Conformity Magazine - March 2008 - (Page 36)

insulator depend on the specific standard used (differences between different FCDM standards will be discussed later). The potential of the field plate can be controlled with a high voltage power supply through a high value resistor. Suspended above the field plate is a ground plane. At the center of the ground plane is a spring loaded pogo pin in the center of a 1Ω disk resistor connected to the ground plane. The pogo pin and ground plane are also connected to a 50Ω coaxial cable. The separation between the field plate and the ground plane as well as the relative position of the ground plane over the field plate is computer controlled. Placing the DUT with the pin side up (dead bug position) produces a capacitance between the DUT and the field plate that scales with the size of the DUT. A low inductance discharge path to ground is formed by moving the ground plane relative to the field plate such that the pogo pin can touch any pin on the DUT. The 1Ω resistor and coaxial cable provide a low inductance current sensor so that discharge waveforms can be conveniently measured. The process of charging and discharging the DUT is where most of the confusion over FCDM occurs. Confusion about the test procedure is understandable because the actual process is opposite from what is expected. It is often stated that in FCDM the DUT is inductively charged and then discharged by contacting the pogo pin to the DUT. In fact field induction does not place any charge on the device. Also, the “discharge” when the pogo pin first touches the DUT is when the DUT is actually charged. In Figure 3 a circuit diagram is overlaid on top of a simplified version of the FCDM simulator. This circuit is a simplification of the full circuit but shows the most important features. A very thorough theoretical treatment of FCDM testing is provided by Atwood et. al.3 CDUT is the capacitance of the DUT to the field plate, CDG is the capacitance of the DUT to the ground plane and CFG is the capacitance of the field plate to the ground plane. Touching the pogo pin to a pin on the DUT has been represented by the switch S. Key to the understanding of FCDM is the series capacitors CDUT and CDG. Assuming no initial charge on the DUT, with the switch S open the DC voltage of the DUT is: Because the separation of the DUT from the field plate is always much less than the separation of the DUT from the ground plane; CDUT is always much larger than CDG. The potential of the DUT will therefore track the voltage on the power supply. The potential of the DUT relative to the ground plane can therefore be controlled without actually putting any net charge on the DUT. There are two procedures for doing FCDM stressing. A positive and a negative stress can be performed with a single charging of the field plate or positive and negative stresses can be done separately. The two procedures start out the same. The single stress method will be described first, followed by the dual polarity stress. 1. With the field plate at zero volts an uncharged DUT is placed on the field plate in the dead bug position and the ground plane is positioned with the pogo pin above the pin to be tested. The field plate is raised to a high potential, for example +500V. The high value resistor insures that the field plate changes potential relatively slowly. The slow change in potential insures that the DUT is not damaged before the CDM event. The potential of the DUT will closely track the field plate, reaching in excess of 450V, although there will be no net charge on the DUT. After the voltage has stabilized the separation between the field plate and Figure 2: Field-induced charged device model simulator 2. 3. Figure 3: Capacitors added to field induced CDM simulator 3 Conformity marCh 2008

Table of Contents Feed for the Digital Edition of Conformity Magazine - March 2008

Conformity Magazine - March 2008 NewsBreaks Editor's Note Adelstein Renominated to the FCC Industry Canada Publishes FAQs On RF Fields FDA Gives Its Scientific Capabilities Poor Marks FDA Offers Guidance On its Product Review Process FDA Publishes Updates to List of Recognized Standards FDA Warns of Patient Risk From Electric Dental Handpieces Complying with the EMC Directive, Second Edition ESD Open Forum Emerging Issues in Standards - Korean "RoHS": Not Like RoHS at All The iNARTE Informer Challenges in Testing - Field-Induced Charged Device Model: What Really Happens Design Fundamentals - EMI Shielding Technologies for Consumer Electronics Equipment Focus On...Compliance Software Buyer's Guide Additional Standards Published for the EU's ATEX Directive Additional Standards for the EU's Medical Devices Directive Also Published China to Require Bar Codes to Enhance Product Safety CPSC Releases 2007 Self-Assessment CPSC Actions in News IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - March 2008

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