Conformity Magazine - January 2009 - (Page 21) a critical difference. Immunity implies the ability to keep functioning in the face of electrical disturbances, such as ESD, transients and RF. But individual devices are typically only tested to determine the voltage level beyond which the device will be damaged.2 Several standards from the Electrostatic Discharge Association (ESDA) and the Joint Electron Devices Engineering Council (JEDEC) exist to qualify devices for their ability to withstand ESD during the handling process. But no standards exist that deal with susceptibility.3 System Testing for Immunity Systems (i.e., finished products) really are tested for immunity. Hardware damage, loss of data and unsafe operation resulting from a test are never allowed. Mandatory compliance testing for the CE mark in the European Union (EU) as well as most other industry and corporate standards include failure criteria similar to that in IEC standards. For example, the performance criterion from the IEC Generic Standard for residential, commercial and light industrial environments is as follows: Performance Criterion A: The apparatus shall continue to operate as intended during and after the test. No degradation of performance or loss of function is allowed below a performance level specified by the manufacturer, when the apparatus is used as intended. The performance level may be replaced by a permissible loss of performance. If the minimum performance level or the permissible performance loss is not specified by the manufacturer, either of these may be derived from the product description and documentation and what the user may reasonably expect from the apparatus if used as intended. Performance Criterion B: The apparatus shall continue to operate as intended after the test. No degradation of performance or loss of function is allowed below a performance level specified by the manufacturer, when the apparatus is used as intended. The performance level may be replaced by a permissible loss of performance. During the test, degradation of performance is however allowed. No change of actual operating state or stored data is allowed. If the minimum performance level or the permissible performance loss is not specified by the manufacturer, either of these may 3 be derived from the product description and documentation and what the user may reasonably expect from the apparatus if used as intended. Performance Criterion C: Temporary loss of function is allowed, provided the function is self-recoverable or can be restored by the operation of the controls. The Disconnect The disconnect between system testing and device testing can be summarized as follows: • The system manufacturer believes that, by requiring the device manufacturers to test to system level standards (IEC 61000-4-2 waveforms for example), the system will likely pass compliance tests. • The device manufacturer believes he’s doing what the system level manufacturer requires. After all, he did a test at several thousand volts4, and the device still functioned afterwards. Of course, both are mistaken. The device may be able to handle a 4kV ESD event during handling. But, because a few volts appear at a reset pin during compliance level testing, the system is considered to have failed, and the finger pointing begins! Although software routines and error correction protocols exist to prevent corruption of data transmissions, these won’t prevent malfunction of the device itself due to ESD or other transients. Methods do exist for testing ICs for RF Immunity in TEM cells. The device is mounted on one side of a circuit board with exposed circuitry on the other side. The board is then mounted in the wall of a TEM cell with the device exposed inside and power applied from the outside. 2 4 The ESD Association Working Group 5.6 is currently working on a Standard Practice that will give device manufacturers some guidance about how to accomplish this testing in the lab. JAnUAry 2009 Conformity 21 http://www.monroe-electronics.com http://www.monroe-electronics.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.