any special considerations for schottky diode selection for buck circuits?

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wight
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any special considerations for schottky diode selection for buck circuits?

Is it just these two considerations? Or are there other things?

  1. Current handling
  2. Reverse voltage

For example, I’m looking at SBR12U100P5 for use in this 17mm QX5241 buck driver. Current and power dissipation look competitive / good to me. DC reverse voltage is high. Beyond that I don’t know what to look at, if anything.

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HKJ
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If a Schottky diode has a high reverse voltage it does also have a high forward voltage, i.e. higher loss.

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wight
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Thanks HKJ. So what I’m looking for is a Schottky with the appropriate reverse voltage (high enough to work when derated for temperature). Any higher and I’m probably sacrificing forward voltage and increasing power dissipation?

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HKJ
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wight wrote:
Thanks HKJ. So what I'm looking for is a Schottky with the appropriate reverse voltage (high enough to work when derated for temperature). Any higher and I'm probably sacrificing forward voltage and increasing power dissipation?

Exactly and also check the capacity of the diode, it will usual increase with current ability and a high capacity does also increase looses.

But, it is better to take a bit higher loss, than a blown diode, i.e. do not use too tight tolerances when selecting a diode.

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wight
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Ah, thanks again. I wondered what affect the capacitance would have. I should have asked specifically since I was wondering, but I wanted to keep the OP simple! Lucky for me you answered the question without me needing to ask :-).

From what I can see, the diode I linked in the OP is very competitive:

  • Vf only gets up to 0.71v at 12A. At lower currents it seems comparable or better than other with other diodes recommended for buck circuits.
  • At relevent voltages (>3v) the capacitance seems OK, maybe 100pF worse (higher) than this random SS54 datasheet I pulled up.

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HKJ
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Try do a search on Digikey or some other place with parametric search:

http://www.digikey.com/product-search/en/discrete-semiconductor-products...

Select voltage (like 20-30V) and current, then sort the result on Vf

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wight
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Thanks. I see that the SBRT15U50SP5 looks much better than SBR12U100P5. Better than most other diodes, it seems.

That seems to have one of the best Vf of any >5A Schottky under $2 USD. Capacitance is presented in a different way in that datasheet, but seems overall to be higher than SBR12U100P5.

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HKJ
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wight wrote:
Thanks. I see that the "SBRT15U50SP5":http://diodes.com/datasheets/SBRT15U50SP5.pdf looks much better than SBR12U100P5. Better than most other diodes, it seems. That seems to have one of the best Vf of any >5A Schottky under $2 USD. Capacitance is presented in a different way in that datasheet, but seems overall to be higher than SBR12U100P5.

The effect of capacity requires a precise circuit simulate to evaluate. A general rule is that capacity is more significant at higher frequencies, i.e. if you are switching at 10-20kHz it is not nearly as significant as a 200-500kHz switcher.

A interesting curve to compare is the "POWER DISSIPATION"/"AVERAGE FORWARD CURRENT" curve. There is a major difference between the two diodes.

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wight
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I see. In order to maintain the small size of this driver I will have to use a low-value inductor. I suspect that the freq will be >500kHz.

You are correct about the power dissipation. We save about 1.5W at 8A by switching to the SBRT15U50SP5!

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup Wink
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