Conformity Magazine - January 2009 - (Page 25) improve field reliability? Nothing’s 100%, but it’s certainly clear that knowing what parts of a circuit are sensitive to transient events makes it possible to take steps to protect or improve that susceptibility. It’s also clear that, if during a system level test upset does occur, it’s pretty likely that some remnant energy got to a sensitive area of the design. Think about how transients get into a system. An ESD event, electrical fast transient or surge comes in from the outside world. Primary and secondary protective devices operate to get rid of the bulk of the energy coming in on the mains or into an I/O port. Even at the device level, protective structures exist to divert any higher than normal voltages preventing hardware damage. What’s left to cause upset are E and H fields that are either radiated into the system or developed as a result of secondary effects. Susceptibility of systems and devices has been shown to be more related to these radiated effects than to the voltage levels of external ESD. Any remnant voltage that does get to a device and causes upset or an unwanted reset doesn’t look anything like the event coming in from the outside world, unless it’s directly connected to the outside world. Even then, it’s likely to be considerably modified by surrounding circuit elements and parasitic effects. Anything that gets to a sensitive area deep inside a product is much more likely to be the result of E and H fields that result from the initial transient. Conclusions Although the gap between system level testing and testing done by device manufacturers does exist, there is a way to bridge this gap. EMC susceptibility testing can and is being done to identify sensitive circuits and devices, to the point of identifying the areas within an individual IC that are sensitive. Using different probes, determination can be made as to whether the sensitivity is to an E field or to an H field, and therefore what kind of steps need to be taken to reduce susceptibility levels. Michael Hopkins is an independent consultant to Amber Precision Instruments, and can be reached at mike@amberpi.com. FAST Link www.conformity.com/2540 Figure 4: A new EMC/ESD qualification process (copyright Amber Precision Instruments. Reproduced with permission.) JAnUAry 2009 Conformity 25 http://www.conformity.com/2540
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