1
\$\begingroup\$

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 Power equation.. 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 ?

| improve this question | | | | |
\$\endgroup\$
2
\$\begingroup\$

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.

| improve this answer | | | | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.