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Texas Instruments Protecting I/O modules from surge events Application notes
Protecting I/O modules from surge events
Cameron Phillips
I/O Modules
VDD
In most industrial applications, the system architecture
consists of a central PLC that controls and
communicates with peripherals like sensors, solenoids,
and valves that are stationed all around the
environment. The PLC communicates with these
through digital signal outputs that are centralized in
PLC Modules called I/O Modules. Ensuring the
reliability of the I/O modules is critical to maintaining a
safe and efficient industrial environment.
Design Challenges for Surge on I/O Modules
• Protection of outputs vs inputs (low impedance vs
high impedance)
• Design considerations for clamping voltages
variations over temp and devices
What is Surge?
Surge is a transient event that happens when a large
energy source is coupled onto at electrical system that
causes a large current and voltage spike. This event is
defined in IEC61000-4-5 as a short circuit current
pulse of 8-µs rise time and 20-µs time to half. The
amount of surge current varies from what environment
the equipment is in and what energy sources can
couple onto the lines. I/O modules are used in
industrial applications that can have surge events
coupled onto them even though they are not directly
touching a place that lightning can strike. For more
information about the causes of surge check out
Demystifying Surge Protection.
What in I/O Modules needs protection?
Input-Output modules have all sorts of circuits that
hang off of their lines. For instance, analog input
modules are normally fed into comparators which,
while they do sometimes have a series resistance that
can limit current, during surge events these resistors
will have to dissipate a lot of heat causing them to
have to be bigger. Another similar example is analog
output modules. These outputs can sometimes see
large surge transients that can harm the small
resistance outputs and especially damage the small
internal ESD cells.
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AT RISK
Surge
DAC
VDAC
+
A1
Absorption
Device
VOUT
TVS1401
RFB
RG1
V SENSE
VREF
RG2
GND
Figure 1. Analog Output Module Components
Digital Output modules need surge protection as their
outputs are typically very low impedance. Digital
Output modules can consist of power deliver modules
like low RON switches that turn on and off relatively
high currents. With these low impedance outputs,
typically source of MOSFET, when a surge event is
applied the current has a very easy path through the
device and can break anything attached. In the figure
below, the light red boxes show what is at risk for
damage during a surge event.
Surge
3.5 to 40V
Supply Voltage
OUT
IN
EN
TPS27S100
TVS2700
DIAG_EN
MCU
General Resistive, Capacitive,
Inductive Loads
FLT
IMON
ILIM
GND
AT RISK
Figure 2. Digital Output Module Components
TI's Flat Clamp
Since these modules need protection, the question is
what can TI's Flat Clamp devices offer for their
protection? TI's Flat Clamp family of devices offer
advantages in the way of clamping voltage, ultra low
leakage, and package size.
Many of these I/O modules have absolute maximum
voltage ratings that are close to the operating voltage.
For instance, the ADS8689 has an input range of ±12
V with an absolute maximum voltage of ±20 V. Using
TI's Flat Clamp TVS1401, during a typical 1-kV Surge
event with 25 A needed to be dissipated, the clamping
voltage would be 19.4 V. This is important for insuring
Protecting I/O modules from surge events Cameron Phillips
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1
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the reliability of the input output module. This is
applicable for the entire Flat Clamp family as all of
them have extremely low clamping voltage relative to
their breakdown voltage. This means they will protect
sensitive equipment at any voltage range.
50
ISURGE [A]
VTVS [V]
45
VTVS3300 [V]
VCLAMP (V)
45
40
40
35
35
30
30
25
25
20
20
15
15
10
10
5
5
0
-1E-5
1E-5
3E-5
5E-5
t [s]
7E-5
SMA: 13mm 2
0
9E-5
D001
Figure 3. Surge Waveform for TVS3300 vs
Conventional TVS
In analog modules, input or output, leakage is
extremely important. These modules work by
measuring the voltage at a certain pin and any leakage
on that pin will change the value being read versus the
actual value on the pin. TI's Flat Clamp device such as
TVS0500 on analog input pins has a leakage of 0.07nA versus a typical SMA TVS diode that would have a
maximum of 1 to 10 µA. A difference of up to 6 orders
of magnitude.
Many of these modules have very strict constraints on
the size of their boards, therefore it is beneficial to
have the smallest footprint possible for each
component. Using TI's Flat Clamp technology, board
designers can get >70% area reduction for the TVS
device. Shown below is the comparison of the DBV
and DRB packages and the industrial standard SMA
and SMB packages. Having these small packages
allow the device to be placed closer to the connector.
This means the surge pulse will be shunted
immediately on the board instead of letting it travel
through the PCB trace that could effect the pulse itself.
2
SMB: 19.1mm
*Drawn to scale
ISURGE [A]
50
TVSxx00: 2x2mm SON
>70% area reduction
from SMA/SMB
2
TVSxx01 : 3x3mm SON
>50% area reduction
from SMA/SMB
Figure 4. Flat Clamp Package Size Comparison
In conclusion, the environment where the I/O modules
are found is susceptible to these surges and can
cause serious damage to the system if not properly
accounted for. TI's Flat Clamp family of devices can
help protect most of the different analog or digital input
and output modules from surge events.
Table 1. Alternative Device Recommendations
Device
Description
TVS0500,TVS0701
Unidirectional and Bidirectional
Device for Surge Protection of 5-V
I/O Modules
TVS1400,TVS1401
Unidirectional and Bidirectional
Device for Surge Protection of 12-V
I/O Modules
TVS2700,TVS2701
Unidirectional and Bidirectional
Device for Surge Protection of 24-V
I/O Modules
Table 2. Adjacent Resources
TVS Surge Rating: Power vs.
Current
Demystifying Surge Protection
Whitepaper
How to Select a Surge Diode
Demystifying Surge Protection
Video Series
Flat-Clamp TVS Layout in
SMA/SMB Footprint
TVS Surge Protection in HighTemperature Environments
Protecting I/O modules from surge events Cameron Phillips
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