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NGHIN CU, THIT K B H TR VAN CHO MCH CHUYN I DC/DC S

DNG CU H

Tng quan:

Mch tr gip van l mch c ni thm vo bn cnh van, gip bo v v tng

hiu qu lm vic ca van. Mch h tr van nhm ci thin cht lng ng m ca

MOSFET khi chuyn mch.

Gim, trit tiu cc xung qu p hoc qu dng a im lm vic ca

van v vng lm vic an ton. iu ny gp phn rt ln trong vic chn la thit b

in t cng sut. V khi s gim c gi thnh mt cch ng k.

Hn ch dU/dt, dI/dt.

Truyn nng lng pht nhit ca van sang in tr ngoi hoc sang

hng khc c li.

Gim tn hao cng sut trong qu trnh ng ct.

t c ht cc tiu ch trn tht s l mt vn nan gii i vi vic thit k.

Tuy nhin vn t ra trc tin l cn phi m bo an ton cho van bn dn trong mich lm vic, nhng tc ng khc c th c tnh n v tha hip trong cc iu kin

c th.

Nguyn l:

Vi cc loi van khc nhau v trong cc iu kin khc nhau th c tnh ng ct

ca cc van bn dn cng hon ton khc nhau. Tuy nhin qu trnh ng ct c th a v

bn dng nh sau:

S bin i xung p ni tip (Buck)

S bin i xung p song song (Boost)

S bin i xung p ni tip song song (Buck boost)

Mch nhnh nghch lu na cu (Inverter pole)

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Cc trng thi chuyn mch trong cc s u tng t nhau v vy ch cn gii

quyt vn thit k mch h tr van cho mt mch l c th thit k c mch h tr van

cho cc mch khc. Di y l s bin i xung p song song.

Hnh 2: S bin i xung p song song

a) Mch khi van m; b) Mch rt gn;

c) Dng sng ca in p v dng trng thi l tng

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Trong trng hp l tng, khi van m, dng in s tng tuyn tnh t 0 n Io.

Khi dng t n gi tr Io it mi kha li, khi in p trn van mi gim t E n 0.

Khi van kha li, qu trnh xy ra theo chiu ngc li. Trn th, c th thy r tn ti

nhng thi im m c in p trn van ln dng in u c gi tr ln gy tn tht trong

qu trnh chuyn mch. Nu tnh n c nhng yu t thc t nh in cm v t in k

sinh th dng in v in p qua van c dng nh sau:

Hnh 3: Dng dng in v in p trn van khi tnh n cc thnh phn in cm v t k

sinh trong s .

Chnh v th, thit k mch h tr van l mt vic ht sc quan trng trong qu trnh

thit k mch in t cng sut. C rt nhiu loi mch h tr van nhng ph bin nht l

hai loi: mch RC v mch RCD.

Mt s loi mch h tr van thng dng

Mch RC:

Gi s c mch van ng ct dng in Io di in p Eo, ni tip van l in cm

Lp, t k sinh ca bn thn van l Cp nh hnh 4a. Vic mc thm mch (R nt C) song song

vi van nh hnh 4b nhm lm gim tn tht trong qu trnh ng ct v gim xung p trn

van.

a) b)

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Hnh 4

dao ng sinh ra trong chuyn mch l tt dn th Cs > Cp, cch la chn tt nht

l bng 2 ln Cp. Rs c chn sao cho Rs = Eo / Io. Nh vy, khi van ng, dng c th

chuyn sang mch tr gip m in p trn van khng vt qu Eo.

Khi van m, t Cs phng in qua in tr Rs qua van. Dng in ny c nh kh

ln tuy nhin thi gian rt ngn nn khng gy nguy hi nhiu v pht nhit trn van.

Nhng dng in phng ca t gy phc tp cho vn di/dt v lm tng thi gian m ca

van.

Cng sut tiu tn trn in tr Rs c th c xc nh qua nng lng tch tr

trong t Cs:

2

0

1W2

c SC E=

Nng lng ny tiu tn trn Rs trong qu trnh ng ct, v vy nu tn s ng ct

l fs th cng sut tiu tn trn Rs s l:

2

0

1

2R S SP C E f =

Mch RC n gin c ng dng rng ri, ch yu trong s cng sut nh v va.

i vi cc phn t khng iu khin nh it hoc iu khin khng hon ton nh tiristo,

triac, mch RC cn c ti u ha bng cch chn gi tr ca Lp bng thc nghim.

Khi cng sut ln hn v nht l i vi cc phn t iu khin hon ton, ch

ng thi im kha ca van nh GTO, MOSFET, BJT, IGBT th cn s dng cc mch

tr gip phc tp hn. Ph bin l mch RCD c gii thiu sau y.

Mch tr gip RCD

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Khc vi mch RC, trong mch RCD, t vn phng in qua tr khi van m nhng

tr khng tham gia vo vn gim xung in p nh trn van v th gi tr ca tr c th

chn linh hot hn. Ngoi ra mch

Mch RCD c nhng u im hn so vi mch RC:

Mch cho php suy gim in p nh v lm gim tn tht ng ct ca van cng

nh tn tht trn mch tr gip.

Van c lm vic trong vng an ton tt hn

T in k sinh trn van cng c s dng nh mt phn ca mch h tr van

Tuy nhin mch c nhc im l dng np t Cs ly thng qua it Ds khng phi

l gi tr ti u do qu in p s ln hn trong mch RC.

Khi van bt u ng, in p trn van tng chm trong khi dng bt u gim v

dng chuyn sang mch it, t v in tr. Nh vy vng an ton v tn hao ng ct

cng gim ng k.

Ty thuc vo gi tr ca Cs m in p E c th tng n Eo trc hay sau khi dng

I v 0. Nu E = Eo trng thi im I = Io th Cs = Cn vi Cn c xc nh nh sau:

0

02

Sn

I tC

E= vi St l thi gian kha ca van.

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C th thy t s Cs/Cn cng ln th tn hao trn mch h tr van cng tng rt

nhanh. V vy tn hao ng ct nh thng thng ngi ta thng chn Cs/Cn = 0,5.

Kh Rs cn chn t phng ht in trong khong thi gian m van, ton. Gi tr ca Rs

c tnh nh sau:

2

onS

S

tR

C=

Thit k:

MOSFET s dng trong n l IRF3205.

T k sinh trn MOSFET Cp = 211 pF

Tn s ng ct van l f = 40kHz

Thi gian m tng van ton = 0,4.ts = 0,4/f

T cc thng s trn, tnh ton c cc gi tr ca RC

Eo = 48 V, Io = 62,5A

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50 0

0 0

62,51,63.10 ( )

2 2 2.48.40000

Sn

s

I t IC F

E E f

= = = =

5 12 61 1 .1, 63.10 211.10 8,15.10 ( ) 8,15( )

2 2S n pC C C F F

= = = =

Chn t 10( )SC F=

6

0, 40,5( )

2 2.40000.10.10

onS

S

tR

C = = =

Chn 2 in tr 1 m mc song song vi nhau

1.0 ; 1.1 ; 1.2 ; 1.5 ; 1.8 ; 2.0 ; 2.2 ; 2.4 ; 2.7 ; 3.3 ; 3.6 ; 3.9 ; 4.3 ; 4.7 ; 5.0 ; 5.1 ;

5.6 ; 6.2 ; 6.8 ; 7.5 ; 8.2 v 9.1

M phng:

Ti liu tham kho:

in t cng sut V Minh Chnh, Phm Quc Hi, Trn Trng Minh

Rudy Severn Design of snubber for power circuit