Dual power supply from two buck converters (LM2596)

[trevortjes]

Hey all,

So the main goal is creating a +-12V power supply (around 1A postive and negative) from a single higher than +12V DC supply. Now I looked around the internet and found out about using cheap LM2596 buck converter modules in a special arrangements (from here).



Only to find out that for some odd reason this just doesn't work for certain types of LM2596 like the JM97HS (goes into short circuit condition). But otherwise others had confirmed that this does work but with other types of LM2596. Now I'm not sure if my question is, what's the difference between those types of IC while all the ICs should be identical in functionality (I might be terribly wrong on this one), or what I can do to actually make it work.

The latter would be more interesting for me tho. So is there any way to make one behave like inverting buck converter? Or are there other ways to deal with this? Or should I just invest in a quite "expensive" center tapped transformer...

[Ian.M]

*Most* of those modules have fake LM2596 chips - many of the fakes are easy to spot in a simple test jig - just check the switching frequency (at pin 2 of the IC) under load.  Genuine LM2596 chips run at 150KHz.  Most of the fakes run at approx 52KHz.   *IF* you can get a fake to work in the inverting configuration, you'll be lucky to get over 300mA out of the negative rail, and they are only good for about 500mA continuous in the normal positive configuration.

A (genuine) LM2596 in the inverting configuration needs up to 5A for up to 2ms to startup correctly.  If its supply is current limited at under 5A, or worse has foldback limiting, it is unlikely to start unless you mod the module by lifting pin 5 and adding the RC network for delayed startup from datasheet fig. 23, (also see fig. 27).

You must also consider if the positive rail coming up first will be problematic for your load.  You may need to mod the positive module to also delay its startup, or worst case, for particularly fussy loads, add a supply supervisor IC and MOSFET switching in both outputs to only connect the load when both rails are up and stable.