Isolators are used to isolate signal interference between circuits and improve the stability and safety of a system. Optocouplers were used to isolate circuits in the past; however, with the progress of CMOS technology, digital isolators have become the preferred technology for isolation. This article will analyze the characteristics of digital isolators and how to choose a suitable one for your needs.
Digital Isolators Gradually Replacing Optocouplers
Isolators are widely used in electronic circuit design, especially in industrial electronic equipment, which usually uses DC isolation to prevent potential high voltage hazards from harming the system and users. In particular, since industrial electronic equipment must operate stably in harsh environments full of unstable factors such as strong electromagnetic fields, surges, fast transients, high noise floors, etc., in order to ensure that a device can operate without faults for a long time in this environment, a reliable isolation circuit has become a must-have.
In the past 40 years, optocouplers have often been used for signal isolation components. After the breakthrough of silicon-based isolation technology, digital isolators that are smaller, faster, and more stable and have a cost advantage have emerged. In many terminal units, this new isolation component has gradually replaced the traditional optocoupler.
Isolators used in the industrial market are mainly used for component protection, user safety, signal level shifting, and adherence to safety regulations. In all cases, the isolation components add value to the system by enabling additional functionality and ensuring safe operation of the system. Adding isolation components usually improves circuit performance and improves component safety. Isolation devices allow multiple power domains to coexist and communicate, which means that sensitive circuits are protected from switching circuits. Modern digital isolation allows for massive integration, which means that circuit component count can decrease. Performance, efficiency, size, and cost are all things that can be affected when adding isolation devices.
The traditional optocoupler is a hybrid device that uses LED light to transmit data across an isolation barrier to a light detector. The LED turns on for logic High and off for logic Low. Optocouplers consume high levels of power, are prone to aging and temperature effects, and provide limited data rates, often below 1Mbps.
Digital isolation devices, on the other hand, were created to meet safety regulations while maximizing the benefits of modern CMOS technology. To do this, digital isolation devices use semiconductor process technology to create either transformers, or capacitors to transfer data instead of light. With this technology, performance and feature integration are both improved.
Optocouplers, although incorporated in many designs, are based on outdated LED-technology that provides significant output variation over input current, temperature, and age. This reduces performance over the device’s lifetime. Digital isolation components easily provide multichannel isolation solutions with a much smaller footprint, increase system reliability due to a lower failure rate, offer twice the electrical noise immunity, operate over a wider temperature range (-40℃ to 125℃), and do not age or degrade over time.
Feature set and isolation performance are both factors to consider in selecting a digital isolator. On the feature set side, consider the number of isolation channels and the channel configurations. Timing specifications, such as propagation delay, should be appropriate for your system. On the isolation performance side, it is important to gain an understanding of the isolation rating your system needs. Transient noise immunity, and electromagnetic emission profile are other considerations related to the isolation structure. With the isolation rating, there may be package options to consider given the system environment.
After a designer has decided to make the switch, the first challenge is to select the correct digital isolator for each application. Once an appropriate device is identified and designed in, the system designer can proceed with their system evaluation in their typical fashion. In addition, these digital isolators must meet appropriate Safety Standards as required by end safety agencies such as UL, CSA, VDE, and CQC. These safety agencies use their component safety standards to qualify and either specify a safety component’s one-minute voltage withstand rating, which is typically 2.5 kVrms, 3.75 kVrms, or 5 kVrms, or its life-time working voltage, which is typically between 125 Vrms to 1000 Vrms. For surge protection, some devices can reach 10 kVpk.
The two most common creepage and clearance requirements required by end systems for basic and reinforced insulation needing up to 250 Vrms working insulation are 3.2 mm and 6.4 mm respectively. Silicon Labs’ narrow body SOIC packages support ~4mm of creepage/clearance and the wide body SOIC packages support ~8mm. The Si80xx, Si83xx, Si86xx, Si87xx, and Si88xx families of one to six-channel bi- and unidirectional digital isolators support isolation voltage ratings up to 5 kV. These CMOS digital isolators give designers the ability to create lower cost, smaller size, higher performance, lower power, and more reliable isolated circuits. With a broad product portfolio, a proven track record of industry innovation and an unwavering commitment to engineering excellence, Silicon Labs’ isolation technology is ready to meet your isolation needs.