Q. What is a heat sink? List the factors, which determine its efficiency.
To reduce the chance of thermal runaway, the rise of temperature at the collector junction to a dangerous level can be checked if we make a suitable provision for rapid conduction of heat away from the collector junction. This can be achieved by the use of a sheet of metal called heat sink. Connecting the heat sink to a transistor increases the area from which heat is to be transferred to the atmosphere.
Heat moves from the transistor to the heat sink by conduction, and then it is removed from the sink to the ambient by convection and radiation. Even if we use a heat sink, which can hold the temperature at ambient temperature the maximum power rating must be considered.
For maximum efficiency the heat sink should
1. Be in good thermal contact with the transistor case.
2. Have the largest possible surface area
3. Be painted black
4. Be mounted in a position such that free air can flow past it.