Surging
Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
Low Capacitance Devices
The junction capacitance of a TVS diode is naturally
large. In high speed data transmission applications,
the extra capacitance introduced by protection devices
needs to be kept to a minimum.
Low capacitance devices are (Figure 1) are manufac-
tured by placing a low capacitance rectifier diode (D
1
) in
series but opposite in polarity with the TVS diode (D
2
).
This has the effect of adding another capacitor (C
1
) in
series with the junction capacitor of the TVS diode (C
2
).
Taking advantage of the relationship for series capaci-
tors (C
T
= C
1
* C
2
/C
1
+ C
2
) means the resulting capaci-
tance will be less than the smallest component in
series. By carefully choosing the rectifier, the effective
capacitance may be reduced by approximately two
orders of magnitude.
Using Low Capacitance Parts
For bidirectional applications, a TVS/rectifier pair is
connected in an anti-parallel configuration across the
line (Figure 2). In this configuration, D
1
& D
2
will con-
duct positive surges while D
3
& D
4
will conduct negative
surges. In unidirectional applications, designers need
to take into consideration the following:
In Figure 3a, if a positive surge occurs on the line, D
1
will conduct in the forward direction and the TVS D
2
will
avalanche and i
1
will flow through the device. If a
negative spike occurs however, surge current i
2
flows,
forward biasing D
2
while D
1
is subject to the full surge
in the reverse bias direction. The power handling
capability of D
1
will be exceeded resulting in heating
and destruction of the device. Adding an additional
diode D
3
in parallel (Figure 3b), ensures D
1
will not be
reversed biased under transient conditions. D
3
will
conduct i
2
under this condition, thus preventing failure.
Semtech has introduced devices for unidirectional
applications which incorporate D
3
as an integrated
part.
SI96-07
Figure 1
Figure 2
Figure 3
Revision 9/2000
1
www.semtech.com
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