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Increase Safety and Reliability of High Voltage Industrial Applications with Galvanic Isolators

By Bill Giovino

Contributed By Digi-Key's North American Editors

Many industrial automation systems, especially those in manufacturing facilities, need to interface to equipment using high voltages ranging from hundreds to thousands of volts. Semiconductor-based isolators are commonly used to separate these high voltages from the much lower 5 volt digital logic voltages used in most control systems. For example, single package, dual-die opto-isolators have been widely used for this purpose due to their high resistance to transient high voltages and immunity to ambient magnetic fields. However, designers need a technology that is more stable over time and temperature extremes, and that is less complex from a manufacturing perspective.

This article explains why and how to use single package galvanic isolators to safely isolate the high voltages used in modern industrial, medical, and electric vehicle (EV) systems. It then looks at two silicon-based galvanic isolators from Texas Instruments that target high voltage, high reliability systems and discusses how to properly lay them out on a pc board to safely insulate high voltages from the digital logic used in programmable logic controllers (PLCs) and human interfaces.

Why isolate high and low voltages?

Many industrial systems are controlled using PLCs, computers, or human machine interfaces (HMIs). These control systems operate using standard digital control voltages of 5 volts or less. When interfacing these systems to manage high voltages of 120 volts or higher, it is important to physically separate and electrically isolate the low digital voltages from the high voltage equipment. Power converters, DC to DC converters, and electric vehicles (EVs) also need to carefully separate digital control voltages from what could be many thousands of volts used in the system.

While power transistors can easily handle these applications, they cannot do so safely. Transistors in these applications have the digital and high voltage control on the same semiconductor substrate. A malfunction or physical damage of the power transistor can quickly result in thousands of volts being injected into the digital logic. Besides destroying the control equipment, this also puts the user at risk.

Optical isolation has historically been the preferred method for physically separating and electrically isolating low and high voltage systems. A typical single package two-die opto-isolator contains an LED on one die which shines its emitted light—usually infrared—across a transparent isolation barrier to a photodiode receptor on a second die. The photodiode converts this to a low voltage signal that is used to control the high voltage circuitry.

For an opto-isolator to safely control thousands of volts, the LED die and photodiode die are both encased in a transparent isolation barrier made of material capable of withstanding the rated voltage of the opto-isolator.

Opto-isolators are resistant to transient electronic noise and are completely immune to ambient magnetic fields, making them the best choice for high voltage motor control applications. Opto-isolators for heavy-duty applications can withstand very high surge voltages of 10,000 volts or more.

However, opto-isolators do not perform well in very high temperature conditions. In addition, the LEDs in opto-isolators degrade with age. Opto-isolators are also two-die devices, which is a more complex manufacturing process compared to single-die semiconductors.

Galvanic isolation

In applications where temperature extremes are likely and where longevity is priority, single package galvanic isolators can be used. Where optical isolation separates two circuits with LEDs and photodiodes, galvanic isolation electrically separates two circuits with charge-coupled components using silicon dioxide (SiO2)-based capacitors or inductors. The effectiveness of the isolation is a function of the SiO2 dielectric.

Galvanic isolators are high speed, long life devices that interface easily to most microcontrollers. Recently introduced examples have been tested to withstand as much as 6,000 volts, operate at temperatures as high as 150°C, and last over 35 years. This improves safety and reliability of the overall system, while reducing maintenance costs.

For example, Texas Instruments’ ISO7762FDWR six-channel general purpose digital isolator can withstand as much as 5,000 volts RMS (VRMS) and has an insulation surge voltage of 12,800 volts (Figure 1). The ISO7762 is available with two options: the ISO7762F has output pins OUT[A:F] default output logic low, while without the F suffix the default output logic state is logic high.

Diagram of Texas Instruments ISO7762F six-channel galvanic isolatorFigure 1: The Texas Instruments ISO7762F is a six-channel galvanic isolator with four forward channels and two reverse channels. (Image source: Texas Instruments)

The ISO7762F has two power domains, one on the left and one on the right, separated electrically and physically by a SiO2 isolation layer. Each power domain has its own independent power and ground pins.

The device has four forward channels and two reverse channels. The two reverse channels (inputs E and F) allow information from the high voltage system to be sent to the digital control system while still maintaining safe isolation of the two power domains. The data transmitted in either direction can be simple digital on/off data, or serial data using a UART or two-wire I2C.

For each channel the ISO7762F uses two SiO2 capacitors in series to separate the two voltage domains. Digital data is transmitted using on-off keying (OOK) modulation where a logic 1 on any input IN[A:F] is represented by an AC signal across the capacitor to the other power domain, and a logic 0 is represented by 0 volts. The data on the corresponding OUT[A:F] reflects the logic state of the input pin. The SiO2 dielectric in the capacitors separates the two power domains to safely isolate the high voltage control electronics from the digital control system.

The designers of the ISO7762F emphasized high isolation resistance for maximum safety. The isolation resistance at 25°C is rated at greater than 1 tera-ohm (TΩ). The ISO7762F isolation resistance at 150°C is greater than 1 giga-ohm (GΩ). To put this in perspective, this resistance is higher than the resistance of the ambient air around the ISO7762F.

The ISO7762F is rated by Texas Instruments to last at least 37 years, but the galvanic isolation insulation layer is rated at a lifetime of over 135 years. While equipment typically does not need to be guaranteed operational over this period of time, these figures indicate the reliability and durability of the device.

For even higher withstand voltages, the Texas Instruments ISO7821LLSDWWR is a dual-channel differential isolation buffer rated at 5700 VRMS with an isolation surge voltage of 12,800 volts (Figure 2). The two channels each go in opposite directions. Each channel is a differential pair transmitter used for low-voltage differential signaling (LVDS) data communications at speeds as high as 150 megabits per second (Mbps).

Diagram of Texas Instruments ISO7821LLS digital isolatorFigure 2: The Texas Instruments ISO7821LLS digital isolator has two differential channels in opposite directions. Each output buffer has an output enable that can disable the output to a high impedance state. (Image source: Texas Instruments)

The SiO2 used for galvanic isolation in the ISO7821LLS is the same as the ISO7762F, except that instead of two capacitors in series for each channel, the ISO7821LLS uses one capacitor for each channel. It also uses the same OOK modulation to transmit digital data across the SiO2 capacitors.

The ISO7821LLS galvanic isolation driver can transmit LVDS data over industrial grade cables such as Belden's 88723-002500 heavy duty, dual twisted pair cable. This is a high quality industrial cable that carries two twisted pairs of 22 AWG wire in a red jacket. It is designed for indoor or outdoor use and can even be buried underground. This cable can handle operating temperature extremes of -70°C to +200°C, making it appropriate for harsh high voltage industrial applications such as solar power inverters in very hot or very cold environments. A control unit can transmit LVDS control data in both directions over this Belden cable to an ISO7821LLS inside the solar inverter box. Any high voltage surge due to a malfunction at the converter box would be stopped at the isolator, protecting the low voltage control unit and any human operators near the unit.

The two outputs on the Texas Instruments ISO7821LLS have independent enable pins that can disable their respective outputs by putting them in a high impedance state. This is useful if the device is on an LVDS bus with more than one driver and needs to cede the bus to another bus master. This is applicable in industrial environments where high voltage equipment needs to be operated by more than one control unit in different locations.

To help designers evaluate the ISO7821LLS, Texas Instruments has the ISO7821LLSEVM evaluation board (Figure 3). It requires a minimum of external components and can be used to evaluate the behavior and performance of the ISO7821LLS and allows monitoring of the LVDS bus communications for test and benchmarking purposes.

Image of Texas Instruments ISO7821LLSEVM evaluation moduleFigure 3: The Texas Instruments ISO7821LLSEVM evaluation module can be used to test and evaluate the LVDS data communications performance of the ISO7821LLS dual-channel differential isolation buffer. (Image source: Texas Instruments)

Because every high voltage application is different, the ISO7821LLSEVM is not intended for use in testing the high voltage isolation behavior of the ISO7821LLS.

Galvanic isolator layout

The layout of a high voltage galvanic isolator must be done very carefully to ensure effective isolation. For a low EMI pc board design, standard layout rules apply, which include using a pc board of at least four layers with high speed traces on top with a solid ground plane below, and the power plane below that. Slower control signals should be on the bottom plane.

It is critical that the low voltage and high voltage components be physically separated on the pc board. For that purpose, the isolators discussed here have separate power domains for the left and right sides of the package. Also, traces for one domain must not be routed near those of the other to prevent signal interference.

If the isolator is located in the high voltage section, it may be safer to place the isolator with the low voltage side facing an edge of the pc board. This helps prevent any high voltages from arcing to the low voltage side, which can severely damage any low voltage electronics at the other end of the isolator.

Conclusion

Industrial equipment using many thousands of volts requires components that can safely isolate these high voltages from the 5 volt or lower digital control logic to protect the equipment and its users. The nature of industrial equipment requires that such isolation be stable and reliable in extreme temperature variations over long periods of time.

As shown, digital isolators based on galvanic isolation have the isolation characteristics and operating temperature specifications suitable for such applications. With proper attention to layout and configuration they can prevent damage or injury.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Bill Giovino

Bill Giovino is an Electronics Engineer with a BSEE from Syracuse University, and is one of the few people to successfully jump from design engineer, to field applications engineer, to technology marketing.

For over 25 years Bill has enjoyed promoting new technologies in front of technical and non-technical audiences alike for many companies including STMicroelectronics, Intel, and Maxim Integrated. While at STMicroelectronics, Bill helped spearhead the company’s early successes in the microcontroller industry. At Infineon Bill orchestrated the company’s first microcontroller design wins in U.S. automotive. As a marketing consultant for his company CPU Technologies, Bill has helped many companies turn underperforming products into success stories.

Bill was an early adopter of the Internet of Things, including putting the first full TCP/IP stack on a microcontroller. Bill is devoted to the message of “Sales Through Education” and the increasing importance of clear, well written communications in promoting products online. He is moderator of the popular LinkedIn Semiconductor Sales & Marketing Group and speaks B2E fluently.

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Digi-Key's North American Editors