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.

I. Introduction

The USB interface is one of the most commonly used high-speed interfaces within electrical devices. Because these connectors are easily accessible to users, they are highly susceptible to electrostatic discharge (ESD). Transient voltage suppression (TVS) diodes are typically added to I/O interfaces to improve the system's immunity to ESD. TVS devices can shunt most ESD current away from sensitive integrated circuits (ICs) during a transient over-voltage event. Ensuring the TVS diode turns on during an ESD event and the on-chip protection device does not take the entire charge can be challenging, however, as many on-chip ESD protection structures will turn on faster at lower voltages than the off-chip TVS [1]. The high data rate of USB interfaces, 480 Mbit/s for USB 2.0 and 5 Gbit/s for USB 3.0, requires substantial attention to signal integrity, making the design of robust ESD protection strategies even more challenging due to the low required capacitance of the device. The USB 3.0 specification requires the total parasitic capacitance be less than 1.1 pF [2], but the sub-pF capacitance ESD structure can create a large voltage overshoot during an ESD strike, which may cause hardware failure [3].