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Meeting isolator safety requirements

Posted: 12 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Isolator? voltage? Optical? magnetic? capacitive?

There are numerous components found in electrical products, some of which are important for protection of the product user. Isolator components are commonly used to protect/separate product users from dangerous voltages, but knowing how to select the correct isolator for safety protection can be confusing. Is it the isolation voltage, the working voltage, the standard, the certification, or ... what's most important? This article will clear up the confusion surrounding the selection of isolators for safety applications in situations where hazardous voltage circuits need to be isolated from product users.

Non-hazardous voltage circuits, 3.3Vdc, 5Vdc, and 12Vdc, are typically user-touchable and must be isolated from hazardous voltages, > 30Vrms/42.4Vpeak/60Vdc, to protect the user from dangerous voltages! Safety isolation is the separation of a section of non-touchable hazardous voltage circuits (> 30Vrms/42.4Vpeak/60Vdc) from user- touchable circuits (? 30Vrms/42.4Vpeak/60Vdc). There are several levels of safety insulation: Basic, Double, and Reinforced. Reinforced insulation is required between hazardous voltage and user-touchable circuits.

Isolator components are integrated circuit semiconductor devices (ICs) which provide safety isolation (insulation). They function by transmission of signal coupling from primary-to-secondary circuits through various mediums including optical (light), magnetic (inductive) and capacitance (charge). Isolator components are referred to as safety-critical components and require special consideration to ensure that they meet the relevant safety standards and are safe for use in products.

The purpose of this article is to highlight the standards, insulation, tests, and ratings for isolators used in hazardous voltage applications where an isolator is used as a safety isolating component. This article will cover important isolator selection parameters, including:
???Component Standards (IEC 60747-5-5, VDE 0884-101,UL 1577)
???Reinforced Insulation (tests and spacings)
???Working Voltage, VIORM (supply voltage); e.g., 150 C 600Vac (210 C 840Vpeak)
???Isolation Voltage, VIOTM (test voltage); e.g., 2,500 C 5,000Vac
???VDE and UL Certifications (International and North America)

1 IEC 60747-17 (VDE 0884-11) to replace digital isolator standard VDE 0884-10 in the future.

Isolator technologies
There are three isolator technologies in use today:

Optical: Uses light (LED) transfer across a transparent nonconductive insulation barrier. Advantage is high electrical isolation values. Uses a silicone polymer insulation.

Magnetic: Inductive coupling transfer using transformer coil design and provides high isolation at high frequencies. Uses polyimide or silicone dioxide insulation.

Capacitive: Uses a changing electric field to transmit energy by means of capacitance. Capable of high speeds and relatively small package. Uses silicon dioxide insulation.

Figure 1: Here is a comparison of optical and digital isolators.

Optical isolators, also referred to as optocouplers or optos, employ an opaque silicone polymer material as the internal insulation. Optos have larger footprints and provide high levels of isolation when a film barrier is used. The newer generation of digital isolators, magnetic and capacitive, offer advantages in performance, size, and power efficiency. Digital isolators utilise polyimide/silicone-dioxide for their insulation. Optical isolators rely mostly on thickness for internal spacing whereas digital isolators have insulation of less than one-thousandth of an inch and rely on safety testing (figure 1).

Product vs component standards
It is very important to understand the difference between product and component standards, and that product and component safety standards have different scopes, tests, and requirements.

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