In my last post, I talked about how integration has simplified 3-wire analog output =
design.=20
In this post, I=E2=80=99ll show you one way to protect these designs =
against dangerous=20
industrial transients that can cause electrical overstress.
Let=E2=80=99s start with a few examples of what we=E2=80=99re =
trying to protect the system=20
against:
- Some systems are installed or calibrated in environments that are =
not=20
ESD-safe, which can lead to ESD damage.
- Industrial control systems are often large systems that span great =
distances and may be exposed to hazards from nature, like lightning=20
strikes.
- Switching transients coupled with environmental parasitics can =
create=20
high-frequency radiated and coupled emissions.
The transients you need to protect the analog output from are very =
dissimilar=20
from the low voltage (< 24V) and low frequency (<10kHz) signals it =
generates. Industrial transients are high voltage, up to 15kV, and high=20
frequency, typically with a period of less than 100ns. Your circuit =
should take=20
advantage of these differences to provide protection, while not =
disturbing the=20
signal integrity of the analog output.
Figure 1. Attenuation and =
diversion=20
overview
Attenuation and diversion can be used to capitalize on the =
high-frequency and=20
high-voltage components of the industrial transients. Figure 2 shows a=20
protection circuit that uses these strategies to protect the =
single-channel,=20
16-bit DAC8760 for 4-20mA current loop applications.
Figure 2. Example protection=20
circuit
Attenuation leverages passive components with frequency-based =
responses, like=20
ferrite beads and capacitors, to attenuate high-frequency signals. In =
Figure 2,=20
the 100nF capacitor on each output terminal interacts with the source =
impedance=20
of the transient generator to attenuate high-frequency =
signals.
I included series pass elements between each stage of the circuit to =
limit=20
current flow between nodes that are clamped to different voltage =
potentials. I=20
used resistors as the series pass elements for the current output and =
nodes that=20
are inside the voltage output. Ferrite beads act as the series pass =
element in=20
the voltage output path outside of the feedback loop to maintain DC =
accuracy=20
while still limiting high frequency current.
Diversion uses diodes to redirect the high-voltage signals away from =
the=20
analog signal chain. You can then either steer the energy to ground by =
using=20
transient voltage suppressor (TVS) diodes or to a supply rail by using =
Schottky=20
clamp-to-rail diodes.
If you=E2=80=99d like to learn more about TVS diodes, I encourage you =
to check out=20
this blog=20
series from my colleague Art Kay, in which he explains several key=20
parameters and provides selection tips.
In short, you should select TVS diodes based on the following;
- Working voltage: The maximum voltage that the diode can be =
exposed=20
to without conducting significant current. This should be high enough =
to=20
ensure the diode does not impact normal circuit operation.
- Breakdown voltage: The voltage that causes the TVS diode to begin =
to=20
conduct. This should be low enough to keep transient voltages within =
the=20
supply rails.
- Power rating: When the diode does breakdown, it will pass =
significant=20
power and needs to be rated accordingly.
Figure 2 also includes a clamp-to-rail stage using Schottky diodes, =
which=20
helps keep transients within the supply rails for two reasons:
- TVS diode breakdown voltages rarely match supply =
configurations.
- The TVS diode breakdown voltage will increase as it passes more=20
current.
While all Schottky diodes should feature low forward voltage, diodes =
used in=20
protection circuits need to maintain low forward voltage even when =
conducting=20
large current.
The circuit in Figure 2 was developed for TIPD153, a CerTIfied TI Precision Design for =
protecting the=20
DAC8760=20
against the IEC6100-4 test suite. For more information on component =
selection,=20
layout guidelines and the IEC6100-4 test results of the design, download =
the reference design guide.
If you have any questions on how to protect 3-wire analog outputs, =
let me=20
know in the comments section below. And be sure to check back next =
month, when=20
I=E2=80=99ll explain how to design a robust 2-wire, 4-20mA =
transmitter.
Related materials: