AC and DC Electrical Strength of Thermally Conductive Insulating Foils under Various Conditions

In this work we present the results of AC and DC breakdown voltage measurements on thermally conductive insulating foil sheets. The aim is to derive suitable voltage values for a nondestructive electrical strength test. The voltages are as high as possible without producing too many rejects. From the different test methods for breakdown measurements, we choose the method that delivers the lowest breakdown voltages. The effect of different voltage rise rates is investigated and a suitable rise rate is derived. The thickness and area dependency of the foils to the breakdown voltage is investigated and discussed. Rectangular electrode surfaces are preferred for testing sheets. Therefore, we investigate the difference between circular and rectangular electrode geometries on the breakdown results. Furthermore, we propose a method for scaling the obtained test voltages to different foil sheet dimensions which is based on the enlargement law. The prerequisite is that breakdowns with extensive random samples are performed on a known arrangement. Because the enlargement law is not valid in all cases, it is necessary to perform breakdown measurements on the scaled arrangement for referencing. For a small number of samples, the method delivers low probability breakdown voltages more accurate than performing statistical analysis on the scaled arrangement.