EMC & Circuit Protection


Doubling up


How is reinforced insulation achieved in optocouplers and digital isolators, Mark Cantrell explains


T


he primary tenet of electric shock safety is that there must be the equivalent of two independent insulation systems between dangerously energized circuits and any conductor that can be accessed by the user of an electrical device. One of these insulation systems could be a safety grounded enclosure paired with a single layer of internal insulation. Another is to use two insulation systems arranged to provide redundant protection. As a result, complex electrical systems using the double insulation approach require galvanically isolated communications across two layers of insulation without losing signal integrity. This creates the need for devices with the equivalent electrical strength and reliability of two redundant insulation systems. These reinforced insulation devices rely on a combination of construction, type testing and continuous monitoring in production to ensure safety equivalence to two independent systems.


Safety isolation Isolation is required in modern systems for many reasons, communication to high side components in battery charging systems or motor drives, breaking ground loops in communications systems or protecting


users from dangerous line or secondary voltages. The level of isolation is determined by the level of safety required for the specific application. Functional isolation provides no protection to a user, only the insulation required for the component to function properly. Basic insulation provides a level of insulation from shock that is adequate for protection of an operator if the insulation is fully intact.


However, to protect people from


hazardous voltages, regulations require that two independent insulation systems be present, Basic insulation for shock protection and a Supplemental layer so that if a fault breaches one insulation system, a redundant system will still provide safety to the operator. This type of arrangement is called Double insulation. When evaluating insulation systems the primary requirement is safety, not electrical functionality, so the failure criterion during evaluation is whether the isolation barrier is intact after the qualification, if the part still functions to the original specifications, it is an added bonus. An example of a reinforced insulation system is the feedback control loop in a power supply. Information about the current output voltage level must flow


from the SELV (Safety Extra Low Voltage) side of the AC/DC converter to the line side of the power supply. Operators can be in contact with the SELV side of the power supply, so two independent isolation systems or a reinforced insulation system must be present in the data paths to protect operators from shock. Passive components like resistors or capacitors can


Internal and external requirements are handled in very different ways. Creepage is the shortest distance along the surface of a component between electrically isolated conductive structures such as component pins. Clearance is also the shortest distance between isolated conductive structures in a component but it is not constrained to be on the surface,


Figure 2. Two common operating conditions


be run in series without significant functional degradation, but putting two data isolators into the path would be impractical for several reasons. First analogue data would lose fidelity and digital data would have long propagation delays and added jitter. Secondly it would create the need for an intermediate power supply to run the coupler interfaces between the two layers of isolation. The impracticality of doubling up data isolation devices created the need for single components that directly connect across a Double Insulation boundary without sacrificing safety. This type of component (Figure 1) is classified as having Reinforced insulation.


Figure 1. Reinforced insulation 22 September 2011 Components in Electronics


Component level requirements Component reinforced insulation is evaluated in two ways; external dimensions of the component such as creepage, clearance and tracking index, and internal electrical performance.


so the path can jump over groves and be suspended over ridges. In simple geometries the creepage and clearance path are often the same. The illustration shows the creepage path for a JEDEC standard SOIC since this style of package is used for many isolation devices. For this style of package, the creepage and clearance have the same path and length. Creepage is always greater than or equal to the clearance. An additional external property of components that is critical to insulation ratings is the Comparative Tracking Index (CTI). This is a measure of how easily an insulating material will erode under electrical discharge. Higher tracking voltages will allow smaller creepage while still maintaining safety. External dimensions must be equivalent to the total distances provided by Basic and Supplemental layers of a double insulation system. In general all creepage and clearance requirements are twice as large for Reinforced components as for Basic/


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