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Achieving reinforced insulation in data isolators

Posted: 12 Sep 2011 ?? ?Print Version ?Bookmark and Share

Keywords:insulation? optocouplers? isolators?

The first rule 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 approach 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 created the need for devices with the equivalent electrical strength and reliability of two redundant-insulation systems. This is called a reinforced insulation device, and it relies on a combination of construction, type testing and continuous monitoring in production to ensure safety equivalence to two independent systems.

We will examine how reinforced insulation is achieved in optocouplers and digital isolators with respect to construction and test requirements under IEC60950 and the related IEC60747-5-5 and VDE-0884-10 as well as differences with other accepted IEC standards for both types of isolators.

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.

 reinforced insulation

Figure 1: This component structure is classified as having reinforced insulation.

An example of a reinforced-insulation system is the feedback control loop in a power supply. Information about the present output voltage level must flow from the Safety Extra Low Voltage (SELV) 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 be run in series without significant functional degradation, but putting two data isolators into the path would be impractical for several reasons. First, analog 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.

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. 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, 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.

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