Texas Instruments | Pushing the envelope with high-performance digital-isolation technology (Rev. A) | Application notes | Texas Instruments Pushing the envelope with high-performance digital-isolation technology (Rev. A) Application notes

Texas Instruments Pushing the envelope with high-performance digital-isolation technology (Rev. A) Application notes
Analog Applications Journal
Industrial
Pushing the envelope with highperformance, digital-isolation technology
By Anant Kamath
Systems Manager, Isolation, Interface group
Introduction
Reinforced isolation
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.
Electronic devices and integrated circuits (ICs) used
for isolation are called isolators. Isolation is required
in modern electrical systems for a variety of reasons.
Some examples include protecting human operators
and preventing damage to expensive processors in highvoltage systems, breaking the ground loop in communication networks, and communicating to high-side devices
in motor-drive or power-converter systems (Figure 1).
Examples of applications that need isolation include
industrial automation systems, motor drives, medical
equipment, solar inverters, power supplies, and electric
vehicles (EVs).
Recent advances in isolation technology are enabling
new solutions, reducing system cost, and allowing customers to push the performance envelope of their equipment.
This article discusses key end applications that are driving
cutting-edge innovations in isolation technology and benefitting from these innovations.
Reinforced isolators are devices that can provide insulation equivalent to two basic isolators in series. By themselves, reinforced isolators are considered sufficient to
ensure electrical safety against high voltage. However,
reinforced isolators must satisfy increasingly tighter
performance requirements. Motor-drive applications have
the most stringent specifications for reinforced isolation
because these systems use very high incoming supply
­voltages and they involve interfaces accessible to human
operators. The requirements for isolation in motor control
are defined in safety standards. For example, the IEC
61800-5-1 electrical, thermal and energy safety standard
for adjustable speed drives. According to this standard,
the requirements on reinforced isolation scale up with an
increase in the system voltage which is defined as the
root-mean-square (rms) voltage between incoming supply
lines and earth.
To guarantee reinforced insulation for drives with
system voltage greater than 600 VAC, an isolator must
withstand a 5-second temporary overvoltage of at least
4400 VRMS. This isolator also must have a surge voltage
Figure 1. Typical isolation configuration for a power drive system
Isolation
Control
Interface
System
Controller
High-Voltage Drivers
with Isolation
Power
Devices
High-Power
Equipment
Isolation
Feedback
Texas Instruments
18
AAJ 4Q 2015
Analog Applications Journal
Industrial
Figure 2. New motor drive architecture
AC Motor Drive
Rectifier Diodes
Isolated IGBT
Gate Drivers
IGBT Module
DC+
Drive
Output
PWM Signals
AC Power
Supply
DC–
DC–
M
DC–
Digital
Isolation
Link
RS-458, CAN,
Ethernet
Controller 1
Controller 2
and Comms
Module
DC–
Microprocessor
Isolated Current
and Voltage Sense
Microcontroller
DC–
DC–
Encoder
capability of at least 12 kVPK, creepage and clearance of at
least 14 mm, and a working voltage in the range of 600 VRMS
to 1000 VRMS, which depends on the supply line voltages
and drive architecture. To make matters more challenging,
new motor-drive architectures are adding local fieldprogrammable gate arrays (FPGAs) and controllers to the
power board referenced to the DC bus (Figure 2, controller 2). This reduces the local isolation requirement on the
power board. However, it increases the data rate and
bandwidth requirement on one, multichannel integrated,
high-speed reinforced link (Figure 2, digital isolation link).
Until recently, isolators meeting these isolation requirements, as well as the timing and data rate requirements,
were not available in the market. The only alternative was
fiber-optic isolation.
Capacitive reinforced isolation solutions are now available that meet the above requirements for high isolation,
wide packages, and high data rates. These solutions are a
good fit for motor-drive applications with system voltages
beyond 600 VAC. Some optocouplers do meet the highisolation requirements, but do not meet the data rate or
multichannel integration requirements. Additionally,
leading magnetic-isolation solutions do not meet the
requirement on working voltage/long-term reliability.
Texas Instruments
Examples of creepage and clearance are illustrated in
Figure 3.
Figure 3: Pictorial representation of
clearance and creepage
Clearance
Creepage
19
AAJ 4Q 2015
Analog Applications Journal
Industrial
High working voltage
waveform is the DC link voltage. For any isolator, its lifetime degrades exponentially with increased voltage stress
across the barrier. Solar and wind inverter systems target
lifetimes in excess of 25 years. It is critical to choose an
isolator that meets the working-voltage requirement with a
liberal margin so that the lifetime of the complete system
is not limited by the isolators.
New capacitive isolators achieve a working voltage of
1500 VRMS (2121 VPK). This voltage is a 50% increase
compared to commonly available devices, which enables
higher-voltage and more efficient inverter systems at a
lower cost. Optocouplers have fair isolation and workingvoltage performance; however, the LEDs they use limit
their lifetimes from an electrical-performance point of
view. As mentioned earlier, leading polyimide-based,
magnetic-isolation technologies have low working voltage
ratings and low long-term insulation reliability, which
limits their use in high-voltage, solar-inverter applications.
Solar and wind energy applications generally use additional isolation barriers (for example, in the path of the
network-communication channel) to achieve reinforced
isolation. Hence, the requirements related to reinforced
isolation are not as high as in motor-drive applications.
However, the requirement for high working voltage can
exceed values seen in motor drives. Power plant centralized solar inverters and wind-energy inverters are trending
to operate with higher DC bus voltages, extending to 1500 V
and beyond. Higher DC-bus voltages enable higher power
ratings without increasing the current levels, which keeps
copper costs the same. This helps reduce the per-unit cost
of energy generated. Another bonus of higher voltage is
increased efficiency because the total power output can
increase with higher voltage, but when current does not
change, the conduction losses also remain the same.
Higher DC bus voltages directly translate to higher
working voltages for the isolators used in switching power
transistors in the inverter. In Figure 4, the isolated gate
drivers (or any digital isolators paired with discrete gate
drivers) continuously see a trapezoidal voltage. These
voltages appear between one side that is connected to
the inverter output, and the other side is connected to
earth reference. The peak-to-peak value of this voltage
High common-mode transient immunity
Common-mode transient immunity (CMTI) is the ability of
an isolator to tolerate high-slew-rate voltage transients
between its two grounds without corrupting signals
passing through it. In electric-vehicle motor drives, as well
as solar- and wind-energy inverter applications, the
Figure 4. Block diagram of a centralized solar inverter
DC/AC Conversion with Isolated
IGBT Gate Drivers
DC+
EMI
Filter
Electric
Grid
PWM Signals
Solar
Panel
LowFrequency
Transformer
DC–
Isolation
Isolation
RS-485,
CAN,
Ethernet
System
Controller
Isolated Current
and Voltage Sense
Microcontroller
Texas Instruments
20
AAJ 4Q 2015
Analog Applications Journal
Industrial
isolated gate drivers or isolators passing gate controls to
power transistors see large ground transients because one
ground is connected to rapidly switching inverter outputs.
Figure 5 shows the ground potential difference experienced by these gate drivers. In these systems, CMTI is a
critical parameter because any bit errors caused by the
transients can result in dangerous short-circuit events.
Recent application requirements are pushing the need
for isolators with higher and higher CMTI. As stated, one
requirement is increasing DC bus voltages. A second is
reducing transition times with faster switching in the
power transistors, which improves inverter efficiency. A
third is increasing switching frequency, which results in
lower-cost and less-bulky magnetics such as inductors,
transformers and motors. Availability of reliable siliconcarbide or SiC-based power transistors, which can switch
faster and tolerate higher voltages versus traditional
IGBTs, is building on the trend for inverters that switch
faster and more efficiently.
Capacitive isolators released in 2014 broke the barrier
for 100-kV/µs CMTI. Capacitive isolators and gate drivers
continue to lead the industry with the highest minimum
guaranteed CMTI, thus enabling faster, more efficient, and
lower-cost inverter designs.
Figure 5. Inverter output switching profile
translates to high CMTI for isolators
~1500 V
dV/dt = 100 kV/µs
Table 1. Creepage requirements depending on working voltage
and package material group
Minimum Creepage (mm)
High-altitude operation
Advanced packaging technology is required for isolators
used in equipment operating at higher altitudes and in a
polluted or high-moisture environments. Improved and
wider packages prevent degradation along the package
surface and arcing through the air between pins, which
ensures isolation quality. Isolators used in solar, windenergy and e-metering applications fall into this category.
Isolator voltages that continuously operate in heavy
pollution can cause package surfaces to degrade and
create a conductive path across the isolator. This phenomenon is called tracking. Choosing a higher-quality packaging mold compound that belongs to a lower material group
with higher comparative tracking index (CTI) can minimize this effect for a given package creepage and working
voltage. Another approach is to choose a wider package
with increased creepage to reduce the risk of tracking.
Table 1 shows the requirements on creepage distances
depending on working voltage, the pollution degree, and
the material group of the isolator’s package mold
compound for reinforced isolation, according to IEC
60664-1. The requirements on creepage increase with
working voltage and pollution degree, however, selecting a
lower material group with a higher CTI can reduce the
creepage requirement.
At higher altitudes of 2000 to 5000 meters above sea
level, the air pressure is lower. Therefore, peak over­
voltages, such as surge or temporary overvoltage, can
more readily cause arching between the isolator pins.
Equipment operating at high altitudes requires greater
Texas Instruments
Working
Voltage
(VRMS)
Pollution Degree 2
Material Group
Pollution Degree 3
Material Group
I
II
III
I
200
2
2.4
4
5
5.6
6.4
400
4
5.6
8
10
11.2
12.6
800
8
11.2
16
20
22
25
1000
10
14.2
20
25
28
32
II
III
Table 2. Multiplication factors for clearances at higher altitudes
Altitude
Multiplication Factor for
Clearances
2000
1
3000
1.14
4000
1.29
5000
1.48
6000
1.7
7000
1.95
spacing between pins (more clearance). Table 2 shows the
multiplication factors by which clearance must be
increased at higher altitudes to prevent arcing per IEC
60664-1.
Traditionally, conformal coating or potting techniques
involving deposition of insulating polymer or other material over the printed circuit board (PCB) have been used
to reduce the pollution degree around the isolator. This
reduces the requirement on creepage and clearance.
However, these methods add cost, are less reliable, and
need additional inspection steps in PCB manufacturing.
Wide-body isolators manufactured with high-quality
molding compounds eliminate the need for conformal
coating or potting, simplify PCB design, and increase
manufacturing reliability.
21
AAJ 4Q 2015
Analog Applications Journal
Industrial
References
New isolators use the best quality mold compound
(CTI material group I) and are available in wide packages
(14.5 mm creepage/clearance). These isolators can enable
high-altitude designs and tolerate higher pollution without
requiring additional steps in PCB manufacturing.
1. IEC 61800-5-1 Ed. 2.0, Adjustable speed electrical
power drive systems, safety requirements, electrical,
thermal and energy. July 2007
2. Anant S Kamath and Kannan Soundarapandian, “Highvoltage reinforced isolation: Definitions and test methodologies,” Texas Instruments White Paper, November
2014 (SLYY063)
3. Anant S Kamath, “Isolation in AC Motor Drives: Under­
standing the IEC 61800-5-1 Safety Standard,” Texas
Instruments White Paper, November 2015 (SLYY080)
4. IEC 60664-1 Ed. 2.0, Insulation coordination for equipment within low-voltage systems, principles, requirements and tests, April 2007
Capacitive digital isolators
Recent advances in capacitive digital isolators place them
at the forefront of technology. These new isolators offer
higher isolation performance, long-term reliability,
increased channel integration, higher data rates and precision timing performance, better quality package mold
compound, wider packages (14.5-mm creepage/clearance),
and CMTI exceeding 100 kV/µs. Combining these features
enables new applications, reduces system cost, and allows
end-equipment manufacturers to push the performance
envelope of their solutions.
Texas Instruments offers the ISO78xx and ISO77xx
families of reinforced digital isolators and the ISO585x and
ISO545x ­families of reinforced isolated IGBT gate drivers.
These isolators offer features and capabilities that can
solve ­difficult isolation problems. These isolators have a
working voltage of up to 1500 VRMS, are rated for 40 years,
have surge voltage capability of 12.8 kV, and withstand a
temporary overvoltage of 5700 VRMS. They offer high data
rates of up to 100 Mbps with low skews and part-to-part
­variations and CMTI exceeding 100 kV/us. They also use
material group I mold compound and are available in
industry-leading wide packages.
Texas Instruments
Related Web sites
Product information:
ISO7821, ISO7831
ISO7840, ISO7841, ISO7842
ISO7721, ISO7731
ISO7740, ISO7741, ISO7742, ISO7761
ISO5851, ISO5852S
ISO5451, ISO5452
22
AAJ 4Q 2015
Analog Applications Journal
TI Worldwide Technical Support
TI Support
Thank you for your business. Find the answer to your support need or get in
touch with our support center at
www.ti.com/support
China:
http://www.ti.com.cn/guidedsupport/cn/docs/supporthome.tsp
Japan:
http://www.tij.co.jp/guidedsupport/jp/docs/supporthome.tsp
Technical support forums
Search through millions of technical questions and answers at TI’s E2E™
Community (engineer-to-engineer) at
e2e.ti.com
China:
http://www.deyisupport.com/
Japan:
http://e2e.ti.com/group/jp/
TI Training
From technology fundamentals to advanced implementation, we offer
on-demand and live training to help bring your next-generation designs to life.
Get started now at
training.ti.com
China:
http://www.ti.com.cn/general/cn/docs/gencontent.tsp?contentId=71968
Japan:
https://training.ti.com/jp
Important Notice: The products and services of Texas Instruments Incorporated and its
subsidiaries described herein are sold subject to TI’s standard terms and conditions of sale.
Customers are advised to obtain the most current and complete information about TI products and
services before placing orders. TI assumes no liability for applications assistance, customer’s
applications or product designs, software performance, or infringement of patents. The publication
of information regarding any other company’s products or services does not constitute TI’s approval,
warranty or endorsement thereof.
A011617
E2E is a trademark of Texas Instruments. All other trademarks are the ­property of
their respective owners.
© 2015 and 2018, Texas Instruments Incorporated. All rights reserved.
Texas Instruments
SLYT649A
AAJ 4Q 2015
IMPORTANT NOTICE FOR TI DESIGN INFORMATION AND RESOURCES
Texas Instruments Incorporated (‘TI”) technical, application or other design advice, services or information, including, but not limited to,
reference designs and materials relating to evaluation modules, (collectively, “TI Resources”) are intended to assist designers who are
developing applications that incorporate TI products; by downloading, accessing or using any particular TI Resource in any way, you
(individually or, if you are acting on behalf of a company, your company) agree to use it solely for this purpose and subject to the terms of
this Notice.
TI’s provision of TI Resources does not expand or otherwise alter TI’s applicable published warranties or warranty disclaimers for TI
products, and no additional obligations or liabilities arise from TI providing such TI Resources. TI reserves the right to make corrections,
enhancements, improvements and other changes to its TI Resources.
You understand and agree that you remain responsible for using your independent analysis, evaluation and judgment in designing your
applications and that you have full and exclusive responsibility to assure the safety of your applications and compliance of your applications
(and of all TI products used in or for your applications) with all applicable regulations, laws and other applicable requirements. You
represent that, with respect to your applications, you have all the necessary expertise to create and implement safeguards that (1)
anticipate dangerous consequences of failures, (2) monitor failures and their consequences, and (3) lessen the likelihood of failures that
might cause harm and take appropriate actions. You agree that prior to using or distributing any applications that include TI products, you
will thoroughly test such applications and the functionality of such TI products as used in such applications. TI has not conducted any
testing other than that specifically described in the published documentation for a particular TI Resource.
You are authorized to use, copy and modify any individual TI Resource only in connection with the development of applications that include
the TI product(s) identified in such TI Resource. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE TO
ANY OTHER TI INTELLECTUAL PROPERTY RIGHT, AND NO LICENSE TO ANY TECHNOLOGY OR INTELLECTUAL PROPERTY
RIGHT OF TI OR ANY THIRD PARTY IS GRANTED HEREIN, including but not limited to any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
regarding or referencing third-party products or services does not constitute a license to use such products or services, or a warranty or
endorsement thereof. Use of TI Resources may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
TI RESOURCES ARE PROVIDED “AS IS” AND WITH ALL FAULTS. TI DISCLAIMS ALL OTHER WARRANTIES OR
REPRESENTATIONS, EXPRESS OR IMPLIED, REGARDING TI RESOURCES OR USE THEREOF, INCLUDING BUT NOT LIMITED TO
ACCURACY OR COMPLETENESS, TITLE, ANY EPIDEMIC FAILURE WARRANTY AND ANY IMPLIED WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUAL
PROPERTY RIGHTS.
TI SHALL NOT BE LIABLE FOR AND SHALL NOT DEFEND OR INDEMNIFY YOU AGAINST ANY CLAIM, INCLUDING BUT NOT
LIMITED TO ANY INFRINGEMENT CLAIM THAT RELATES TO OR IS BASED ON ANY COMBINATION OF PRODUCTS EVEN IF
DESCRIBED IN TI RESOURCES OR OTHERWISE. IN NO EVENT SHALL TI BE LIABLE FOR ANY ACTUAL, DIRECT, SPECIAL,
COLLATERAL, INDIRECT, PUNITIVE, INCIDENTAL, CONSEQUENTIAL OR EXEMPLARY DAMAGES IN CONNECTION WITH OR
ARISING OUT OF TI RESOURCES OR USE THEREOF, AND REGARDLESS OF WHETHER TI HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
You agree to fully indemnify TI and its representatives against any damages, costs, losses, and/or liabilities arising out of your noncompliance with the terms and provisions of this Notice.
This Notice applies to TI Resources. Additional terms apply to the use and purchase of certain types of materials, TI products and services.
These include; without limitation, TI’s standard terms for semiconductor products http://www.ti.com/sc/docs/stdterms.htm), evaluation
modules, and samples (http://www.ti.com/sc/docs/sampterms.htm).
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