ALL OF HEAT IN ELECTRICAL COMPONENTS TO MEMORIZE Flashcards
What is joule heating?
The process by which the passage of an electric current through a conductor releases heat.
What is power in electric components dissipated as?
Heat
What technology are many modern electronic components based on?
CMOS - complementary metal oxide semi conductor technology
What strengths do CMOS transistors have?
transistors based on this technology only consume significant power when they are switching between their on and off states.
Why is thermal management in electrical components becoming a critical issue?
As switching speed and transistor densities have continued to increase, meaning there is more power generated at at a faster rate and higher density. Increased temps in electrical components give a lower mean time to failure rate.
What is thermal runaway phenomena and what material has this?
(Silicon) Electrical resistance increases with increasing temp up to a certain temperature where past this temp the electrical resistance starts to decrease. This decrease in electrical resistance causes more current to flow through the component and thus a further increase in temp occurs… and so the cycle repeats
Why is it desirable to spread the heat around a system as equally as possible?
Spreading the heat will mean that one particular area will not end up being at an extremely high temp compared to everything else but instead the whole system will be at an equal, acceptable temp. This means the likelihood of damage to an electrical component due to overheating is lowered.
What is a heat sink?
DRAW DIAGRAM. A heat sink is a highly conductive component which can be attached onto an electronic component which is dissipating heat in order to conduct the heat away from the component and thus prevent overheating.
How does a heat sink work?
A heat sink is made up of two main parts: the base and the protruding fins. The heat sink base is in contact with the electrical component and it is at a lower temp than the electrical component in order for it to conduct heat away from the component. The protruding fins are long and thin and there are many of them as this increases the surface area over which convection with the cooling fluid (e.g. air) can occur. Heat sinks must have a temp higher than their surroundings in order for heat transfer to occur.
What are some factors that effect the performance of a heat sink?
- Approach air velocity
- choice of material
- fin design
- surface treatment
What are TIM’s and how do they work?
Thermal interface materials are highly conductive materials which displace the air trapped in the voids which are present due to imperfect contact between two objects.
Why must care be taken when selecting a TIM?
Most manufacturers give a value for the thermal conductivity of the TIM. However, thermal conductivity does not take into account the interface resistances. Therefore, if a TIM has a high conductivity, it does not mean that the interface resistance will be low.
In what ways is aluminium a better material choice for a heat sink than copper and vice versa?
Aluminium has a lower overall thermal conductivity than copper. However, aluminium is much cheaper, lighter and can be extruded (cu cannot and heat sinks made from copper have to be machined).
Why does heat transfer rate decrease from the base to the end of the fin?
Because the combination of thermal resistance of the heat sink impeding the flow, heat lost by convection and the temp of the fin.
What is the eqn for fin efficiency?
Fin efficiency = (actual heat transferred)/(heat that would be transferred if entire fin area were at base temp) = tanh(mLc)/mLc mLc = sqrt(2*h/k*t)L Where: h - convection coefficient k - conductivity t - fin thickness L - fin height
