9.2 Heat Sink/Thermal Considerations

In many cases, no heat sink is required to keep the LM2585 junction temperature within the allowed operating 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 LM2585). For a safe, conservative design, a temperature approximately 15°C cooler than the maximum junction temperature should be selected (110°C).

4) LM2585 package thermal resistances θ_JA and θ_JC (given in Thermal Information).

Total power dissipated (P_D) by the LM2585 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 0.5V 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

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 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:

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.