Download Thermal MANAGEMENT Electronic equipment power density is on

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THERMAL MANAGEMENT
Electronic equipment power density is on the increase 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.
D1.1 Efficiency
Power processing being the objective of power electronics it is not surprising that power is one of the
most important electrical quantities.
System efficiency, given by equation D1.1, is never 100%
 
Pout
Pin
 
1
P
1  loss
Pout
  1
Ploss
Pin
(D1.1)
Power losses occur in resistive elements (intended components and parasitic) and in the power
semiconductors where
Power Loss
EET307
POWER ELECTRONICS
Thermal Management
Heat
1
Temperature rise
Prof R T Kennedy
2005-2006
D1.2
Thermal Characterization - Power Semiconductors v Temperature
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 / parameter.
In power electronic applications devices are usually subjected to pulses of current resulting in cyclic
junction temperature as shown in Fig. D1-1. When a device is energized and de-energized the
temperature of the cells rise and cool respectively creating stress that can lead to thermal fatigue.
P
0
T max
T
T
av
0
FIGURE D1-1
A plot of device temperature is analogous to the output of a dc power supply in which there is a steady
state dc (average value) and superimposed voltage ripple. In the case of temperature, where there is the
steady state average temperature plus temperature ripple, devices should be rated at the maximum
junction temperature and not the average.
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'.
EET307
POWER ELECTRONICS
Thermal Management
2
Prof R T Kennedy
2005-2006
D1.3 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.
Heat removal by transferring energy through a fluid that could be
mix of hot and cold air and by natural or forced convection.
h, the convection efficiency, is complex and includes the effects
of fluid viscosity, volume , thermal resistivity & geometry.
CONVECTION:
Qconv  hA(Tsurface  Tam b)
Heat transfer by contact .
Liquid cooling techniques are mainly by conduction.
CONDUCTION:
dQconduction
dT
  th A
dt
dL
RADIATION:
Qradiation  sA (Tsurface  Tam b )
4
4
Flow of heat in the infra red section of the e.m. spectrum.
at extreme altitude radiation is predominant.
s Stefan Boltzmann constant
 emissivity :
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
Ambient temperatue(TA)
is defined as the mass surrounding a material or device.
is defined as the temperature surrounding the device but not influenced by
the device heat dissipation.
Under steady state conditions (thermal equilibrium) the heat generated at the junction has to be
conducted through the layer structure and via internal package materials to the device body (case) then
either directly or through a heat exchanger to the ambient.
The first heat propagation mode encountered is conduction and this is represented by Fourier's Heat
transfer Law, modified for thermal equilibrium, in equation D1.2.
Q  hc  A  T
(D1.2)
Q
=
rate of heat transfer
hc
=
convection heat transfer coefficient
A
=
cross sectional area
T
=
temperature difference between the regions of heat transfer
Thermal ability of the device requires heat dissipation at a rate  the heat generated.
Combining this with equation D1.2 yields equation D1.3 that describes the ability of a device to effect
the transfer of thermal energy by conduction in terms of the device material thermal conductivity  th ,
high conductivity being desirable.
 A
 l
dPD
 hc  A  th
 Rth  th
dT j
l
A
where
(D1.3)
dPD
is the device thermal conductance.
dT j
EET307
POWER ELECTRONICS
Thermal Management
3
Prof R T Kennedy
2005-2006
D1.4 Thermal-Electrical Analogy
D1.4.i 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 equation D1.4
Rth, j c 
th  l
(D1.4)
A
The dimension of thermal resistance o C / W (some data sheets use K / W) is more obvious from equation
D1.5
Rth, j c 
T j c
(D1.5)
PD
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.
ELECTRICAL


THERMAL
voltage
V
T
temperature
potential difference
T
temperature difference
current
V
I
PD
power dissipated
power
P
Q
heat
conductivity

thermal conductivity
resistivity

 thermal
 thermal
ANALOGY
thermal resistivity
Tjunction
Rth,j-c
An equivalent thermal circuit is shown in Fig.D1-2
Tcase
FIGURE D1-2
Low thermal resistance is desirable hence higher current devices with a larger die area are sometimes
selected to optimize the thermal effects of current rather than the need for current carrying capability.
EET307
POWER ELECTRONICS
Thermal Management
4
Prof R T Kennedy
2005-2006
D1.4.ii 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 structure and an equivalent thermal circuit are shown in Fig. D1.3 and Fig. D1.4
respectively
Tjunction
Rth,j-c
Tcase
Rth,c-a
FIGURE D1-3
Tambient
FIGURE D1-4
The typical high values of junction-ambient thermal resistance as indicate the poor performance of a
device to remove heat from the junction directly to the ambient.
D1.5 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 RthCA the high thermal resistance from case to ambient.
Mounting the semiconductor on a heat sink reduces the effective value of RthCA by providing a low
parallel thermal resistance path, RthSA , as shown in Fig. D1-5.
PD
Tjunction
Rth,j-c
Tsink
Tcase
Rth,s-a
Rth,c-a
PD
FIGURE D1-5
Tambient
the heat flow divides, as would current flow, with the majority taking the lower thermal resistance path
via the heatsink. In practice Rth, s  a is << Rth, c  a and is a common approximation to the parallel path.
EET307
POWER ELECTRONICS
Thermal Management
5
Prof R T Kennedy
2005-2006
D1.5.i Heat Sinks
Heatsinks are available in a large variety of materials and colours as well as shapes and sizes as shown in
Fig.D1-6
FIGURE D1-6
the first task is conducting the heat from the device (base, case, stud ) to the heat sink fins, the major
dissipating area.
Poor thermal conduction results in a hotter case and a cooler heatsink. High thermal conductivity is the
primary consideration in the selection of heat sink material, however weight, cost and machineability are
some of the other factors that may decide the final choice.
Copper, although having better thermal properties, is less popular than the moderate cost aluminium that
has the best thermal performance per lb.wt. A copper heat sink would be 3.3 x as heavy as a thermally
equivalent aluminium heat sink .
D1.5.ii contact thermal resistance
Proper contact between the mating surfaces of the device and heat sink is essential if the thermal
properties are to be optimized. allowance for contact is provided by RthC , the contact thermal resistance,
as shown in Fig. D1-7.
Tjunction
Rth,junction-case
Theat sink
Rth,contact
Tcase
Rth,case-ambient
Rth,heat sink
Tambient
FIGURE D1-7
Mounting contact pressure as provided by the manufacturer is extremely important.
Contact pressure too small increased contact thermal resistance .
Contact pressure too large internal mechanical stress that damages the device materials.
EET307
POWER ELECTRONICS
Thermal Management
6
Prof R T Kennedy
2005-2006
D1.5.iii Contact Interface materials
good as surface flatness may appear to the naked eye there will be voids, as shown in Fig. D1-8
FIGURE D1-8
"Avoid the voids - there's no substitute for good mating"
The air that fills the voids has a high thermal resistivity (1200 0C.in / W ) and when compared to copper
( 0.1 0C.in / W ) and aluminium ( 0.19 0C.in / W ) is a severe impediment to the conduction of heat.
The comparatively low thermal resistivity of silicon grease ( 204 0C.in / W) wrt air provides a suitable
though messy thermal compound interface.
an alternative to thermal compounds is provided by thermal washers, as shown in Fig. D1-9.
A typical washer may comprise a 0.001" aluminium foil with a double sided coating of high thermal
conductivity silicone rubber to provide low thermal resistance (0.1 0C /W ).
TJ
PD
RthJC
R thW
TC
R thC
RthSA
RthCA
TA
FIGURE D1-10
FIGURE D1-9
As shown in fig. D1-10 the thermal washer and thermal compound reduce the effect of voids by
introducing RthW , an element of lower thermal resistance in parallel with the contact thermal resistance.
washer thermal resistance may appear in data sheets as 0C -in2 /W. This includes the active heat transfer
area hence the value should be divided by area to provide a figure in 0C / W .
factors that influence the efficiency of thermal washers are
w
flatness / smoothness of the mating surfaces
w
contact pressure
w
washer thickness
w
washer thermal conductivity
w
conformability : ability to provide a tight fit to both even and uneven surfaces.
Thermal interface washers designed to optimize heat transfer will, in general, not provide good electrical
isolation between the semiconductor case and the heat sink.
EET307
POWER ELECTRONICS
Thermal Management
7
Prof R T Kennedy
2005-2006
D1.5.iv
Electrical isolation
Direct contact between a device and heat sink means that the larger heat sink surface area will be at the
same potential as one of the device terminals. this is potentially dangerous at the voltage levels
encountered in power electronic circuits.
Direct contact also prevents mounting more than one device on the same heat sink, unless the contact
terminals are at the same electrical point in the circuit.
early insulating techniques were a two component format using a low cost mica washer, having good
electrical properties and capable of operation at high temperatures, with thermal compound. Mica is
however rigid (doesn't conform) and is brittle.
A number of vendors provide a range of thermally conductive electrically insulating elastomers that
offer performance choice (trade-off) between the conflicting requirements of electrical isolation and heat
conduction and provide a single component solution without the need for 'grease'.the elastic properties
also provide some shock protection.
Thermal insulating washers comprise
elastomer binding: Silicone rubber, or polyester based if silicone is a problem, to provide flexibility
which under clamping pressure conforms to the mating surfaces thereby providing
a good thermal interface.
filler:
CERAMIC:
Beryllium oxide which has 10x the thermal conductivity of
aluminium ceramic, but it does have safety implications
COMPOSITES: Alumina ( Rth ~ 0.3 0C / W; 2.5 kV BDV ) which is cheaper than
but also less conformable than Boron Nitride ( ~ 0.2 0C / W; 4 kV)
reinforced fibreglass cloth to provide strength against assembly problems.
backing:
A polyimide film in conjunction with boron filled silicone rubber provides excellent thermal
performance
and a tough dielectric barrier, a typical product being shown in fig. D1-11.
FIGURE D1-11
FIGURE D1-12
A replacement 'friendly' alternative product having a high tack pressure sensitive adhesive layer(s) and
which does not rely on contact pressure is shown in Fig. D1-12 .
EET307
POWER ELECTRONICS
Thermal Management
8
Prof R T Kennedy
2005-2006
D1.5.v Electromagnetic Interference ( E.M.I. )
High frequency, fast switching circuits, typical to switched- mode power supplies can result in the
transfer of unacceptable levels of 'noise' conducted from the device case to the heat sink via the thermal
interface capacitance. A mica washer and TO3 case present a capacitance of the order of 10 pf. as shown
in Fig. D1-13. Insulating washers with a low dielectric constant and increased thickness are desirable to
minimize capacitance, but there will be trade-offs.
FIGURE D1-13
FIGURE D1-14
FIGURE D1-15
A solution, shown in Fig.D1-14, uses a bonded laminate of thermally conducting, electrically isolating
thermal pad with a copper shield between the layers that has a pretinned solder point for easy grounding
as shown in fig. D1-15. Connection of the shield's solder point to the emitter, as shown in Fig. D1-15,
prevents the unwanted interference from returning via the heat sink ground to the supply.
D1.6 Modules
the task of the heat sink can be made easier if the device is efficient in heat transfer.Transfer of heat
when isolation is required is a major problem for the electronics packaging industry but technology
changes have resulted in devices being packaged in modules with improved thermal and isolation
characteristics, the latter providing the capability to mount devices on the same heat sink.
the principal changes are
PASSIVATION
ENCAPSULATION
advances in passivation techniques that protect junctions against
contamination with a covering layer of glass,varnish or alternative.
.
advances in the chemistry of encapsulation resins
METALLIZATION
improvements in the processing and metallization of ceramics
FIGURE D1-16
FIGURE D1-17
The limitations in heat dissipation of conventional alumina ceramic substrates, shown in fig. D1-16, can
be overcome by the direct bonded copper (DBC) substrate, shown in fig. D1-17, in which there is no
thermal barrier between the copper and the ceramic.
EET307
POWER ELECTRONICS
Thermal Management
9
Prof R T Kennedy
2005-2006