Isolation FAQ · Opening | Having trouble designing digital isolators? Everything you want to know is here!

Regarding digital isolation technology, we continue to collect problems that everyone encounters during design. We know these questions and answers are critical for designers who already use digital isolation or want to convert from traditional optocoupler isolation to digital isolation, so we’re publishing a series of articles to answer your questions and provide learning opportunities resource.

In this first article, we'll discuss issues surrounding the core isolation technology itself . See if your question has been answered below. The applet at the end of the article also comes with a collection of digital isolation white papers for learning . If you have any other questions that need to be answered, please leave us a message in the comment area.

Q: What is the difference between capacitive digital isolation and optocoupler isolation?

A: Optocouplers work by sending light over a physical distance to communicate while providing physical isolation. The logic input generates input-side current and the LED output transmits light through the molded composite layer to the receiving photodetector and output. Digital isolators use a silicon-based semiconductor technology with two integrated circuit chips and a high-voltage dielectric capacitor built into the silicon process. At Texas Instruments (TI), the digital signal is converted to the high-frequency domain and sent through a capacitor-based SiO2 dielectric isolation layer.

Q: What technical improvements does capacitive isolation technology have compared to previous isolation technologies?

A: Capacitive isolation technology brings many advantages compared to traditional technologies such as optocouplers. Including but not limited to: longer life, stronger high-voltage performance, space saving, higher operating temperature, better integration capabilities, reliable life specifications, and low power consumption.

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Q: What are the dielectric materials used in digital isolation devices?

A: Capacitive digital isolators from Texas Instruments (TI) use silicon dioxide (SiO2) material as the dielectric to create an isolation barrier within the device.

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Q: What are the failure modes of isolation devices?

A: All isolation techniques may fail in "open circuit" and "short circuit" modes, depending on the conditions of the high voltage event the isolator is exposed to. To prevent failure, be sure to operate within the safety limits found in the isolator data sheet.

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Q: Will the characteristics of optical coupling devices change at high and low temperatures?

A: A key parameter affected by temperature is the current transfer ratio (CTR), which is the ratio of output current to input current. Ideally, CTR is 100%, but it varies with temperature, as shown in the figure below. Therefore, in order to maintain a certain CTR, designers must "over-design" so that the same performance level can be maintained throughout the life of the system. This is why optocouplers usually require higher input currents (about 10mA, forward current) to compensate for the degradation of CTR with temperature and time. Driving optocouplers with high forward currents also shortens their lifetime, which has an additional adverse effect on reliability.

Q: Some isolation devices usually require power supplies. If these power supplies are designed using this type of technology, can the isolation effect still be achieved?

A: There are many ways to build an isolated power supply, from fully discrete to fully integrated. Our transformer driver products, such as the SN6505, can be used in conjunction with a transformer to create a small isolated power supply with an output current of up to 1A. In this case, the transformer plays an isolating role in the system. Texas Instruments (TI) also offers single-chip isolated power solutions, such as the ISOW7741 solution, which enables communications by combining on-chip transformers to create an isolated power supply with capacitive digital isolation channels.

Q: Will digital isolation cost less? How much will the price of optocouplers increase with digital isolation?

A: Digital isolation technology has made great progress in the past decade, reducing costs while improving performance. At the same time, digital isolators have advantages in integration and reliability, making the cost gap between the two technologies at the isolation design system level smaller and smaller.

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Q: In order to increase the withstand voltage, are multiple capacitors connected in series?

A: Yes! This is one of many ways TI is improving the high-voltage capabilities of its isolators. New isolation devices use capacitive isolation layers on both sides of the device and provide higher isolation strength and more isolation margin.

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Stay tuned for our next article. We will soon have answers to your questions about choosing the right isolator, isolation interface, and digital isolator design.

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