To determine how low the thermal impedance path needs to be for safe and reliable operation, start with the largest permissible temperature rise. If the maximum ambient operating temperature of the application is 50°C and the silicon junction must remain at 125°C or less, the largest permissible temperature rise is 75°C (calculated as 125°C minus 50°C). Next, calculate the largest permissible thermal impedance between the junction and ambient air. If the largest permissible temperature rise is 75°C and the watts dissipated in the TO-220 package is 2.78W, the largest permissible thermal impedance would be 27°C/W (calculated as 75°C divided by 2.78W). Finally, add up all the thermal impedance paths from the silicon junction to the ambient air and ensure they sum up to less than the largest permissible thermal impedance; 27°C/W in this example.