Datasheet

LM2585
SNVS120F –APRIL 2000–REVISED APRIL 2013 www.ti.com
• N is the transformer turns ratio
• D is the duty cycle
• I
LOAD
is the maximum load current (and ∑I
LOAD
is the sum of the maximum load currents for multiple-output
flyback regulators) (7)
The duty cycle is given by:
where
• V
F
is the forward biased voltage of the diode and is typically 0.5V for Schottky diodes and 0.8V for fast
recovery diodes
• V
SAT
is the switch saturation voltage and can be found in the Characteristic Curves (8)
When no heat sink is used, the junction temperature rise is:
ΔT
J
= P
D
× θ
JA
. (9)
Adding the junction temperature rise to the maximum ambient temperature gives the actual operating junction
temperature:
T
J
= ΔT
J
+ T
A
. (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:
ΔT
J
= P
D
× (θ
JC
+ θ
Interface
+ θ
Heat Sink
) (11)
Again, the operating junction temperature will be:
T
J
= ΔT
J
+ T
A
(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, Texas Instruments is making available computer
design software to be used with the Simple Switcher line of switching regulators. Switchers Made Simple
available on a 3½″ diskette for IBM compatible computers from a Texas Instruments sales office in your area or
the Texas Instruments Customer Response Center ([phone]).
24 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated
Product Folder Links: LM2585