EMC & Circuit Protection
withstand test. In the case of IEC60747- 5-5 or equivalent certifications, a partial discharge insulation quality test is also performed on each device.
Table 1. IEC60747 type testing
Supplemental rated components. An example is shown in Figure 2 of two common operating conditions and the creepage and clearances required. This approach is taken since the external
environment and surface properties determines the external spacing requirements. These include the amount of contaminants expected, the air pressure, and the tendency of the outer surface of a component to be eroded by surface discharges, called tracking. For internal properties of components is that the quality of the insulation is more important than having a specific quantity or thickness of insulation. The manufacturer can demonstrate that the part has the required electrical properties to withstand the voltage stresses both long and short term. The requirements of IEC60950 standard
are for office and telecom equipment and to a large extent for medical devices. The external dimensions and materials are readily verified with a micrometer and some bulk material testing for tracking index. For internal requirements there are three approaches for qualifying the component.
• The component can be evaluated as if it contained only solid insulation. This is the simplest approach since it requires that all of the internal distances through the insulation or along cemented joints are greater than 0.4mm. No further type testing is required. However it is difficult to make a high performance data coupler that meets these requirements. It is widely believed that the 0.4mm minimum insulation thickness applies to all reinforced isolation devices; this is not the case and is a point of confusion for many engineers.
• If the component is an Optocoupler, then the IEC60747-5-5 standard must be applied. This is a rigorous standard specifically designed to qualify optocouplers for reinforced insulation and has a battery of type tests and life tests with isolation withstand verification tests after each one. • The component can be treated as a semiconductor device. This category of devices has a set of type tests similar to the IEC60747-5-5 requirements. This approach is used by digital isolators since the testing requirements of the Optocoupler standard are specifically tailored to Optocoupler structures. Qualification to and maintenance of a
reinforced rating is accomplished in three phases. 1) Materials and dimensions are evaluated and electrical type testing is conducted. Testing includes thermal cycling, limited life testing and electrical overstress that would cause heating or catastrophic insulation failure. The integrity of the isolation is checked with a voltage withstand test after each environment or test. IEC60747 type testing is summarized in Table 1.
2) After the part has been approved based on its dimensions and type testing, insulation integrity is checked for each device as it is manufactured by a voltage
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3) Periodic audits are conducted by the certifying body to verify material sets and dimensions have not changed, that all assembly line tests are being properly conducted with calibrated equipment. Some of the type tests periodically repeated on a sample basis and reviewed by the auditor.
Isolation requirements The previous discussion centered around one of the most widely applied standards. Different standards can have disparate requirements at the component level. It even varies from edition to edition within a single standard. This is becoming less problematic as IEC is trending toward a unified approach. This will likely take a significant amount of time to achieve since the individual standards committees still have significant independence. A unifying trend in application of system level standards is the availability component level standard such as IEC60747-5-5. If such a standard exists for a component, it can be applied instead of the particular requirements of a system level standard as in point two in the Component Level Requirements section. Currently this standard only applies to optocouplers, not other newer digital isolators; however VDE has created a draft standard VDE0884-10 which applies the insulation tests of the IEC60747-5-5 standard to digital isolators. These standards both set a high bar for
reinforced insulation including surge testing at levels of 10kV or higher. Very thin insulation layers will not pass this test and it has proven to be the discriminating test for many optocouplers and digital isolators for qualification as reinforced insulation. Components that cannot meet the requirements usually fall back to the IEC60747-5-2 standard which can be applied to basic insulation. This is yet another confusing point for designers of isolated systems where it is assumed that an IEC60747-5 qualification automatically confers reinforced status. The IEC committees are currently working to revise the IEC60747-5-5 standard to include digital isolators. The next unified standard will be applicable across all IEC system level standards and should help to eliminate confusion in the future. Reinforced insulation in data isolators is designed and qualified to provide the protection of double insulation systems while providing the data transmission performance available with a single isolation barrier. Externally, the components have a creepage and clearance requirement that is equivalent to twice the basic insulation requirement. Internally, insulation either meets the requirements of solid insulation including through the insulation minimum distance, or it receives extensive type testing and assembly line testing during production. The availability of a reinforced insulation rating which is verified by test rather than detailed structural requirements allows innovation in insulation technology to be qualified without rewriting the standards for each new technology.
Analog Devices |
www.analog.com Mark Cantrell is an Applications Engineer with Analog Devices
Components in Electronics September 2011 23
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