Chuong 2 Linh Kien DTCS

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  • Ch 2 Linh kien DTCS.doc Page 1 of 20

    Chng 2: LINH KIEN CONG SUAT

    Trong chng 2, ta se khao sat ac tnh cua cac linh kien ien t cong suat thong dung, bao gom diod, transistor, SCR va cac linh kien cung ho. Vi nhng linh kien a c khao sat chi tiet trong mach ien t, chng nay ch bo sung nhng noi dung lien quan en hoat ong ong ngat la ac tnh c s dung trong cac bo bien oi nang lng ien.

    II.1 DIODE:

    1. ac tnh phan loai:

    Diod la linh kien ien t quen thuoc, cau tao bang mot moi noi ban dan p-n. Diod co kha nang dan ien mot chieu khi c phan cc thuan VAK > 0 Khi b phan cc ngc, diod co dong ro ngc, thng co tr so khong ang ke va khi VAK co tr so ln hn VBR , co hien tng gay ngc (Reverse Breakdown) lam h hong linh kien. Trong mach ien t cong suat, ta s dung diod cong suat, gom cac loai sau:

    - Diod dung vat lieu silic thong thng, gom hai loai: diod dung trong cac s o lam viec tan so li ien va diod phuc hoi nhanh lam viec tan so cao, sut ap thuan VF t 0.7 en 1 volt.

    Hnh II.1.1: Ky hieu.

    Hnh II.1.2: ac tnh Volt Ampe cua diod.

    - Diod Schottky c khuech tan nhieu chat dan ien nen co the lam viec tan so cao, sut ap thuan thap (khoang 0.2 0.4V). Nhng v the ma diod Schottky ch co the chu ap khoa thap.

    2. ac tnh phuc hoi (recovery) cua diod:

    ac tnh phuc hoi cua diod thuoc ve mot trong hai ac tnh ong (dynamic) mo ta qua trnh qua o, chuyen oi gia hai trang thai ong hay dan ien (ON) va ngat (OFF) cua diod. No mo ta qua trnh chuyen t trang thai dan ien sang trang thai khoa nh trong hnh II.1.3.

    Hnh II.1.3: Hai kieu phuc hoi.

    Khi bat au phan cc ngc, co khoang thi gian ngan diod dan dong am goi la thi gian

  • Ch 2 Linh kien DTCS.doc Page 2 of 20

    phuc hoi ngc trr (rr: reverse recovery) tng ng vi viec xa va nap ngc lai cac ien tch

    cua moi noi p-n. Thi gian phuc hoi ngc trr tnh t khi dong qua diod ao chieu cho en khi no con bang 25% gia tr dong ngc cc ai IRR (hnh II.1.3). Thi gian nay kha ln (vai chuc micro sec) diod tan so cong nghiep, be hn nhieu (nho hn microsec) diod phuc hoi nhanh (fast recovery).

    - ien tch phuc hoi ngc: Qua khao sat, ac tnh phuc hoi ngc cua diod phu thuoc vao ien tch phuc hoi ngc

    RRRR Ft

    Q i dt= . cua moi p n diod. ien tch nay phu thuoc vao cong nghe che tao diod va dong dan ien thuan trc o IF . T hnh II.1.3, ta co quan he sau khi cho dong ien qua diod tang, giam tuyen tnh:

    12

    .RR rr RRQ t I= hay 2 RRRRrr

    QIt

    = ,

    Vi ta >> tb (thng ung trong a so trng hp, do di/dt kha be), suy ra:

    2. . .RR a rr RRdi di diI t t Qdt dt dt

    = =

    Cong thc nay cho phep ta tnh dong ien ngc cc ai IRR theo ien tch phuc hoi ngc QRR va toc o tang dong ngc di/dt. QRR c cho trong cac so tay tra cu cua nha che tao, di/dt quyet nh bi mach ien.

    - Anh hng cua ien tch phuc hoi ngc: Sau khi dong ngc at gia tr cc ai, dong qua diod giam ve zero trong thi gian tb. Thi gian nay ch phu thuoc ac tnh diod, co tr so kha nho, lam toc o giam dong ngc di/dt luc nay rat cao, cac t cam co trong mach ien co the gay ra qua ap. Qua trnh nay co the mo ta tren hnh II.1.4.a khi thay diod bang ien tr R co tr so thay oi sao cho dong giam ve zero trong thi gian tb theo qui luat ng thang:

    ; RR RRD Lb b

    I Idi diu U u U L U Ldt t dt t

    = = = = +

    Vi tb thng rat be, ap cam ng nay rat cao co the lam h hong phan t mach hay gay nhieu. Ta co the chong lai bang cach s dung RC song song vi diod (hnh II.1.4.b) va mach ien dung e tnh toan la hnh II.1.4.c khi thay the diod la khoa K va khong e y en ap nguon U. Ap ban au tren tu ien uC (0) = 0, khoa K m ra khi dong qua L at gia tr IRR. Phan t RC c chon sao cho mach LCR la khong dao ong va ap cc ai tren khoa K la be nhat (bai tap).

    KC

    i (0) = IU

    L

    RU

    _

    L

    LRR

    DD

    C

    (b)

    R

    RRLL i (0) = I

    (c)

    m ra t = 0

    +

    u

    +

    R

    (a)

    _

    Hnh II.1.3: Anh hng cua ac tnh phuc hoi ngc cua diod va tac dung cua mach snubber:

    Nh c khao sat trong phan tiep theo, thyristor cung gap van e tng t.

    Diod Silicon Carbide:

  • Ch 2 Linh kien DTCS.doc Page 3 of 20

    T 2001, ngi ta a che tao cac diod Schottky dung vat lieu Silicon Carbide vi cac ac tnh: ap ngc cao hn diod Schottky truyen thong rat nhieu (1KV), nhiet o hoat ong cho phep co the vt qua 200o trong khi hau nh khong co qua trnh phuc hoi ngc (hnh II.1.4). ieu nay co the lam tang hieu suat ang ke cac BB flyback (chng 4) khi lam giam ton hao cua qua trnh ong ngat va tang c tan so lam viec cua mach.

    Gan ay cac mosFET dung cong nghe SiC a ra i vi nhieu ac tnh vt troi: chu ap cao vi Ron rat thap, tan so va nhiet o lam viec cao cho thay cong nghe Silicon Carbide ha hen lam tang hieu suat cac bo bien oi len mot bac na, nh trng hp thay oi t BJT qua mosFET cong suat trc ay.

    Hnh II.1.3: So sanh qua trnh phuc hoi cua diod SiC va diod Si phuc hoi nhanh

    II.2 TRANSISTOR CONG SUAT:

    Transistor la nhom linh kien ieu khien lam viec nguon mot chieu, bao gom BJT khi ieu khien bang dong cc B va MosFET hay IGBT vi ap cc cong G. Trong mach TCS, ta s dung cac linh kien cong suat ln, la cac transistor chu c dong, ap cao, lam viec trong che o ong ngat. Transistor cong suat gom co:

    - BJT: transistor hai moi noi, la linh kien ieu khien bang dong cc B, mach ien t cong suat s dung hai che o lam viec:

    - IB = 0 => BJT khoa, khong dan ien

    - IB u ln (IB >> IC / ) BJT bao hoa, dan dong tai IC ch phu thuoc mach tai.

    Hnh II.2.1: Cau tao cua BJT cong suat: Cc B phan bo eu tren toan bo dien tch, cung cap kha nang ieu khien hieu qua hn (he so khuech ai ln, toc o ong ngat nhanh).

    Nh vay che o khuech ai ch ton tai trong qua trnh qua o gia 2 trang thai nay. Mot lu y la trong che o bao hoa, he so khuech ai dong cua BJT giam ang ke (con t 10 -20), ac bie la khi dong, ap cao. e giam dong ieu khien linh kien, cac nha san xuat che tao cac transistor Darlington vi he so khuech ai dong t vai tram en hang nghn.

    G

    S

    D

    S

    G

    D

    G

    C

    E G

    C

    E Transistor Darlington MosFET kenh n va ky hieu quen dung Ky hieu IGBT Mach nguyen ly IGBT

    Hnh II.2.2: Ky hieu cac transistor (ngoai BJT)

    - MosFET: la transistor trng co cc cong cach ien, bao gom kenh dan ien DS c kiem

  • Ch 2 Linh kien DTCS.doc Page 4 of 20

    soat bi cc cong G cach ien bang lp oxit kim loai (hnh II.2.2). MosFET la transistor trng loai tang (enhancement), no dan ien khi co ap phan cc cong VGS. Giong nh BJT, mach TCS s dung MosFET hai che o:

    - VGS 0 : transistor khoa, kenh dan ien DS h mach. - VGS > VTH (ien ap them VTH t 3 .. 5 volt): mosFET bao boa - ien tr kenh dan ien rDon rat be (sut ap thc te t 1 2 volt dong ien nh mc).

    - IGBT (Insulated Gate BJT): Cong nghe che tao MosFET khong cho phep tao ra cac linh kien co nh mc dong ln, IGBT co the xem la s ket hp gia MosFET ngo vao va BJT ngo ra (hnh II.2.2) e co c linh kien ong ngat dong mot chieu en hang nghn Ampe, ieu khien bang ap cc cong G. Nh vay mosFET va IGBT co cung phng phap ieu khien.

    Transistor noi chung khong cho phep phan cc ngc nen ngi ta hay noi mot diod song song ngc e bao ve. Trong nhieu trng hp diod song song ngc la co san trong MosFET hay IGBT (goi la diod ky sinh tao ra trong qua trnh che tao). Tuy nhien, cac diod ky sinh nay thng co ac tnh ong ngat khong tot, ngi ta van phai thay the bang cac diod phuc hoi nhanh e am bao BB lam viec tot. Tren th trng, cac nha san xuat a che tao ra nhng module gom transistor va cac phan t phu lam thanh mot ngat ien hoan chnh, e tien dung khi thc hien BB cong suat ln.

    GHi NH - Mach TCS ch s dung transistor trong che o ong ngat

    - MosFET va IGBT co cung phng phap ieu khien.

    a. ac tnh ong ngat cua transistor:

    Lam th nghiem vi cac mach ien tren hnh II.2.3, nguon VBB co dang song tren hnh II.2.4.a va bien o u ln e transistor Q bao hoa, ta co nhng nhan xet sau:

    - Khi ong (chuyen t khoa sang bao hoa) BJT mat thi gian tON co tr so khoang 1 micro giay, va thi gian tOFF co tr so vai micro giay e khoa (dang song hnh II.2.4.a).

    Hnh II.2.3: mach th nghiem qua trnh ong ngat cua BJT.

    Hnh II.2.4.a va b:

    - Qua trnh chuyen trang thai khoa bao hoa khong xay ra tc thi, co thi gian e ap vCE va iC thay oi tr so (tng ng che o khuech ai). V du khi tai tr:

    vCE = VCC Rt . iC Hnh II.2.4.c

  • Ch 2 Linh kien DTCS.doc Page 5 of 20

    ap vCE cua BJT tang dan theo qua trnh giam cua iC trong qua trnh khoa.

    Nh vay co thi gian, du rat be, BJT lam viec trong che o khuech ai vi dong ln va ap cao, dan en co ton hao P trong BJT khi ong ngat (hnh II.2.4.b). V du khi ap tren BJT bang 200 volt va dong 20 ampe, cong suat tc thi tren moi noi CE luc o la 200*20 = 4000 watt so vi vai chuc watt khi dan bao hoa.

    Bi

    QR2

    VBB

    C

    DR

    R1

    Hnh II.2.5: cum BJT ong ngat vi cac linh kien phu

    Hien tng nay ac biet nghiem trong khi tai la RL va co diod phong ien (hnh II.2.3): dong qua tai cuon day khong thay oi tc thi trong khi diod phong ien D ch dan ien khi BJT tat han. Ket qua la moi noi CE se chu nguyen dong tai cho en khi vCE = VCC.(hnh II.2.4.c) Nh vay ton hao trong qua trnh ong ngat tai RL se cao hn trng hp tai R (dang dong ap tren hnh II.2.4.a).

    Ton hao trong qua trnh ong ngat se tr nen ang ke khi transistor lam viec tan so cao, trong thc te no la nguon nhiet chu yeu lam phat nong transistor cong suat va ieu nay gii han tan so lam viec cua mach.

    e han che s phat nong do ong ngat, can chon ung loai transistor ong ngat (loai SWitching), dung