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THERMAL MANAGEMENT
Electronic equipment power density is on the increase and can lead to greater failure rates hence there is a
need for effective thermal management that is introduced early in the design cycle rather than as an after
thought. Failure to do so will compromise the design and the possible trade-offs between the
semiconductor devices and the cooling mechanism. It can sometimes be an advantage to use a higher rated
semiconductor device and in return reduce the size and cost of heat exchangers.
In the early days when the cost of power semiconductors was high the cost of heat removal was perhaps
not so important, however as device costs have reduced and package sizes have become smaller a global
approach to thermal management is now appropriate for efficient cooling to prolong equipment and
component life and increase reliability.
Individual power semiconductors although capable of controlling large amounts of power have low 'heat'
capability. Devices are very thin and consist of a number of layers hence it is not possible to differentiate
between the regions. It is generally assumed that all the losses in the device are converted into heat which
is dissipated at junctions producing a uniform laterally distributed temperature across the junction area
resulting in the importance of Junction Temperature as a critical rating.
Device current carrying capability is limited by the permissible maximum junction temperature and the
current density through the active silicon wafer. Junction temperature affects device current and voltage
capability. Device parameters deteriorate if the maximum junction temperature is exceeded and can
contribute to failure mechanisms. Manufacturers provide upper and lower operating and storage
temperatures.
The upper operational temperature limit is to contain excessive temperature rise due to current.
The upper storage temperature may be greater than the operating temperature and is based on 'no electrical
connection' and is limited by the reliability and stability of device characteristics.
Although it is the device junction temperature that is the limiting value, device characteristics are often
related to a measurable reference that is usually the case or mounting base.
The lower temperature limits are set at levels to avoid fracture of the semiconductor material due to
differences in thermal expansion and contraction of the semiconductor material and the various other
materials that connect the semiconductor die to the external 'world'.
Heat removal
Convection, conduction and radiation are the three modes of heat propagation although some may be more
predominant than others dependent on the system and its mode of operation.
CONDUCTION:
Heat transfer by contact.
Thermal energy at the junction as a result of power dissipation requires
the device to have a capability of transferring this heat to the outside
'world' referred to as the Ambient.
Ambient
is defined as the mass surrounding a
material
or
device.
1
EET307: POWER ELECTRONICS
Prof R T KENNEDY
Ambient
temperatue(T
)
is
defined
as
the
temperature
surrounding
A
THERMAL MANAGEMENT -intro
the device but not influenced by
Thermal energy at the junction as a result of power dissipation requires the device to have a capability of
transferring this heat to the outside 'world' referred to as the Ambient.
Ambient
is defined as the mass surrounding a material or device.
Ambient Temperatue(TA)
is defined as the temperature surrounding the device but not influenced by
the device heat dissipation.
Junction - Case Thermal Resistance
There is an analogy between electrical and thermal conduction that allows the use of circuit theory in
thermal analysis and it is therefore not surprising that, like the use of electrical resistance, heat removal is
discussed in terms of the device material thermal resistivity  th .
This introduces device thermal resistance from junction to case, Rth,j-c , as given by Rth, j c 
 th  l
A
The dimension of thermal resistance 0C / W (some data sheets use K / W)
Thermal resistance, or effective thermal resistance, can be defined as the temperature rise of a designated
junction above the reference point per unit power dissipation, under conditions of thermal equilibrium.
Rth, j c 
ELECTRICAL

voltage
potential difference
current
power
conductivity
resistivity
V
V
I
P
T

P
TJ Tc
PD

ANALOGY
T

PD
Q

 th
 th

THERMAL
temperature
temperature difference
power dissipated
heat
thermal conductivity
thermal resistivity
Low thermal resistance (high thermal conductivity is desirable)
TJ
An equivalent thermal circuit
PD
RthJC
TC
EET307:
POWER ELECTRONICS
THERMAL MANAGEMENT -intro
2
Prof R T KENNEDY
Junction - Ambient Thermal Resistance
Having conducted the heat from the junction to the case reasonably efficiently due to the comparatively
low junction-case thermal resistance it must now be transferred to the ambient.
The thermal resistance from case to ambient, RthCA , is the ability of the device alone to transfer heat from
the case to the ambient.
A typical device equivalent thermal circuit
TJ
PD
Rth,j-c
PD
TC
Rth,j-a
Rth,ca
PD
Heat Exchangers
To overcome the inability of the power semiconductor device to transfer heat efficiently to the ambient it
must be helped. The problem to be overcome is Rth,c-a the high thermal resistance from case to ambient.
Mounting the semiconductor on a heat sink reduces the effective value of Rth,c-a by providing a low parallel
thermal resistance path, Rth,s-a
TJ
PD
Rth,j-c
the heat flow divides, as would current flow,
with the majority taking the lower thermal resistance
path via the heatsink.
PD
PD
TC
Rth,s-a
Rth,c-a
PD
EET307:
POWER ELECTRONICS
THERMAL MANAGEMENT -intro
3
Prof R T KENNEDY
Thermal Analysis - Steady State
" A semiconductor device is only as good as its heat exchanger"
Thermal - Electrical Analogy
A simplified version in which all heat transfer from the device case / base / substrate is represented by a
single element Rth,s-a that includes thermal washers and contact is shown
The thermal performance of the device and heat exchanger can be represented by equations
TJ
PD
DEVICE
TJ  TC

Rth, j c  PD
PD

 TC
TJ

Rth, j c Rth, j c
Rth,j-c
PD
TC
Rth,j-a
HEATSINK
Rth,c-a
TC  T A
PD
EET307:

Rth,s a  PD

TC
Rth,s a

PD
TA
TA
Rth,s a
POWER ELECTRONICS
THERMAL MANAGEMENT -intro
4
Prof R T KENNEDY
Power Dissipation versus TC
A numerical solution is relatively straightforward, however if the equations are translated into graphs,
which being straight lines are easy to deal with, then a design aid is created
TJ
Device:
RthJC
The graphical representation of the equation
slope =
P
D
1
RthJC
there are however 2 constraints that must be applied
0
w negative PD or TC > TJmax is not possible
T
w the graph implies that as TC becomes more negative
PD can continue to increase
P
T
Jmax
C
Dmax
P
D
there is a maximum limit, PDmax ,to device power dissipation
based on device TJmax and Rth,j-c .
0
T
Heat Exchanger:
the graphical representation of the equation
C
slope =
P
D
T
Jmax
1
R thSA
0
Device + Heat exchanger
T
-TA
Superimposing permits a 'load line' approach to
finding PD(op) , the permissible operational power
dissipation, and TC(op) ,the operational case
temperature.
R
T
A
C
thSA
P
D
P
Dmax
PDop
T
C
0
TA
Power Derating Curve
TCop
T
Jmax
Device power capability reduces as the case temperature increases at a rate dependent on the device
thermal resistance & maximum junction temperature. The slope of the curve is the power derating in W/ oC
R
=0
th,s-a
P
Dmax
0
slope = W / C
P
D
0
T
A
EET307:
POWER ELECTRONICS
THERMAL MANAGEMENT -intro
T
C
5
T
Jmax
Prof R T KENNEDY