Environmental Engineering Reference
In-Depth Information
integrated cold plate in the power inverter is used along with external coolant pumps,
condenser and relevant plumbing. This adds significant complexity to the vehicle,
motivating to use the same coolant as used for the hybrid M/G, and potentially, to
use engine coolant for both the M/G and the power electronics. Systems design,
available power semiconductor devices, microcontrollers, bus capacitors and asso-
ciated components are not rated for temperatures exceeding 65-85
C.
A typical transistor stack and thermal model is illustrated in Figure 6.19 for a
power MOSFET chip. The transistor stack consists of a double bonded copper
substrate and solder layer to the chip. Thermal grease is used to mount the module
to the inverter heat sink.
For power elect. module (per cm
2
)
Rth_jc
Rth_cs
Rth_s-h/s
Rth_js
Diagram shows junction-to-case resistance Re
jc
for various surface-mount power MOSFET packages
P
0.25 °C/W, per cm
2
Si
0.34 °C/W, 25 µm grease
0.005 °C/W, Al cold plate
0.595 °C/W-cm
2
T
j
Junction
Re
jc
T
c
Silicon chip
Case
Heat sink
Die attach
Re
jc
T_inlet
65 °C, max inlet temp.
T
s
Thermal grease
Tjmax
125
°
C, max j unction
Re
jc
d T
60
°
C,
d T
j-coolant
T
A
MOSFET steady-state thermal resistance model
(a)
(b)
Figure 6.19 Transistor stack and thermal model: (a) schematic of transistor stack
and (b) thermal model parameters
The simplest model accounts for thermal resistances, which is sufficient for
steady state dissipation limits. The table of parameter data in Figure 6.19 is typical
of a hybrid propulsion system. The coolant system is constrained to inlet tem-
peratures and junction maximum temperatures as shown. Other than exceeding
these thermal limits, the most important consideration is the temperature excursion
that the transistor die makes during power cycling shown as
d
T
in the list in
Figure 6.19. Temperature excursion of the semiconductor die relative to the coolant
plate should be restricted to
<
40
C for automotive grade durability. This durability
constraint immediately sets the lower bound on required semiconductor device
active area, hence cost. For lower cost systems it is possible to permit temperature
excursions up to 60
C only if the system operating modes have been well defined.
Recall that inverter cold plate coolant inlet temperatures are specified at or below
65
C, so for a 60
C temperature rise the transistor junction temperatures will be at
125
C, a high value for power processing.
If an assessment of transient thermal performance is necessary then a more
complete model that accounts for thermal capacities of each layer must be used; for
example, if a detailed assessment of power semiconductor junction temperature is
needed when the hybrid propulsion system is operating over a standard drive cycle.
Figure 6.20 shows typical thermal models that account for thermal resistances,
capacitances and interface thermal impedances.
