Conformity Magazine - June 2008 - (Page 38)

a number of simulator channels together and also had a wire loop that could accommodate a high speed current probe. The method is illustrated in Figure 4. Figure 4 shows two wire loops and current probes to show how the short module could be used to measure current coming into the short module (the A side) or leaving the short module (the B side). Short modules from 14 pins to 256 pins were used in a combination of DIP and PGA form factors. It was found that, as the number of shorting pins increased, there was very little difference in the A side current, while there was significant reduction in the peak B side current and a large increase in the rise time as seen in Figure 5. Figure 6 illustrates how the A and B currents can differ. Each of the “unused” channels of the HBM simulator includes parasitic capacitances across the open relays. The connection between the relays and the ground terminal is also not zero Ohms and the deviation is likely much larger than wiring resistance. HBM simulator manufacturers build machines that provide the waveforms specified in the HBM standards. They are not required to reproduce the HBM schematic as shown in Figure 1. This gives the manufacturer the design space to meet the waveform requirements. One of the challenges is to prevent excess ringing during the stress. To do this it is likely that termination resistors have been added between the relays and the system ground. Reference [7] determined that the relay capacitance on modern high pin count HMB simulators ranged from 4.5 to 9 pF, and the termination resistance was likely in the range of 50 to 100 Ohms. As an example, we will assume 7.5pF of relay capacitance and 75 Ω of termination resistance. The circuit for the simulation is shown in Figure 7. The 1pF capacitor and 5μH inductor simulate realistic parasitics in the HBM simulator pulse source. [8] The 1500 Ω HBM resistor has been reduced to 1425 Ω to account for the 75 Ω termination resistor. If n +1 pins are shorted together by the device under test there are n parallel parasitic current paths. Each of these paths is a series combination of a single termination resistor and an open relay capacitance. The n parallel paths can be replaced by a series combination of a single resistor of value 75Ω/n, and a single capacitor, of value n×7.5pF. Figure 8 shows a comparison of no parasitics and for 30 simulator channels shorted together by the device under test. Without the parasitic path the current looks very similar to the example in Figure 2. With the parasitic paths there is only a Figure 5: Measurement of B Side current as a function of the number of pins Figure 7: Schematic for circuit simulation Figure 6: Modified HBM Schematic showing relay capacitance and termination resistors 38 Conformity JUnE 2008 Figure 8: Simulated HBM waveforms with and without parasitic paths for a device under test with 30 shorted pins

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Conformity - June 2008 Contents Editor's Note 700 MHz Auction Raises a Record $20 Billion FCC Modifies PCS and AWS Power Limit Rules for Broadband Wireless Commission Designates Spectrum Test Bed FCC Releases Report on Wireless Competition Commission Proposes $5 Million Fine for Slamming Using EDX for Non-Destructive Detection of Lead in Consumer Products ESD Open Forum Challenges in Testing - Improving Election Security and Accuracy Part 1 Challenges in Testing - Human Body Model: The Hidden Challenges High-Speed Signal Integrity Considerations for ESD Components Focus On...EMC Components Buyer's Guide FCC Levies $2.6 Million Fine for Junk Faxes FDA Updates Guidance on its Product Review Process EU Issues Updated Energy Star Regulations EU Repeals Directive on Veterinary Electro-Medical Equipment New Product Announcements Updated Standards List for the EU's PPE Directive CPSC Announces New Effort to Keep Out Hazardous Products CPSC Actions in the 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 - June 2008

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