noise and electromagnetic compatility
DESCRIPTION
Màn chắn điện từ (Shield)TRANSCRIPT
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B GIO DC V O TO
TRNG I HC BCH KHOA H NI
VIN IN ------- -------
BO CO TIU LUN [1]
ti:
MN CHN IN T
Ging vin ging dy : TS. Nguyn Vit Sn
Hc vin : Nguyn An Ton
SHHV : CA140027
Chuyn ngnh : iu khin v T ng ha
Lp : 14AKTH.KH
H Ni, thng 02/2015
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MN CHN IN T
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MC LC
Chng 1:TNG QUAN V MN CHN ................................................................................ 3
Chng 2:HIU SUT CHE CHN .......................................................................................... 9
Chng 3:HIU SUT CHE CHN VI NGUN TRNG XA .......................................... 12
3.1. Tnh chnh xc ................................................................................................................ 12
3.2. Tnh gn ng ................................................................................................................ 13
Chng 4:HIU SUT CHE CHN VI NGUN TRNG GN ....................................... 17
4.1. Khc nhau gia trng gn vi trng xa: ...................................................................... 17
4.2. Ngun in trng: ........................................................................................................ 21
4.3. Ngun t trng: ............................................................................................................ 22
Chng 5:CHE CHN T TRNG TN S THP ............................................................ 23
Chng 6:HIU NG KHE H ............................................................................................... 28
TI LIU THAM KHO ......................................................................................................... 31
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MN CHN IN T
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Chng 1
TNG QUAN V MN CHN
Thut ng shield thng cp n mt v1 kim loi m bao quanh hon ton mt
thit b in t hoc mt phn ca thit b .
Mt mn chn c 2 mc ch, nh minh ha trong hnh 1.1. Th nht, nh th hin
trong hnh 1.1a, mn chn l ngn chn nhng bc x t cc thit b in t hoc mt
phn ca cc thit b in t, bc x ra bn ngoi thit b. Mc ch y l ngn chn
1 enclosure
(a) (b)
(c)
Hnh 1.1 Minh ha vic s dng v bc (mn chn)
(a) ngn bc xa ra ngoi v (b) ngn bc x bn ngoi vo; (c) phng bc mn chn.
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MN CHN IN T
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nhng bc x gy ra bi thit b m khng tun th cc gii hn v bc x hoc ngn
chn cc thit b gy nhiu cc thit b in t khc. Mc ch th hai ca mn chn, nh
th hin trong hnh 1.1b l ngn cc bc x t bn ngoi n thit b t cc thit b
in t khc, ci m c th gy nhiu cho thit b. Mn chn c th c s dng gim
nhy vi cc tn hiu bn ngoi nh cc raa cng sut cao, i radio v TV. Bc nh
v mt phng bc mn chn c s dng th nghim v EMC1 c th hin trong
hnh 1.1c.
Nh vy, mn chn l mt ro cn i vi vic lan truyn trng in t. Ta c th
nh gi hiu sut ca mt mn chn qua t l gia ln cng in trng (t
trng)2 n3 mn chn vi ln cng in trng (t trng) truyn qua mn
chn. Ngoi ra, chng ta c th xem y l t s gia cng in trng (t trng)
truyn n thit b in t vi cng in trng (t trng) b mn chn chn li4.
Theo ngha th hai ny, chng ta c th lng t ha hiu sut ca mn chn nh mt gi
tr suy gim trung gian5. Cc khi nim trn mang tnh nh lng v ngha ca thut
ng mn chn v s c nh lng chnh xc hn nhng mc sau.
Vic tnh ton gi tr l tng ca hiu sut mn chn s c trnh by trong cc
mc sau v s c c mt vi gi tr tng i ln ca hiu sut mn chn n hng
trm dB. Hiu sut mn chn 100dB ngha l cng in tng (t trng ) n b
gim 100.000 ln khi n ra khi mn chn! c c hiu sut mn chn l tng v
cc ln nh trn, th mn chn phi bao quanh hon ton thit b in t v phi khng
b xuyn qua6 bi l trng, ng ni, khe h hoc cp. Bt k mt s xuyn qua no
khng ng cch, u c th lm gim mnh m hiu qu ca mn chn.V d, xt mt
hp kim loi kn (mn chn) c mt dy dn xuyn qua n nh th hin trn hnh 1.2a.
Gi s mt ngun bc x bn cnh nh mt ng-ten bc x mt trng in t. Trng
in t ny s mc vng qua7 dy dn, to ra mt dng in trong dy dn. Dng in
ny s chy t do qua v thit b v mc vng qua cc linh kin in t bn trong. V
ngc li, nhiu bn trong mn chn s mc vng qua dy dn, chy trn dy dn ra
ngoi v thit b, ri bc x. Mt mn chn b xuyn qua kiu ny s loi b hon ton 1electromagnetic compatibility 2 electric (magnetic) field 3 incident 4shield removed to that with the shield in place 5 insertion loss 6 penetration 7 coupled
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MN CHN IN T
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hiu qu ca mn chn. Vic xuyn cp nh vy phi c x l ng cch m bo
hiu qu ca mn chn. Mt s phng php ph bin nh dng b lc ti im vo / ra
hoc s dng cp c v bc vi mn chn bc bn ngoi v c ni vi mn chn ca
thit b nh th hin trong hnh 1.2b [2]. Ta thy, cp s c dng in cm ng bi trng
bn ngoi1. n gin ch cn ni mn chn ca cp vi mn chn ca thit b bng mt
dy dn nh trong hnh 1.2b th dng in trn mn chn cp s c dn vo b
1external field
Hnh 1.2 Nhng nguyn nhn nh hng n hiu sut mn chn trong thc t
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MN CHN IN T
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mt ca thit b, v tr m chng c th, mt ln na bc x n cc linh kin bn dn bn
trong thit b, nh vy gim bt hiu qu ca mn chn thit b.
Mt ln na, iu ngc li cng ng; nu mn chn cp khng c ni bn
ngoi vi v thit b (mn chn), dng in nhiu bn trong v bc c th chy qua v ra
bn ngoi mn chn, ni m chng c th bc x. Loi b mt mn chn ton phn1 t
cp ngoi vi (bn ngoi) thc s c th lm gim s bc x ca cp! Mt dn chng cho
nhn nh ny l cp my in Centronics m ni gia PC vi my in ny. Nh mt quy
tc chung, mt mn chn bc xung quanh si cp cha hn l gim bc x ca si cp.
Gii thch hin trng ny c minh ha trong hnh1.2c. c hiu qu che chn ca
mn chn si cp trong vic gim bc x ca si cp, th mn chn si cp phi c
ni vi mt im in p khng (mt im t l tng)2. Nu in p ca im cui3 ca
mn chn v im t logic ca PCB4 l khc nhau, nh minh ha trong hnh 1.2c, th
chng ta s to ra mt ng-ten n cc (mn chn cp). Nu chiu i ca mn chn cp
hn 01
4 , mn chn tr thnh mt b bc x hiu qu. Cp ngoi vi nh cp my in cho
PC thng c chiu di yu cu 1,5m, khong mt phn t bc sng 50MHz. Cng
hng trong cc bc x ca mt thit b do dng common-mode5 trn cc loi cp ngoi
vi thng xuyn c quan st trong phm vi tng s 50-100MHz. Ngt kt ni cp
ngoi vi vi thit b thng loi b nhng cng hng ny v lm ni bt6 bc x ca
chng.
S thm nhp ph bin khc l t mt l trng hay khe h trn tng chn.
Trng bn trong hoc bn ngoi mn chn7 s bc x qua khe h ny, nh vy lm gim
hiu qu ca mn chn. iu ny c minh ha bi mt nh l quan trng bit l
nguyn l Babinet [3]. Trong phn minh ha nguyn l ny, xt mt khe m c ct
trn mt mn dn in l tng nh th hin trn hnh 1.3a. Mt dy dn c ni qua
vo hai im ca khe h, v kch thch n. Trng xa8 m c bc x bi ng-ten khe
1 overall shield 2 a zero-potential point (an ideal ground) 3 pigtail 4Printed Circuit Board 5 common-mode: Chokes (mc 5.9) (nhiu tc ng vo) 6accentuated 7 fields interior to the shield or exterior 8near field: l trng bc x (ca ng-ten) trong vng bn knh 1 bc sng tnh t ng-ten pht. far field: l vng xa hn trng gn ca mt ng-ten.
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MN CHN IN T
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h c k hiu l sE , sE , sH v sH . By gi th xt cu trc b th hin trn hnh
1.3b, n bao gm vic thay th mn kim loi bng khng gian t do v thay th khe h
bng mt vt dn1 l tng c hnh dng ging nh khe h. ng-ten c kch thch mt
ln na, dy dn c ni n 2 na ca cu trc b. Trng xa bc x bi cu trc
b ny c k hiu l cE , cE , cH and cH .
Nguyn l Babinet: Trng xa bc x bi mn ban u vi khe h v trng xa
bc x bi cu trc b c quan h vi nhau nh sau [3]:
s cE H (1.1)
s cE H (1.2)
20
cs
EH
(1.3)
1 conductor
Hnh 1.3 Minh ha v hiu qu ca mt khe h trong che chn
(minh ha nguyn l Babinet)
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MN CHN IN T
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20
c
s
EH
(1.4)
iu ny minh ha rng khe h c th c bc x hiu dng1 bng ng-ten c kch
thc dn l kch thc ca khe h. iu ny cung cp ci nhn su sc2 v khe h hiu
dng trong mn chn lm gim hiu qu ca mn chn. V d, gi s rng khe h trong
hnh 1.3a c kch thc bng mt na bc sng, 0 / 2L . Nguyn l Babinet quy nh
rng cc tnh cht bc x ca khe h ny s ging nh (s hon i gia E v H) mt
ng-ten lng cc cng chiu di khe h nh minh ha trong hnh 1.3b. Ta c bit
rng na bc sng di ca ng-ten lng cc nh trong hnh 1.3b vi 0 / 2L l cu
trc bc x rt hiu qu. Do , mt khe h c kch thc nh th cng bc x rt hiu
qu. Ta thy rng nu chiu rng w khe h nh hn, th khe h (v lng cc b3) vn
bc x hiu qu. Do , ta khng nn c suy ngh rng4 Nu ta khng th nhn thy nh
sng qua khe h, th n s khng bc x tt. iu ny tt nhin khng ng. Khe h
xut hin nhiu ni trn v bc (mn chn)5. Np to ng dn vo bn trong v bc
(mn chn) v chng c khe h xung quanh. Do , khe h ca chng c gia cng
bng cch t min lt dn in6 ti khe h hoc beryllium finger stock.
Tm li, mn chn thc t khng th c tin cy hon ton loi b cc bc x
ca thit b. C nhiu v d ca cc thit b in t m khng dng v bc (mn chn),
nhng chng vn c th tun th cc quy nh gii hn v bc x v dn bc x7. Cc
my nh ch in t v my in c bc trong v nha. Chi ph v tnh khng thc t
ca mt v chn kim loi bc b vic s dng chng trong cc thit b. Mt khc, my
tnh c nhn v cc siu my tnh ln s dng v bc kim loi. X l mt cch hiu qu
tt c cc thm nhp cung cp hiu qu s dng ca cc mn chn cho cc loi thit b.
Nh mt nguyn tc c bn, nh thit k EMC s kt hp cc nguyn tc thit k EMC
cng nhau trong mt thit b d cho thit b c c che chn hay khng.
1 effective radiators 2 considerable insight 3 complementary dipole 4 we should not be caught in the trap of thinking that 5shielded enclosure 6 placing conductive gasketing material 7 conducted emissions
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MN CHN IN T
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Chng 2
HIU SUT CHE CHN
Trong mc ny s cp n hiu sut che chn ca mt mn chn kim loi. Cc
khi nim chung v hiu qu che chn c cp trn. Vi cc khi nim ny, ta
xt cc vn chung ca ro chn kim loi c dy t, in dn sut , hng s in
mi tng i 1r , v t thm tng i r , th hin trn hnh 2.1. Khi c sng
in t ti ro chn ny, th sng phn x c to ra, v mt phn sng ti truyn qua
ro chn. Hiu sut che chn ca ro chn c nh ngha theo cng in trng,
n vi decibel (dB) l:
10
20log
i
t
ESE
E (2.1)
Ch rng, cng thc ny s cho mt kt qu dng, v cng in trng ti
lun ln hn cng in trng ra khi ro chn. V d, hiu sut che chn l 120dB
ngha l ln ca cng in trng truyn qua b gim so vi ln ca cng
in trng ti l 106. Mt s nh ngha v hiu sut che chn l t s gia cng
in trng truyn qua trn cng in trng ti. nh ngha ny s cho kt qu m
Hnh 2.1 Minh ha hiu qu che chn ca ro chn dn in c sng phng ng nht n b mt
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MN CHN IN T
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v n v l dB, n l m ca (2.1). V cng t trng, hiu sut che chn, c th
c nh ngha l:
10
20log
i
t
HSE
H (2.2)
Nu cng t trng ti l sng phng ng nht v mi trng mi bn ro
chn l ng nht, th hai nh ngha l ging nhau, nn cng in trng v t
trng c lin quan bi tr khng c trng1 ca mi trng cho sng phng ng nht.
i vi trng gn hoc l mi trng khc nhau hai bn bin th chng khng bng
nhau. Tuy nhin, nh ngha v hiu sut che chn trong quan h cng in trng
(2.1) thng c ly lm tiu chun cho c hai tnh hung.
C mt vi hin tng lm gim cng in trng ti khi n i qua ro chn.
Hy xt s trn hnh 2.2, th hin kt qu ny.
Kt qu u tin l phn x trn mt bn tri ca ro chn. Mt phn ca cng
in trng ti m b phn x l do h s phn x b mt. Mt phn sng m vt qua b
mt ny th tip tc i qua tng chn. Khi n i qua mi trng dn ny, th bin ca
n gim dn theo hm ze , trong l hng s suy gim ca vt liu. iu ny c
gi l tn tht hp th. i vi vt liu mn chn m dn in tt, hng s suy gim
lin quan n dy lp mt ca vt liu , l 1 / v 1 / f , trong
1 intrinsic impedance
Hnh 2.2 Minh ha v phn x nhiu ln trong mt mn chn
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MN CHN IN T
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l t thm ca ro chn v l in dn sut ca n. V vy, cng ca in
trng b suy gim l /ze . Nu dy t ca ro chn ln hn nhiu so vi dy lp
mt1 ca vt liu ro chn tn s sng ti, th sng m c truyn qua mt u tin
th b suy gim rt ln khi n p vo mt bn phi. Mt phn ca sng ti ny b phn
x, v mt phn c truyn qua ro chn vo mi trng bn phi ca ro chn. Phn
phn x ca sng ny b truyn tr li ro chn v p vo mt u tin, c th ti t kim
loi. Mt ln na mt phn ca sng ny c truyn qua mt bn tri v thm vo tng
cng in trng phn x bn tri mi trng, v mt phn b phn x v tip tc
n bn phi. Phn ny li tip tc suy yu khi n truyn qua ro chn. Mt ln na, n i
qua ro chn v p vo mt bn phi, mt phn b phn x v mt phn c truyn qua
mt bn phi. Phn truyn qua mt bn phi thm vo tng cng in trng m
c truyn qua mn chn. Qu trnh c tip tc nh vy, nhng cng t trng
phn x v truyn qua b sung b suy yu dn bi s di chuyn ca chng trong ro chn
dn in. Nu mn chn c thit k c dy m n ln hn nhiu so vi dy
lp mt ca vt liu tn s ca cng in trng ti cho trc, th c mt vi h
qu n s phn x lin tc b mt bn trong ca ro chn. S phn x v truyn qua
nhiu ln ny c c th b qua do dy mn chn l ln hn nhiu so vi dy lp
mt, v ch c phn x v truyn qua ban u bn tri v phi b mt l cn c xem
xt. Tuy nhin, hiu sut che chn a ra (2.1) c th c chia thnh ba thnh phn
tiu biu l tn tht phn x, tn tht hp th v phn x nhiu ln ( n v dB):
dB dB dB dBSE R A M (2.3)
Trong : R c trng cho tn tht phn x gy ra bi phn x b mt bn tri v
phi, A c trng cho tn tht hp th ca sng khi n i qua ro chn, v M c trng
cho hiu ng ph do phn x v truyn qua nhiu ln. Ta thy, cc phn x li s to ra
cng in trng truyn qua b mt bn tri v s thm vo cng in trng
ban u. V vy, vic phn x nhiu ln M s l mt s m v s lm gim hiu sut che
chn (v R v A s dng). By gi, ta bt tay vo vic nh lng nhng yu t gp phn
vo hiu sut che chn ca mt ro chn.
1 skin depth
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MN CHN IN T
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Chng 3
HIU SUT CHE CHN VI NGUN TRNG XA
Trong mc ny, ta s gi s ngun trng in t m ti ro chn l xa so vi
mn chn th trng in t ti xem nh sng phng ng nht1. Trc tin, ta s xc
nh cch tnh chnh xc hiu sut che chn, ri s xc nh mt cch gn ng m vn
mang li kt qu tng t cho mn chn m c to t vt dn tt, vi dy t rt ln
so vi dy lp mt tn s sng ti.
3.1. Tnh chnh xc
tnh chnh xc hiu sut che chn ca ro chn kim loi, ta xem li hnh 2.1.
Mn chn dn in c dy t, in dn sut , hng s in mi 0 , t thm .
Mn chn dn in c sng phng ng nht ti mt bn tri ca n. Gi s mi trng
hai bn ca mn chn l khng gian t do2. Chn h ta vung gc nh hnh 2.1, vi
b mt bn tri cha hai trc xy c z = 0 v b mt bn phi c z = t.
Vi gi thit rng ro chn c to t vt liu dn in tt, nn tr khng c
trng ca vt dn l rt nh so vi khng gian t do: 0 . V chng ta cng gi thit
rng dy lp mt nh hn nhiu so vi dy t ca ro chn. Khi , hiu sut che
chn (n v dB) theo (2.1) l:
/010 1020log 20log4dB
dB
tdB dB
A
R
SE e M
(3.1)
Trong : 0 l tr khng c trng ca vng khng gian t do:
000
(3.2)
l tr khng c trng ca mn chn:
0
j
j
(3.3)
1uniform plane wave 2free-space
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MN CHN IN T
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l dy lp mt vt liu ro chn tn s sng ti:
1
f
(3.4)
MdB l tn tht phn x nhiu ln:
2 / 21020log 1t j t
dBM e e (3.5)
Trong cng thc (3.1) th cc thnh phn do phn x, do hp th v do phn x
nhiu ln c tch bit vi nhau nh cng thc (2.3).
3.2. Tnh gn ng
By gi, ta xt cc kt qu trc (3.1) vn vi gi thuyt rng ro chn c to
t vt liu dn in tt 0 , v b dy ca ro chn l rt ln so vi dy lp mt
tn s ca sng ti t ,
T cng thc (3.1), ta c tn tht phn x:
01020log 4dBR
(3.6)
Ta tnh xp x tr khng c trng ca vt liu dn in tt nh sau:
1
1 ( / )
j
j
j
j
j
(3.7)
45
(ro chn l vt liu dn in tt / 1 )
V: 000
(3.8)
Thay vo (3.6), ta c:
100
120log
4dB
r
R
(3.9)
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MN CHN IN T
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Trong , ta gi s 0 r v 0 . Vi in dn sut ca kim loi, n thng
c tham chiu vi ng, cht m c in dn sut 75,8.10Cu S/m. V vy, in dn
sut ca cc kim loi khc c vit l Cu r , trong , r l in dn sut tng
i so vi ng. Thay vo (3.9), ta c:
10168 10logr
dB
r
Rf
(3.10)
Ta thy rng, tn tht phn x rt ln tn s thp v in dn sut ca kim loi
ln. Tng t, vt liu t tnh 1r , lm gim tn tht phn x. Tn tht phn x gim
vi tc -10dB/decade vi tn s. V d, xt mt mn chn c lm t ng ( 1r ).
Tn tht phn x 1kHz l 138dB. 10MHz, tn tht phn x l 98dB. Mt khc, l
thp c 1000r v 0,1r . 1kHz, tn tht phn x l 98dB, v 10MHz th n b
gim xung 58dB.
T cng thc (3.1), ta c tn tht hp th:
/1020logt
dBA e (3.11)
Ta cng c th c n gin ha (3.11). Ta c dy lp mt:
1
f
0,06609
( )r r
mf
(3.12)
Thay (3.12) vo (3.11), ta c:
1020
logdBt
A e
8,686t
(3.13)
131,4 r rt f
Cng thc (3.13) cho thy rng tn tht hp th tng theo f thang o dB khi
tn s tng. V vy, tn tht hp th tng kh nhanh khi tn s tng. Vt liu st t c
1r nn tn tht hp th ln hn ng (gi thit rng 1r r ). Tn tht hp th c
th c hiu l t l gia b dy ca mn chn v dy lp mt, t (3.13) ta c:
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MN CHN IN T
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8,686
dB
tA
8,7 dB (khi 1t
)
17,4dB (khi 2t
) (3.14)
iu ny cho thy tm quan trng ca dy lp mt trong tn tht hp th.
Ta thy rng, tn tht phn x l mt hm t l vi /r r , trong khi tn tht hp
th li l mt hm t l ca r r . Bng 3.1 th hin cc h s ca chng khi vt liu thay
i.
Hnh 3.1 th hin cc thnh phn trong hiu sut che chn ca mt tm ng c b
dy 20mil khi tn s thay i t 10Hz n 10MHz. Ta thy rng, tn tht hp th tri hn
hn tn s trn 2MHz. Hnh 3.2 th hin cc thnh phn trong hiu sut che chn ca
mt tm thp c b dy 20mil. D liu ny c v t 10Hz n ch 1MHz. Ch rng
tn tht phn x ca kim loi ny tri hn hn tn s di 20kHz. Nhng s liu ny
cho thy rng tn tht phn x l thnh phn ng gp ch yu vo hiu sut che chn
tn s thp cho c mn chn bng vt liu hp kim ca st hoc kim loi mu. cc tn
s cao hn th vt liu hp kim ca st tng tn tht hp th v hiu sut che chn cng
tng. Tm li, cng in trng truyn qua ch yu qua mt th hai, trong khi
cng t trng truyn qua ch yu mt th nht, v vy hp th l quan trng nht
Bng 3.1
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MN CHN IN T
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gim cng t trng.
Hnh 3.2 Hiu sut che chn ca mt tm thp c dy 20 mil
Hnh 3.1 Hiu sut che chn ca mt tm ng c b dy 20mil
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Chng 4
HIU SUT CHE CHN VI NGUN TRNG GN
cc phn tch trc v hiu sut che chn gi thit l sng phng ng nht
n b mt ca mn chn. iu ny l do gi thit rng mn chn t vng trng xa
ca ngun trng in t ti1. Trong mc ny, ta s xt ngun trng gn2. Ta s kho
st cc k thut da vo mn chn vi loi ngun ny; d l ngun cng t trng
hay ngun cng in trng. Phi nhn mnh rng, trng gn th cu trc phc tp
hn nhiu so vi trng xa (trng m n gin v sng phng ng nht ging nhau).
Do , phn tch hiu ng ca tm phng, ro chn dn in trng gn l mt qu
trnh rt phc tp.
Tht khng hp l khi mong i c mt cng thc n gin v chnh xc cao cho
che chn trng gn nh t c (chnh xc) cho che chn trng xa m cp
mc trc. Kt qu sau l kt qu xp x gn ng (kt qu rt phc tp). Trung tm ca
phng php gn ng ny l thay th tr khng c trng ca khng gian t do
0 0 0/ , bng tr khng sng3 wZ , da vo lng cc Hertz (lng cc in)
4 v
vng t trng nh (lng cc t)5. Mt d y l mt xp x hi th, nhng n c
nghin cu v cho kt qu kh chnh xc nu chng ta lm mt s thay i n gin.
4.1. Khc nhau gia trng gn vi trng xa6:
iu quan trng l phi phn bit gia cc loi ngun khi chng gn mn chn.
Ta thy rng, im cch ngun ny xa, trng in t ging nh sng phng trong
(1) cc thnh phn ca vng trng xa E v H l vung gc vi nhau v (2) t s
gia hai vector trng in t l tr khng c trng ca mi trng 0/E H . Trong
vng trng gn ca ngun ny th khng cn tha mn nhng iu kin trn na. c
bit, cn phi v tr hn 03 th mi gi c hai tnh cht trn. Ni chung, vng trng
gn c nhiu thnh phn trng in t hn vng trng xa. Hn na, cc thnh phn
1incident field 2 near-field sources 3 wave impedence 4 Hertzian (electric) dipole 5 small magnetic loop (dipole) 6 near field versus far field
-
MN CHN IN T
Trang18
trng in t khng bin i n gin nh nghch o khong cch 1/ r , nhng cng
ph thuc vo 21 / r v 31 / r .Mt ch tiu hp l xc nh bin trng gn / trng xa
s l ni m t s E trn H xp x bng 0 . T s ny c gi l tr khng sng:
wE
ZH
(4.1)
Tr khng sng c tnh theo cng thc:
2 3
0 0 00
20 0
1 1
( ) ( )1
( )
w
j
r r rZ
j
r r
(4.2)
Trong hng s bc sng 0 0 0 v v tr ca mt im trong h trc ta
cu c nh ngha l khong cch radial ti im r.
ln ca tr khng sng theo khong cch ti ngun c th hin trn hnh
4.1a. Ti trng xa th 1 / r tri hn nn 0
wZ . Cng thc 4.2 c rt gn trong
trng gn l:
00
1wZ j
r
(trng gn, 0 1r )
0
0
90r
(4.3)
Trong vng trng gn th cng in trng t l vi 31 / r , trong khi cng
t trng th t l vi 21 / r :
3
2
1
1
Er
Hr
(Ngun in trng, trng gn) (4.4)
Ngoi ra, trong vng trng xa ca tr khng sng lng cc in1 l rt ln so
vi tr khng tng ng ca mi trng. Cho nn lng cc in c xem l ngun
tr khng cao2. ln tr khng sng ca mt ngun in trng l:
1 electric dipole 2 high-impedance source
-
MN CHN IN T
Trang19
0
12
we
Zf r
060r
(4.5)
Trong , ch s di e th hin mt ngun in trng (lng cc Hertz1). Cng
thc (4.3) c vit li theo bc sng trong khng gian t do 0 0 /v f vi 0 120
v 0 02 / .
Lng cc t nguyn t (vng t trng nguyn t)2 l i ngu ca lng cc
in nguyn t m ta c th chuyn i cng in trng v cng t trng s
nhn kt qu tng ng. Cc thnh phn trng xa ca lng cc t nguyn t l E v
1 Hertzian dipole 2 elemental magnetic dipole (loop)
Hnh 4.1 Tr khng sng ca lng cc Hertzian (a) v lng cc t (b)
-
MN CHN IN T
Trang20
H . Tr khng sng ca ngun ny c nh ngha l:
wE
ZH
(4.6)
Hay: 2
0 00
2 30 0 0
1
( )1 1
( ) ( )
w
j
r rZ
j
r r r
(4.7)
ln ca tr khng sng theo khong cch ti ngun c th hin trn hnh
4.1b. Ti trng xa th 1 / r tri hn nn 0
wZ . Cng thc 4.7 c rt gn trong
trng gn l:
0 0
wZ j r (trng gn, 0 1r )
0 0 90r (4.8)
Cng t trng trng gn t l vi 31 / r trong khi cng in trng
li t l vi 21 / r :
3
2
1
1
Hr
Er
(Ngun t trng, trng gn) (4.9)
Ngoi ra, trong trng gn ca lng cc t th tr khng sng rt nh so vi tr
khng c trng ca mi trng. Cho nn lng cc t c xem l ngun tr khng
thp. ln tr khng sng ca mt ngun t trng l:
0
2wm
Z f r
0
2369r
(4.10)
Trong , ch s di m th hin mt ngun t trng (vng t trng1) v thay
0 0 0/ , 0 0 0 v 0 02 / vo (4.8).
S phn bit gia ngun in trng v t trng s cho php chng ta chuyn i
nhiu kt qu c trong trng xa cho trng hp ngun trng gn. C nhiu v d
cho cc ngun nh vy.
1 magnetic loop
-
MN CHN IN T
Trang21
V d nh mt my bin p c to ra vi nhiu vng dy qun quanh li t.
Trng in t ln cn ngun ny phn ln l t trng. Nh vy, my bin p ging
vi vng t trng1. Vi ngun ny th trng gn c tnh cht l tr khng sng nh hn
rt nhiu so vi 0 , v cng in trng t l vi 21 / r trong khi cng t trng
t l vi 31 / r .
V d nh ngun in trng l b phng in v im cn xt l im cui ca
h quang, nh trong chi than ca ng c in mt chiu. i vi ngun ny th trng
gn c tnh cht l tr khng sng rt ln so vi 0 , v cng t trng t l vi 21 / r
trong khi cng in trng t l vi 31 / r .
4.2. Ngun in trng:
K thut c bn ca mn chn m c xt vi ngun trng xa th thng thy
vi ngun trng gn, nhng loi ngun ny th gii hn phng php xc nh hiu qu
che chn. Mt gii php chnh xc cho vn ny l kh khn hn nhiu so vi ngun
sng phng ng nht. Nn mt cch gn ng l ta vit hiu sut mn chn di dng
cc khi nim phn x, hp th v phn x nhiu ln, v s dng cc kt qu trc nhng
thay tr khng c trng 0 bng tr khng sng
wZ trong cc cng thc. Khi nim tn
tht hp th khng b nh hng bi loi ngun ny.
Tn tht phn x thu c bng cch thay tr khng c trng bng tr khng
sng ca khng gian t do trong (3.6):
10
20log4w
dB
ZR
(4.11)
Thay tr khng sng i vi ngun in trng t (4.5) v xp x vi vt dn tt
t (3.7) ta c:
, 10 3 2322 10logr
e dB
r
Rf r
(4.12)
Tn tht phn x i vi mt ngun in trng c v trn hnh 4.2 vi mn
chn ng v thay i khong cch t ngun n mn chn. Tn tht phn x i vi
ngun sng phng ng nht c th hin ng hi t tim cn khi khong cch t
ngun n mn chn tng. Ta thy rng trn th th tn tht phn x i vi trng gn
-
MN CHN IN T
Trang22
ca ngun in t cao hn ng k so vi mt ngun sng phng ng nht. N cng
tng khi m gim khong cch gia ngun v mn chn.
4.3. Ngun t trng:
Tn tht hp th ging vi ngun sng phng ng nht. Tn tht phn x i vi
trng gn ngun t trng thu c bng cch thay tr khng sng t (4.10) vo (4.11),
ta c:
2
, 1014,57 10logr
m dB
r
frR
(4.13)
Hnh 4.2 cng th hin tn tht phn x i vi trng gn, ngun t trng khi
thay i khong cch gia ngun ti mn chn v thay i tn s vi mn chn bng
ng. Ta thy rng tn tht phn x gim khi gim tn s v rt nh so vi tn tht phn
x ca sng phng. Do , tn tht phn x thng c b qua tn s thp. Ta cng c
tn tht hp th cng nh tn s thp, nn cc k thut khc phi c s dng i vi
mn chn i ph vi tn s thp, trng gn ca ngun t trng.
Hnh 4.2 Tn tht phn x trng gn ca ngun in trng v t trng
-
MN CHN IN T
Trang23
Chng 5
CHE CHN T TRNG TN S THP
Trong cc phn trc, ta thy tn tht phn x vi ngun trng xa (sng
phng ng nht) th mn chn chim u th nhng tn s thp, trong khi tn tht hp
th th mn chn chim u th nhng tn s cao. i vi trng gn, ngun in
trng, th ln ca chng cng khng thay i vi trng hp sng phng ng nht:
Tn tht phn x vn chim u th nhng tn s thp, v tn tht hp th cng chim
u th nhng tn s cao. Vi trng gn, ngun t trng, th chng khc nhau nhiu
nhng tn s thp. Tn tht hp th trn mn chn l ch yu trng gn, ngun t
trng ti tt c cc tn s. Tuy nhin, c tn tht phn x v tn tht hp th l kh nh
trng gn, ngun t trng ti nhng tn s thp, v vy cn phi c phng php
chng li ngun t trng tn s thp.
C hai phng php c bn mn chn chng li ngun t trng tn s thp:
to ng trnh1 cho ng sc t vi vt liu c t thm cao v to ng sc t i
lp2 theo nh lut Faraday, m ta bit nh l phng php vng ngn mch3. ng
trnh ca ng sc t i vi ng t tr thp4 ( t thm cao) c minh ha trn
hnh 5.1a. Gi s mi trng bn ngoi l khng gian t do vi 0 v mn chn
c to t vt liu st t s c 0r vi 1r , t trng s c xu hng tp trung
vo ng st t c t tr thp v nh vy s b lm lch hng khi nh hng n khu
vc bn trong ca mn chn. Trong phng php vng ngn mch minh ha trn hnh
5.1b vng dn in nh mt dy dn c t sao cho t trng i ti v xuyn qua b
mt th b chn bi vng ngn mch, do theo nh lut Faraday, trong vng s xut
hin mt dng in cm ng indI , v to ra ng sc t ind . ng sc t cm ng
ny c cc hoc hng chng li t trng ti ban u, vy nn t trng tng ln cn
ca vng ngn mch b trit tiu. C v s ng dng ca hai phng php trn. Nht l
trong nhng trng hp mn chn b gim hiu sut do t trng tn s thp.
1 diversion 2 opposing flux 3 shorted-turn 4 low-reluctance
-
MN CHN IN T
Trang24
C hai yu t c th lm gim hiu qu ca k thut ng trnh t thng m ta
phi nh l:
t thm ca vt liu st t gim khi tng tn s.
t thm ca vt liu st t gim khi tng cng t trng.
Nh sn xut vt liu st t thng ghi r t thm tng i ca vt liu tn
s thp nh l 1kHz nn gi tr ny r c th rt ln. V d nh, hp kim Mui1 c t
thm tng i hn 10,000 khi tn s bng 0 (DC) n 1kHz, nh th hin trn hnh 5.2.
Trn 1kHz, t thm tng i ca hp kim Mui gim t ngt, v trn 20kHz th n
khng ln hn thp cn ngui2. V vy, vt liu c t thm cao nh hp kim Mui th
hiu qu hn vi t trng so vi vt liu r hn nh thp khi tn s di vi chc kHz.
mn chn chng li c t trng trn 20kHz, th thp c hiu sut bng vt liu c
t thm cao. y l l do ti sao ro chn cho b cp ngun chuyn mch3 c to
t thp nhiu hn l t hp kim Mui. Thp th r hn hp kim Mui v cng hiu qu tn
s chuyn mch c s (20 100kHz) v sng hi chuyn mch. Mn chn ca ngun
in ngn chn tn s thp, t trng bc cao ca bin p cho b chuyn mch4 khi bc
x n cc mch in hay dy dn khc ca thit b, ni m chng c th gy ra nhiu vn
hoc c dn ra ngoi qua dy ngun, ri gy ra bc x.
1 Mumetal 2 cold-rolled steel 3 switching power supplies 4 switching transformer
Hnh 5.1 Hai phng php quan trng ca mn chn chng li t trng tn s thp:
(a) S dng vt liu st t c t thm cao lm lch hng t trng;
(b) S dng "vng ngn mch" dn in to t trng i lp.
-
MN CHN IN T
Trang25
Mt khc, mn chn chng li nhiu 60Hz hiu qu hn vi hp kim Mui nu
cng t trng khng qu ln chng lm bo ha vt liu. Hin tng bo ha
ny ca vt liu st t bi t trng bc cao c minh ha trn hnh 5.3a. dc ca
ng cong B-H t l vi t thm tng i ca vt liu. Cng t trng t l vi
ampere vng1 nu nh ni tip cc vng dy m mang dng in I qun quanh vt liu.
Nh vy, dng in ln thng to t trng bc cao ni m dc ca ng cong
phng hn, kt qu l r gim i vi t trng bc cao. V vy, c v nh hp kim Mui
c th l vt liu che chn hiu qu vi t tng 60Hz v t thm tng i ca n cao
tn s thp ny, nhng tht ra khng ng nh vt, v rng dng in ln thng c tn
s 60Hz. Nh sn xut vt liu st t thng ch ra trng thi c trng ca t thm
tng i ban u ca vt liu ca h ti (1) tn s thp, c trng 1kHz; v ti (2) t
trng bc thp2. Mt phng php tiu biu gim thiu hiu ng bo ha l dng hai
mn chn nh th hin trn hnh 5.3b. Mn chn u tin c r thp v cm ng t
thp bo ha t. Mc ch ca mn chn ny l gim t trng ti n khng bo
1 ampere turns 2 low field levels
Hnh 5.2 Minh ha s ph thuc vo tn s ca cc vt liu khc nhau
-
MN CHN IN T
Trang26
ha mn chn th hai, mn chn m c r cao v c kh nng cao i vi bo ha t. Tiu
biu l lp th nht gy ra tn tht phn x cho t trng.
Hnh 5.4 S dng hiu ng vng ngn mch kh bc x t trng ca t thng r ca my bin p:
(a) hnh chiu cnh; (b) hnh chiu bng.
Hnh 5.3 Minh ha hin tng bo ha t trong vt liu st t:
(a) ng cong B-H;
(b) dng mn chn t nhiu lp gim hiu ng bo ha.
-
MN CHN IN T
Trang27
ng dng thng thng ca vng ngn mch gim bt t trng ca my bin
p cp ngun cho b chuyn mch. Vng dn in bao gm di bng ng c qun
quanh my bin p nh th hin trn hnh 5.4a v b. Mc ch ca vng ngn mch hay
di bng1 ny l kh t trng bc x ca t thng r2 ca my bin p.
Hnh nh ca my bin p cp ngun cho b chuyn mch vi vng ngn mch
qun quanh chng c th hin trn hnh 5.5.
1 band 2leakage flux
Hnh 5.5 Hnh nh v vng ngn mch ca my bin p chuyn mch dng kh t trng bc x
-
MN CHN IN T
Trang28
Chng 6
HIU NG KHE H
Nh ch ra trc , c nhiu trng hp l trng trn mn chn l khng th
trnh khi bi nhng l do thc t. Mt trong nhng trng hp l vic thng gi.
Qut c s ng chuyn hi nng bn trong mn chn ra ngoi. Ta thy rng cc
loi l trng th thng s lng ln ca loi l nh th ln hn loi l ln. Nguyn nhn
quan trng ca vn ny c minh ha trong hnh 6.1.
Xt mn chn rn th hin trong hnh 6.1a. Dng in cm ng chy trong mn
chn ny v t thng lin i1 ca chng m to ra t thng tn2, t thng m chng li
hay kh t thng ti. T thng ti to ra dng in cm ng trn b mt, dng in m
c th xem l to ra t thng phn x. T thng phn x c cc tnh sao cho kh c
t thng ti p ng iu kin bin l tng in trng tip tuyn vi vt dn in tt
phi bng khng. m mn chn c th kh c nh trn, th dng in cm ng phi
c php chy qua m khng b cn tr. Gi s ta to mt khe h trn mn chn v n
vung gc vi hng ca dng in cm ng. Khe h s lm gin on dng in chy
qua v s lm gim hiu sut che chn. rng ca khe h khng nh hng ng k
n iu ny, th hin trn hnh 6.1b, c. Mt khc, nu ta hng ca khe h song song
vi hng ca dng in cm ng, th khe h s nh hng t hn n vic che chn, nh
th hin trn hnh 6.1d. Tht l khng kh thi xc nh hng ca dng in cm ng
v t hng ca khe h cho thch hp, v vy s lng ln ca cc l nh c thay th
1 associated fields 2 scattered fields
Hnh 6.1 Minh ha hiu ng khe h ln dng in cm ng ca mn chn.
-
MN CHN IN T
Trang29
cho khe h di nh minh ha trn hnh 6.1e. Ni cch khc l nhiu l nh to ra nhiu l
thng gi bng khe h di, nhng lm nhiu lon dng in cm ng th b hn nhiu, v
bng cch cng gim h.
Cng tht s cn thit khi to np y trn v thit b (mn chn). Ti im ng
np th mt khe h c to ra quanh np y m c th xem nh mt ng-tenna khe1,
nh cp trc . Mc d khong h c th kh nh, nhng kh nng bc x ca
khe h c th kh ln, nh minh ha bng nguyn l Babinet, ni cp trc.
V d, gi s rng c mt np y pha trn v thit b (mn chn) nh minh ha trn
hnh 6.2a.
Nguyn l Babinet th hin rng khi chuyn i khe h bng vt dn rn cng kch
thc vi khe h s to ra trng bc x ging vi t khe h. Trong minh ha ny th
chiu di ca khe h quan trng hn b dy ca n khi xc nh bc x ca khe h. Nu
chiu di khe h bng mt na bc sng, th bc x tng t vi ng-ten lng cc mt
na bc sng. iu ny gii thch ti sao cn thit dng nhiu c vt quanh np l
ph v cc ng-ten khe h tim nng ny, minh ha trong hnh 6.2b, v rng cc ng-ten
1 slot antenna
Hnh 6.2 Minh ha hiu ng khe h ti np y:
(a) bc x khe h theo nguyn l Babinet;
(b) nhiu c vt nm gn nhau ph v chiu di khe h;
(c) minh ha v tr thch hp v khng thch hp ca ming m nhm gim gim pht x t c vt.
-
MN CHN IN T
Trang30
tuyn tnh ngn hn thng hiu qu bc x km hn so vi ng-ten di. Ming m kim
loi thng c s dng lm kn khe h nh minh ha trn hnh 6.2c. N thng c
dng li thp hoc ng beryllium finger stock. Ming m ny s c t pha bn
trong so vi cc c vt, v rng nu chng c t pha bn ngoi cc c vt th bc x t
cc l c vt s khng b ngn chn.
Cc ro chn h cng c th c bo v bng cch s dng ng dn sng above
cutoff1. Mt ng sng vung vi cnh bng d th suy gim ca ng dn sng theo
chiu di l th t l vi l
de
. V vy, suy gim hay hiu sut che chn nh vo ng dn
sng l:
10
10
20log
.20log
ld
dBSE e
l ed
27,3l
d (6.1)
Ta thy hiu sut che chn t l thun vi chiu di ng dn sng. iu ny l c
s s dng ng dn sng below cutoff2 cho php lung khng kh i vo trong v
thit b (mn chn) v ng thi ngn chn s lan truyn tn s thp hn tn s cutoff vo
trong mn chn.
[4][5]
1 waveguide above cutoff 2 waveguide below cutoff
-
MN CHN IN T
Trang31
TI LIU THAM KHO
[1] C. R. Paul, Introduction to Electromagnetic Compatibility. John Wiley & Sons,
2006.
[2] E. F. Vance, Electromagnetic-interference control, IEEE Trans. Electromagn.
Compat, vol. EMC-22, pp. 319 328, 1980.
[3] C. A. Balanis, Antenna Theory. Wiley, 2005.
[4] P. A. Laplante, Electrical Engineering Dictionary. CRC Press LLC, 2000.
[5] S. Gibilisco, The Illustrated Dictionary of Electronics. The McGraw Hill, 2001.