The internal damage traces of the light-pipe-type high-isolation-voltage optocoupler after breakdown were studied. It was indicated that the continuous ablation caused by partial discharge under high voltage leads to breakdown between the front and rear stages. After analyzing partial discharge positions and considering the air gaps defects nearby, the electric field intensity distribution near the air gap was calculated with ANSYS software, which showed that the partial discharge was caused by the low dielectric strength of air in the air gap, and was enhanced by conductive adhesive infiltrating. After taking structure and process measures to avoid air gap defects and conductive adhesive infiltrating, partial discharge was eliminated, and minimum breakdown voltage (VISO) of the optocoupler was increased from 23.8kV to 33.5kV.
