Characterization of ESD Risk for Wearable Devices

The core difference between the posture assumed for IEC 61000-4-2 human metal discharge and a discharge to a wearable device is the impedance between the charged body and the grounded structure discharged to. Especially for a waist-worn device, a larger portion of the body is close to the grounded structure; thus the geometry forms much lower impedance which will lead to higher currents. Despite the variability for the air discharge, in most cases, the current will be higher than 3.75 A/kV as specified for contact mode electrostatics discharge (ESD) calibration. Even for the most slowly rising discharges having a spark length equal to the value given by Paschen's law, a 10 kV ESD from the waist will surpass 37.5 A for a waist-worn metal part discharged (e.g., to a door frame). Modeling the wearable device discharge provides predictions on the current derivative and the transient field a wearable device is subjected to. Observed failure levels of a wearable electronic device and comparing discharges according to the IEC 61000-4-2 test standard against discharges from the device while wearable showed that the IEC set-up may be insufficient to ensure the robustness of the wearable devices.

I. Introduction

The electrostatics discharge (ESD) scenario specified by the IEC 61000-4-2 standard is based on a human holding a metal piece in the hand and discharging to the ground plane [1], referred to as the human metal model (HMM). However, for wearable devices the ESD scenario is different: a wearable device may spark to a door frame if the person brushes by the door frame. In a “brush-by” situation currents can reach levels twice as high as a human metal discharge at the same voltage. This posture related reliability risk has been shown to possibly endanger medical equipment [2], e.g., during patient movement on a transfer board. The impedance formed between the ground and the wearable device is much lower compared to the impedance between the hand-held metal and ground in the HMM scenario, as shown in Fig. 1. Fig. 1.

Different ESD scenarios for Brush-by and HMM.