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3 - Electrical overstress in a nut shell

Part 3 - Electrical overstress in a nut shell

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Part 3 - Electrical overstress in a nut shell

Art Kay

Jul 10 2014 11:31 AM

Comments 0

In the last electrical overstress blog we introduced the
specifications for a Transient Voltage Suppressor (TVS).  In this blog we
will go through a step by step procedure for selecting the best TVS for your
design.

1.
Consider the package size and maximum power dissipation.  The power
rating for a TVS is generally calculated as the maximum clamping voltage
multiplied by the maximum peak pulse current (PPP =
IPP∙VC).  The maximum power dissipation is for
typically for a 1ms pulse.  One advantage to using a TVS with a high power
rating is that the slope of the V-I curve in the (breakdown) region is very
steep.  We will discuss the V-I characteristic in the clamp region in more
detail later, but the important point is that the voltage regulation during a
fault condition is better for the device with the higher power rating for a
given fault current.

Notice
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
package.  However, if may not be necessary to have a TVS for each
amplifier.  One TVS may be used for the entire supply bus, or perhaps a few
could be used for a large PCB.  In this example, we will choose the 400W
package to minimize the TVS size.  In later calculations will see how the
fault voltage increases for large fault
currents.

Figure
1: Example package size and power dissipation for TVS

2.
Choose the reverse standoff voltage to be equal to the maximum normal operating
supply voltage for your application.  In this example, the largest
supply voltage is equal to the specified maximum (±18V fromTable 3), so
we will select the standoff voltage to be 18V.  As long as the supply is
less than the reverse standoff voltage, the TVS leakage current will remain
relatively low (typically micro-amps) and is generally specified by over the
full temperature range in the TVS data sheet.  Table 1 shows some examples
of a TVS with an 18V standoff voltage and their associated 5uA and 10uA leakage
currents.

Table
1: Example specifications for a Transient Voltage Suppressor (400W and 5000W
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
Table for OPA192

VALUE

UNIT

Supply voltage

±20 (+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

±10

mA

Output short circuit

Continuous

Operating temperature

-55 to +150

℃

Storage temperature

-55 to +150

℃

Junction temperature

+150

℃

Electrostatic discharge (ESD) ratings

Human Body Model (HBM)

4

kV

Charged device model (CDM)

1

kV

Table 3: Operating
Specifications for OPA192

ELECTRICAL CHARACTERISTICS

At TA = +25C, VCM = VOUT = VS/2, and
RLOAD = 10kΩ 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 = 0A

1

1.2

mA

TA = -40C to 125C, IO = 0A

1.5

mA

3. Determine the
relationship between voltage and current when the TVS device is turned on and
protecting the amplifier.  To understand this relationship, you will
need to consider the breakdown voltage (VBR) and the clamp voltage
(VC).  The breakdown voltage (VBR) is the point at
which TVS protection begins to breakdown or “turn on” and starts to draw
significant current.  Increasing current through the TVS will cause the
voltage across it to increase somewhat.  This increase happens from series
resistance and self heating of the device.  The clamp voltage is the
voltage across the TVS for a specified large current.  In the example, the
clamp voltage is 29.2V for a 13.7A current (see Table 1, 400W option).  The
goal of the TVS is to limit the voltage to less than the amplifiers absolute
maximum rating.  The actual voltage across the TVS will depend on the
current through it, and the voltage will be between the breakdown voltage and
the clamp voltage.

Referring to Figure 2, you can see that the V-I
characteristic between the breakdown voltage and clamp voltage is linear.
It is possible to use these two points to derive an equation for the line.
This equation can be used to determine the reverse voltage for any current
between the breakdown current and the maximum peak pulse current.  This
equation will tell us the maximum voltage that our application will see based on
the expected fault current.  Equations (1) to (3) show the linear equation
as well as the derivation of the slope and intercept for this example.

Figure 2: V-I Curve for a TVS
Device

4. Estimate the worst case current that will
flow in the TVS during a fault, and calculate the corresponding fault
voltage. The ultimate goal is to limit any over voltage conditions to a safe
level (i.e. below the absolute maximum supply voltage).  In order to
estimate the fault voltage we really need to know what fault current we can
expect.  The expected fault current depends on the application.  If
the supply is connected to an inductive load, or the application is in the
presence of strong electromagnetic fields the fault current may be high.
If the supply is well behaved and the application is in a shielded environment
the fault current may be low.  Making this estimate may involve some
engineering assumptions, and will depend on how conservative you want to
be.  For our example problem I will choose a fault current if 2A.
Using the linear equation, the fault voltage is limited to 23.14V, see Equation
(4).

5.
Compare the fault voltage to the amplifiers absolute maximum voltage.
The absolute maximum voltage is ±20V for this example, see Table 2.
The maximum fault voltage in this example is 23.14V for a 2A fault, see Equation
(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
this requirement.  Below are three options to consider if the fault voltage
is greater than the absolute maximum voltage.

a. Use a lower nominal operating supply
voltage.  In this example, we set the nominal supply voltage to ±18V,
so the standoff voltage (VR) was also set to 18V.  The absolute maximum for
this device ±20V.  There is only a 2V difference between the operating
voltage and the absolute maximum voltage.  If you intentionally use a lower
nominal supply voltage (e.g. ±15V) , the margin between nominal supply and
absolute maximum is larger (e.g. Vmax – Vnominal = 20V –
15V = 5V).  This wider margin makes it easier to find a TVS that meets the
requirements.

b. Look for a better TVS.  Some TVS
diodes may be better suited to your application than others.  In this
example, we selected a TVS with a 400W rating.  Choosing a device with a
higher power rating would reduce the fault voltage at 2A.  In this example,
using a 5000W rather than a 400W TVS would reduce the fault voltage from 23.14V
to 22.59V.  Nevertheless, you will never find a device with ideal
characteristics, because there are limits to the manufacturing process.

c. Use the TVS even though the fault voltage is
greater than the absolute maximum supply voltage.  On the surface, it
sounds like a really bad idea to allow for fault voltages that exceed the
absolute maximum specification.  However, some protection is always better
than no protection.  In many cases it is not practical to find a TVS that
is off over the operating voltage range but turns on just before the absolute
maximum supply limit.  One thing to keep in mind is that there is some
margin in the absolute maximum specification.

6.
Consider the cost.  Once you have narrowed down your search, cost can
be used to make the final selection.

References:

Walters, Kent. “How To Select Transient Voltage Suppressors”, MicroNote
125, July 1999. http://www.ti.com/hpa-pa-opamp-thehub-20140711-part3-externalpage1-en

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

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OPA192,
art
kay, op
amps

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