SLYY063A October   2014  – May 2026 ISO5451 , ISO5452 , ISO5851 , ISO5852S , ISO6420 , ISO6421 , ISO6441 , ISO6463 , ISO7810 , ISO7820 , ISO7821 , ISO7830 , ISO7831 , ISO7841 , ISO7842 , ISOW1050 , ISOW3080 , ISOW6441 , UCC5310 , UCC5320 , UCC5350 , UCC5390

 

  1.   1
  2.   FAD
  3.   Understanding the definitions of high-voltage isolation parameters, their relevance to real applications, and the methodologies used to test them, allows systems engineers to pick the right isolator for their design need.
  4.   Maximum transient isolation voltage and isolation withstand voltage
  5.   Maximum repetitive peak voltage and working voltage
  6.   Maximum surge isolation voltage
  7.   Comparative tracking index
  8.   Distance through insulation
  9.   Conclusions
  10.   References
  11.   Appendix

Comparative tracking index

When an isolator is placed on a system board as part of end equipment in addition to its internal isolation parameters, the mold compound used in its package is important. This is because when high voltage is applied across the isolator, electric discharges on or close to the surface of the package, can cause localized deterioration in the mold compound, resulting in a partially conducting path from one side of the isolator to the other. This phenomenon is called tracking. The ability of a material to withstand tracking is quantified by a comparative tracking index (CTI).

IEC 60664-1 classifies materials into four material groups according to their CTI values:

Material group I: 600 V < CTI
Material group II: 400 V < CTI < 600 V
Material group IIIa: 175 V < CTI < 400 V
Material group IIIb: 100 V < CTI < 175 V

CTI plays a major role in determining the minimum creepage, or shortest distance along the surface of the isolator from pins on one side of the isolator to pins on the other side. A minimum creepage is required for a given working voltage depending on the extent of the pollution present in the system environment. Using a mold compound with a higher CTI allows the use of smaller packages, and saves board space. For example, as per IEC 60664-1, a package with 8 mm creepage using a CTI-I mold compound can withstand up to 1600 Vrms of working voltage, whereas the same package using a CTI-IIIa mold compound can withstand only 800 Vrms.

The ISO7842 uses a CTI-I mold compound. This implies that it can actually enable a 1500 Vrms working voltage at the system level with a standard 8 mm creepage SOIC-16 package. In contrast, competing isolators using a CTI-IIIa mold compound in the same package can only enable a working voltage of 800 Vrms at the system level, even though they may claim a higher value of VIORM/VIOWM at the component level.