Distance Through Insulation: How Digital Isolators Meet Certification Requirements
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1 Introduction
When it comes to high voltage safety certifications, the following three distances are of utmost importance to determine insulation ratings of semiconductor components: clearance, creepage, and DTI. Clearance is the shortest distance in air between two conductive parts and creepage is the shortest distance along the surface of a solid insulation material between two conductive parts across the isolation barrier. DTI, on the other hand, is the shortest distance within an insulating material interposed between two conductive parts. In other words, DTI is the distance inside a solid insulation whereas clearance and creepage are distances outside the solid insulation. See Figure 1-1 for typical depiction of clearance, creepage, and DTI.
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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. Digital isolators 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.
2 Solid Insulation
Solid Insulation is defined as insulation consisting entirely of solid material. The intrinsic material characteristics of solid insulation directly impact its insulation behavior. As the electric strength of solid insulation is considerably greater than that of air, the distances through solid insulation are much smaller than the clearance so that high electric stresses result. In insulation systems, gaps or voids may occur between electrodes and insulation and between different layers of insulation. Partial discharges can occur in these voids at voltages far below the level of puncture and this influences the service life of the solid insulation. As opposed to air, solid insulation is not a renewable insulating medium and therefore high voltage peaks which may occur infrequently can have a very damaging and irreversible effect on solid insulation. This situation can occur while in service and during routine high-voltage testing. The physical and geographical location of the equipment can affect the insulation system significantly. Environmental factors such as altitude, temperature, vibrations and humidity require consideration to ensure that the insulation remains reliable over the life time of the equipment.
3 Why International Standards for High Voltage Components Have Minimum DTI Requirements
Many International Electro-technical Commission (IEC) system-level safety standards have 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 digital isolators use newer isolation technologies and high-quality dielectric material, such as silicon dioxide (SiO2), they do not need to be thick to provide high-quality insulation.
