Osamu Fujiwara

An experimental study of electrostatic discharge immunity testing for wearable devices

Wearable electronic devices have proliferated in recent years. Since such devices are used by attaching to a human, every time the human is electrically charged or discharged, the electric potential of such a wearable device abruptly changes. Thus the device is exposed in close proximity to the resultant transient electromagnetic fields, which may cause malfunctions in the device. Electrostatic discharge (ESD) immunity test methods that assume this situation are not yet developed in the present international standards. In this study, to investigate such ESD immunity testing, we considered the worst case as an ESD event which occurs when the body-mounted wearable device approaches a grounded conductor. In lieu of actual devices, we measured discharge currents caused by air discharges from a charged human through a hand-held metal bar or through a semi-sphere metal attached to the head, arm or waist. As a result, we found that at a human charge voltage of 1 kV, the peak current from the semi-sphere metal is large in order of the attachment of the waist (15.4 A), arm (12.8 A) and head (12.2 A), whereas the peak current (10.0 A) from the hand-held metal bar is the smallest. It was also found that the discharge currents from the semi-sphere metal decrease at around 50 ns regardless of the attachment positions, however, the current from the handheld metal bar continues to flow at over 90 ns. This suggests that ESD immunity testing for wearable devices needs different specifications of test equipment and test method from the conventional testing.

An alternative air discharge test in contact discharge of ESD generator through fixed gap

It is notable that the air discharge method specified in the International Eelectrotechnical Commission (IEC) standard provides unstable testing reproducibility. In this paper, to achieve stable testing reproducibility, we proposed an alternative air discharge test method using an electrostatic discharge (ESD) generator in contact discharge mode. Measurements of air discharge currents through a fixed gap to sphere or conical electrodes in view of various metal curvatures inside equipment covered with a non-conductive case were conducted with respect to different gap lengths from 1 mm to 6 mm, and their current peaks and rise time were investigated in conjunction with the statistical variations. Comparison with air discharge current waveforms was also made to reveal better testing repeatability for the proposed method.