Bo co tt nghip Nguyn Vn Hiu TH46

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Lp pht E c cng tp cht ln nht, lp gc B c nng tp cht

nh nht. phn bit vi cc loi tranzito khc, tranzito PNP v NPN cn

gi l tranzito lng ni vit BJT (Bipolar Juntion Tranzito).

1.2.1 Nguyn tc hot ng

Trong in t cng sut ngi ta dng ph bin nht loi tranzito NPN.

tranzito cng sut c dng ng ngt dng in mt chiu cng

tng i ln, v vy chng ch lm vic hai trng thi ng v trng thi

m.

tranzito lm vic ngi ta phi a in p mt chiu ti cc cc B

ca tranzito gi l phn cc cho tranzito.

a) b)

Hnh 1.10: S phn cc ca tranzito npn (a) v pnp (b) ch khuch i

phn tch nguyn l lm vic ta ly tranzito pnp lm v d. Do JE

phn cc thun nn cc ht a s (l trng) t min E phun qua JE to nn

dng emit (IE). Chng ti vng baz to thnh ht thiu s v tip tc khuch

tn su vo vng baz hng ti JC. Trn ng khuch tn mt phn nh b

ti hp vi ht a s ca baz to nn dng in cc baz (IB). Do cu to

min baz mng nn gn nh ton b cc ht khuch tn ti c b ca JC v

b trng gia tc (do JC phn cc ngc) cun qua ti c min colect to

nn dng in colect (IC). Qua vic phn tch trn ta c mi qua h v dng

in trong tranzito: IE= IB + IC (1-3)

nh gi mc hao ht dng khuch tn trong vng baz ngi ta

nh ngha h s truyn t dng in ca tranzito.

Bo co tt nghip Nguyn Vn Hiu TH46

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= CE

II

(1- 4)

H s xc nh cht lng ca tranzito v c gi tr cng gn mt vi cc tranzito loi tt.

nh gi tc dng iu khin ca dng in IB ti dng colect (IC),

ngi ta nh ngha v h s khuch i dng in ca tranzito. = C

B

II

(1- 5)

thng c gi tr trong khong vi chc n vi trm. T cc biu thc trn ta c mi quan h gia cc h s:

IE= IB(1+) (1- 6) v =

1+ (1- 7)

u im ni bt ca tranzito l ch cn iu khin dng IB l c th iu

khin cho tranzistor ng ngt d dng.

1.2.3 Cch thc iu khin tranzito

Gi IC l dng colect chu c in p bo ho VCEsat khi tranzito dn

dng bo ho IB= IBbh v khi kho IB= 0; VCEsat=VCE.

+ Mch tr gip tranzito m

Khi tranzito chuyn t trng thi ng sang trng thi m. Mch tr

gip bao gm cc phn t t in (C), in tr (R2), it(D2)

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Hnh 1.11: Mch tr gip tranzito m

tf: thi gian cn thit IC t gi tr max gim xung 0

Dng in ti I l thi gian chuyn mch ca tranzito rt ngn vy cho

nn dng ti = const.

S kin: VCE = 0

IC = I ID = 0 (1- 8)

Khi cho xung p m tc ng vo cc gc baz ca tranzito dng IC

gim xung khng trong khong thi gian tf. Nu khng c mch tr gip ta

c: I = IC + ID = const (1-9)

Khi gim IC th ID Tng ln ngang D1 s lm ngn mch ti nng lng

tiu tn bn trong tranzito s l:

. .2

fT

U I tW = (1-10)

Chnh v vy ta phi mc thm mch tr gip m cho trazito.

I= IC ID = const Khi IC bt u gim th I1 cng bt u tng(IC v I1 phi tuyn vi nhau,

lc ny t in C c np in)

Vc Ct

d I Id C

= (1-11)

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Khi t = tf ; Ic = 0 Vc (tf)= V0 = VCE

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nng hn ch s tng vt ca dng IC trong khong thi gian ng Ton ca

tranzito.

Ton: l thi gian cn thit VCE gim t in p ngun VCC xung VCE 0. Thi gian tng cng cho qa trnh ng l tf.

in cm L c tnh theo cng thc:

di i I URL U L L Ldt t R I

= = = = (1- 14) chn L ta chn thi gian ng tr trong khong: 2ton < tr < 5ton

in tr R4 c tc dng hn ch dng do sc in ng t cm trong

cun cm (L) to ra trong mch L; D5; R4 trong khong thi gian tc chuyn

sang trng thi m ca tranzito.

Nh vy tc phi tho mn iu kin.

4

citR

> (1-15) in tr R5 c tc dng hn ch dng in phng ca t in C trong

mch vi khong thi gian ng tf.

Ta c D6: To mch i vi xung p dng t vo cc gc baz

D5: hn ch dng iu khin cho cc gc (baz)

D4: Dng chng bo ho

1.2.4 ng dng ca tranzito cng sut

+ Mch khuch i

Hnh: 1.13: Tranzito lm vic ch khuch i

- Trong thc t tranzito thng c lm vic ch kho

- Khi dng cc gc bng khng dng in cc ghp bng khng,

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tranzito lc ny h mch hon ton.

- Khi dng in cc gc c gi tr bo ho th tranzito tr v trng thi

dn hon ton.

1.2.5 Cc thng s k thut c bn ca tranzito

- khuch i dng in c tr s thay i theo dng IC. Khi dng IC nh th thp, dng IC

tng th tng n gi tr cc i nu tip tc tng IC n mc bo ho th gim.

= CB

II (1-16)

- Dng in gii hn

Dng in qua tranzito phi c gii hn mc cho php nu qu tr

s th tranzito s b h.

ICmax : l dng in ti a cc colect

IBmax: l dng in ti a cc baz

- in th gii hn

in th nh thng BV (breakdown Voltage) l in th ngc ti a

t vo gia cc cp cc.

- Tn s ct

Tn s thit on (f cut- off) l tn s m tranzito ht kh nng khuch

i lc in th ng ra bng in th ng vo.

Bng 1.2 Gii thiu mt s loi tranzito

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VCE VCE0 VCE,sat IC I tf ton ts Pm M hiu

V V V A A s s s W BUV, (BUX)20

21

22

23

24

BUT 90

91

BUX 47

47A

48

48A

98

98C

ESM 3000

3001

3002

3004

3005

3006

3007

160

250

300

400

450

200

300

850

1000

850

850

850

1200

200

200

250

600

600

1000

1000

125

200

250

325

400

125

200

400

450

400

400

400

700

100

150

200

400

500

600

700

1,2

1,5

1,5

1

1

1,2

1,2

1,5

1,5

1,5

1,5

1,5

1,5

1,5

1,5

1,5

1,5

1,5

1,5

1,5

50

40

40

30

20

50

50

9

9

15

15

30

30

150

150

140

120

120

50

50

5

3

2,5

3,2

2,4

7

4

1,2

1

2

2

4

3

15

15

28

13

10

7

6

0,3

0,12

0,5

1,2

1,4

0,4

0,3

0,8

0,8

0,8

0,8

0,8

0,8

0,5

0,5

0,7

1

1

1,5

1,5

1,5

1,8

1,3

1,3

1,6

1,2

1

1

1

1

1

1

1

1,5

1,5

1,5

1,5

1,5

1,5

1,5

1,2

1,2

2

2,5

3

1,5

1,5

3

3

3

3

3

3

1,8

1,8

2

3,5

3,5

5

5

250

(50)

50

50

50

50

250

250

125

125

175

175

250

250

400

400

400

400

400

300

300

1.3 Thyristor

1.3.1 Cu to

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Thyristor cn c gi l SCR (Silicon controlled Rectifier) b nn in

c iu khin bng cht silicum.

Thyristor l linh kin bn dn gm bn dn gm 4 lp P- N- P- N ghp

ni tip to nn 3 cc Anode k hiu l A dng cc, Catode k hiu l K m

cc v cc Gate k hiu l G l cc iu khin hay cc ca.

J1, J2, J3 l cc mt ghp.

Hnh 1.14: Thyristor a- S cu trc bn trong

b- K hiu c- Cc loi thyristor

1.3.2 Nguyn l lm vic

Ty theo cch ni ca A v K ca thyristor vi ngun in mt chiu

m thyristor c th c phn p ngc hay phn p thun.

Khi phn p ngc (ant ni vi cc m ngun, catt ni vi cc dng

ngun) nh hnh1.15 th lp phn cc J2 phn cc thun (in tr rt nh)

nhng cc lp tip xc J1 v J3 li phn cc ngc (in tr rt ln) khng c

dng in qua t K sang A. Ph ti (bng n) khng c dng in chy qua

v khng sng. Thc s th vn c mt dng in r rt nh, khng ng k c

vi mA. c tnh V- A khi phn p ngc l nhnh thuc gc phn t th III.

c

b

a

c

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Khi in p ngc tng n mt tr s no ln (Uct) th thyristor

b chc thng ging nh trng hp ca it v kt qu l dng in ngc

tng ln rt nhanh v mnh.

Khi phn p thun (ant ni vi cc dng ngun, catt ni vi cc m

ngun) nh hnh 1.15 th cc lp J1 v J3 c phn cc thun, in tr rt

nh, nhng lp J2 li b phn cc ngc, c in tr rt ln. Do vy, trng

hp ny cng ch c mt dng in r rt nh chy qua lp J2 (thuc gc phn

t th I).

Hnh 1.15: S phn p ngc v thun ca mt thyristor

Thyristor khc vi it ch: it dn in ngay sau khi phn p thun,

cn thyristor c phn p thun cng cha dn in. Mun cho thyristor thng

khi c phn p thun cn phi c iu kin. iu kin g? l phi cp mt

xung p dng vo cc iu khin G khi thyristor c phn p thun. Xung

dng iu khin c th c to ra mt cch n gin nh ng cng tc K

Hnh 1.16

Bo co tt nghip Nguyn Vn Hiu TH46

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Hnh 1.16: S nguyn l iu khin thyristor

Khi , lp tip xc J3 c phn p thun thm trc tip bi ngun Eg

nn dng in qua lp J3 tng mnh. Cc in t t cc ngun ngoi qua N2

chuyn dch sang P2 vi ng nng ln. Mt phn v cc G hnh thnh dng

iu khin Ig, phn khc ln hn, vt qua lp J2 vo N1 ri qua P1 v ngun

to ra dng Ia. Khi cc in t lp J2 vi ng nng ln s bn ph cc nguyn

t trung ha trong lp tip xc, to ra cc in t t do khc. S in t mi

li bn ph tip cc nguyn t trung ha khc.... c nh th, s in t t do

tng ln rt nhanh, s cc phn t dn in tng vt, in tr trong cng in

trng ro th gim mnh v dng in qua thyristor tng vt. im lm vic

chuyn t T1 sang T2 ri T hnh 1.17. Thyristor trng thi thng.

Tr s dng in Ia ph thuc vo in tr trong mch ph ti ( hnh:

1.16 dng Ia ph thuc vo in tr ca bng n).

Khi thyristor thng in tr trong R13 ca n rt nh (c vi phn trc

hoc phn trm ca mt m) nn st p U13 khng ng k (khng qu 1V). Khi thyristor thng, dng iu khin khng cn tc dng g v c ct

dng iu khin th thyristor vn thng. Nguyn do v dng Ia qua lp J2 s

tip tc lm in tr lp J2 gim thp v duy tr s dn in. Qua lp ny t N1

sang P2.

Nu khi cho xung dng iu khin vo cc G kch thng thyristor

m in p thun gim thp, on OT1 tr thnh OT1, OT1.... Th cn phi

tng dng iu khin ln hn Ik1 > Ik1> Ik1. Khi dng iu khin tng ti