10 Heat Sink/Thermal Considerations

In many cases, a heat sink is not required to keep the LM2586 junction temperature within the allowed operating temperature range. For each application, to determine whether or not a heat sink will be required, the following must be identified:

1) Maximum ambient temperature (in the application).

2) Maximum regulator power dissipation (in the application).

3) Maximum allowed junction temperature (125°C for the LM2586). For a safe, conservative design, a temperature approximately 15°C cooler than the maximum junction temperature should be selected (110°C).

4) LM2586 package thermal resistances θ_JA and θ_JC (given in the Electrical Characteristics).

Total power dissipated (P_D) by the LM2586 can be estimated as follows:

Equation 7.

where

• V_IN is the minimum input voltage
• V_OUT is the output voltage
• 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)

The duty cycle is given by:

Equation 8.

where

• V_F is the forward biased voltage of the diode and is typically 0.5 V for Schottky diodes and 0.8 V for fast recovery diodes
• V_SAT is the switch saturation voltage and can be found in the Characteristic Curves

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

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

If the operating junction temperature exceeds the maximum junction temperature 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:

Again, the operating junction temperature will be:

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 is 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).