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Disadvantages of paralleling diodes?
Disadvantages of paralleling diodes?
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Old 12th October 2017, 04:10 PM   #11
Chris888 is offline Chris888  United Kingdom
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http://www.st.com/resource/en/applic...dm00098381.pdf

This is for smaller devices but shows the principle?
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Old 12th October 2017, 06:21 PM   #12
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Mark Johnson View Post
I think you might have omitted a factor of approximately-two?
  • in the first design (single diode), each diode carries I amperes
  • in the second design (diodes in parallel), each diode carries I/2 amperes

Since the current goes down by 2X but the forward voltage goes up by less than 2X, . . . . what can we conclude?
I was figuring, the total current remains the same, but because the diode gets less hot, the Vf goes up. But I see what you mean, the lower current in itself lowers the Vf.
You think the net effect is lower Vf?
Most probably depends on the specific situation. With a lot of current, you are high on the (steep) Vf curve and the net effect might be higher Vf due to lower temp, no?

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Old 13th October 2017, 09:59 AM   #13
Kay Pirinha is offline Kay Pirinha  Germany
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What about parallelling both diodes in those double common cathode Schottky diodes that are very common in these days? The manufacturers' data sheets seem to imply this.


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Old 13th October 2017, 11:50 AM   #14
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Kay Pirinha View Post
What about parallelling both diodes in those double common cathode Schottky diodes that are very common in these days? The manufacturers' data sheets seem to imply this.


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These are probably quite well matched in Vf due to being manufactured together (same die?).

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Old 16th October 2017, 08:29 AM   #15
SemperFi is offline SemperFi  Wake Island
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I know some designs have paralleled more diodes than necessary just to ensure current is diverted between enough diodes so no diode gets too much. But beware the added diode parasitic capacitances!

Quote:
Originally Posted by Chris888 View Post
http://www.st.com/resource/en/applic...dm00098381.pdf

This is for smaller devices but shows the principle?
Thnx for link. Very interesting. I was tought paralleling diodes is poor design, but evidently not always.

So spread in Vf is more important than thermal differences (within reason). And the lower reverse voltage rating the lower the spread in Vf, to the point that schottkys with Vreverse < 60V can safely be paralleled.

Obviously layout is critical since spread in path inductance will also unbalance current sharing.
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Old 16th October 2017, 09:32 AM   #16
trobbins is offline trobbins  Australia
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Nice info from a manufacturer and their statistical test results.

Carel, it seems like a quick static test is worthwhile to minimise the difference between paralleling pairs.

Also indicates that unless matching is carried out on any ballast resistors, then they may not assist.
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