Application Note AN-1059
DirectFET Technology
Thermal Model and Rating Calculator
Table of Contents
Page
Introduction............................................................ 2
Equivalent circuits ................................................. 3
Thermal resistance values ............................... 3
Analysis ............................................................ 3
Rating Calculator ................................................... 5
Notes on use .................................................... 5
Rating Calculator inputs ................................... 5
Rating Calculator outputs................................. 5
Types of cooling ............................................... 6
Validation of results ............................................... 6
Model................................................................ 6
Values .............................................................. 7
Thermal resistance values .................................... 8
With equipment chassis or case cooling .......... 8
With no additional DirectFET can cooling ........ 8
Example of use of Rating Calculator................ 9
Summary ............................................................... 9
Appendix A .......................................................... 10
Equation 1 ...................................................... 10
Equation 2a .................................................... 10
Equation 2b .................................................... 10
Appendix B .......................................................... 10
Appendix C .......................................................... 11
Appendix D .......................................................... 11
DirectFET’s thermal properties are fundamentally different from industry-standard, encapsulated power
semiconductors. Its construction encourages heat to disperse from the die in opposite directions,
cooling through both the substrate pad connections (source and gate) and the metal can on top of the
device. The can-to-ambient thermal interface can be maximized by fitting a heat sink.
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AN-1059
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Thermal Model and Rating Calculator
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Introduction
Encapsulated power semiconductors in packages
such as the TO-220 or D-Pak are fairly easy to model
thermally, using only one thermal parameter. The
assumption is that most of the power generated in the
silicon chip travels in one direction. The assumption is
reasonable because the silicon chip is soldered (or
epoxy-attached) to a lead frame that provides the
main cooling path to the environment. Heat flow in the
opposite direction is limited because the die is
insulated with a layer of encapsulation ‘plastic’. The
dissipation of heat from the lead-frame into the
environment is often enhanced by fitting a heat sink.
DirectFET
®
is fundamentally different. Its construction
encourages heat to disperse from the die in opposite
directions, cooling through both the substrate pad
connections (source and gate) and the metal can on
top of the device. The can-to-ambient thermal
interface can be maximized by fitting a heat sink. The
design of the can also provides a parallel or shunt
thermal path from the can to substrate.
Figure 1 shows the direction of heat flow from a
DirectFET device and an approximate thermal
equivalent circuit for such a device in use.
Measuring the thermal resistance of a DirectFET
device inevitably produces a composite result based
on the temperatures measured at the junction, can or
substrate using the total power dissipated by the
silicon. While this gives the effective thermal
resistance under particular cooling conditions, it does
not give a value that applies under other conditions.
When determining a value for dual-sided cooling
conditions, the most significant factors are the thermal
resistance of the heat sink and substrate. To assess
these correctly, the power flow through each path
must be known. This requires a method of predicting
the proportion of power flow through the paths. The
temperature of the can and the substrate will change
with different levels of can and substrate cooling.
Indeed, it is this feature of DirectFET construction –
which enables cooling from both sides of the silicon
die – that gives the devices their particular benefits.
This application note provides an easy method for
assessing the proportion of power flow from each of a
DirectFET device’s surfaces, so that the appropriate
thermal resistance figures are used and the true rating
is accurately determined.
Tambient
Rth chassis / heat sink
T chassis / heat sink
Rth gap filler
Tcan
Rth junction-can
Rth can-substrate
(can)
Tjunction
Rth junction-substrate
(source)
Tsubstrate
Rth substrate
Rth heat sink
(substrate attachment,
if present)
Tambient
can heat sink
junction
heat sink
interface material
can (drain)
substrate
gate pad
source pads
Figure 1a Directions of heat flow (indicated by the red arrows)
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Figure 1b Approximate thermal equivalent circuit
Thermal Model and Rating Calculator
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AN-1059
Version 3, September 2010
Equivalent circuits
Thermal resistance values
With the thermal modeling software available today, it
is possible to predict temperatures at any chosen point
in an entire cooling system. However, many circuit
designers do not have access to such software nor
time to spend on numerous lengthy computations for
each possible set of cooling arrangements.
With this in mind, International Rectifier provides
equivalent thermal resistance values for DirectFET
devices. These can be entered into an online Rating
Calculator that rapidly returns the maximum permitted
power generation by the device in any combination of
substrate and can cooling conditions.
Figure 2 shows how the three parameters (R1, R2 and
R3) relate to the physical construction of a DirectFET
device. Their values are derived using a combination
of the dimensions and conductivity of the materials
used in constructing the device.
The table at the right shows these equivalent thermal
resistance values for the current range of DirectFET
devices (both lead and lead-free variants).
Analysis
The analysis is carried out on the equivalent thermal
circuit of the DirectFET device (the green circuit in
Figure 3). It also includes the additional cooling
resistance paths to the can and to the substrate (the
black circuits above and below it in Figure 3).
Thermal resistance R
C
represents the total thermal
resistance of the path from the surface of the can to
the ambient cooling medium. This must include any
interface resistances resulting from isolation or
mounting materials fitted in this path.
Thermal resistance R
S
represents the total thermal
resistance of the path from the bottom of the
DirectFET device through the substrate. This may
include a substrate heat sink.
Using the information shown in Figure 3, International
Rectifier has generated an equation that satisfies all
the various power flow constraints. This is shown in
Appendix A as Equation 1. In summary:
T
J
= fn (W, T
A
,
α, β, δ, γ, ς, Φ)
Outline
Can size
R1
R2
R3
Non-PbF devices
SH
SQ
ST
MN
MQ
MT
MX
SH
SJ
SQ
ST
MN
MP
MQ
MT
MU
MX
MZ
S1
S2
SB
M2
M4
L4
L6
L8
small
small
small
medium
medium
medium
medium
small
small
small
small
medium
medium
medium
medium
medium
medium
medium
small
small
small
medium
medium
large
large
large
1.39
1.14
1.08
0.43
0.99
0.33
0.50
2.96
2.05
2.43
2.36
0.91
2.26
2.07
0.71
1.91
1.04
1.62
4.18
2.35
2.68
2.09
1.27
1.06
0.80
0.65
3.47
3.47
2.58
0.97
2.60
0.97
1.50
3.48
2.22
3.48
2.58
0.97
2.58
2.58
0.97
2.58
1.18
0.97
3.43
4.55
2.47
1.03
0.68
0.56
0.44
0.25
0.98
0.98
0.98
0.80
1.10
0.80
0.80
0.98
0.98
0.98
0.98
0.80
1.54
1.54
0.80
1.54
0.98
0.80
1.53
1.60
1.05
1.33
0.80
1.06
0.56
0.49
PbF devices
Two options for ratings then become available:
For applications where conduction and switching
losses are both significant (such as DC/DC buck
circuits and other high-frequency circuits), the
expression may be arranged to give the maximum
permitted power rating. This is shown in Appendix
A as Equation 2a.
For low-frequency applications (say lower than
30–50 kHz), only conduction losses need be
considered and the expression may be arranged
to give the maximum permitted current rating.
This is shown in Appendix A as Equation 2b.
Note: Some International Rectifier data sheets provide more
information about switching power losses. For example,
PD-94574A for the IRF6607 DirectFET:
www.irf.com/product-info/datasheets/data/irf6607.pdf
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mean heat flux
path through can
can
R3
Tcan
R3
Rth can-substrate
R2
R1
R2
R3
R1
Tsubstrate
Rth junction-can
Rth junction-source
Tjunction
thermal
conductive compound
solder
silicon
gate
footprint
can footprint
x4 (drain)
source footprint
source footprint
drain
footprint
Figure 2 Physical realization of parameters R1, R2 and R3
Tambient
T
A
W is the total
junction power
Rth can-ambient
W5
R
C
Tcan
W1 through W5
are the individual
branch powers
T
C
W2
W3
Rth can-substrate
(can)
Rth junction-can
R2
W
Tjunction
R3
W1
T
J
Rth junction-
substrate (source)
R1
Tsubstrate
W=W1+W2
W4=W1-W3
W5=W3+W2
W3=(T
s
-T
c
)/R3
W4=(T
s
-T
A
)/R
S
W=(T
C
-T
A
)/R
C
basic branch
expressions
T
s
Rth substrate-ambient
W4
Tambient
R
S
T
A
Figure 3 Thermal equivalent circuit with basic assumptions
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AN-1059
Version 3, September 2010
Thermal Model and Rating Calculator
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Rating Calculator
The DirectFET Rating Calculator is available at:
www.irf.com/product-info/hexfet/thermalcalc.html
The Rating Calculator applies the equations from
Appendix A. It uses a simple tabular entry format to
facilitate fast and accurate calculations.
Notes on use
For high-frequency applications, use the value
returned for
maximum permitted power.
For low-frequency applications (where switching
losses are not significant), enter a worst-case
value for R
DS(on)
and use the value returned for
maximum permitted current.
Obtain values for the thermal resistance of the
substrate from the manufacturer’s material data
but make sure it is correct for the device footprint,
board thickness and power rating.
Obtain values for the thermal resistance of the
heat sink from the manufacturer’s curves – include
the mounting interface as appropriate.
Appendix B gives guidance for applications where
the can is cooled through the chassis or case.
Appendix C gives guidance for small and medium
can devices that have no additional heat sinking.
Where fan or forced cooling is used, thermal
resistance tends not to vary with power. This
makes it easier to use the Rating Calculator.
Where thermal resistance varies considerably with
power, estimate the relative power flow through
the can and substrate. Run the Rating Calculator
and use the result to review the resulting power
flow, checking if thermal resistance values are still
correct for those power levels. If not, adjust the
inputs, repeat the calculation and check the power
flow again. Repeat until the power flows through
the substrate and heat sink align with their thermal
resistances for those flows. Appendix D
demonstrates this procedure.
Rating Calculator inputs
Enter:
R1, R2 and R3 for the appropriate DirectFET
device from the table on page 3
Rth substrate-ambient for the appropriate cooling
conditions and power (estimate the power flows
through this path for the first iteration)
Rth heat sink if one is used, taking into account
the cooling conditions and power flow (estimate
the power flows through this path for the first
iteration) – if no heat sink is fitted to the can, enter
the figures for Rth can-ambient from Appendix C.
Ambient temperature of the environment
Maximum permitted junction temperature
The hot R
DS(on)
of the device – enter this only if a
low-frequency current rating is required.
Rating Calculator outputs
The Rating Calculator returns maximum current or
maximum power. If Rth substrate-ambient or Rth can-
ambient varies significantly with power, adjust the
values to reflect the actual power returned by the
Rating Calculator. Refer to the last of the notes on
using the Rating Calculator (left) and to Appendix D.
DirectFET Technology
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AN-1059
Version 3, September 2010
Thermal Model and Rating Calculator
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