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Safety certification of digital isolators

20 Feb 2015  | Alec Makdessian

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Digital isolators provide signal isolation and the level shifting necessary for the correct operation of many circuits. Equally important, they insulate the user from electric shock. With basic human safety considerations so pertinent here, these isolators must undergo extensive testing and certification to ensure user safety. This article briefly summarises the international safety standards and certifications that apply to digital isolators. An example exercise using the MAX1493x family shows how a board or system designer must use a datasheet and the standard's specification tables to determine which digital isolator will be optimal for an application.

Basics of international safety standards
The International Electrotechnical Commission (IEC)1 is the organisation that publishes the international standards for electrical safety. Regional specifications are handled by bodies such as the UL (U.S.A.), CSA (Canada), VDE (Germany), and CNCA (China); actual testing and certifications are done by the UL, CSA, VDE and TUV in Germany, and CQC in China.2

Safety standards can be separated into three broad categories: equipment standards, support standards, and component certification standards. Developed over time in many geographic areas and across many industries, safety certifications evolved into a plethora of standards. In recent years great progress has been made towards harmonizing these standards. When you see multiple prefixes on a standard, it indicates a harmonized standard that meets the requirements of those entities. For example, a mixed standard identified as BS EN IEC meets the British Standards Institute (BS), European Norms (EN), and IEC. This harmonisation of standards is a very slow process but very much needed. Until this is achieved, component suppliers will have to meet as many of the requirements as possible to be able to sell their devices worldwide.

Equipment standards
Equipment safety standards apply, as you would expect, to end equipment which is assembled from multiple components. These standards cover a variety of safety concerns including insulation, pressurized gasses, and the strength of electrical cords. The IEC 61010-1 (Safety requirements for electrical equipment for measurement, control, and laboratory use) is concerned with the transient voltages that equipment will experience and the characteristics of the insulation used. This standard does not specifically address isolators. In fact, neither the word "isolator" nor "isolation" is found anywhere in this IEC standard. It is, nonetheless, relevant to isolators because it specifies both the transient voltage and the minimum creepage and clearance distances for the insulation. The insulation of a marketed isolator must, therefore, be compatible with these IEC requirements.

Support standards
Support standards provide definitions, methods, or requirements that are applicable to numerous other standards. Support standards help with harmonisation. An example is IEC 60664-1 (Insulation coordination for equipment within low-voltage systems). Once again this standard does not specifically mention isolation or isolator.

Component certification standards
Component certification standards apply to specific components that can be used in a variety of applications. These standards define the required tests, sample size, and preconditioning. They may also define terms and information that are required for the datasheet. With a few exceptions, these standards do not set performance levels. Instead, the performance requirements are "according to the manufacturer's specifications."

Component-level certifications
It will be useful now to take a quick look at the specifications that directly impact the manufacture of digital isolators.

The UL1577 specification is for optical isolators, but is also used to qualify capacitive and inductive isolators. The rating is based strictly on the voltage breakdown and does not include any requirements on clearance or creepage. To be certified under this standard, a device must withstand the isolation voltage, VISO (specified by each manufacturer, usually 2.5kVRMS or 5KVRMS), for 1 minute. In addition, the specification allows a production test of 120% of the isolation voltage for 1 second. Devices passing these requirements (plus the 150% of Viso Overload Test and the Thermal Aging Test) are certified with a single protection rating. A double protection rating requires survivability to a 20kV discharge test (applied 50 times) and a 1-second production test of the rated isolation RMS voltage or 2.5kVRMS, whichever is greater.

IEC60747-5 and VDE0884-10
The IEC is working on a capacitive and magnetic isolator specific spec (IEC60747-17), which is still in draft mode at the time of this composition. For now, the IEC60747-5 and the German VDE0884-10 standards are used to certify digital isolators.

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