“Regarding digital isolation technology, we continue to collect the difficulties you encounter when designing. We know these questions and answers are critical for designers already using digital isolation or looking to transition from traditional optocoupler isolation to digital isolation, so we’re publishing a series of articles to answer your questions and provide follow-up lessons resource.
Regarding digital isolation technology, we continue to collect the difficulties you encounter when designing. We know these questions and answers are critical for designers already using digital isolation or looking to transition from traditional optocoupler isolation to digital isolation, so we’re publishing a series of articles to answer your questions and provide follow-up lessons resource.
In the 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 is also accompanied by a collection of digital isolation white papers for learning. If you have other questions to answer, 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 isolation physically. The logic input generates input-side current, and the LED output transmits light through the molded compound layer to the receiving photodetector and output. Digital isolators use a silicon-based semiconductor technology in which two integrated circuit chips and a high-voltage dielectric capacitor are built into the silicon process. At Texas Instruments (TI), digital signals are converted to the high frequency domain and sent through a capacitor-based SiO2 dielectric isolation layer.
Q: What are the technical improvements in capacitive isolation technology compared to previous isolation technologies?
A: Capacitive isolation technology brings many advantages over traditional technologies such as optocouplers. Including but not limited to: longer life, stronger high voltage performance, space saving, higher operating temperature, better integration capability, reliable life specification, low power consumption.
Q: What dielectric materials are used in digital isolation devices?
A: Texas Instruments (TI) capacitive digital isolators use silicon dioxide (SiO2) material as the dielectric to create an isolation layer within the device.
Q: What are the failure modes of isolation devices?
A: In both “open” and “shorted” modes, all isolation techniques may fail, depending on the high voltage event the isolator is exposed to. To prevent failure, it is important to keep operating within the safe limits in the isolator data sheet.
Q: Do the characteristics of optocouplers change at high and low temperatures?
A: A key parameter affected by temperature is the current transfer ratio (CTR), which is the ratio of the output current to the input current. Ideally, the CTR is 100%, but it varies with temperature as shown in the graph below. Therefore, in order to maintain a certain CTR, designers must “over-engineer” in order to maintain the same level of performance over the life of the system. This is why optocouplers typically require higher input currents (about 10mA, forward current) to compensate for the degradation of CTR with temperature and time. Driving the optocoupler with high forward current also shortens its lifetime, which has an additional adverse effect on reliability.
Q: Some isolation devices usually require power. If these power supplies are designed with this type of technology, can isolation still be achieved?
A: There are many ways to build an isolated power supply, from fully discrete to fully integrated. Our transformer driven products such as the SN6505 can be used in conjunction with a transformer to create a small isolated power supply with output currents up to 1A. In this case, the transformer acts as an isolation in the system. Texas Instruments (TI) also offers single-chip isolated power solutions, such as the ISOW7741 solution, which enables communication by combining an on-chip transformer to create an isolated power supply with capacitive digital isolation channels.
Q: Will the cost of digital isolation be lower? How much will the price of optocouplers go up with digital isolation?
A: Digital isolation technology has come a long way over the past decade, increasing performance while reducing costs. At the same time, digital isolators have more advantages in integration capability and reliability, making the cost gap between the two technologies at the system level of isolation design smaller and smaller.
Q: In order to increase the withstand voltage, will multiple capacitors be connected in series?
A: Yes! This is one of many ways TI has increased the high voltage capability of its isolators. The new isolation device uses capacitive isolation layers on both sides of the device and provides higher isolation strength and more isolation margin.
Stay tuned for our next article. Questions about choosing the right isolator, isolation interface, and digital isolator design will be answered shortly.