fgure 8) s lnear o lne volage. The 377 samles o realze he lne comensaon. ndc (35) The comensaed volage of lne comensaon ( C_E ) can be calculaed by he followng formula, C _ E k E (36) Ths s desgned o comensae he addonal volage of C nroduced by delay, whch s he delay me of nernal drvers of C and rmary sde swch. delay dela ndc (37) cs Then E can be adjused o acheve excellen lne regulaon of ouu curren. E delay 0.8 ( cs) /( ) (38) 670k.6 Cable Comensaon Desgn Three versons of C are desgned o mee dfferen requremen for cable volage comensaon. s we know, an ncrease volage a ( _ CBE ) wll nroduce an ncrease volage a UT ( UT _ CBE), whch s a lnear funcon of he ouu load ( UT ). Then n alcaon of 377, CC n deecs he load nformaon and a corresondng dela volage s added o o comensae he volage dro a ouu cable. s defned n daashee below, for examle, n 377, here s a oal volage ncrease of 6% a when he ouu s a full load ( UT _ MX ). nd f he ouu s a 0%* UT _ MX, he ncrease volage of s 0.6%. roer verson of C can be chosen accordng o he ressance of he ouu cable. CBE CMET ECT Cable Comensaon olage % 377B 3 4 % _CBE/ % 377C 0 % ssume % _ CBE (39) Then from Fgure 6, UT _ CBE % _ MX CBE (40) Then afer % s calculaed, roer verson of 377 can be chosen accordngly. % _ MX CBE /( ) (4) Desgn Examle (for 5/. alcaon): ecfcaon: nu volage: 85 C o 65 C uu cable: _CBE 5 uu curren:. uu CB: 5.3, (WG Cable, engh of cable00cm) r. 0 ev.. 0 BCD emconducor Manufacurng med 8

9 lcaon oe 08 her seng by users: wchng frequency: f W 65kHz Forward volage of secondary dode: d 0.4 Forward volage of auxlary dode: da. CC volage: CC 4 Core_ye: M5 (e3.7mm ), Bmax<3000G dc_ske 50 (wh snubber crcu) Desgn es: ) Calculae urn rao of ransformer ( ) ndc_mn η k _ MX (.) 5.8 (4) ndc_mn 40 (43) nac_mn Consderng some margn for Flyback conrol, we choose 5.5. ) Check sress volage of rmary sde swch and reverse volage of secondary dode ccordng o formulas (3) (4) and he seleced, roer ower devces could be chosen. ds _ swch dc_ske ndc_max 50 < 700 (44) ndc_max 9 < (45) dr 40 3) Calculae he eak curren of rmary sde and curren sense ressor ( K & C ) s k 330m (46) η η C C.5 Ω (47) 4) Calculae he nducance of rmary sde--- f. η 9 K W mh (48) 5) Calculae he urns of rmary, secondary and auxlary (,, ) K K T e B e B max 89.8 (49) We choose 93T 6 T (50) 6 T (5) 6) Check he maxmum duy cycle of rmary sde The maxmum duy cycle of rmary sde s calculaed as followng, ( d ) 0.4 D 0.49 (5) η ndc 7) Check reverse volage of auxlary dode ndc_m ax da r 79 (53) 8) Feedback essors o D.56 (54) 4.9kΩ, 9.85kΩ 9) ne Comensaon essors E delay ( cs ) /( 0) Cable Comensaon Choce 0.8 ) k k Ω (55) 4.04, he same n hree versons of 377. Then, % _ MX CBE /( ).4% (56) ccordng o daashee nformaon, 377B s a beer choce. _ F _ ( % ) _ MX CBE (57) Where _ 5. Therefore, he ouu volage a cable ermnal a full load s a lle hgher han he volage a no load. r. 0 ev.. 0 BCD emconducor Manufacurng med 9

10 lcaon oe 08 Desgn esuls ummary:.maxmum eak curren of rmary sde and C K 330 m eak curren of rmary sde C.5 Ω Curren sensed ressor.transformer.90 mh nducance of rmary sde 5.5 Turn rao of rmary and secondary 93 T Turns of rmary sde 6 T Turns of secondary sde 6 T Turns of auxlary sde D MX 0.49 Maxmum duy cycle of rmary sde a DC rmary ower swch and dode ds_swch 50 olage sress of rmary ower swch dr 9 Maxmum reverse volage of secondary dode dar 79 Maxmum reverse volage of auxlary dode 4. olage feedback ressors 4.9k Ω Feedback ressor a usde from auxlary sde o n 9.85k Ω Feedback ressor a downsde from n o GD 5. ne comensaon ressor E 3.4k Ω ne comensaon ressor 6.Cable Comensaon C verson 377B _ 5 uu cable no load _F 5.03 uu cable full load 3. ummary n order o ge good erformance of 377, s moran o correcly desgn sandby ower, swchng frequency, ransformer arameers, feedback ressance and lne comensaon ressance. Ths alcaon noe only gves a relmnary desgn gudelne abou hese asecs and consders deal condons, so some arameers need o be adjused slghly on he bass of he calculaed resuls. r. 0 ev.. 0 BCD emconducor Manufacurng med 0

11 lcaon oe lcaon of 377 wh 4340 F T D C BG T C s C C T UT CC D GD C UT Da a C 5/. C CC Q a U 377 CC CC UT C CC GD C E C Fgure 9. Tycal lcaon Crcu of 377 wh 4340 n rmary de egulaon of 377 alcaon, f 4340 s used a secondary sde as he ouu volage regulaor, excellen dynamc resonse and low sandby ower can be acheved. When deecng he ouu volage lower han a ceran level, he 4340 ouus erodcal sgnals whch wll be couled o auxlary sde and deeced by 377. By fas resonse and cooeraon, 4340 and 377 can effecvely mrove he ransen erformance for rmary de egulaon ower sysem. Besdes, dummy load s no needed a secondary sde and as a resul sandby ower wll be decreased. For more dealed oerang rncles, lease refer o lcaon oe of 4340 (lcaon oe 078_BCD). r. 0 ev.. 0 BCD emconducor Manufacurng med