Conformity Magazine - December 2007 - (Page 36) Stress Conditions Now that the environment for the EUT has been established, the actual stress to the system must be defined. There are two types of stress, each with two subsets. • Direct Discharge o Air discharge o Contact discharge To understand the difference, it is useful to return to the schematic of an ESD gun as it was presented in the first article in this series (see Conformity, October 2007) and is reproduced in Figure 2. In contact discharge, the relay S2 starts in the open position, the 150pF capacitor is charged to the test voltage through the relay S1 and the high value resistor, the gun is placed against the EUT, the gun’s trigger is pulled and relay S1 opens and a short time later relay S2 closes and a stress occurs. In air discharge the sequence is different. The relay S2 stays closed throughout the testing. The gun is held away from the EUT and the gun’s trigger is pulled. The gun charges the capacitor through relay S1 and then opens the relay. The gun is then moved toward the EUT. As the gun approaches the EUT a discharge can occur, resulting in an ESD stress. Discharge may not occur if the surface being stressed is an insulator and the test voltage is not high enough to cause an arc to a conducting surface. An additional difference between the air discharge and contact discharge is the type of tip used on the ESD gun. In contact discharge, a pointed tip is used. This allows the tip of the • Indirect Discharge o Discharge to the HCP o Discharge to the Vertical Coupling Plane (VCP) Direct air discharge is the test that is most intuitive; a charged ESD gun is brought close to the EUT until an air discharge occurs. This test closely resembles a real world event. Air discharges are not, however, the most reproducible of events. An arc in air depends on many things, including air pressure, altitude, temperature, humidity, geometry and speed of approach. A more reproducible discharge occurs for contact discharge, in which the stress event is initiated not by an arc in air but by the closing of a relay. Figure 1: Test setup for ESD testing of small or table top systems as specified by IEC 61000-4-2. 36 Conformity DeCember 2007
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