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AN1826
APPLICATION NOTE
®
TRANSIENT PROTECTION SOLUTIONS:
Transil™ diode versus Varistor
A. BREMOND / C. KAROUI
Since the seventies, electronic modules are more and more present in our life. This is the case for our
entertainment, automotive, telecommunication, production tools, house access equipment, and it is not
really a secret that these smart functions are very sensitive to transients coming from human body itself
(ESD1), electromagnetic part switching and industrial transients (EOS2), atmospheric effects (lightning).
These surges are more often lethal for the electronic functions and the constant decrease of silicon
elementary cell size (transistor) makes them more and more compact but more and more fragile.
To assume a correct definition of a protection stage, we have to ask the two following questions :
- what we have to protect?
- against what we have to be protected?
The answer to the first question depends on both the technology used to realize the part to be protected
and normal operating waveform of the signal on the line to be protected. Needed parameters are the
maximum voltage that technology can withstand, the type of normal operating signal (unidirectional : 0-Vcc
or bi-directional : ±Vcc), the maximum and minimum voltages of this signal and its frequency. The answer to
the second one is closely linked to the environment where modules are operating. In each application we
have to focus on one type of disturbances, i.e. in telecom we are faced to atmospheric effects, in domestic
and industrial worlds, disturbances are EOS while in computer or mobile phones, the main surge cause is
due to ESD.
To avoid dramatic consequences on systems submitted to over voltage transients, the best way is to use
protection devices based on clamping action.
In this kind of products we have the choice between two technologies :
- silicon devices named Transil™ - so called TVS3
- and ceramic components - Varistor for instance.
Figure 1 shows the electrical characteristics of a
clamping device, this curve has two areas.
Fig. 1: Electrical characteristics of bi-directional
clamping devices.
I
The first one located between 0V axis and VRM4 is
the normal operating zone; in this area the signal is
not be affected by the transient suppressor (low
leakage current and negligible intrinsic capacitance).
IPP
The second one is the surge suppression zone in
which the surge is clamped at VCL5.
IRM
V CL = V BR + Rd × I PP
VBR6: knee voltage of the I/V curve
Rd: dynamical resistance.
IPP: current value during surge.
AN1826/0104
VRM VCL
VBR
V
1/5
AN1826 - APPLICATION NOTE
In this zone, the main property of the protection device is to have a clamping voltage as close as possible to
the operating voltage. This property is defined by the clamping factor CF.
CF =
Fig. 2: Electrical characteristics of unidirectional
clamping devices.
I
IPP
IRM
IR
VRM VCL
VBR
V
VR
V CL
V BR
When the signal moving through the line to be
protected is unidirectional (0 to +Vcc or -Vcc to 0)
the surge suppressor has to be unidirectional,
otherwise the protection function is not optimized.
Figure 2 shows the electrical characteristics of a
unidirectional clamping device. In this case the
positive behavior of the device is still the same
while the negative one is totally different.
Negative over voltage occurring on the protected
line is clamped to a typical forward drop voltage of
a diode.
Figure 3 gives the electrical characteristics of both
Transil™ diode and Varistor measured on curve
tracer. Both tested components have been chosen
in the same power and voltage (14V) ranges in
order to make an accurate comparison.
200
200
150
150
100
100
50
0
Varistor
-50
Current (µA)
Current (µA)
Fig. 3: Electrical characteristics measured on curve tracer.
50
0
-50
-100
-100
-150
-150
-200
-20
-10
0
Voltage (V)
10
20
Transil
-200
-20
-10
0
10
20
Voltage (V)
The first remark we can do, regarding these curves, is that the Varistor operates uniquely in bi-directional
mode while the Transil™ diode can be both uni or bi-directional (here this is a unidirectional type). The
second point is the leakage current which is higher for the Varistor particularly close to the knee of the
curve. The third point to focus on is the slope of these characteristics in the clamping zone (vertical part of
the curve after VBR), here one can easily see that the Transil™ diode slope is tighter than the Varistor one.
1
, so for the same surge current the clamping
And taking into account that the curve slope represents
Rd
voltage will be lower for the Transil™ and then its clamping factor will be better.
To verify this point we performed measurements on both Transil™ and Varistor during EOS and ESD
surge test conditions. The EOS tests are based on IEC61000-4-5 1.2/50µs (8/20µs current) composed
waveform while ESD surges were performed in accordance with IEC61000-4-2 system standard.
2/5
AN1826 - APPLICATION NOTE
Fig. 4: Varistor and Transil™ diode EOS electrical behaviors.
Tek Run: 10.0MS/s
[
Sample
T
Trig?
Tek Run: 10.0MS/s
]
[
Sample
T
Trig?
Varistor
[email protected]=37.2V
]
Transil™ diode
[email protected]=24.6V
Voltage
Voltage
Current
Current
2
1
2
1
Ch1
10.0V
Ch2
200mV
M 5.00µs
Ch2
96mV
Ch1
5.0 V
Ch2
200mV
M 5.00µs
Ch2
96mV
Figure 4 shows the behavior of both Varistor and Transil™ when submitted to EOS surges. Tests were
performed with 10A peak current, in such a condition the Varistor clamps at 37.2V while remaining voltage
across the Transil™ in the same condition is only 24.6V. That means in such a surge condition the
protected IC's input will see 12.6V less in case of Transil™ diode use; that leads to a better clamping factor
for Transil™ diode versus varistor.
Table 1: Clamping factor comparison.
Varistor
Transil™
VCL typ.(V) @ IPP = 10A
37.2
24.6
VBR typ. (V) @ 1mA
18.4
15.2
CF = VCL/VBR
2.02
1.62
Fig. 5: Varistor and Transil™ diode ESD electrical behaviors.
Varistor
Transil™ diode
3/5
AN1826 - APPLICATION NOTE
Figure 5 gives their behavior when submitted to ESD strikes. In this case the Transil™ diode is much faster
than the Varistor to clamp and to derivate surge energy. Turn-on time measured on a Transil is typically
below the 10 picosecond level, while a Varistor will be in the range of 500 to 1000 ps at the best. In the first
few ns, the parasitic inductance and reaction time generate >70V peak for the Varistor while this value is
limited to 55V only for the Transil™. After this phase, the maximum voltage is 30V for the Varistor while we
see only 15V across the Transil™ diode (very close to VBR).
Another point to take into account is the system reliability. So ensuring device reliability and avoiding
ageing phenomena are two major concerns. To check these two points, repetitive ESD tests have been
performed (100 pulses versus IEC61000-4-2 system standard 15kV contact).
Fig. 6: Varistor and Transil™ repetitive ESD test results.
TM
Varistor
Transil
200
200
150
150
100
Before
0
After
-50
Current (µA)
Current ( µA)
100
50
50
Before
0
after
-50
-100
-100
-150
-150
-200
-20
-200
-10
0
10
20
-20
-15
-10
-5
0
5
Voltage (V)
Voltage (V)
Figure 6 shows after these tests, Varistor leakage current drastically increases, while Transil™ one
remains the same. This phenomena is called ageing . So the integrity of the protected system will be
greatly dependent on the choice of the protection component.
Transil™ diode technology can be easily used in more complex integration where dedicated ESD
protection is required and space saving is a major concern. For instance this is particularly true in computer
and mobile phone applications. This is the case for USB link, which is based on plug and play connection.
This kind of surges is always omnipresent. Figure 7 gives downstream port dedicated circuit USBDF01W5,
which integrates either the ESD protections, the line matching resistors (Rt), the filtering capacitors (Ct)
and the pull down resistors. All this internal component are required by the USB standard.
Table 2: Comparison sum-up.
Fig. 7: USBDF01W5 circuit.
Rt
Transil™
Leakage current
--
++
ESD ruggedness
+++
+++
Protection efficiency
--
++
CF = VCL/VBR
-
++
Ageing
---
+++
Security
--
+++
D+ Out
D+ In
Ct
15k
Gnd
15k
Ct
D- In
D- Out
Rt
4/5
Varistor
AN1826 - APPLICATION NOTE
CONCLUSION
Transil™ devices assume better efficiency in terms of ESD protection and can be uni and bi-directional
suppression functions. The MOV7 are uniquely bi-directional devices and then are not optimized to protect
sensitive ICs inputs, because these are 90% of the time dedicated to uni-directional normal operating
signals. So IC inputs to be protected will be submitted to high voltages in the negative way during surge
clamping.
Remaining voltage (VCL) for both EOS and ESD across the line is lower in the case of Transil™ than for
Varistor.
Low leakage current for Transil™ device is an important parameter to take into account when portable
equipment (as mobile phone or mobile computer) are concerned. This point will improve autonomy of
these kinds of equipment. In term of reliability Transil™ device has no ageing phenomena regarding ESD
events while Varistors are subjected to it.
In many applications safer failure are requested, this characteristic consists on a failure in short circuit
when an over sized surge occurs across the protection device. This is the failure mode of Transil™ which
assumes total security of the protected equipment.
1
ESD: ElectroStatic Discharge
EOS: Electrical Over Stress
3
TVS: Transient Voltage Suppressor
4
VRM: Stand off voltage
5
VCL: Clamping voltage
6
VBR: Breakdown voltage
7
MOV: Metal Oxyde Varistor
2
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