Schottky Diodes in parallel

Question

I was looking at a reference design and I saw a symbol I didn't recognize:

So I looked up the part. It's a dual Schottky rectifier like this:

What is the purpose of having two diodes in parallel in this case?

Thanks

Answer 1

The current is split between two junctions, thus slightly reducing thermal dissipation on each one and improving reliability/MTBF. Since the diodes are in the same package, the risk of unbalanced current is likely to be very low, so considerations about paralleling discrete diodes don't apply.

Furthermore, SOT23 double diodes seem to be very common and somewhat standard in larger firms (thus less expensive). Given that connecting together anodes of a double diode doesn't involve a significant increase in the probability of failure, it makes sense to do so instead of not.

However, it is important to note that such approach leads to double leakage current. This has to be considered for applications where leakage current can raise any problem.

Take a look at this German forum topic, about BAT54C (very similar to BAR43C, indeed): Warum zwei parallele Dioden? (Google translation)

Answer 2

Diodes shall never be put in parallel, unless they are in the same package. (More or less true) :

From a thermal perspective: Having both of them in the same package guarantee the differents junctions are at the same temperature. The risk of unbalanced current, is still present, but every rise in temperature of one diode will impact other one. Which make a thermal runaway of one diode less likely.

From a current perspective: Whit that said, you shall never put diodes in parallel to increase the max current. Max current shall still be less that the individual maximum current of one diode. Because even in the same package diode can have different forward voltage, meaning an unequal current spread.

If the diodes are not in the same package : The false theory, is that the current will be spread accros the two diodes.

In fact what will happen is that the diode whith the lesser forward voltage with let pass more current. Provoking a rise in temperature, this rise in temperature cause the forward voltage to decrease even more due to the negative thermal coefficient of diode junctions. This is thermal runaway, where the diode will be destroyed due do this positive feedback loop.

This is explained partly in the following document : AN4381 Application note