5. Cooling Flashcards
What are heatsinks?
Specially shaped pieces of metal that are designed to remove heat efficiently from electronic components
Book 4 page 65
What is the formula for thermal resistance?
Where
θ = thermal resistance (°C W-1)
TH = temperature at hotter end (°C)
TC = temperature at cooler end (°C)
P = power flow (W)
Book 4 page 66
What is convection?
Process in which heat is transferred away from a hot object by air or other fluid moving past it
Book 4 page 68
What kind of convection is utilised in a normal radiator, where warm air rises up from the radiator and cooler air from floor level takes its place?
Natural convection
Book 4 page 68
What does the Stefan-Boltzmann law state?
That power emitted is proportional to the fourth power of the absolute temperature
Book 4 page 69
Why will you often find that heatsinks have a matt black finish?
For good radiation emission
Book 4 page 70
What is encompassed by the single figure often quoted by heatsink manufacturers of thermal resistance?
It includes conduction away from the point where the component is mounted and convection from the surface under natural cooling conditions
Book 4 page 70
What is being referred to when talking about the maximum temperature of a component such as a transistor?
Temperature of the actual silicon die
Book 4 page 71
What two resistances are usually quoted for a transistor under the header ‘operating junction temperature’?
Junction-to-case
Junction-to-ambient/junction-to-air
Book 4 page 71
How can the thermal resistance between mating surfaces be reduced?
Thermally conductive grease
Book 4 page 71
What is the usual value of contact resistance between a heatsink and a component?
0.5-1 °C W-1
Book 4 page 72
Describe a heat pipe?
Sealed assembly where one end is the evaporator section and is heated by the device to be cooled, while the other end is the condenser section and is kept cool through fins usually with forced convection
Works by evaporating and condensing a fluid
Book 4 page 74
