Electronic – Is paralleling diodes a bad idea


I'm trying to repair an 800W power supply (see my previous question on this.) One thing that gets me is that the design has two schottky diode packages (in TO-220) in parallel. I was always told this was A Bad Idea, but since they are thermally coupled to the same heatsink, does it present a problem in this instance? I've also noticed the same for the input bridge rectifier, two are used in parallel.

Best Answer

The issue with putting diodes in parallel is that as they heat up, their resistance decreases. As a result, that diode ends up taking on more current then the other diode, resulting in it heating up even more. As you can probably see, this cycle will cause thermal run away causing the diode to eventually burn if you give it enough current.

Now the fact that you couple them to the same heatsink will reduce this effect some, but I still would not recommend it. There are far too many unknowns that will affect this to not ever trust it, especially in a commercial product.

Now for the case of this power supply you are looking at, it may very well be that they spent the time to get the diodes matched as closely as possible and allow the heatsink to keep them at about the same temperature.

It may also be that they are running the diodes far under their capacity and they put the second one in parallel so that they aren't always running them near max capacity, but I find this unlikely.