LI NI U...................................................................................................................2 Chng 1. Tm hiu v PFC Power Factor Correction...................................................2 1.1. Th no l Power Factor:........................................................................................2 1.2. Ti sao cn phi iu chnh h s cng sut:..........................................................3 1.2.1. Gim gi thnh nng lng in v ph truyn ti:.........................................3 1.2.2. Ti u ha kinh t k thut:.............................................................................3 1.3. Nguyn tc c bn iu chnh h s cng sut PFC:.........................................4 1.3.1. iu chnh h s cng sut tuyn tnh:............................................................4 1.3.2. iu chnh h s cng sut phi tuyn tnh:......................................................4 Chng 2. Tm hiu vi mch PFC UC3854......................................................................9 2.1. Cc c im k thut chnh:.................................................................................9 2.2. S khi v s ni chn:..............................................................................10 2.3. S in hnh s dng vi mch UC3854:..........................................................11 Chng 3. Th nghim, phn tch mt b ngun thc t................................................12 3.1. Nhim v:.............................................................................................................12 3.2. M t cu trc:......................................................................................................12 3.3. Lp rp ti:............................................................................................................14 3.3. Th nghim v nh gi kt qu:.....................................................................15 TI LIU THAM KHO...............................................................................................20
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LI NI UHin nay, do ngun ti nguyn thin nhin ngy cng cn kit do s khai thc v nhu cu ca con ngi nn vn tit kim nng lng ang c t nn hng u i vi ngnh cng nghip. B ngun hiu sut cao cng l mt cc tit kim nng lng. Trong dp thc tp tt nghip em c lm v ti Tm hiu v bngun hiu sut cao cho thit b chiu sng dng LED. ti ny gip em hiu r thm ngnh in hn cng nh gip em b p thm mt s kin thc thiu ht. Em xin chn thnh cm n thy Nguyn Danh Huy v cc thy c trong b mn gip em hon thnh bo co ny ! H ni ngy 21, thng 2, nm 2012. Sinh vin: Nguyn Th Dip.
Chng 1. Tm hiu v PFC Power Factor Correction.1.1. Th no l Power Factor:-Power factor l ch vit tt ca thut ng H s cng sut, v c k hiu l cos. - Trong h thng in xoay chiu AC c 3 loi cng sut: + Cng sut tiu th thc P (W). + Cng sut phn khng Q (VAR). + Cng sut biu kin S (VA). 2
Hnh1.1. Tam gic cng sut. - T l gia cng sut tiu th tht v cng sut phn khng c gi l h s cng sut. Trong trng hp dng in AC l dng sng sin thun ty, h s cng sut l cosine ca gc pha () gia dng in v in p ca dng sng sin, v l do ny trong cc ti liu k thut ngi ta thng vit tt h s cng sut l cos. H s cng sut khng c n v ring, gi tr ca n c th hin t 0 n 1. c th hin bng cng thc: Cos = P/S - H s cng sut bng 1 khi dng in v in p cng pha. H s cng sut bng 0 khi in p v dng in lch pha nhau 900, y cng sut phi c th hin dng in nhanh hay chm pha hn so vi in p. - Mun nng cao cng sut tht P th cn nng cao h s cos.
1.2. Ti sao cn phi iu chnh h s cng sut:1.2.1. Gim gi thnh nng lng in v ph truyn ti: - Nng cao h s cng sut em li nhng u im v k thut v kinh t, nht l gim tin in. Thc t cho thy cng ty cung cp in bn in cho ngi dng di hai gi tr l in p v dng in (VA), nhng ha n li c tnh bng Watt. Nu h s cng sut ca thit b c gi tr thp hn 1 th cn phi c nhiu cng sut VA c truyn i c th p ng c cng sut Watt tht, ngoi ra n cn lm tng chi ph thc hin vic truyn dn in. 1.2.2. Ti u ha kinh t k thut: - Ci thin h s cng sut cho php ngi s dng my bin p, thit b ng ct v cp nh hn vvng thi gim tn tht in nng v st p trong mng in. - H s cng sut cho php ti u ha cc phn t cung cp in. Khi y cc thit b in khng cn nh mc d tha. V mt k thut, h s cng sut l t s gia cng sut tc dng v cng sut biu kin. Cng sut tc dng l cng sut thc, cn cng 3
sut biu kin thng ln hn cng sut thc, l cng sut truyn ti trn ng dy in n thit b s dng. - H s cng sut cng ln th cng sut tc dng cng gn bng cng sut biu kin, hiu qu s dng cng cao hn.
1.3. Nguyn tc c bn iu chnh h s cng sut PFC:- B PFC c lp t ti v tr gia ngun cp ( sau chnh lu) v ti mt chiu, c tc dng theo di h s cng sut ca ti v t ng iu chnh in p v dng in lun ng pha (cos = 1). ng thi n cn c tc dng n nh in p u ra, lm tng tnh n nh ca h thng, x l cc thay i din ra pha ngun cp v pha ti mt chiu, thng bo v tc ng khi xy ra s c. - Trong mch PFC, vi in p u vo l mt pha th sau khu chnh lu ta dng cc mch bm p iu chnh in p u ra, h s cng sut ca mch.
1.3.1. iu chnh h s cng sut tuyn tnh: - iu chnh PFC truyn tnh p dng cho cc thit b tiu th trc tip in p li. Vic iu chnh c th t c bng vic thm vo hay bt ra cc cun dy hay t in cho thit b. Nh ng c mang tnh cm khng c th iu chnh PFC bng vic u thm mt t song song cun dy vn hnh nhm gip trit tiu cng sut phn khng, lm gim cng sut biu kin v tng h s PF. - Thit b iu chnh h s cng sut thc cht l mt thit b cung cp mt cng sut phn khng tng ng v i nghch li vi cng sut phn khng c to ra ca thit b. Thm t in hay cun dy vo qu trnh hu b i hiu ng cm ng hay in dung tng ng c to ra. ng c c tnh cm ng c th c b bng cc t lc, l h quang in c tnh in dung c th b bng cc cun dy. - Khi thm vo hay ly ra cc thit b b cng sut phn khng c th to ra s bin ng in p hay to ra cc mo hi, trong trng hp xu nht cc thnh phn b cng sut phn khng c th to ra hin tng cng hng vi h thng c b, lm cho in p tng cao v gy mt n nh cho h thng. Do vy vic iu chnh h s PFC khng th n gin l vic thm hay bt cc thnh phn, m n cn c tnh ton k ph hp vi tng mc cng sut ti trn thit b. 1.3.2. iu chnh h s cng sut phi tuyn tnh: - Ti phi tuyn thng l dng ti chnh lu, khng s dng trc tip t in xoay chiu m nn li thnh dng in mt chiu-chnh lu nh cc b ngun my tnh (PSU), adaptor,hay cc thit b s dng nng lng gin on-lin tc nh my hn, bng n hunh quanh,..,cc thit b ny trong qu trnh tiu th nng lng cn to ra cc dng sng hi c tn s l bi s ca tn s in li, chn vo tn s in li. Cc thnh phn linh kin tuyn tnh nh cun dy v t in khng th loi b c cc di 4
tn s mi c to ra ny, v vy n phi dng cc b lc hay b iu chnh h s cng sut c th lm phng dng in ra trn mi chu k nhm gim dng hi. - Trong cc loi ti phi tuyn tnh th PSU c s dng nhiu nht, vi thit k chuyn i nng lng theo kiu ng/ct (switching). Trc y cc b ngun ny ch n gin c thit k vi mt cu nn in chnh lu ton sng np mt mc in p di mc chu ng c ca t in. iu ny s to ra mt dng in np ban u rt cao, h s cng sut rt thp, ng thi to ra cc sng hi khng c li. 1.3.2.1. iu chnh h s cng sut th ng Passive PFC: -Phng php Passive PFC n gin ch l s dng mt b lc, b lc ny ch cho qua dng in c tn s bng vi tn s in li (50Hz hoc 60Hz) v chn khng cho cc tn s sng hi i qua. Lc ny ti phi tuyn tnh c th xem nh mt ti tuyn tnh, h s cng sut c nng cao hn. -Tuy nhin yu cu cn phi c cun cm c gi tr cm khng ln lm cho b lc cng knh v c gi thnh cao, nhng thc t vi mch Passive PFC c cun dy tuy ln hn cun dy ca mch iu chnh h s cng sut tch cc Active PFC nhng gi thnh chung li r hn. y l mt phng php n gin v r tin iu chnh h s cng sut v lm gim sng hi tuy nhin n li khng hiu qu bng phng php iu chnh h s cng sut tch cc Active PFC v cun dy c kch thc c nh nn gii in p hot ng khng cao. 1.3.2.2. iu chnh h s cng sut tch cc Active PFC: -L mt h thng in t cng sut c chc nng kim sot nng lng cung cp cho ti, iu chnh h s cng sut mc tt nht trn mi mc ti. Trong thit k thc t, mch Active PFC iu khin dng np cho ti sao cho dng sng ca dng vo cng pha vi dng sng u vo ( y l sng sin). Thc cht n chnh l mch bm p c th dng trong b PFC. V c bn c 3 dng mch Active PFC c s dng, l; Boost, Buck v Buck-Boost. - Mch Boost ( Boost PFC):
Hnh1.2. S mch Boost. 5
Khi van ng : Diode D kha do b phn cc ngc bi t C0. Nng lng trong t C0 x qua ti, dng in t dng ngun qua cun L qua van ri tr v m ngun. Khi van ct: Diode D phn cc thun dn dng t ngun qua cun L np cho t C 0 vi cc tnh nh hnh v. Cun L i cc tnh, in p np cho t C0 l VC0 = Vs + VL. Do vy in p ra ca mch Boost cao hn in p u vo.
Hnh1.3. Dng sng in p v dng in vo b Boost PFC.
- Mch Buck (Buck PFC):
Hnh 1.4. S mch Buck. Khi van ng: Diode kha do b phn cc ngc, dng in t dng ngun qua L np cho t . C0 Khi van ct: Cun L o cc tnh, diode D dn, dng in qua ti bng tng dng in qua L v dng in qua t C0: Iti = IL + IC0in p trn t C: Uc = Vs - VL = Uti. in p ra ca b Buck lun nh hn in p ngun cp.
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Hnh 1.5. Dng sng in p v dng in vo b Buck PFC. - Mch Buck/Boost (Buck/Boost PFC):
Hnh 1.6. S mch Buck/Boost. Khi van ng: in p trn cun L bng in p ngun. Diode D kha do b phn cc ngc. Dng in qua ti c cung cp bi t C0. Khi van ct: in p trn L o cc tnh, diode D dn dng khp mch L C 0 D L. T C0 c np chun b cho chu k phng khi van ng. in p ra trong trng hp ny c th ln hoc nh hn in p ngun cp. -Trong PSU, dng mch c s dng thng dng nht l Boost. Mt mch chuyn i c chn vo gia cu nn in v t lc chnh. N to mt in p DC n nh u ra v duy tr dng in vo lun ng pha vi tn s ca in p vo. Phng php ny i hi phi thm mt s linh kin chuyn mch bn dn cng sut v mch iu khin nhng b li n c kch thc nh hn mch Passive PFC. -Dng mch iu chnh h s cng sut Active PFC c th hot ng trn mt di in p vo rt rng, t 90VAC n 264VAC, nn c s dng rng ri, ngi dng khng cn quan tm ti mc in p ph hp vi PSU ti khu vc mnh ang , ngoi ra n cn gip PSU hot ng c nhng khu vc c in p AC khng n nh.
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Hnh 1.7. Mch Passive PFC thc t trong PSU.
Hnh 1.8. Mch Active PFC thc t trong PSU.
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Chng 2. Tm hiu vi mch PFC UC3854.2.1. Cc c im k thut chnh:a. c im: - S dung phng phap iu ch rng xung PWM tng h s cng sut ln ti 0,99. - Giam thiu meo dong xung con nho hn 5%. - iu khin toan mach ma khng cn n b ngt mach. - iu chinh s cp dong. - Kim soat ch dong in mc trung binh. - Nhay cam vi ting n nho. - Cp dong khi ng thp. - Phng phap PWM gi tn s n inh. - B chia/b nhn bu tin hiu tng t mc thp. - Chun in ap chnh xc. b. M ta chung v UC3854: - UC3854 cho phep iu chinh h s cng sut cua ngun cp ma nu khng thi dong nhn c se khng sin t ngun dong sin. Thit bi nay thc hin tt ca cac chc nng iu khin cn thit tao nn ngun cp co cng sut ti a t ngun cp sn co, ng thi giam thiu ti mc thp nht co th s meo dong. lam c iu nay, UC3854 phai co mt b khuych ai in ap, mt khi b chia/b nhn tin hiu tng t, mt b khuych ai dong in, mt b gi tn s n inh s dung phng phap PWM. Ngoai ra, UC1854 con phai co mt b iu khin cng tng thich ngun cho MOSFET vi in ap chun la 7,5V, b oan trc dong, b so sanh tai cho phep, b do ngun cp thp, b so sanh qua dong. -UC3854 s dung phng phap iu khin dong trung binh at c s iu khin dong in va tn s n inh vi n inh cao va meo thp. Khng ging vi ch dong in cc ai, phng phap iu khin dong trung binh duy tri chinh xac dong in hinh sin ma khng cn bu h s goc phan hi nho giam qua trinh qua . -UC3854 co chun in p cao va bin dao ng cao lam giam thiu ti mc thp nht nhay cua ting n, trong khi o, b phn iu bin rng xung PWM cho phep bm tn s trn 200kHz. UC3854 co th s dung ngun 1 pha hoc 3 pha vi in ap khoang t 75 275V va dai tn s lam vic la 50400Hz. giam thiu tai trong trong mach ma ngun cp phai cp, thi c im cua UC1854 la dong khi ng thp. - Nhng chi tit tao nn UC3854 c tich hp sn trong mt khi ni tip nhau gm 16 chn nha va s.
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2.2. S khi v s ni chn:
Hnh 2.1 - S khi.
Hnh 2.2 - S chn.
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2.3. S in hnh s dng vi mch UC3854:
Hnh 2.3 S in hnh s dng vi mch UC3854.
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Chng 3. Th nghim, phn tch mt b ngun thc t.3.1. Nhim v:Nghin cu, phn tch mt b ngun thc t Tm hiu v b ngun hiu sut cao cho thit b chiu sng LED c sn xut hng lot trn thc t.
3.2. M t cu trc:
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3.2. Lp rp ti T mch nguyn l ta thit k c mch nh sau
Hnh 3.1. S mch cu trc. Cu trc mch gm hai tng: - Mch Boost vi khu AC DC: Ngun c cp t in li xoay chiu, qua cu chnh lu thnh ngun mt chiu, in p mt chiu c san phng nh t C1. Sau c a qua cun dy th nht (1 3), qua diode D3 np cho t C5. Khu to ngun cp ngun cho vi x l qua chn 8. Khi (C2 // R5) phn hi in p v chn s 3 ca vi x l. Khi in tr shunt ( R7 // R8) o dng phn hi v chn s 4 ca vi x l. Khi [ ( R11 ni tip R12) ni tip ( R9 // R10)] l mch phn hi in p u ra v chn s 1. Khi ( C3 // ( R4 ni tip C4 ) l cm mch lc. Mch Buck vi khu DC DC:
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Khi ( R13 // R14 // R15) l khi in tr shunt o gi tr dng in ri phn hi v chn s 4 bo v qu in p cho Mosfet Q2. Mosfet Q2 iu khin chn s 7 ca vi x l. Khi (C7 // C8) lc ngun u vo chn 8 ca vi x l. Khi [ (C6 // R18 ) ni tip(C7 // R17) ] l mch to tn s bm xung PWM.
3.3. Lp rp ti:T mch nguyn l ta thit k c mch nh sau
Hnh 3.2 Mch thc t.
Hnh 3.3 Ti. in p ra ti l 350V, dng in qua ti trong khong 0,20,3A nn gi tr in tr c tnh theo cng thc:R1 = 350 = 1750 () 0.2 R2 = 350 = 1167 () 0.3
Gi tr in tr trong khong 11671750 . Ta chn 17 in tr 10W, 100 nn tng tr ti l 1700 14
Khi dng in thc t trn ti o c l 0,234A. 3.3. Th nghim v nh gi kt qu: 3.3.1. in p u vo v u ra ca cun dy
Hnh 3.4 . Cun dy tng boost v dng in p vo cun dy (sau chnh lu).
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Hnh 3.5. Cun dy tng buck.
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in p ra cun dy. 3.3.3. in p iu khin ca Mostfet:
Hnh 3.6. Xung iu khin ca Mostfet.
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3.3.4. in p trn ti:
Hnh3.7. Ti v dng in p ra trn ti. in p ra trn ti l dng mt chiu, nhng khi o chu k T = 2ms (tn s cao) nn dng in p khng l ng thng m hi ln sng. 3.3.5. in p ca t:
Hnh 3.8. T. in p ca t l in p mt chiu c gi tr 403404 V 3.3.5. in p v dng in u vo ngun 19
in p v dng in u vo ngun cng chn ni t nn c im chung nh hnh di y:
Hnh 3.9. Dng in p v u vo ngun.
TI LIU THAM KHO1. http://en.wikipedia.org/wiki/Power_factor. 20
2. Datasheet v UC3854 ca hng Texas Instrument Incorporeted.
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