FAQ: Isolation >> Opto-Emulators >> Current FAQ
Short Answer
This is a common question we receive, as some International Electro-technical Commission (IEC) standards state a minimum Distance Through Insulation (DTI) of 0.4 mm to be used in those applications. However, these same standards also provide exceptions for the cutting-edge thin-film SiO2-based insulation, that allow for a lower DTI. Opto-emulators use digital isolation technology for data transfer and they have a minimum DTI of 17 um, however this DTI still provides superior level of insulation compared to optocouplers’ epoxy-based insulation at 0.4 mm.
What is Distance through Insulation (DTI)?
DTI is the shortest distance within a solid insulating material interposed between two conductive parts across the isolation barrier as shown below in Figures 1 and 2. Figure 1 is a typical optocoupler with epoxy-based insulation between the LED and photo-detector die. In this case, DTI is the minimum distance between the LED and the detector die. Figure 2 is an opto-emulator with a thin-film, SiO2-based insulation technology. In this case, DTI is the shortest distance across insulation between transmit and receive digital circuits of the opto-emulator.
Many IEC equipment safety standards list minimum DTI requirements for components that provide high voltage insulation. This is because dielectric thickness and quality determine the electric field withstand capability of isolators. Historically, optocouplers have been used as isolators in high voltage applications and therefore many safety standards ended up with much thicker DTI requirement, such as a minimum of 0.4 mm, 0.6 mm, or 1 mm for increasing working voltage ratings. These DTI thickness requirements correspond to the voltage withstand capability of the lower quality insulation material used by optocouplers and other legacy isolators. Since opto-emulators use newer digital isolation technology and high-quality dielectric material, such as silicon dioxide (SiO2), they do not need to be as thick to provide high-quality insulation.
IEC Equipment Certification Requirements
There are a lot of misconceptions in the market about DTI requirements of digital isolators. These misconceptions arise due to confusion caused by the complexity of the international end-equipment standards and multiple clauses of these standards that deal with DTI requirements and exceptions. Some equipment manufacturers believe that the large DTI requirements outlined in certain clauses of their equipment standards strictly apply to digital isolators; this is simply not true. Opto-emulators use a much thinner, yet more robust, insulation material when compared to optocouplers and other legacy isolators, and therefore many international standards have different test criteria for isolators with thin DTI compared to legacy isolators.
Typically, IEC equipment standards have clauses that require minimum DTI for reinforced insulation but at the same time, there are other clauses within the standard which makes it absolutely clear that the minimum DTI requirements don’t apply if the component meets certain stringent test criteria. For example, IEC 62368-1 and IEC 60601-1 are standards for Information Technology and Medical equipment respectively. Both these standards require 0.4 mm minimum DTI for reinforced insulation, but they also provide an option to bypass this DTI requirement for components if they pass thermal cycling, humidity conditioning test, followed by AC withstand voltage that is 60% higher than the rated voltage. Similarly, IEC 61010-1 is a standard for measurement, control, and laboratory use equipment. This standard also has minimum DTI of 0.4 mm, 0.6mm, or 1 mm for DC or RMS working voltages of up to 300 V, 600 V, or 1000 V respectively. So based on these requirements, it would seem that semiconductor components with thinner dielectric might not be certified to this standard but what most observers do not realize that clause 14.1a of the standard allows applicable safety requirements of a relevant IEC standard, such as IEC 62368-1, to bypass the minimum DTI requirements of this standard. Since IEC 62368-1 allows thermal cycling and humidity preconditioning tests followed by dielectric test with 60% margin to bypass the minimum DTI requirements, the same is allowed for IEC 61010-1 according to clause 14.1a of this standard.
