Texas Instruments | Considerations for Selecting Digital Isolators | Application notes | Texas Instruments Considerations for Selecting Digital Isolators Application notes

Texas Instruments Considerations for Selecting Digital Isolators Application notes
Considerations for Selecting Digital Isolators
Luke Trowbridge, Product Marketing Engineer, Isolation, Interface Group
Introduction
Isolation is a means of preventing DC and uncontrolled
AC currents between two parts of a system, while
allowing signal and power transfer between those two
parts. This isolation can be required in order to protect
human operators and prevent damage to expensive
processors in high voltage systems, break ground
loops in communication networks, and to communicate
to high-side devices. Digital isolators are a common
choice when looking to achieve galvanic isolation for
interfaces such as SPI, UART, I2C, RS-485, and RS232 in many different system applications, including
industrial automation systems, motor drives, medical
equipment, solar inverters, power supplies, and hybrid
electric vehicles. This application brief identifies key
considerations when selecting the right digital isolator
for a given application and provides a guide for
understanding the different choices from Texas
Instruments (TI) broad portfolio of digital isolators.
TI Isolation Technology
TI isolators use silicon-dioxide (SiO2) based, highvoltage capacitors to serve as the signal insulation and
dielectric in digital isolators. The digital isolator product
families later discussed use two thick SiO2 capacitors
in series - one on each side of the isolation barrier to
achieve high-voltage isolation. A graphical
representation of this barrier technique can be seen in
Figure 1. Compared to inductor based (polyimide)
insulators and traditional optocouplers, SiO2 provides
the highest dielectric strength, does not degrade with
exposure to ambient moisture, and can offer an
isolation barrier lifetime >100 years. For a deeper
explanation of TI's isolation technology refer to
Enabling high voltage signal isolation quality and
reliability.
VCCI
Isolation
Capacitors
VCCO
INx
High-voltage isolation performance of a digital isolator
is quantified at the component level by parameters
such as maximum transient isolation voltage (VIOTM),
isolation withstand voltage (VISO), maximum surge
isolation voltage (VIOSM), maximum repetitive peak
voltage (VIORM), working voltage (VIOWM), and commonmode transient immunity (CMTI). These parameters
represent a digital isolator's capability to handle highvoltage stresses of different magnitude and transient
profiles and are key to selecting the right digital
isolator for specific system requirements.
• Maximum transient isolation voltage (VIOTM):
Defined by IEC 60747-5-5 and VDE 0884-11 as the
peak transient voltage that the isolator can handle
for up to 60 seconds without breaking down. Arcing
or load changes on a system power supply can
cause disturbances where the voltage could briefly
become several times that of the line voltage. An
isolator must be able to handle these over voltages
without damage.
• Isolation withstand voltage (VISO): Similar to the
VIOTM, isolation withstand voltage is defined per UL
1577 as the root mean square (rms) value of
voltage that the isolator can handle without
breakdown for 60 seconds. The difference is the
value is given in an rms instead of a peak voltage.
• Maximum surge isolation voltage (VIOSM):
Quantifies the ability of the isolator to withstand
very high voltage impulses of a certain transient
profile. This waveform is shown in Figure 2. This
parameter represents direct and indirect lighting
strikes. As per IEC 60747-5-5 and VDE 0884-11,
an isolator claiming a certain VIOSM must pass the
surge test at a peak voltage of 1.3 times VIOSM for
basic isolation, and 1.6 times VIOSM for reinforced
isolation. A digital isolator can be called reinforced
at the component level, only if it passes the surge
test at a level greater than 10kV.
OUTx
GNDI
GNDO
Figure 1. Series Capacitor Isolation
Key Isolation Specifications
Before choosing the right digital isolator, it is important
for designers to know the isolation specification
requirements for their given system application. Once
this is known, how does a designer know how much
protection a device can provide, and what are the
maximum voltages an isolator can withstand?
SLLA426 – July 2018
Submit Documentation Feedback
Figure 2. Surge Transient Waveform
Considerations for Selecting Digital Isolators Luke Trowbridge, Product Marketing Engineer,
Copyright © 2018, Texas Instruments Incorporated
Isolation, Interface Group
1
www.ti.com
•
•
•
Maximum repetitive peak voltage (VIORM):
Defined in IEC 60747-5-5 and VDE 0884-11 as the
maximum repetitive peak voltage that the isolator
can withstand. This specification is intended to
qualify the ability of an isolator to handle high
voltage across its barrier on a continuous, day-today basis.
Working Voltage (VIOWM): Similar to the VIORM,
working voltage is the maximum rms, or equivalent
dc voltage, that the isolator can withstand over a
specified long lifetime. Again, the difference is the
value is given in an rms instead of a peak voltage.
CMTI: Common-mode transient immunity is the
ability of an isolator to tolerate high-slew-rate
voltage transients between its two grounds without
corrupting signals passing through it, which could
potentially cause bit errors. In some applications,
these bit errors caused by the transients can result
in dangerous short-circuit events. Higher CMTI
indicates a more robust isolation channel.
Packages also dictate the isolation channel counts and
solution size of a digital isolator. Drawn to scale
images of TI's digital isolator package options can be
seen in Figure 3 and the measurements, creepage,
and channel counts of each package can be found in
Table 1.
Figure 3. Packages
Table 1. Package Properties
Package
Identifier
Length (mm)
Width
(mm)
Creepage
(mm)
Channels
8-pin SOIC
D
4.90
3.91
4
1,2
16-pin SSOP
DBQ
4.90
3.90
3.7
3,4,6
8-pin SOIC
DWV
5.85
7.50
8.5
2
16-pin SOIC
DW
10.30
7.50
8
1,2,3,4,6
16-pin SOIC
DWW
10.30
14.00
14.5
1,2,3,4
Additional explanation on each of these isolation
parameters can be found in High-voltage reinforced
isolation: Definitions and test methodologies. The
isolation certifications that have been mentioned
ensure that your applications meet worldwide industry
standards. Use these tables to check which TI devices
meet each certification requirement.
Since the package of a digital isolator has a direct
effect on the isolation performance of an isolator,
packaging specifications must be considered when
selecting the right device for a given application. The
isolation capabilities of each product family and
package can be found in Table 3.
Package Options
Conclusion
Creepage and clearance are the distance along the
surface of the package and through the air between
pins on one side of the isolator to the pins on the other
side. This distance is mandated by system level
standards based on parameters such as isolation
voltage requirements, material group of the isolator's
package mold compound, comparative tracking index
(CTI) and altitude. CTI indicates the ability of the
package mold compound to handle steady high
voltage without surface degradation. A higher CTI
allows the use of smaller packages for the same
working voltage. TI offers package options with
creepage distances of up to 14.5-mm and the isolator
families shown in Table 3 both guarantee a CTI of
>600 V.
This application brief serves as an introduction to
some of the key considerations important to selecting
and narrowing down TI's portfolio of digital isolators for
use in industrial and automotive designs. These
considerations are also useful when evaluating TI's
additional portfolio of isolated interface devices. For a
deeper explanation and analysis of the topics covered,
consider the related technical documents in Table 2.
Video series covering both these and similar isolation
topics can be found on TI's isolation overview page
and also in the Precision Labs training center.
Table 2. Related Technical Documents
Literature Number
Document Title
SSZY028
Enabling high voltage signal isolation quality and reliability
SLYY063
High-voltage reinforced isolation: Definitions and test
methodologies
SLLA284A
Digital Isolator Design Guide
SLYT649
Pushing the envelope with high-performance, digital-isolation
technology
Table 3. Digital Isolator Families
2
Device
Family
VIOTM (VPK)
VISO (VRMS)
VIOSM (VPK)
ISO77xx
4242, 7071,
8000
3000, 5000
4000, 5000,
8000
ISO78xx
8000
5700
8000
VIORM (VPK)
VIOWM (VRMS)
566, 637, 1414 400, 450, 1000
2121, 2828
1500, 2000
Considerations for Selecting Digital Isolators Luke Trowbridge, Product Marketing Engineer,
Isolation, Interface Group
Copyright © 2018, Texas Instruments Incorporated
Min CMTI
(kV/µs)
CTI (V)
Package
Options
85
>600
DBQ, D,
DWV, DW
100
>600
DW, DWW
SLLA426 – July 2018
Submit Documentation Feedback
IMPORTANT NOTICE AND DISCLAIMER
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE
DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”
AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY
IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD
PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate
TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable
standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you
permission to use these resources only for development of an application that uses the TI products described in the resource. Other
reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third
party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims,
damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on
ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable
warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2018, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising