What is Flip Flop:

A bistable multivibrator. A circuit which has two output states and is switched from one to the other by means of an external signal (trigger).

How to Design Heat sink for IGBT?

Any fined aluminum extrusion with a flat smooth surface can be used then your IGBT is mounted on it with a layer of silicon grease in-between, but remember the mounting surface is also your collector, if your heat sink is going to touch other metal parts it is beter to electrically insulate the IGBT with a silicon insulator strip and insulating sleeves for the mounting screws, then the heat sink can be mounted on a box that is fitted with a cooling fan with the fins inside the box, else it can be mounted open with the fins in a vertical position for natural airflow over the fins

Lenz's Law