Modeling an ESD Gun Discharge to a USB Cable

When an electrostatic discharge (ESD) gun discharges to a USB cable, the routing and quality of the cable impacts the waveform seen at the printed circuit board (PCB) connected to the cable and the ability of an on-board transient voltage suppressor (TVS) to protect sensitive electronics. The impact of cable configurations during ESD gun contact discharge tests was investigated for multiple cable configurations. Injection to a cable pin whose shield is 'floating' at the injection site can cause a double-peak in the ESD waveform at the PCB and a lower maximum stress level than when the cable shield is connected to the return plane. Poor shielding of the USB connector can further induce a pre-pulse effect, where a smaller ESD pulse arrives at the PCB before the main pulse. This pre-pulse can result in poor firing of the TVS device and thus worsen ESD stress at a sensitive IC. Circuit models were developed to anticipate and explain both of these phenomena. These models were incorporated into a system-level transient simulation including models of a PCB with a TVS and a pair of on-chip diodes. This system-level model was able to predict the quasi-static and peak voltages and currents at the on-chip diode during 1-8 kV ESD contact-discharge tests with various USB cable configurations to within less than 30%. These models were used to develop test and design guidelines to account for the impact of the quality and configuration of a USB cable during an ESD discharge.