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Safety certification of digital isolators

20 Feb 2015  | Alec Makdessian

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Together, the pollution degree classification and the highest working voltage in which the system will be operating determine the creepage requirements for IEC certification per the IEC specs noted above. For example, table 4 lists the minimum creepage requirements in mm under IEC60950-1.


Table 4: Basic Insulation Creepage Requirements in mm per IEC609501-1.


The values in table 4 are for Basic Insulation certification. Creepage requirements for Reinforced Insulation certification are 2x the values listed in table 4.

The IEC60950-1 also specifies the voltage used for testing electrical strength, held for 60 seconds, based on the equipment's working voltage and the appropriate grade of insulation certification sought (i.e., Functional, Basic, Supplemental, or Reinforced). Table 5 summarises the test voltages required, assuming an Overvoltage Category II (portable equipment supplied from a wall outlet).


Table 5: Test Voltages for Electric Strength Tests.


Applying the data to an application
The minimum creepage/clearance for digital isolators certified by one of the standards noted above must be listed in their datasheet. We will use the above information as an example, Assume that you are considering two isolators, available in both narrow-body and wide-body SO packages. Your application is telecom equipment powered by 220VAC. You want to determine which component package is optimal for the application. From table 4, the designer will need 8.0mm of creepage (250VRMS, pollution degree 3, and Reinforced insulation certification category) for Material Group III or 6.4mm of creepage for Material Group I. The wide-body SO package would meet either requirement. If the narrow-body device was preferred for the application, some form of conformal coating may be needed. Finally, table 5 indicates that the device will need to pass the 3kV test voltage for a Reinforced insulation rating.


Conclusion
Signal isolation is a necessity in today's circuits not only for functionality, but equally important, to provide the user with the required protection from electric shock. The designer is aided—or confused—today by the slew of international and regional standards and certifications that are available. While a digital isolator is not specifically mentioned in IEC standards today, the UL1577, IEC60747-5, VDE0884-10 standards are the key component-level certifications required for digital isolators. Additionally, certification under the IEC60950-1, IEC61010-1 and IEC60601-1 standards may be required, depending on the end application.


References
1. For the IEC standards, start with their website at http://www.iec.ch/index.htm.

2. For basic information on these entitles, see: UL in the U.S.A. at http://www.ul.com; CSA in Canada at http://www.csagroup.org/; VDE in Germany at http://www.vde.com/en/; CNCA in China at http://www.cnca.gov.cn/cnca/cncatest/20040420/column/227.htm; TUV Rheinland in Germany at http://www.tuv.com; TUV Sud in Germany at http://www.tuv-sud.com; and CQC in China at http://www.cqc.com.cn/english/.


About the author
Alec Makdessian is the Business Director at Maxim Integrated.


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