Mch cu H

I. Mch cu H (H-Bridge Circuit). Gi s bn c mt ng c DC c 2 u A v B, ni 2 u dy ny vi mt ngun in DC (c qui in battery). Ai cng bit rng nu ni A vi cc (+), B vi cc (-) m ng c chy theo chiu thun (kim ng h) th khi o cc u dy (A vi (-), B vi (+)) th ng c s o chiu quay. Tt nhin khi bn l mt control guy th bn khng h mun lm cng vic ng tay ng chn ny (o chiu u dy), bn t s ngh n mt mch in c kh nng t ng thc hin vic o chiu ny, mch cu H (H-Bridge Circuit) s gip bn. Nh th, mch cu H ch l mt mch in gip o chiu dng in qua mt i tng. Tuy nhin, ri bn s thy, mch cu H khng ch c mt tc dng tm thng nh th. Nhng ti sao li gi l mch cu H, n gin l v mch ny c hnh ch ci H. Xem minh ha trong hnh 1.

Hnh 1. Mch cu H.

Trong hnh 1, hy xem 2 u V v GND l 2 u (+) v (-) ca c qui, i tng l ng c DC m chng ta cn iu khin, i tng ny c 2 u A v B, mc ch iu khin l cho php dng in qua i tng theo chiu A n B hoc B n A. Thnh phn chnh to nn mch cu H ca chng ta chnh l 4 kha L1, L2, R1 v R2 (L: Left, R:Right). iu kin bnh thng 4 kha ny m, mch cu H khng hot ng. Tip theo chng ta s kho st hot ng ca mch cu H thng qua cc hnh minh ha 2a v 2b.

Hnh 2. Nguyn l hot ng mch cu H. Gi s bng cch no (ci cch no chnh l nhim v ca ngi thit k mch) m 2 kha L1 v R2 c ng li (L2 v R1 vn m), bn d dng hnh dung c mt dng in chy t V qua kha L1 n u A v xuyn qua i tng n u B ca n trc khi qua kha R2 v v GND (nh hnh 2a). Nh th, vi gi s ny s c dng in chy qua i tng theo chiu t A n B. By gi hy gi s khc i rng R1 v L2 ng trong khi L1 v R2 m, dng in li xut hin v ln ny n s chy qua i tng theo chiu t B n A nh trong hnh 2b (V->R1->B->A->L2->GND). Vy l r, chng ta c th dng mch cu H o chiu dng in qua mt i tng (hay c th, o chiu quay ng c) bng mt cch no . Chuyn g s xy ra nu ai ng ng thi 2 kha cng mt

bn (L1 v L2 hoc R1 v R2) hoc thm ch ng c 4 kha? Rt d tm cu tr li, l hin tng ngn mch (short circuit), V v GND gn nh ni trc tip vi nhau v hin nhin c qui s b hng hoc nguy him hn l chy n mch xy ra. Cch ng cc kha nh th ny l iu i k i vi mch cu H. trnh vic ny xy ra, ngi ta thng dng thm cc mch logic kch cu H, chng ta s bit r hn v mch logic ny trong cc phn sau. Gi thit cui cng l 2 trng hp cc kha phn di hoc phn trn cng ng (v d L1 v R1 cng ng, L2 v R2 cng m). Vi trng hp ny, c 2 u A, B ca i tng cng ni vi mt mc in p v s khng c dng in no chy qua, mch cu H khng hot ng. y c th coi l mt cch thng ng c (nhng khng phi lc no cng c tc dng). Ni chung, chng ta nn trnh trng hp ny xy ra, nu mun mch cu khng hot ng th nn m tt c cc kha thay v dng trng hp ny. Sau khi c bn nm c nguyn l hot ng ca mch cu H, phn tip theo chng ta s kho st cch thit k mch ny bng cc loi linh kin c th. Nh ti trnh by trong phn trc, thnh phn chnh ca mch cu H chnh l cc kha, vic chn linh kin lm cc kha ny ph thuc vo mc ch s dng mch cu, loi i tng cn iu khin, cng sut tiu th ca i tng v c hiu bit, iu kin ca ngi thit k. Nhn chung, cc kha ca mch cu H thng c ch to bng r le (relay), BJT (Bipolar Junction Transistor) hay MOSFET (Metal Oxide Semiconductor Field-Effect Transistor). Phn thit k mch cu H v vy s tp trung vo 3 loi linh kin ny. Trong mi cch thit k, ti s gii thch ngn gn nguyn l cu to v hot ng ca tng loi linh kin bn c d nm bt hn. II. Mch cu H dng r le. R le l mt dng cng tc (switch) c in (electrical mechanical device, khng phi c in t u nh :) ). Gi l cng tc c in v chng gm cc tip im c c iu khin ng m bng dng in. Vi kh nng ng m cc tip im, r le ng l mt la chn tt lm kha cho mch cu H. Thm na chng li c iu khin bng tn hiu in, ngha l chng ta c th dng AVR (hay bt k chip iu khin no) iu khin r le, qua iu khin mch cu H. Hy quan st cu to v hnh dng ca mt loi r le thng dng trong hnh 3.

Hnh 3. Cu to v hnh dng r le. Hnh 3a (pha trn) m t cu to ca 1 r le 2 tip im. C 3 cc trn r le ny. Cc C gi l cc chung (Common), cc NC l tip im thng ng (Normal Closed) v NO l tip im thng m (Normal Open). Trong iu kin bnh thng, khi r le khng hot ng, do lc ko ca l xo bn tri thanh nam chm s tip xc vi tip im NC to thnh mt kt ni gia C v NC, chnh v th NC c gi l tip im thng ng (bnh thng ng). Khi mt in p c p vo 2 ng kch Solenoid (cun dy ca nam chm in), nam chm in to ra 1 lc t ko thanh nam chm xung, lc ny thanh nam chm khng tip xc vi tip im NC na m chuyn sang tip xc vi tip im NO to thnh mt kt ni gia C v NO. Hot ng ny tng t 1 cng tc chuyn c iu khin bi in p kch Solenoid. Mt c im rt quan trng trong cch hot ng ng m ca r le l tnh cch li. Hai ng kch nam chm in hon ton cch li vi cc tip im ca r le, v v th s rt an ton. C 2 thng s quan trng cho 1 r le l in p kch Solenoid v dng ln nht m cc im im chu c. in p kch solenoid thng l 5V, 12V hoc 24V, vic kch solenoid chnh l cng vic ca chip iu khin (v d AVR). V tip xc gia cc C v cc tip im l dng tip xc tm thi, khng c nh nn rt d b h mch. Nu dng in qua tip im qu ln, nhit c th sinh ra ln v lm h tip xc. V th chng ta cn tnh ton dng in ti a trong ng dng ca mnh chn r

le ph hp. Hnh 3a (pha di) l k hiu ca mt r le m bn c th gp trong cc phn mm thit k mch in t. Trong k hiu ny, chn 1 l chn C, chn 2 l tip im NC v chn 3 l tip im NO, trong khi hai chn 4 v 5 l 2 u ca cun solenoid. Chng ta s dng k hiu ny khi v mch cu H dng r le. S mt mch cu H u dng r le c minh ha trong hnh 4.

Hnh 4. Mch cu H dng r le. Trong mch cu H dng r le hnh 4, 4 diode c dng chng hin tng dng ngc (nht l khi iu khin ng c). Cc ng kch solenoid khng c ni trc tip vi chip iu khin m thng qua cc transistor, vic kch cc transistor li c thc hin qua cc in tr. Tm thi chng ta gi t hp in tr + transistor l mch kch, ti s gii thch r hn hot ng ca mch kch trong phn tip theo. Mch cu H dng r le c u im l d ch to, chu dng cao, c bit nu thay r le bng cc linh kin tng ng nh contactor, dng in ti c th ln n hng trm ampere. Tuy nhin, do l thit b c kh nn tc ng/m ca r le rt chm, nu ng m qu nhanh

c th dn n hin tng dnh tip im v h hng. V vy, mch cu H bng r le khng c dng trong phng php iu khin tc ng c bng PWM. Trong phn tip theo chng ta s tm hiu cc linh in c th thay th r le trong mch cu H, gi l cc kha in t vi kh nng ng/m ln n hng nghn hoc triu ln trn mi giy. II. Mch cu H dng BJT cng sut. BJT l vit tt ca t Bipolar Junction Transistor l mt linh kin bn dn (semiconductor device) c 3 cc tng ng vi 3 lp bn dn trong cu to. Trong tt c cc ti liu v in t c bn u gii thch v bn dn v BJT, trong ti liu ny ti ch gii thiu khi qut cu to ca transistor v ch yu l cc ch hot ng ca transistor. Bn dn l cc nguyn t thuc nhm IV trong bng tun hon ha hc, Silic (Si) l mt v d in hnh, cc nguyn t ny c 4 electron lp ngoi cng. trng thi thng, Si l cht dn in km (gn nh khng dn in), khi nhit tng, cc electron dao ng mnh v d dng b bt ra khi tinh th v do tnh dn in ca bn dn s tng. Tuy nhin, bn dn c dng ch to linh kin in t khng phi l cc tinh th thun khit m c pha tp cht. Nu pha nguyn t nhm V (nh Photpho) vo Si, 4 electron lp ngoi cng ca P to lin kt cng ha tr vi Si v c 1 electron ca P b tha (v P c 5 electron lp ngoi cng). Cht bn dn c pha Photpho v th rt d dn in v c tnh cht m nn gi l bn dn loi n (Negative), ht dn trong bn dn loi n l electron (e tha). Trng hp nguyn t nhm III, nh Bo (Boron), c pha vo Si, 3 electron lp ngoi cng ca Bo kt hp vi 4 electron ca Si tuy nhin vn cn 1 ch trng sn sng nhn electron. Ch trng ny c gi l l trng v c tnh cht nh 1 loi ht dn dng. Bn dn loi ny v th gi l bn dn loi p (Positive). Mc pha tp cht quyt nh dn ca bn dn. Tuy nhin, bn dn c pha tp cht d ci thin tnh dn in vn khng c nhiu tc dng, iu k diu ch xy ra khi ghp chng li vi nhau. Khi ghp bn dn loi p v loi n vi nhau to thnh tip xc p-n (p-n junction), y chnh l cc diode. c im ca tip xc p-n l ch c dng in chy qua theo 1 chiu t p sang n. Khi ghp 3 lp bn dn s to thnh transistor, ph thuc vo th t bn dn c ghp chng ta c transistor npn hay pnp. Ti s chn transistor npn gii thch hot ng ca transistor v loi ny c dng ph bin trong cc ng dng iu khin (v c trong mch cu H). Hnh 5 l m hnh v k kiu ca transistor npn.

Hnh 5. Transistor npn. Ba lp bn dn n, p v n kt hp to thnh 3 cc C (cc thuCollector), cc B (nn Base) v cc E (pht Emitter). Ty theo cch mc transistor m ngi ta c cc loi phn cc khc nhau, trong hnh 6 ti trnh by cch phn cc rt c bn m chng ta s dng sau ny, phn cc E chung (CE- Common Emitter).

Hnh 6. Phn cc E chung cho npn BJT. Tuy l c to nn t cc bn dn tp cht nhng nng tp cht ca cc lp trong npn BJT rt khc nhau. Lp E rt giu ht dn, k n l lp C v lp B th li rt t ht dn v rt mng. Khi in p cc B ln hn in p cc E, tip xc p-n gia B v E c phn cc thun. Dng electron t E (vn c rt nhiu do cch pha tp cht) o t chy v B, trong khi lp B (bn dn loi p) vn rt mng v ngho ht dn (l trng), nn phn ln electron t E s trn qua cc C v i v ngun Vc nh m t trn hnh 6. Ch trn hnh 6 ti v chiu di chuyn l chiu ca dng electron, chiu dng in s ngc li (v theo nh ngha chiu dng in ngc chiu electron). Din gii n gin, dng din t cc B gy ra dng in t cc C v E. Quan h ca cc dng in nh sau: IE=IB+IC (1) Mt c im th v l dng electron trn qua cc C s t l vi

dng electron n cc B. mi quan h nh sau: IC=hfeIB (2) Thng s hfe gi l h s khuych i tnh (DC Current Gain) ca BJT v l hng s c ghi bi cc nh sn xut, n chnh l c tnh phn bit tng loi BJT, gi tr ca thng rt ln, t vi chc n vi trm. Chnh v c im ny m transistor c dng nh l mt linh kin khuych i. Hy quan st phn mch in bn phi trong hnh 6 (pha Vc), nu gi s on CE ca BJT l mt in tr, xem li cng thc (2), nu tng dng in IB th dng IC s tng theo trong khi in tr RCv ngun VC li khng i, r rng in tr EC ang gim. Ni cch khc, dng IB s lm gim in tr gia 2 cc CE ca BJT. Tip tc tng IBth iu g xy ra, in tr gia 2 cc CE s gim n gi tr nh nht c th ca n (thng gn bng 0, gi tr ny c ghi trong datasheet mi loi ca BJT). Khi in tr CE t gi tr min, phn mch in bn phi gn nh c nh (VC, RC, RCE) nn dng ICcng t gi tr max v gn nh khng thay i cho d c tng IB. Quan h gia IBv IC khng cn ng nh cng thc (2). Hin tng ny gi l bo ha, y l hin tng rt quan trng ca transistor, n l c s cho s pht trin ca cc mch in t s (iu ny gii thch ti sao ngi ta hay cp n s lng transistor trong cc chip s, nh vi x l cho my tnh chng hn). Mt cch tng qut, iu kin BJT ri vo trng thi bo ha l ICmax < hfeIB. Khi BJT bo ha n s hot ng nh mt kha in t, hy xem hnh 7.

Hnh 7. Kha in t BJT. Gi s trong mch in hnh 7 RB=330, RC=10K , h s khuych i tnh ca transistor l 100. Khi in p ng vo Vi=0V, BJT khng hot ng, dng in qua RCbng 0 (hoc rt nh), in p ng ra Vo=12V. Khi Vi c kch kch bi in p 5V, dng IB=(5 0.7)/330=0.013A trong 0.7 l in p ri trn BE. Dng IC t gi tr ln nht khi VCE=0V, khi ICmax =12/10K=0.0012A. R rng IC< hfeIB v BJT s bo ha. Khi BJT bo ha, VCE=0V v ng ra Vo c ni vi GND nn Vo=0V . Tm li, bng cch thay i mc in p Vi t 0V sang 5V, in p ng ra s c switch t 12V sang 0V. Hot ng ca BJT khi bo ha i khi cn c gi l khuych i in p. V ch bo ha, BJT c th c dng lm cc kha in t trong mch cu H. Bn hy dng ch bo ha ca BJT t gii thch hot ng ca 4 BJT 2N3904 dng trong mch cu H hnh 4. Mch in trong hnh 7 gi l E chung. Mch E chung ca BJT hot ng rt tt trong ch kha in t. Nu chng thay in tr bng ng c th mch ny tng ng vi phn pha di ca mch cu H (BJT tng ng vi kha L2 hoc R2 trong hnh 1). Cu hi t ra

l c th dng thm 1 BJT npn nh trn lm phn trn ca mch cu H. Hy xt mch in trong hnh 8.

Hnh 8. Mch C chung. Mch in trong hnh 8 gi l mch C chung, im khc bit duy nht ca mch in ny so vi hnh 7 l in tr RC c di xung pha di cc E nn gi l RE. Khng cn kho st phn cc C hy kho st mch Vi -> B ->E -> RE -> GND. Khi Vi=5V, do in p ri trn BE lun l 0.7V (c im ca tip xc pn khi dn in) nn in p ri trn in tr RE lun l 4.3V mc d in p cc C l 12V, nh th in p gia 2 cc CE l 12 - 4.3 = 7.7V. iu ny c hiu l gia CE c mt in tr kh ln, kha in t khng hot ng tt i vi mch C chung. Nu RE l mt motor DC loi 12V th r rng motor khng hot ng tt v in p ri trn n ch c 4.3V. Mc khc in p CE qu ln c th gy hng BJT. V l do ny nu bn dng BJT npn lm phn trn ca mch cu H, BJT ny s rt mau hng (rt nng) v mch khng hot ng tt. Nh vy, mt ch khi thit k kha in t dng BJT l ti phi c t pha trn BJT tc l nn dng mch E chung nh trong hnh 7. Quay li mch cu H, gii php vt qua nhc im cp

trn l s dng BJT loi pnp cho phn trn ca mch cu H. Nguyn l hot ng ca BJT pnp cng na n npn nhng chiu dng in th ngc li. Vi cc kha in t dng BJT loi pnp, kch kha th in p cc B c ko xung thp thay v ko ln cao nh trong hnh 7. Chng ta hy kho st mt mt v d trong hnh 9.

Hnh 9. Mch E chung dng BJT pnp. Mi tn trong k hiu ca BJT pnp hng t E vo B, ngc li vi BJT npn. Nu in p Vi=12V=VEhoc ng Vi khng c kt ni th BJT khng hot ng, khng c dng in qua RC v dng IB =0 nn dng IC=0. Khi Vi=0V th dng IBxut hin v xut hin dng IC (t cc E) , nu dng IB ln s gy bo ha BJT v in p VEC gn bng 0V hay in p ri trn RCgn bng 12V, kha hot ng rt tt. Do , BJT pnp thng c dng lm phn trn trong cc mch cu H. Mt iu th v l mch in trong hnh 9 cng l mt mch E chung. C l n lc chng ta di thit kt mt mch cu H hon chnh dng BJT. Trong hnh 10 ti gii thiu mt cch thit k, y khng phi l cch duy nht nhng ti s dng mch ny trong vic gii thch

v v d iu khin (nu c). Bn c th ch li ty thch min sao m bo tt c cc BJT phi ri vo trng thi bo ha khi c kch.

Hnh 10. Mch cu H dng BJT. Ti chn 2 loi BJT cng sut trung bnh TIP41C v TIP42C lm mch cu. in p cao nht m 2 loai BJT ny chu c l 100V v dng ti a l 6A (ch l danh ngha, thc t c th thp hn). BJT npn TIP41C c th kch trc tip, ring BJT pnp TIP42C cn dng thm 1 BJT loi npn 2N3904 lm mch kch. Khi in p ng L1 mc thp, BJT Q0-1 khng hot ng, khng tn ti dng IC ca BJT ny, ngha l khng c dng IB ca BJT Q1, Q1 v th khng hot ng v tng ng mt kha Q1 m. Khi L1 c ko ln mc cao, 5V, BJT Q0-1 bo ha (mch E chung), dng IC ca Q0-1 xut hin v cng l dng IB ca BJT Q1. Q1 v th cng bo ha v tng ng mt kha ng. Nh vy, chng ta c th dng cc mc in p chun 0V v 5V kch cc BJT dng trong mch cu H cho d in p ngun c th ln vi chc hay trm Volt. Cc ng L1, L2, R1 v R2 s c vi iu khin (AVR) iu khin. Do BJT c th c kch

tc rt cao nn ngoi chc nng o chiu, mch cu H dng BJT c th dng iu khin tc motor bng cch p tn hiu PWM vo cc ng kch (tho lun sau). Nhc im ln nht ca mch cu H dng BJT l cng sut ca BJT thng nh, v vy vi motor cng sut ln th BJT t c s dng. Mch in kch cho BJT cn tnh ton rt k a BJT vo trng thi bo ha, nu khng s hng BJT. Mt khc, in tr CE ca BJT khi bo ha cng tng i ln, BJT v vy c th b nng Trong phn tip theo ti gii thiu mt loi linh kin khc thng dng lm mch cu H, MOSFET. IV. Mch cu H dng MOSFET. MOSFET l vit tt ca cm Meta Oxide Semiconductor FieldEffect Transistor tc Transisor hiu ng trng c dng kim loi v oxit bn dn. Hnh 11 m t cu to ca MOSFET knh n v k hiu ca 2 loi MOSFET knh n v knh p.

Hnh 11. MOSFET. MOSFET c 3 chn gi l Gate (G), Drain (D) v Source (S) tng ng vi B, E v C ca BJT. Bn c th nguyn l hot ng ca MOSFET cc ti liu v in t, y ch m t cc kch hot MOSFET. C bn, i vi MOSFET knh N, nu in p chn G ln hn chn S khong t 3V th MOSFET bo ha hay dn. Khi in tr gia 2 chn D v S rt nh (gi l in tr dn DS), MOSFET tng ng vi mt kha ng. Ngc li, vi MOSFET knh P, khi in p chn G nh hn in p chn S khong 3V th MOSFET dn, in tr dn cng rt nh. V tnh dn ca MOSFET ph thuc vo in p chn G (khc vi BJT, tnh dn ph thuc vo dng IB),

MOSFET c gi l linh kin iu khin bng in p, rt l tng cho cc mch s ni m in p c dng lm mc logic (v d 0V l mc 0, 5V l mc 1). MOSFET thng c dng thay cc BJT trong cc mch cu H v dng m linh kin bn dn ny c th dn rt cao, thch hp cho cc mch cng sut ln. Do cch thc hot ng, c th hnh dung MOSFET knh N tng ng mt BJT loi npn v MOSFET knh P tng ng BJT loi pnp. Thng thng cc nh sn xut MOSFET thng to ra 1 cp MOSFET gm 1 linh kin knh N v 1 linh kin knh P, 2 MOSFET ny c thng s tng ng nhau v thng c dng cng nhau. Mt v d dng 2 MOSFET tng ng l cc mch s CMOS (Complemetary MOS). Cng ging nh BJT, khi dng MOSFET cho mch cu H, mi loi MOSFET ch thch hp vi 1 v tr nht nh, MOSFET knh N c dng cho cc kha pha di v MOSFET knh P dng cho cc kha pha trn. gii thch, hy v d mt MOSFET knh N c dng iu khin motor DC nh trong hnh 12.

Hnh 12. Dng MOSFET knh N iu khin motor DC. Ban u MOSFET ko c kch, ko c dng in trong mch, in p chn S bng 0. Khi MOSFET c kch v dn, in tr dn DS rt nh so vi tr khng ca motor nn in p chn S gn bng in p ngun l 12V. Do yu cu ca MOSFET, kch dn MOSFET th in p kch chn G phi ln hn chn S t nht 3V, ngha l t nht 15V trong khi chng ta dng vi iu khin kch MOSFET, rt kh to ra in p 15V. Nh th MOSFET knh N khng ph hp lm cc kha pha trn trong mch cu H (t nht l theo cch gii thch

trn). MOSFET loi P thng c dng trong trng hp ny. Tuy nhin, mt nhc im ca MOSFET knh P l in tr dn DS ca n ln hn MOSFET loi N. V th, d c thit k tt, MOSFET knh P trong cc mch cu H dng 2 loi MOSFET thng b nng v d hng hn MOSFET loi N, cng sut mch cng b gim phn no. Hnh 13 th hin mt mch cu H dng 2 loi MOSFET tng ng.

Hnh 13. Mch cu H dng MOSFET. Ti dng 2 MOSFET knh N IRF540 v 2 knh P IRF9540 ca hng International Rectifier lm cc kha cho mch cu H. Cc MOSFET loi ny chu dng kh cao (c th n 30A, danh ngha) v in p cao nhng c nhc im l in tr dn tng i ln (bn tm c datasheet ca chng bit thm). Phn kch cho cc MOSFET knh N bn di th khng qu kh, ch cn dng vi iu khin kch trc tip vo cc ng L2 hay R2. Ring cc kha trn (IRF9540, knh P) ti phi dng thm BJT 2N3904 lm mch kch. Khi cha kch BJT 2N3904, chn G ca MOSFET c ni ln VS bng in tr 1K, in p chn G v th gn bng VS cng l in p chn S ca IRF9540 nn MOSFET ny khng dn. Khi kch cc line L1 hoc R1, cc BJT 2N3904 dn lm in p chn G ca IRF9540 st

xung gn bng 0V (v kha 2N3904 ng mch). Khi , in p chn G nh hn nhiu so vi in p chn S, MOSFET dn. Vi iu khin c th c dng kch cc ng L1, L2, R1 v R2. ...