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Chapter 3
3.18.7 Temperature Rise
The vendor has stated that the inductor is such that 380 mW dissipation corresponds to
50°C rise in temperature. In effect this tells us that the thermal resistance of the core I is
Rth
T
W

50
038
131 6
.
. °C/W (any topology)
The inductor was originally designed for a total loss of
PP P
CORE CU
mW (any topology)385 18 8 403 8..
This would have given a temperature rise of
TRthP 131 6 0 404 53.. ( )°C any topology
In our application
PP P
CORE CU
mW389 2 391
This will give a temperature rise of
TRthP 131 6 0 391 51.. °C
Provided we accept this temperature rise in our application (that will depend on our
maximum operating ambient temperature), we can validate the chosen inductor. We have
already confi rmed it does not saturate in our application, and further, the current ripple
ratio it provides is acceptable too.
This completes the general inductor design procedure.
3.19 Calculating the Other Worst-case Stresses
Having validated our choice of inductor, we can look a little more closely at the important
issue of how the wide-input range impacts the other key parameters and stresses in our
proposed converter. This also helps in correctly selecting the other power components.
3.19.1 Worst-case Core Loss
In the above so-called general inductor design procedure, we have actually been working
at V
INMAX
for a buck, and at V
INMIN
for a boost or buck-boost. The reason was that the
inductor sees the highest peak current, at this voltage end, so we have to insure the
magnetics design at this particular point. But this point may not be the worst-case point for
the other stresses in the power supply, and we need to start understanding that clearly now.

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