In the last electrical overstress blog we introduced the =
specifications for a Transient Voltage Suppressor (TVS). In this =
blog we=20
will go through a step by step procedure for selecting the best TVS for =
your=20
design.
1.=20
Consider the package size and maximum power dissipation. The =
power=20
rating for a TVS is generally calculated as the maximum clamping voltage =
multiplied by the maximum peak pulse current (PPP =3D=20
IPP=E2=88=99VC). The maximum power =
dissipation is for=20
typically for a 1ms pulse. One advantage to using a TVS with a =
high power=20
rating is that the slope of the V-I curve in the (breakdown) region is =
very=20
steep. We will discuss the V-I characteristic in the clamp region =
in more=20
detail later, but the important point is that the voltage regulation =
during a=20
fault condition is better for the device with the higher power rating =
for a=20
given fault current.
Notice=20
in Figure 1 that the power dissipation is related to package size. =
Unfortunately, the package size of a TVS can be large compared to the =
amplifier=20
package. However, if may not be necessary to have a TVS for each=20
amplifier. One TVS may be used for the entire supply bus, or =
perhaps a few=20
could be used for a large PCB. In this example, we will choose the =
400W=20
package to minimize the TVS size. In later calculations will see =
how the=20
fault voltage increases for large fault=20
currents.
Figure=20
1: Example package size and power dissipation for =
TVS
2.=20
Choose the reverse standoff voltage to be equal to the maximum normal =
operating=20
supply voltage for your application. In this example, the =
largest=20
supply voltage is equal to the specified maximum (=C2=B118V =
fromTable 3), so=20
we will select the standoff voltage to be 18V. As long as the =
supply is=20
less than the reverse standoff voltage, the TVS leakage current will =
remain=20
relatively low (typically micro-amps) and is generally specified by over =
the=20
full temperature range in the TVS data sheet. Table 1 shows some =
examples=20
of a TVS with an 18V standoff voltage and their associated 5uA and 10uA =
leakage=20
currents.
Table=20
1: Example specifications for a Transient Voltage Suppressor (400W and =
5000W=20
options)
Part Number |
Reverse Standoff Voltage
VR |
Leakage
IR @ VR |
Breakdown Voltage
VBR |
Breakdown Current
IBR |
Maximum Clamping Voltage
VC |
Maximum Peak Pulse Current IPP |
Peak Power PPP |
Min |
Max |
SMAJ18A |
18V |
5uA |
20.0V |
22.1V |
1mA |
29.2V |
13.7A |
400W |
5.0SMDJ18A |
18V |
10uA |
20.0V |
22.1V |
1mA |
29.2V |
172A |
5000W |
Table 2: Absolute Maximum =
Ratings=20
Table for OPA192
|
VALUE |
UNIT |
Supply voltage |
=C2=B120 (+40, single supply) |
V |
Signal input terminals |
Voltage |
Common-mode |
(V-) - 0.5 to (V+) + 0.5 |
V |
Differential |
(V+) - (V-) + 0.2 |
V |
Current |
|
=C2=B110 |
mA |
Output short circuit |
Continuous |
|
Operating temperature |
-55 to +150 |
=E2=84=83 |
Storage temperature |
-55 to +150 |
=E2=84=83 |
Junction temperature |
+150 |
=E2=84=83 |
Electrostatic discharge (ESD) ratings |
Human Body Model (HBM) |
4 |
kV |
Charged device model (CDM) |
1 |
kV |
Table 3: =
Operating=20
Specifications for OPA192
ELECTRICAL CHARACTERISTICS
At TA =3D +25C, VCM =3D VOUT =
=3D VS/2, and=20
RLOAD =3D 10k=E2=84=A6 connected to Vs/2, unless otherwise noted.
PARAMETER |
TEST CONDITIONS |
MIN |
TYP |
MAX |
UNIT |
POWER SUPPLY |
VS |
Specified voltage range |
|
+4.5 |
|
+36 |
V |
IQ |
Quiescent current per amplifier |
IO =3D 0A |
|
1 |
1.2 |
mA |
TA =3D -40C to 125C, IO =3D 0A |
|
|
1.5 |
mA |
3. Determine =
the=20
relationship between voltage and current when the TVS device is turned =
on and=20
protecting the amplifier. To understand this relationship, you =
will=20
need to consider the breakdown voltage (VBR) and the clamp =
voltage=20
(VC). The breakdown voltage (VBR) is the =
point at=20
which TVS protection begins to breakdown or =E2=80=9Cturn on=E2=80=9D =
and starts to draw=20
significant current. Increasing current through the TVS will cause =
the=20
voltage across it to increase somewhat. This increase happens from =
series=20
resistance and self heating of the device. The clamp voltage is =
the=20
voltage across the TVS for a specified large current. In the =
example, the=20
clamp voltage is 29.2V for a 13.7A current (see Table 1, 400W =
option). The=20
goal of the TVS is to limit the voltage to less than the amplifiers =
absolute=20
maximum rating. The actual voltage across the TVS will depend on =
the=20
current through it, and the voltage will be between the breakdown =
voltage and=20
the clamp voltage.
Referring to Figure 2, you can see that =
the V-I=20
characteristic between the breakdown voltage and clamp voltage is =
linear. =20
It is possible to use these two points to derive an equation for the =
line. =20
This equation can be used to determine the reverse voltage for any =
current=20
between the breakdown current and the maximum peak pulse current. =
This=20
equation will tell us the maximum voltage that our application will see =
based on=20
the expected fault current. Equations (1) to (3) show the linear =
equation=20
as well as the derivation of the slope and intercept for this =
example.
Figure 2: V-I Curve for a TVS=20
Device
4. Estimate the worst case current =
that will=20
flow in the TVS during a fault, and calculate the corresponding fault=20
voltage. The ultimate goal is to limit any over voltage conditions =
to a safe=20
level (i.e. below the absolute maximum supply voltage). In order =
to=20
estimate the fault voltage we really need to know what fault current we =
can=20
expect. The expected fault current depends on the =
application. If=20
the supply is connected to an inductive load, or the application is in =
the=20
presence of strong electromagnetic fields the fault current may be =
high. =20
If the supply is well behaved and the application is in a shielded =
environment=20
the fault current may be low. Making this estimate may involve =
some=20
engineering assumptions, and will depend on how conservative you want to =
be. For our example problem I will choose a fault current if =
2A. =20
Using the linear equation, the fault voltage is limited to 23.14V, see =
Equation=20
(4).
5.=20
Compare the fault voltage to the amplifiers absolute maximum =
voltage. =20
The absolute maximum voltage is =C2=B120V for this example, see =
Table 2. =20
The maximum fault voltage in this example is 23.14V for a 2A fault, see =
Equation=20
(4). Ideally the fault voltage should be less than or equal to the =
absolute maximum to prevent damage to the device. This TVS does =
not meet=20
this requirement. Below are three options to consider if the fault =
voltage=20
is greater than the absolute maximum voltage.
a. Use a lower nominal operating =
supply=20
voltage. In this example, we set the nominal supply voltage to =
=C2=B118V,=20
so the standoff voltage (VR) was also set to 18V. The absolute =
maximum for=20
this device =C2=B120V. There is only a 2V difference between the =
operating=20
voltage and the absolute maximum voltage. If you intentionally use =
a lower=20
nominal supply voltage (e.g. =C2=B115V) , the margin between nominal =
supply and=20
absolute maximum is larger (e.g. Vmax =E2=80=93 =
Vnominal =3D 20V =E2=80=93=20
15V =3D 5V). This wider margin makes it easier to find a TVS that =
meets the=20
requirements.
b. Look for a better TVS. =
Some TVS=20
diodes may be better suited to your application than others. In =
this=20
example, we selected a TVS with a 400W rating. Choosing a device =
with a=20
higher power rating would reduce the fault voltage at 2A. In this =
example,=20
using a 5000W rather than a 400W TVS would reduce the fault voltage from =
23.14V=20
to 22.59V. Nevertheless, you will never find a device with ideal=20
characteristics, because there are limits to the manufacturing =
process.
c. Use the TVS even though the fault =
voltage is=20
greater than the absolute maximum supply voltage. On the =
surface, it=20
sounds like a really bad idea to allow for fault voltages that exceed =
the=20
absolute maximum specification. However, some protection is always =
better=20
than no protection. In many cases it is not practical to find a =
TVS that=20
is off over the operating voltage range but turns on just before the =
absolute=20
maximum supply limit. One thing to keep in mind is that there is =
some=20
margin in the absolute maximum specification.
6.=20
Consider the cost. Once you have narrowed down your search, =
cost can=20
be used to make the final selection.
References:
- Walters, Kent. =E2=80=9CHow To Select Transient Voltage =
Suppressors=E2=80=9D, MicroNote=20
125, July 1999. http://www.ti.com/hpa-pa-opamp-thehub-20140711-part3-ex=
ternalpage1-en
- STMicroelectronics, =E2=80=9CESDA-1K Data Sheet=E2=80=9D, Doc ID =
17883 Rev 1, September=20
2010, V-I curve, Page 2.=20
http://www.st.com/web/en/press/c2747