LM2587
SNVS115D –APRIL 2000–REVISED APRIL 2013
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(8)
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.
When no heat sink is used, the junction temperature rise is:
ΔTJ = PD × θJA.
(9)
Adding the junction temperature rise to the maximum ambient temperature gives the actual operating junction
temperature:
TJ = ΔTJ + TA.
(10)
If the operating junction temperature exceeds the maximum junction temperatue in item 3 above, then a heat
sink is required. When using a heat sink, the junction temperature rise can be determined by the following:
ΔTJ = PD × (θJC + θInterface + θHeat Sink
)
(11)
Again, the operating junction temperature will be:
TJ = ΔTJ + TA
(12)
As before, if the maximum junction temperature is exceeded, a larger heat sink is required (one that has a lower
thermal resistance).
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 calculate the heat sink thermal resistance required to maintain the regulator junction temperature
below the maximum operating temperature.
To further simplify the flyback regulator design procedure, TI is making available computer design software.
Switchers Made Simple software is available on a (3½″) diskette for IBM compatible computers from a TI sales
office in your area or the TI WEBENCH Design Center team.
http://www.ti.com/ww/en/analog/webench/index.shtml?DCMP=hpa_sva_webench&HQS=webench-bb
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Dunmore Road
Tuam Co. Galway, Ireland Phone: +353 93 24 107 Fax: +353 93 24 459
24
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