2. •
•
Electronic components depend on the passage of electric current
to perform their duties, and they become potential sites for
excessive heating, since the current flow through a resistance is
accompanied by heat generation.
Continued miniaturization of electronic systems has resulted in
a dramatic increase in the amount of heat generated per unit
volume
INTRODUCTION
3. • The failure rate of electronic equipment increases
exponentially with temperature.
4. COOLING LOAD OF
ELECTRONIC EQUIPMENT
• The first stage of selection and design of a cooling system is the
determination of heat dissipation.
• The current flow through a resistance is always accompanied by
the heat generation in the amount of Q= 𝐼2Rt.
5. • The duty cycle is another important consideration in the
design and selection of a cooling technique.
6. METHODS OF COOLING
Depending upon the load the cooling method is generally chosen.
The methods are:-
Natural convection cooling.
Forced convection cooling with air.
Immersion cooling with natural convection.
Immersion cooling with boiling.
Forced circulation of water.
Heat Pipe.
7. METHODS OF COOLING
The choice of the cooling method can be decided by using the
chart if we know the heat flux.
8. The component manufacturers provide the data about the heat dissipation
rate and maximum allowable temperature for each of the component. The
heat dissipation rate divided by the component area gives the heat flux. For
a heat flux of 0.1 W/cm2 and allowable temperature difference of 60°C
natural convection with radiation can be chosen. If direct forced
convection is adopted for this flux the temperature rise will be limited to
about 15°C. For a heat flux of 1 W/cm2 forced convection will lead to
more than 100°C temperature rise and so the next suitable method,
immersion with fluorocarbons may be the choice.
METHODS OF COOLING
9. METHODS OF COOLING
Depending upon the load the cooling method is generally chosen.
The methods are:-
Natural convection cooling.
Forced convection cooling with air.
Immersion cooling with natural convection.
Immersion cooling with boiling.
Forced circulation of water.
Heat Pipe.
10. •Electronic components or PCBs placed in enclosures such as a
TV or DVD player are cooled by natural convection by
providing a sufficient number of vents on the case to enable
the cool air to enter and the heated air to leave the case freely
AIR COOLING:
NATURAL CONVECTION & RADIATION
11. AIR COOLING:
NATURAL CONVECTION & RADIATION
Heat transfer coefficient can be determined by equation of the
form below for natural convection.
where K depends on the geometric shape and positioning of the
body, L is the flow length on the body and ΔT = TS – T∞
ℎ = 𝑘
∆𝑇
𝐿
0.25
12. • When hot surfaces are surrounded by cooler surfaces such
as the walls and ceilings of a room or just the sky, the
surfaces are also cooled by radiation
13. AIR COOLING:
FORCED CONVECTION
•
•
When natural convection cooling is not adequate, we
simply add a fan and blow air through the enclosure that
houses the electronic components.
By doing so, we can increase the heat transfer coefficient by a
factor of up to about 10.
15. Hollow core cooling
In cases where air is not allowed to pass over the electronic components,
hollow core method is adopted. Cooling air flows through the hollow space
between the PCBs, collecting heat from the devices mounted on the circuit
boards. The flow area is a rectangle of sides equal to width of PCB and the
depth of the hollow core. Heat generated by the components is conducted
through the PCB and a thin layer of epoxy board to reach the cooling air.
Heat picked up by air is given by
Q = mcp (Tair out – Tair in)
The heat flow can be also calculated by
Q = hAs ΔTmean
where As is the heat transfer area. Convection coefficient h, is to be calculated
using the correlations for internal flow. As the temperature difference may
vary, ΔTmean is used for property values. In the case of PCBs heat is
generated uniformly along the length. So in the hollow core cooling the
correlation for constant heat flux should be used.
