The world is a hazardous place, and electronic systems are just as=20
susceptible to mechanical failures as they are to unseen electromagnetic =
radiation. Let=E2=80=99s take a look at how to protect circuit designs =
from some of=20
these visible and invisible perils.
A world inhospitable to electronics
It isn=E2=80=99t enough that the complexity of systems is =
ever-increasing; designers=20
now need to worry about protecting their circuitry from both man-made =
and=20
naturally occurring phenomenon such as electrostatic discharge (ESD), =
lighting,=20
electromagnetic interference (EMI), mechanical failure (such as =
equipment drops)=20
and many more. However, the good news is that for a small additional =
cost, you=20
can circumvent many of these issues and protect the end user=E2=80=99s =
investment by=20
protecting the circuitry.
The first, most obvious problem is the users themselves. People can =
become=20
very large capacitors, especially in dry climates indoors during winter =
or=20
desert conditions elsewhere. ESD, or electrical overstress (EOS), is a =
leading=20
cause of electronic death for many systems. ESD may occur when a =
buildup=20
of electric charge on an object or person discharges into an electronic =
system.=20
The charge naturally wants to dissipate, and that can happen extremely =
rapidly.=20
A classic example of an electrostatic discharge is when you are scuffing =
across=20
the floor, and you touch a grounded conductor such as a screw on a wall =
switch=20
or another person. The electric field intensity gets so high between =
these two=20
points that it ionizes the air, allowing a flow of electrons. It takes =
about=20
70,000V to breach one inch of dry air =E2=80=A6 and only several hundred =
volts to damage=20
some semiconductors.
By placing a simple low-cost device called a transient voltage =
suppressor=20
(TVS) diode at any location where a connector leads into the circuitry =
can help=20
prevents ESD damage to the system. Many of these TVS devices are =
designed for=20
specific applications, such as the SN65220-Q1=20
designed for USB ports. Others are designed for higher-speed =
interfaces=20
where capacitive loading can be an issue.
Another problem with users is that they are prone to dropping things =
like=20
handheld equipment or smartphones. Most systems can handle mechanical =
shock, but=20
some cannot, such as a rotating media hard drive in a laptop. Excessive =
G-force=20
can damage hard drives and in many cases cause data loss. As strange as =
it=20
sounds, detecting a fall is extremely beneficial: adding a simple =
accelerometer=20
can detect the beginnings of a fall via a system management processor =
which can=20
quickly park drive heads in a position to prevent damage and data =
loss.
Following a crash or other system disruption, additional damage may =
occur=20
when there is an upset to the normal operating parameters (a latch-up =
condition=20
caused by an electromagnetic wave, ESD or other event) or damage to the =
device=20
caused mechanically which can cause open or short circuits. A simple but =
very=20
effective method for protecting the system is to monitor the power =
supplies by=20
sensing the output voltage and current. If the voltage sags and the =
current=20
rises, it=E2=80=99s a good bet that something is drawing current that =
shouldn=E2=80=99t, such as=20
when a paper clip pokes into a keyboard causing a short circuit.
Monitoring the voltage is fairly straightforward, but monitoring =
current is a=20
bit harder. If you sense the return current in the power supply, it is=20
ground-referenced and easy to measure (Figure 1). But if you want to =
sense=20
current in the supply (high) side, you must do a differential =
measurement across=20
the sensing resistor. Most designers want to use a very small value for =
this=20
resistor so as to not impact efficiency. The problem is that you need =
high gain=20
=E2=80=93 along with a very low input offset and a high common-mode =
voltage =E2=80=93 to=20
accurately measure the current that the power supply is providing.
Figure 1: Low-side sensing with a precision =
amplifier to=20
protect a load from overcurrent conditions
Fortunately, there are devices designed to measure the current in the =
high=20
side of a power supply. One example is the INA219,=20
which includes a programmable gain amplifier (PGA), analog-to-digital =
converter=20
(ADC) and I2C digital interface.
Figure 2 shows a typical high-side application. By simply reading =
registers=20
within the device, you can obtain the output voltage, current and power. =
If the=20
readings go out of range, a system monitor can turn off the =
power=20
supply before damage occurs. If a faster response is required, an analog =
output=20
version such as the LMP8480,=20
along with a high-speed comparator, can shut down the power supply.
![3D"]()
=
Figure 2: High-side intelligent sensing example =
using the=20
INA219 with a digital interface
Conclusion
Protecting circuitry from the environment can be pretty easy, and the =
choices=20
available to designers for protection devices have increased =
significantly. So=20
if you don=E2=80=99t want your designs ending up as a paperweight, =
remember to build in=20
circuit protection from the power supplies out to the =
connectors=E2=80=A6 till next=20
time!
Additional resources