Precision labs series: Introduction to isolation

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Hello and welcome to TI Precision Labs. TI's Precision Labs video program is a comprehensive online curriculum for engineers. This video covers the basic questions you may have about galvanic isolation. More videos and topics can be found by going to ti.com/precisionlabs. This Precision Lab topic will answer the following questions-- what is galvanic isolation? When is galvanic isolation needed? What are the methods of isolation? What are isolation technologies? And how do I know if my system needs isolation?

What is galvanic isolation? When two devices or circuits are in communication, DC currents and AC signals typically flow freely. In low voltage systems, this is a safe way for two parts of the system to work. But when high voltage enters into one or more parts of the system, freely-flowing DC current and some AC signals can be dangerous.

The presence of high voltages can introduce significant potential differences, which may cause damaging DC currents or unwanted AC currents to flow to other parts of the system. This may cause errors or create hazardous conditions of operation. In these circumstances galvanic isolation is needed.

Galvanic isolation is a means of preventing DC and unwanted AC currents between two parts of a system while still allowing signal and power transfer between those two parts. Isolators are electronic devices and semiconductor ICs that are used for isolation. When is isolation needed? Isolation is required in modern electrical systems for a variety of reasons. Some examples include preventing electrical shock to human operators, protecting expensive processors, AC or FPGAs from risk of damage in a high voltage system, and breaking ground loop and communication networks, such as motor drives or power converter systems.

Let's look at the three main reasons galvanic isolation is used between circuits. First, galvanic isolation is used for safety. Isolation prevents current from flowing from high voltage potential elements to ground through a person's body. Environments where human operators are present and equipment operates under a high voltage or is exposed to high voltage, including the risk of a potential lightning strike, requires galvanic protection.

By galvanically isolating circuits, operators and other circuitry are protected from potentially lethal or damaging current flow. The second reason that a galvanic isolation is used is to address ground potential differences, also called ground loops, which can cause inaccuracies or disruptions between communicating subsystems.

Ground loops occur when an unintended physical connection in a system's grounding scheme results. This forms multiple ground paths between circuits. In this example, an RS485 interface is used to communicate with a microprocessor or MCU. While the RS485 interface is intended to handle a defined range of negative 7 to 12 volts referenced to a known ground, the reality is that ground potentials between the two circuits can vary.

This variation in ground potential from one circuit to the next creates a voltage difference, which across a long cable length can cause current to flow. When current flows through a ground loop, significant voltage differences can occur, causing an error in the data communication. Ground links can also provide paths that can act as antenna, causing disturbances from environmental noise.

The most common example of environmental noise would be 50/60 hertz noise, which can pick up and induce unwanted currents in the system grounds. Digital isolators are used to break the ground loop, thereby preventing the noise pickup and maintaining communication integrity. The third reason galvanic isolation is most commonly used is to improve circuit noise immunity.

While in many cases the influence of ground loops could fall into a category of noise sources, a primary source of noise interference is transient behavior within the system. For example, when transients from motor control switching occurs, a high slew rate transient voltage can result on the signal path. This often creates a common mode voltage transient that requires an isolator with high common mode transient immunity, or CMTI.

This noise immunity is used to maintain signal integrity. CMTI is specified in the vendor datasheet and the higher the CMTI specification, the better noise immunity of the device. For circuit isolation, there are two methods of isolation, analog or digital. There are multiple topology options available to isolate either an analog or digital input. And choosing the right solution is determined by the system design priorities.

Analog isolation isolates an analog signal in front of an analog-to-digital converter, or ADC input, which then digitizes the signal. Isolated amplifiers or isolated ADCs are most often used to isolate analog signals, typically from a shut resistor or sensor input. Because the isolation barrier is in front of the ADC, it's important to note that any error in the input signal resulting from the input gain amplifiers would also be digitized by the ADC.

This must be accounted for when determining the accuracy needed to achieve a target design resolution. Amplifier gain error can be avoided with analog isolation by choosing an isolated data converter like an isolated delta sigma modulator, which directly samples analog input signals. These solutions achieve high resolution isolated inputs by being optimized for direct connection to shunt resistors or other low-voltage level signal sources.

You can learn more about isolated amplifier and data converters in isolated amplifiers and modulators section of the precision lab series. Digital isolation is the method of isolating digital input signals. The isolator transfers digital communication across an isolation barrier following the ADC between microprocessors and FPGAs and then onto FET and gate drivers.

There are three primary technologies used for analog and digital isolation of signals today-- optical, inductive, and capacitive. Each technology uses a different insulator material with different dielectric strengths. Dielectric strength is a measurement used to describe the maximum applied electric field that a material can withstand without undergoing electrical breakdown and becoming electrically conductive.

This is measured in volts RMS per micrometer. The higher the value of the dielectric strength, the more robust the isolator. An optical isolator or optocoupler is shown here and consists of an input LED, a receiving photodetector, and an output driver. The driver circuit and LED circuits are typically built using Complementary Metal Oxide Semiconductor technology, or CMOS technology.

The isolation barrier of the optocoupler is typically built using air, epoxy, or mold compound. Both the input and output of the optocoupler require a separate voltage supply connected through an anode and collector pins and separate grounds, typically connected through a cathode or emitter pin, in order to maintain signal isolation between input and output.

Communication within an optocoupler occurs when it applies CMOS logic and generates an input side current, which then creates a proportional LED output for transmission through the mold compound barrier and then to receiving photodetector and output. Because optical isolation relies on light transfer, the communication rate of an optocoupler is typically less efficient than its capacitive or inductive counterparts.

This is primarily because the rate of transfer is limited to the LED switching speed. As with all LEDs, use of the LED contributes to a weakened signal over time, creating a limitation to the long-term functionality of the communication. The current transfer ratio, or CTR parameter, describes the behavior of output current to input current over time.

For systems needing long lifetimes, one must either calibrate the system to account for the CTR or over design the system to ensure that the light intensity is strong enough for the required operating lifetime. Inductive isolators are based on a transformer technology using an insulation material called polyimide. The logic inputs are used to generate an electromagnetic field and to transfer proportional energy signals across the inductive transformer barrier.

Capacitive isolation is based on energy transfer across the silicon dioxide, or CMOS barrier, through a high frequency carrier. A digital input signal is applied and modulated, then communicated across the isolation barrier. A proportional output is then produced to the level of the measured signal at the input.

Because capacitive isolators are designed with the highest dielectric strength material for insulation, they offer high data rates, low thermal profiles, and long lifetime operation. To learn more about digital isolation technology and architectures, watch the precision lapse video, "What Is a Digital Isolator?"

Component-level isolation requirements are most commonly determined by high voltage ratings of the system itself, and it's important to note that while component isolation standards and system-level standards are complementary, they are not the same. Component-level standards pertain to the device and its level of isolation certification, while system-level standards are determined by the industry standards bodies with guidelines that include environmental, regional, and international regulations, as well as end equipment specific requirements.

To determine which level of component isolation is required for your system, start with the system-level certification requirements that determine the component-level ratings needed. Component-level certifications and ratings are available on the vendor websites. This concludes the precision lab's introduction to galvanic isolation. We discussed the definition of galvanic isolation, when galvanic isolation is needed, methods and types of galvanic isolation, and a brief introduction to the standards and certification levels for galvanic isolation.

Thank you for your time. You can browse this and other isolation topics at www.ti.com/isolation. Please continue watching to take the galvanic isolation quiz.

Question number 1, True or False-- Galvanic isolation prevents signals from passing between two circuits. False. Galvanic isolation is used to enable signals to pass, but to prevent DC currents and unwanted stray AC currents.

Question 2. What is the number one reason galvanic isolation is used within a system? Safety and protection from high voltages.

Question 3. For systems with high potential voltage differences, how can isolation be used to minimize unwanted current flow through the system? Isolators can be used to break ground loops, which are a source of unwanted noise and current flow.

This concludes the "What Is Galvanic Isolation?" Precision Labs quiz. You can browse more topics at ti.com/precisionlabs.