Generating -12V - Page 1
Forum section: electronics / projects

author: Dajgoro
author_rank: Frequent Contributor
timestamp: July 17, 2019, 09:04:57 am
content: Hi,I've been doing some analog projects, which require a symmetrical power supply, but since I want the devices to run from a single 12V adapter I need to generate the -12V on the board itself. I used to do that with the (in)famous MC34063 which I really want to avoid because it has funky behavior. To start with the recommended component values to generate -12V from its datasheet don't work at all, and even when they do sometimes it has some weird issues. Another reason I want to replace it is to get a more modern part with a higher frequency so its more efficient and I don't have to waste much board space with it and that can source about an amp.My first attempt was super simple, it was the AP3211 which works fine, except that is too much voltage for it, so it tends to overheat.Then I tried with the AOZ1284PI and that gives me a dead short. I might had screwed up the compensation network, or something else.Are there any other low cost / easily available parts that might work for this that I might try?P.S. I know about those ready made modules which generate +/-15V, but I don't want them, they are too big, too expensive and I'd need a lot of them.

author: Buriedcode
author_rank: Super Contributor
timestamp: July 17, 2019, 04:11:44 pm
content: What are the current requirements? If you have your 12V and want a negative rail up to about 50mA, then one of the popular (but old) charge pump IC's like the ICL7660 will do. There are plenty of specific IC's for inverting converters, but if you want ultra cheap then a 555 with a couple of stage sof inverting charge pump will do. It all depends on the current required, allowable output ripple, and switching frequency. Note higher switching frequency doesn't necessarily mean higher efficiency - just smaller inductors/caps and "easier" filtering.Generally the options are: inverting switcher (inductor), charge pump, DC/DC module, push-pull converter driving a tapped transformer, all the way to a full mains transformer with dual outputs.The MC34063A is indeed a bit poo, but I've found it to get the job done, albeit with pretty poor efficiency and output ripple - how did you build these? Breadboard is a nono for SMPS.The AP3211 appears to be a buck converter, these can sometimes be configured as inverting switchers, but not all are capable. Schematic for your setup?You mention size restrains too - so what are they? And how many "units" ?

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 17, 2019, 04:21:01 pm
content: If I read correctly between the lines, the OP wants a current output up to 1A? (What about max ripple/noise?)That pretty much rules out any kind of charge pump, at least of reasonable size and efficiency...Regarding the AOZ1284PI, yeah, that would help to see your schematic. If you don't want to use a ready-made module, your only reasonable option wil probably be to use an inductor-based switching converter mounted as an inverter.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: July 17, 2019, 05:43:41 pm
content: I want this for more than one project.My idea was to make a little submodule that I can just solder to the actual pcb I'm doing.Charge pumps won't do, I need more current, I have plenty of TL074 opamps that will require quite some power.As for the ripple, one of the reason I wanted a better part with higher frequency is to reduce the noise for the TL074.It does not have to be perfect, its not audiophile grade, it just has to be below audible levels when using audio.One error I found after I broke it is that EN pin is max 6V in, but I'm now out of IC to try a new one.