LM2585

SNVS120F – APRIL 2000 – REVISED APRIL 2013

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•

N is the transformer turns ratio

•

D is the duty cycle

•

flyback regulators)

(7)

The duty cycle is given by:

where

•

recovery diodes

•

(8)

When no heat sink is used, the junction temperature rise is:

(9)

Adding the junction temperature rise to the maximum ambient temperature gives the actual operating junction

temperature:

(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:

(11)

Again, the operating junction temperature will be:

(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 ((800) 477-8924).

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