In the last electrical overstress blog we introduced the =
absolute=20
maximum specification table and described how a series resistor can be =
used to=20
protect the inputs from electrical overstress. Exceeding the power =
supply=20
voltage is another common overstress issue. One possibility is =
that a=20
large transient voltage is coupled into the power supply. This can =
happen=20
from inductive kickback of a load such as a motor start up. Large=20
transient voltage on the supply is a common problem in many real world =
systems,=20
and you should always design to protect your application from this=20
problem.
The most common way to protect =
against power=20
supply transients is to have a transient voltage suppressor (TVS) on =
each=20
supply. The TVS will limit the supply voltage to a safe level so =
that the=20
maximum supply voltage is not exceeded. We introduced the TVS in =
the=20
previous blog. In this blog we will continue the discussion by =
explaining=20
the TVS specifications.
Figure 1 illustrates the typical =
V-I=20
characteristics for a TVS device. As was mentioned previously, the =
device=20
behaves much like a zener diode except that it is optimized to react =
quickly to=20
large transient currents. Also, the specifications of a TVS =
highlight key=20
characteristics that are important to protection against transient=20
overvoltage. Table 1 gives an example TVS =
specification. =20
Notice that all the specifications correspond to key points on the V-I=20
curve. As we describe the specifications refer to the V-I curve to =
help=20
clarify the meaning of the specification.
Figure=20
1: V-I Characteristics for a=20
TVS
Table =
1:=20
Example specifications for a Transient Voltage Suppressor=20
The reverse=20
standoff voltage (VR) is the normal operating voltage for the =
TVS=20
device. At this voltage level, the device is effectively =
=E2=80=9Coff=E2=80=9D and will=20
have a specified leakage current. The leakage current =
(IR) is=20
normally in the micro-amp level. This current adds to the total =
power=20
supply current and can be a concern for low power applications. =
Increasing the=20
supply beyond the reverse standoff voltage will cause leakage to =
increase, and=20
the TVS will eventually breakdown when the breakdown voltage =
(VBR) is=20
reached. Figure 1 shows where VR is on the TVS V-I =
curve. =20
From Table 1 you can see that VR =3D 18V, and IR =
=3D 5=C2=B5A in=20
the example TVS specification.
The=20
breakdown voltage (VBR) is the point at which TVS protection =
begins=20
to breakdown or =E2=80=9Cturn on=E2=80=9D and starts to draw significant =
current. After it=20
breaks down, the voltage across the TVS will =E2=80=9Cclamp=E2=80=9D to =
a relatively constant=20
voltage. From Table 1 you can see that VBR ranges from =
20V to=20
22.1V, with a 1mA breakdown current (IBR) for our =
example TVS=20
specification.
Referring=20
to figure 1, notice that although the voltage is relatively constant =
after=20
breakdown, there will be some voltage increase with increasing =
current. =20
The clamp voltage (VC) is defined to help understand how =
voltage=20
increases across the TVS after breakdown. The clamp voltage is the =
voltage=20
drop across the TVS when it is on and drawing significant current. =
For=20
some devices a few different clamp voltages are given at different =
current=20
levels. In this example, the TVS maximum clamp voltage is given as =
VC =3D 29.2V at a current of IPP =3D 13.7A. =
In the next=20
blog we will learn how to estimate the clamp voltage for different =
current=20
levels.
In=20
this blog, we discussed the key specifications for TVS devices. In =
the=20
next blog we will show how to select a TVS device that will protect your =
application. The first post in this series is here.=20
References:
1. =20
Walters, Kent. =E2=80=9CHow To Select Transient Voltage =
Suppressors=E2=80=9D, MicroNote 125,=20
July 1999. www.microsemi.com
2. =20
STMicroelectronics, =E2=80=9CESDA-1K Data Sheet=E2=80=9D, Doc ID 17883 =
Rev 1, September 2010,=20
V-I curve, Page 2. http://www.st.com/web/en/press/c2747