Reinforced Isolation: When 1 is greater than 2

This article was updated on January 16, 2020.

Industrial system designers working with applications that have exposure to high voltages are required to comply with system-level IEC standards to ensure their end equipment has adequate insulation. These standards, such as IEC 61800-5-1 for adjustable speed drives, regulate how well the equipment insulates against steady high voltages over its lifetime, occasional overvoltage transients and extreme high-voltage peak surges. Reinforced isolation is the ultimate protection against dangerous high voltages in applications like industrial automation, motor drives, human-machine interfaces and medical electronics. Think of reinforced isolation as giving your system double the insulation protection in a single device. So, how exactly is 1 greater than 2?

Isolation standards mandate one primary “basic” isolation barrier between humans and hazardous voltage sources, fortified with an independent, additional “supplementary” insulation barrier, to ensure that if one barrier fails, sufficient insulation is still present to protect users and components. This is called double insulation.

Reinforced isolation provides the same level of high-voltage protection as double insulation, but it does not have insulation layers that can be independently tested as basic or supplementary. In highly integrated isolation systems where components of insulation are often inside ICs, use of materials that have superior insulation properties (such as silicon dioxide, SiO2) provide insulation levels equivalent to or higher than double insulation. Since it is impractical to identify or test these integrated isolators as basic or supplementary insulation layers from outside the IC package, standards such as VDE-0884-11 and later the IEC 60747-17 give minimum requirements for a component to comply and certify as a reinforced isolator.

VDE-0884-11 and the later IEC 60747-17 will govern the performance, test and certification requirements for capacitive and magnetically coupled isolators. Key isolation requirements for reinforced isolators include voltage surge tolerance of >10 kVPK and assurance of 37.5 years of continuous lifetime with <1ppm defect levels at rated working voltage. The latest capacitive and magnetic isolator standards mandate a combination of fully accelerated lifetime testing, audit testing on manufacturing lines and 100% high-voltage production testing of every device to ensure the quality of isolation. In addition to the standard requirements, as very high levels of voltage are involved, accelerated barrier stress testing on every production lot ensures higher confidence for reinforced levels of performance.

Any margins over the levels of surge, lifetime and working voltage mandated by the standard are significant benefits to the safety application. For example, the inherent high-voltage surge-withstand capability of an isolation barrier at 12 kVPK provides at least 2kV of margin. Similarly, in a given application with a certain operating voltage, the actual lifetime of an isolator with a rated working voltage of 1500 Vrms will be more than an order of magnitude higher than that of an isolator with 650-Vrms rated working voltage.

Inherent isolation strength notwithstanding, external package dimensions are a limiting factor for high-voltage protection, since creepage and clearance distances of a digital isolator generally limit the maximum allowed system voltage rails that can be accommodated. However, newer packages with higher creepage and clearance distances provide high-voltage performance that far exceeds any isolation level requirements defined in existing component standards for reinforced isolation. In applications like motor control or medical devices, these margins directly translate to higher reliability levels and much more robust performance, giving system designers confidence that the isolation components they choose support their end equipment and meet safety-level reliability requirements for the lifetime of the system.

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