Schottky Diode with high current

Hi,

I have an offline, 22W, flyback SMPS with Vout = 36V.

There is a very high value energy storage capacitor on the output.

When this SMPS is switched ON, there is a flow of current through the output Schottky Diode of 4Amps RMS for 1 second, into this capacitor

This diode is only rated for one Amp…….

Schottky = HSM1100J by microsemi.com:-

Schottky datasheet:-
http://www.datasheetcatalog.org/datasheet/microsemi/msc1527.pdf

Do you think this diode will survive this high current episode unscathed?

hi again,

here is a picture of the unloaded circuit, just starting up and charging up the 30mF output capacitor, up to 36V....
-the blue is the flyback secondary diode current...........

A.**Blue = flyback secondary diode current; Green = V(out)
https://i56.tinypic.com/1039wnc.jpg

here is a close up of this current at the beginning of the start-up period, when the output voltage is still low, just 1V...

B.**Blue = Flyback secondary diode current; Green = V(out)
https://i55.tinypic.com/1z212zs.jpg

..as you can see this is about 6 Amps RMS flowing through the secondary diode, which is not rated to carry this current level continuously.

...as the output voltage rises, so the secondary diode current decreases, but when the output is 5V.........

C.**Blue = Flyback secondary diode current; Green = V(out)
https://i52.tinypic.com/xqcf0n.jpg

...as you can see from picture C, 34ms after switch-on there is still at least 4A rms current flowing through the secondary diode.

...now lets look at when the output has risen to 34V.......

D.**Blue = Flyback secondary diode current; Green = V(out)
https://i56.tinypic.com/4l64ud.jpg

...so the secondary diode current is lower now, but do you think that overall, this level of start-up current through the secondary 1A rated diode will kill this diode?

1. You can use the simulator to calculate the average power and total absorbed energy during the oparation cycle. It would be much more precise than guessing based on a number of screenshots.

2. Some manufacturers have dynamic thermal impedance diagrams, that allow to estimate the maximum junction temperature from the total absorbed energy determined in step 1. I see it e.g. for IRF 10BQ040, an 1A SMB schottky diode.

3. To further refine the analysis, you can derive a thermal resistance equivalent circuit from the manufacturer diagrams and and "couple" it to your electrical transient analysis. It can give you an estimated junction temperature waveform.