48
w w w . l i t t e l f u s e . c o m
SMT50
The electrical characteristics of a SMT50 device are similar to
that of a self-gated Triac, but the SMT50 is a two terminal
device with no gate. The gate function is achieved by an
internal current controlled mechanism.
Like the T.V.S. diodes, the SMT50 has a standoff voltage (Vrm)
which should be equal to or greater than the operating
voltage of the system to be protected. At this voltage (Vrm)
the current consumption of the SMT50 is negligible and will
not affect the protected system.
When a transient occurs, the voltage across the SMT50 will
increase until the breakdown voltage (Vbr) is reached. At this
point the device will operate in a similar way to a T.V.S.
device and is in avalanche mode.
The voltage of the transient will now be limited and will only
increase by a few volts as the device diverts more current. As
this transient current rises, a level of current through the
device is reached (Ibo) which causes the device to switch to a
fully conductive state such that the voltage across the device
is now only a few volts (Vt). The voltage at which the device
switched from the avalanche mode to the fully conductive
state (Vt) is known as the Breakover voltage (Vbo). When the
device is in the Vt state, high currents can be diverted
without damage to the SMT50 due to the low voltage across
the device, since the limiting factor in such devices is
dissipated power (V x I).
Resetting of the device to the non-conducting state is
controlled by the current flowing through the device. When
the current falls below a certain value, known as the Holding
Current (Ih), the device resets automatically.
As with the avalanche T.V.S. device, if the SMT50 is subjected
to a surge current which is beyond its maximum rating, then
the device will fail in short circuit mode, ensuring that the
equipment is ultimately protected.
SELECTING A SMT50
COMPLIES WITH THE
PEAK SURGE
VOLTAGE
CURRENT
ADMISSIBLE IPP
NECESSARY
FOLLOWING STANDARDS
VOLTAGE
WAVEFORM
WAVEFORM
RESISTOR
(V)
(
S)
(
S)
(A)
(
)
(CCITT) ITU-K20
1000
10/700
5/310
25
-
(CCITT) ITU-K17
1500
10/700
5/310
38
-
-
VDE0433
2000
10/700
5/310
50
VDE0878
2000
1.2/50
1/20
50
-
IEC-1000-4-5 10/700
5/310
50
-
level 4
level 3
1.2/500
8/20
100
-
FCC Part 68, lightning surge
1500
10/160
10/160
75
12.5
type A
800
10/560
10/560
55
6.5
FCC Part 68, lightning surge
1000
9/720
5/320
25
-
type B
Bellcore TR-NWT-001089
2500
2/10
2/10
150
11.5
first level
1000
10/1000
10/1000
50
10
Bellcore TR-NWT-001089
5000
2/10
2/10
150
11.5
second level
CNET I31-24
1000
0.5/700
0.8/310
25
-
V
R
V
BR
V
RM
V
BO
I
RM
I
BO
I
H
I
pp
I
V
1. When selecting a SMT50 device, it is important that
the Vrm of the device is equal to or greater than the
the operating voltage of the system.
2. The minimum Holding Current (Ih) must be greater
than the current the system is capable of delivering
otherwise the device will remain conducting following
a transient condition.
V-I Graph illustrating symbols
and terms for the SMT50 surge
protection device.
ELECTRICAL CHARACTERISTICS
w w w . l i t t e l f u s e . c o m
49
ELECTRICAL CHARACTERISTICS (Tamb 25C)
ABSOLUTE MAXIMUM RATINGS (Tamb 25C)
THERMAL RESISTANCE
SYMBOL
PARAMETER
SYMBOL
PARAMETER
V
RM
Stand-off Voltage
V
BO
Breakover Voltage
I
RM
Leakage Current at Stand-off Voltage
I
H
Holding Current
V
R
Continuous Reverse Voltage
I
BO
Breakover Current
V
BR
Breakdown Voltage
I
PP
Peak pulse Current
C
Capacitance
SYMBOL
PARAMETER
VALUE
UNIT
R
TH
(J-I)
Junction to leads
20
C/W
R
TH
(J-I)
Junction to ambient on printed circuit
100
C/W
(with standard footprint dimensions)
SYMBOL
PARAMETER
VALUE
UNIT
P
Power dissipation
T
lead
5
W
I
PP
Peak pulse current
10/1000S
50
A
8/20S
100
A
I
TSM
Non repetitive surge peak on-state current
tp + 20ms
30
A
dV/dt
Critical rate of rise of off-state voltage
V
RM
5
KV/S
T
stg
Storage temperature range
-55 to +150
C
T
j
Maximum junction temperature
150
C
T
L
Maximum lead temperature for soldering during 10s
260
T
stg
Type
SMT50-62
SMT50-68
SMT50-100
SMT50-120
SMT50-130
SMT50-180
SMT50-200
SMT50-220
SMT50-240
SMT50-270
Marking
Laser
A062
A068
A100
A120
A130
A180
A200
A220
A240
A270
I
RM
@ V
RM
MAX
(A)
(V)
I
RM
@ V
R
MAX
(A)
(V)
62
68
100
120
130
180
200
220
240
270
50
50
50
50
50
50
50
50
50
50
800
800
800
800
800
800
800
800
800
800
82
90
133
160
173
240
267
293
320
360
56
60
90
180
117
162
180
198
216
243
2
2
2
2
2
2
2
2
2
2
V
BO
@ I
BO
MAX
(V)
(mA)
I
H
MIN
(Note 1)
(mA)
150
150
150
150
150
150
150
150
150
150
C
MAX
(pF)
150
150
100
100
100
100
100
100
100
100
All parameters are tested @ 25
C except where indicated.
Note 1: Measured @ 1V bias, 1M
HZ
All parameters are tested using a FET TEST
TM
model 3600
SMT50
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