FE150H Datasheet, PDF (6/8 Pages) Tyco Electronics – Thermal Management for High-Power Board-Mounted Power Modules

Thermal Mangement for
High-Power Board-Mounted Power Modules
Technical Note
August 1997
Detailed Thermal Model (continued)
Detailed Thermal-Resistance Model
Thermal resistances can be represented in an electri-
cal analogy as resistances in parallel (see Figure 13).
This model is valid for forced convection. Natural
convection can be estimated using v = 0.25 m/s
(50 ft./min.) for open environments with no additional
heat sources.
RADIATION EDGE BOTTOM TOP
BMPM
PD
R
E
B
T
15
10
15
8
V
V
V
From Figure 13, the equivalent resistance to heat flow
from other than the top surface of the module is:
θother = θR || θE || θB
θother = 1/(1/θR + 1/θE + 1/θB)
θother = 1/(1/30 + 1/20 + 1/30)
θother = 8.6 °C/W
The thermal resistance for heat flow off of the top
surface of the module should be:
θT = 1/(1/θtotal – 1/θother)
θT = 1/(1/1.9 – 1/8.6)
θT = 2.4 °C/W
And the heat sink requires a thermal resistance of:
θH = θT – θI
θH = 2.4 – 0.15
θH = 2.25 °C/W
Note: v is measured in m/s.
8-699(C)
Figure 13. Detailed Thermal-Resistance Model
The following examples illustrate how this detailed
model can be used to solve specific thermal problems
and to analyze unusual thermal applications.
Example C. Custom Heat-Sink Design
An FE150H is operated at PO =150 W with natural
convection and TA = 50 °C. θR ≈ 30 °C/W represents
the radiation thermal resistance from the sides and
back, and θI = 0.15 °C/W is a conservative value for the
contact resistance between the heat sink and the
mounting surface. Determine the thermal resistance of
the smallest heat sink required for this application.
Using the data sheet for the FE150H, PD ≈ 24 W is
calculated. In order to maintain TC, max = 95 °C, the
overall module resistance must be:
θ=
(TC, max – TA)
------------------_P----_D------------------
θ=
(95 – 50)
---------₂----₄----------
Example D. Contact Resistance
Using the module and heat sink selected in Example C,
determine the temperature drop from the surface of the
module to the heat sink.
First determine the heat flow through the top surface of
the module. This is given by:
PD, top = (TC, max – TA)/θT
PD, top = (95 – 50)/2.4
PD, top = 18.8 W
The temperature drop is heat flow multiplied by the
resistance:
∆T = PD, top(θI)
∆T = 18.8(0.15)
∆T = 2.8 °C
Therefore, to keep the module from overheating, do not
allow the temperature on the top surface of the heat
sink to exceed:
TH = TC, max – ∆T
TH = 95 – 2.8
TH = 92.2 °C
The contact resistance between the top surface of the
module and the heat sink should not be allowed to
exceed 0.2 °C/W. Typically, with an appropriate dry
pad, θI < 0.15 °C/W.
θ = 1.9 °C/W
6
Tyco Electronics Corp.