Conformity - November 2008 - (Page 28) capacitor is made from RR-4 PCB. The R-C-R structure also forms a loop, such that its inductance improves the low pass filter function. • Main RC elements: The ESD standard asks for a 330 Ohm/150 pF main time constant of the ESD generator. This is implemented as a combination of capacitors connected in series (due to the maximal voltage allowed for the capacitors) and a series connection of resistors. • Discharge tip: The discharge tip is a metal extension. • Structural elements of the ESD generator and electronics: The electronic circuit boards, display and battery pack are modeled as metallic blocks. • Ground strap: Its main function is allowing the return of low frequency components of the current (charge return). However, in most ESD generators the ground strap is not decoupled for RF, from the pulse creating parts of the ESD generator. Thus, strong RF currents (e.g., created by the initial peak of the breakdown) will flow on the ground strap. The ground strap is modeled as a metallic connection. • Additional details: Additional details, such as the impedance looking into the high voltage supply, have been measured and implemented in the model. Details of the pulse forming parts are shown in Figure 3. The step function source is located within the relay. Verification The current injected into a large ground plane is a main 2.5 criterion for ESD generator performance. The current has been measured using a ESD current target (> 4 GHz) and a 4 GHz, 20 GS/sec oscilloscope. The fast rising and possibly ringing behavior of the first peak in the discharge current waveform dominates the disturbances in electronic system. The data shown in Figure 4 indicate a good match for the initial pulse. However, the ringing of the pulse is at present not well modeled. However, from the point of view of induced noise, the initial pulse often dominates the DUT’s response. Hence, the deviation shown has been accepted for now. Results: Voltage Induced in a Small Loop The ESD standard IEC 61000-4-2 is presently being revised to address the problems of ESD test result repeatability. One measure to better characterize an ESD generator is the voltage induced in a loop close to the ESD generator. To avoid possible coupling into cables, a ground plane mounted semicircular loop is used. It is unshielded, thus, it is sensitive to the E and the H field. Its diameter is 28 mm, and it is placed at a distance of 10 cm from the point of discharge. Such a loop represents a wire loop within an electronic system, and is a better predictor for system level disturbances than the discharge current. The standard will provide intended induced loop voltage waveforms such that the actual ESD generator manufacturers can compare their instruments against the optimal waveforms given by the standard. We used the semi-circular loop to verify the simulation model of the ESD generator against measurements. The results are shown in Figure 5. It is significantly more difficult to achieve a good match between the simulation and the measurement for an induced Figure 1: Induced loop voltage of previous ESD generator model (Centola/Pommerenke/Caniggia) compared to measurements Figure 2: ESD generator full wave model overview 28 Conformity november 2008
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.