Included in the Switchers Made Simple design software is
a more precise (non-linear) thermal model that can be used
to determine junction temperature with different input-output
parameters or different component values. It can also calcu-
late the heat sink thermal resistance required to maintain the
regulator junction temperature below the maximum operat-
ing temperature.
Application Hints (Continued)
Boost:
To further simplify the flyback regulator design procedure,
National Semiconductor is making available computer de-
sign software. Switchers Made Simple software is available
on a (31⁄
") diskette for IBM compatable computers from a
2
where VF is the forward biased voltage of the diode and is
typically 0.5V for Schottky diodes and 0.8V for fast recovery
diodes. VSAT is the switch saturation voltage and can be
found in the Characteristic Curves.
National Semiconductor sales office in your area or the Na-
tional Semiconductor Customer Response Center
(1-800-272-9959).
When no heat sink is used, the junction temperature rise is:
European Magnetic Vendor
Contacts
Please contact the following addresses for details of local
distributors or representatives:
=
∆TJ PD x θJA
.
Adding the junction temperature rise to the maximum ambi-
ent temperature gives the actual operating junction tempera-
ture:
=
TJ ∆TJ + TA.
If the operating junction temperature exceeds the maximum
junction temperatue in item 3 above, then a heat sink is re-
quired. When using a heat sink, the junction temperature rise
can be determined by the following:
Coilcraft
21 Napier Place
Wardpark North
Cumbernauld, Scotland G68 0LL
Phone: +44 1236 730 595
Fax: +44 1236 730 627
=
∆TJ PD x (θJC + θInterface + θHeat Sink
)
Again, the operating junction temperature will be:
=
TJ ∆TJ + TA
As before, if the maximum junction temperature is exceeded,
a larger heat sink is required (one that has a lower thermal
resistance).
Pulse Engineering
Dunmore Road
Tuam
Co. Galway, Ireland
Phone: +353 93 24 107
Fax: +353 93 24 459
www.national.com
24
NSC [ National Semiconductor ]
