Effect of Approach Speed on Spark Length determining Air Discharge Current from ESD Generator in Environment with Different Temperature and Humidity

An air discharge current waveform for immunity testing is largely affected by various factors such as test voltage, approach speed, ambient temperature (T) and relative humidity (RH). For investigating the combined effect of the approach speed and climatic conditions on the air discharge current, we previously measured air discharge currents from an electrostatic discharge (ESD) generator with test voltages from 2 kV to 15kV at two approach speeds of 80 mm/s and 20 mm/s under 6 combinations of T and RH in the IEC specified and non-specified climate range. The result showed that the effect of the approach speed on air discharges significantly differ depending on whether the climate condition is within the IEC specified range or the IEC non-specified range. Under the IEC specified climate conditions, at a test voltage of 15 kV, the 80 mm/s causes the discharge current peak that decreases as the absolute humidity (AH) increases, whereas the 20 mm/s gives the peak less affected by the AH. Under the IEC non-specified climate conditions, the current peak decreases regardless of the approach speed when the AH increases. In this study, to examine why the above findings occurred at different approach speeds in particular, we estimate the spark lengths for air discharges of the ESD generator from our previously measured air discharge currents. As a result, under the IEC specified climate conditions, at a test voltage of 15 kV, the 80 mm/s gives the spark length that increases as the AH increases, whereas the 20 mm/s causes the longer spark length than the 80 mm/s, which is almost unaffected by the AH. Under the IEC nonspecified climate conditions, the spark length increases regardless of the approach speed when the AH increases. A spark length dependence on the current peak is exhibited to be on a specific curve according to the test voltages from 2 kV to 15 kV regardless of the approach speeds and climatic conditions. This property is quantitatively analyzed by using a simplified equivalent circuit of ESD generator based on the IEC standard.