slots in pwb

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I have a question regarding the slots show in the PWB under the bridge rectifier for the switch mode power supply reference designs. I notice that they have a slot pattern that resembles and E cut-out under the solidstate bridge rectifier chip. What is this for? As I understand it UL requires creepage and clearance between high voltage and low voltage but not between primary side circuit conductors. Would someone please help shine some light on this for me?

Are you referring to a PI reference design?

Slots like you describe are often used for improving lightning surge performance of a PSU. Sometimes arcs like to occur from one point to a nearby one on the PCB along the surface, wherever there is a high voltage potential (during the surge event). A slot often prevents said arcing. In and around the bridge rectifier is one such area.