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Creepage distance requirements for medical applications
2022-06-16 Source:Shenzhen Tenen Technology Co., Ltd.
The international standard for medical safety equipment, IEC 60601, covers the electrical aspects of isolation components and differs from other standards in several ways, including creepage distance and clearance requirements and defibrillation tolerance testing. Creepage requirements under many other standards allow the relationship between creepage and operating voltage to be smoothly interpolated between the values listed in the specification sheets. For example, if the creepage distance is slightly lower than the value in the table, the operating voltage can be scaled down and the device will retain most of its operating range. In contrast, the IEC 60601 standard explicitly prohibits interpolation of creepage and working voltages. 125 V rms working voltage requires > 6 mm of creepage distance, while 250 V rms working voltage requires > 8 mm of insulation creepage distance to provide both means of patient protection - no exception for parts.
A common package for reinforced isolator components is the JEDEC standard SOIC16-W, which is 7.4mm wide and 2mm thick. The shortest path along the surface of this package is usually near the end, usually only 7.6 mm. This is shorter than one might expect because, like most leaded devices, it includes a piece of metal along the creepage path, which must be deducted from the creepage distance according to the rules for measuring creepage distances.
These metal tabs are the tie bars that secure the package lead frame during the lead forming step in manufacture. These tie bars are typically not connected to the inner leadframe, but are exposed as floating metal at each end of the package. Even so, the creepage distance measurement must take into account the connecting rod, resulting in 7.6mm*, which for medical applications cannot meet the creepage distance requirement of 8mm. Therefore, the JEDEC standard SOIC-16W package can only be used for operating voltages up to 125 V rms. Since the world mainly operates at 220 V rms to 240 V rms, this package is not suitable for medical applications in most parts of the world. Fortunately, there are several ways to meet the 8 mm creepage distance requirement for medical applications worldwide. However, for specialty packages, it is best to keep such packages close to the JEDEC standard dimensions to reduce cost and risk to the designer. Oddly-shaped packages incur high costs because assembly lines must be modified to handle them, and they create high risk because custom assembly equipment can create a single-source bottleneck if it fails as it becomes more difficult to maintain or replace, thereby lead to production downtime. Maintaining JEDEC compliant packaging allows the cost of parts to be minimized by using standard tooling and does not lock components in a specific location in the event of a facility issue. The challenge for ADI was to create a JEDEC compliant package.