electrical coupling
DESCRIPTION
Electrical couplingTRANSCRIPT
1
CouplingOutline
• Capacitive coupling:– frequency domain– time domain
• Inductive coupling:– frequency domain– time domain
2
Capacitive Coupling• Wherever there are two circuits, there is
mutual capacitance. Voltages in one circuit create electric fields which affect the second circuit.
• A mutual capacitive coupling between two circuits 1 and 2 is simply a parasitic capacitor C12 connected from circuit 1 to circuit 2.
• A mutual capacitance C12 injects a current into circuit 2 proportional to the rate of change of voltage in circuit 1
3
Capacitive Coupling: Frequency Domain
C12 = parasitic capacitance between conductors 1 and 2
U2NC12
C2G
R
U1
Conductor 1
Conductor 2
4
Capacitive Coupling: Frequency Domain
+U2N
U1
C12
RC2G
Equivalent circuit
( )
UU
jj
R C C
CC C
N
GG
21
12 2
1212 21=
++
⋅+
ω
ω
Voltage noise U2N induced in conductor 2
5
Capacitive Coupling: Frequency Domain
UU
N21
ω
CC C G
1212 2+
Voltage noise U2N induced in conductor 2
( )G212c CCR
1+
=ω
6
Capacitive Coupling: Time DomainAssumptions:
• The coupled current flowing in C12 is much smaller than the primary signal current in circuit 1.
• The coupled signal voltage in circuit 2 is smaller than the signal on circuit 1.
• A capacitor C12 has a large impedance compared to the impedance to ground of circuit 2.
• The impedance to ground of circuit 2 is R
7
Capacitive Coupling: Time Domain
1 2 3 ns0
U1(t)Tr=1ns
∆U1
Max dv/dt of driving waveform:
r
11TU
dtdU ∆
=
Injected current in circuit 2:
r
112C T
UCI ∆=
r
12
1
N2T
RCU
UCrosstalk =∆=
8
Capacitive Coupling: Time DomainExample:
∆U1 = 1V, Tr = 1ns, C12 = 1pF, R = 50 Ω
050TRC
UUCrosstalk
r
12
1
N2 .==∆=
9
Capacitive Coupling: Time Domain
10⋅U2N
0 1 2 3 4 5 6 7 8 9 [ns]00.10.20.30.40.50.60.70.80.9
1
U1
Estimation of C12 from measured noise voltage:
pF1150105
URAreaC
11
112 =⋅
⋅=∆=−
10
Capacitive Coupling: Time Domain
10⋅U2N
U1
U2Npk
10.9
0.70.8
0 1 2 3 4 5 6 7 8 9 [ns]00.10.20.30.40.50.6
05080UU
CrosstalkMeasured1
Npk2 .=∆=
11
Capacitive Coupling: Example
Cgd
Cgs
RG
RDriver
uDS
+
- 100 200 ns0
uDS(t)trv=100ns
UOFF = 100V
For a 100V, 20A device Cgs≈1nF, Cgd≈100pF
A1.0t
UCdt
duCI
rv
OFFgd
gdgdN =≈=
12
Capacitive Coupling: Example
100
200
UOFF = 100V
t [ns]
IN = 0.1A
trv=100ns Equivalent circuit:uDS(t)
Cgs
iN(t)uGS
+
-RT
t [ns]iN(t)
( )
−=
−gsTCR
t
NTGS e1IRtu
DriverGT RRR +=
0
uGS(t)<UTH=5V RT < 50Ω
13
Parasitic Capacitances in Transistor Package
EB
C
BHeat-sink &
chassis Mica insulation
Metallic case
airgapC E
For normal thickness
mica insulation
Cparasitic ≈ 100 pF for TO-3≈ 80 pF for TO-5≈ 30 pF for TO-220
For isolate package, Cparasitic depends on chip area. For a 600V, 75A IGBT, C ≈ 25pF.
14
Inductive Coupling• Wherever there are two loops, there is
mutual inductance. Current in one loop creates a magnetic field which affects the second loop.
• A mutual inductive coupling between two circuits 1 and 2 is simply a parasitic mutual inductance LM between circuit 1 and circuit 2.
• A mutual inductance LM injects a noise voltage into circuit 2 proportional to the rate of change of current in circuit 1
15
Inductive CouplingLoop 1
loop 2
U2N(t)+-
I1(t)
• Current in loop 1 produces a pattern of magnetic field energy
• The total magnetic field strength over the area of loop 2 (magnetic flux) is a function of the distance, physical proportions, and relative orientations of the loops as well as of the current in loop 1
16
Inductive CouplingLoop 1
loop 2
U2N(t)+-
I1(t)
( ) ∫ ⋅−=φ
−=2area
12N2 sdBdt
ddt
dtU rrFaraday’s law:
Mutual inductance:
1
12M IL φ
=
( ) dtdILtU 1
MN2 =
17
Inductive Coupling
U2NLMi1
R2
R
L2
+
u1
+
18
Inductive Coupling: Frequency Domain
U2N
LM
I1
R2 RL2+
Equivalentcircuit-I2
T
N2
2
N22
1M22N2
RU
RRUI
ILjILjU
−=+−=
ω+ω=
Voltage noise U2N induced in conductor 2
1
T
2
MN2 I
RLj1
LjUω+
ω=
19
Inductive Coupling
Ui
N21
ω
2
TLR
MLjω
2
TMLRL
20
Inductive Coupling: Time DomainAssumptions:
• The induced voltage across LM is much smaller than the primary signal voltage in circuit 1.
• The coupled signal current in circuit 2 is smaller than the current in circuit 1.
• The secondary impedance is small compared to the impedance to ground of circuit 2.
• The impedance to ground of loop 1 is R1
21
Inductive Coupling: Time Domain
1 2 3 ns0
I1(t)Tr=1ns
1
1RU∆
Max di/dt of driving waveform:
r1
11TR
UdtdI ∆
=
Injected voltage in circuit 2:
r1
1MN2 TR
ULU ∆=
r1
M
1
N2TR
LU
UCrosstalk =∆=
22
Inductive Coupling: Time DomainExample:
∆U1 = 1V, Tr = 1ns, LM = 10nH, R1 = 50 Ω
2.0TRL
UU Crosstalk
r1
M
1
N2 ==∆=
Among high-speed digital circuits, mutual inductance is often a worse problem than mutual capacitance
23
Summary• High signal rise and fall times increase both capacitive
and inductive noise coupling• A high receptor impedance to ground increases its
susceptibility to capacitive noise coupling• Magnetic coupling depends on loop areas (mutual
inductance)• Capacitive coupling injects a noise current into the
receptor circuit• Inductive coupling injects a noise voltage into the
receptor circuit• Unlike mutual capacitive coupling, mutual inductive
coupling is capable of inducing crosstalk with a polarity opposite that of the driving signal.