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2 - Electrical overstress in a nut shell

Part 2 - Electrical overstress in a nut shell

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Art Kay

Jun 05 2014 21:38 PM

Comments 0

In the last electrical overstress blog we introduced the absolute
maximum specification table and described how a series resistor can be used to
protect the inputs from electrical overstress.  Exceeding the power supply
voltage is another common overstress issue.  One possibility is that a
large transient voltage is coupled into the power supply.  This can happen
from inductive kickback of a load such as a motor start up.  Large
transient voltage on the supply is a common problem in many real world systems,
and you should always design to protect your application from this
problem.

The most common way to protect against power
supply transients is to have a transient voltage suppressor (TVS) on each
supply.  The TVS will limit the supply voltage to a safe level so that the
maximum supply voltage is not exceeded.  We introduced the TVS in the
previous blog.  In this blog we will continue the discussion by explaining
the TVS specifications.

Figure 1 illustrates the typical V-I
characteristics for a TVS device.  As was mentioned previously, the device
behaves much like a zener diode except that it is optimized to react quickly to
large transient currents.  Also, the specifications of a TVS highlight key
characteristics that are important to protection against transient
overvoltage.   Table 1 gives an example TVS specification.
Notice that all the specifications correspond to key points on the V-I
curve.  As we describe the specifications refer to the V-I curve to help
clarify the meaning of the specification.

Figure
1: V-I Characteristics for a
TVS

Table
1:
Example specifications for a Transient Voltage Suppressor

The reverse
standoff voltage (VR) is the normal operating voltage for the TVS
device.  At this voltage level, the device is effectively “off” and will
have a specified leakage current.  The leakage current (IR) is
normally in the micro-amp level.  This current adds to the total power
supply current and can be a concern for low power applications. Increasing the
supply beyond the reverse standoff voltage will cause leakage to increase, and
the TVS will eventually breakdown when the breakdown voltage (VBR) is
reached.  Figure 1 shows where VR is on the TVS V-I curve.
From Table 1 you can see that VR = 18V, and IR = 5µA in
the example TVS specification.

The
breakdown voltage (VBR) is the point at which TVS protection begins
to breakdown or “turn on” and starts to draw significant current.  After it
breaks down, the voltage across the TVS will “clamp” to a relatively constant
voltage.  From Table 1 you can see that VBR ranges from 20V to
22.1V, with a  1mA breakdown current (IBR) for our example TVS
specification.

Referring
to figure 1, notice that although the voltage is relatively constant after
breakdown, there will be some voltage increase with increasing current.
The clamp voltage (VC) is defined to help understand how voltage
increases across the TVS after breakdown.  The clamp voltage is the voltage
drop across the TVS when it is on and drawing significant current.  For
some devices a few different clamp voltages are given at different current
levels.  In this example, the TVS maximum clamp voltage is given as
VC = 29.2V at a current of IPP = 13.7A.  In the next
blog we will learn how to estimate the clamp voltage for different current
levels.

In
this blog, we discussed the key specifications for TVS devices.  In the
next blog we will show how to select a TVS device that will protect your
application. The first post in this series is here.

References:

1.
Walters, Kent. “How To Select Transient Voltage Suppressors”, MicroNote 125,
July 1999. www.microsemi.com

2.
STMicroelectronics, “ESDA-1K Data Sheet”, Doc ID 17883 Rev 1, September 2010,
V-I curve, Page 2. http://www.st.com/web/en/press/c2747

$core_v2_language.FormatString($ti.GetResource('Blog_PostQuestionAnswerView_CommentsCountFormatString'),
$post.CommentCount)

operational
amplifier, art
kay, op
amps, overstress,
power
supply transients, SMAJ18A,
transient
voltage suppressor, series
resister

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