schottky diode power rating

Question

I am trying to calculate the power rating of a schottky diode. The diode in question is the 1N5822U from ST Micro (a rad hard schottky diode). The datasheet is - Diode datasheet

Now, I am using in a circut that has about 2Amps. Now, the power dissipated throgh this diode is given by the equation below . It is given in the page 2 of the datasheet.

Now, i am assuming If(av) = 2Amps. What would If(rms) be ? This is a dc system so the RMS can be assumed to be just a dc level current ?

Also, now from this value I get the power dissipated by my circuit and not the max rated power of this diode. For, this I am making use of the Thermal resistance value = 7degree Celcius/Watt. The max operating or junction temp possible is 150degrees celcius. So, taking this into pic the difference from normal operating room temp and the max (150-25 = 125). So, the max is 125degrees celcius rise. This means that 125/7 = 17W. Approx 17W is the max power rating of trhis schottky. Is my assuption correct ?

Answer 1

If the current is continuous at 2A with no variation then 2A is the RMS current. If it's unidirectional (say 20A pulses at 10% duty cycle for average 2A) then the RMS current would be more like 6.3A.

As to your second question, the datasheet states the thermal resistance as 7°C/W junction to case. If you have to design for a maximum ambient temperature of (say) 70°C, and the thermal resistance case-to-ambient is 20°C/W (just picking a number out of the air) and you are really willing to allow the junction to go to 150°C under those conditions, then you could dissipate almost 3W. It would be more conservative to limit the junction temperature to a much lower level such as 100°C, which would allow only 1.1W of dissipation.

Fortunately, your dissipation at 2A is only about 0.84W, so it's not a problem, if the assumptions above on case-to-ambient and maximum ambient temperature requirement are correct. The second number should be provide to you, the first is going to depend on the mechanical design and may require building a prototype or FEA thermal simulation.

17W is kind of a theoretical maximum if you had some chilled water cooled heatsink directly bonded to the case and holding it at 25°C. Not really related to reality.