Creating -15 VDC out of +15VDC - Page 1
Forum section: electronics / beginners

author: Phil Smith
author_rank: Regular Contributor
timestamp: March 24, 2025, 08:42:05 pm
content: Hi guys,I'm looking for a design advise please.Currently I'm building a circuit which requires -15VDC supply.I have +15VDC on board and I'm looking for a different options of creating that negative rail stated above.After a quick Mouser search, I've found the following IC:https://www.mouser.co.uk/ProductDetail/579-TC7662BCOA713Looks like great candidate, but max current seams to be quite low, around 160uA, which might be not enough to supply -15v to couple of op-amps.Is there a better IC for that application? Or perhaps is better to take a look into a discrete voltage inverter?Appreciate any help.

author: TimFox
author_rank: Super Contributor
timestamp: March 24, 2025, 09:06:59 pm
content: Look for an “isolated DC to DC converter” module.Mouser has 15 V output (unregulated) modules with, for example 67 mA output for roughly $5.50 in single quantity.If you need regulation, use a 24 V output device and a 78L15 regulator, connecting the isolated output for negative voltage w.r.t ground.

author: MAS3
author_rank: Regular Contributor
timestamp: March 24, 2025, 09:13:17 pm
content: Maybe something like this :https://www.mouser.co.uk/c/?q=CMR0515S3CIt's a through hole part as opposed to your SMD part, i don't know if that'll be a hurdle to you.

author: Phil Smith
author_rank: Regular Contributor
timestamp: March 24, 2025, 09:38:17 pm
content: Quote from: TimFox on March 24, 2025, 09:06:59 pmQuote from: MAS3 on March 24, 2025, 09:13:17 pmGee whiz! Totally forgot about those small dc-dc converter modules!.. They totally will do the jobCheers guys

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 25, 2025, 07:40:02 am
content: You can find lots of options on Aliexpress for this:https://www.aliexpress.com/w/wholesale-boost-converter-%2B15V-%252F-%2525252d15V.html?spm=a2g0o.home.search.0

author: jwet
author_rank: Frequent Contributor
timestamp: March 25, 2025, 04:41:19 pm
content: Just for completeness, you've misunderstood the 7662- its much more capable. The 160 uA that you've keyed on is the max "supply current" for the parts in the unboosted mode- they can provide 20 mA with some caveats.The 7662 charge pump type parts can make 20 mA- though it will droop a bit, they have a finite output resistance. For an op-amp negative supply, this is not likely to matter depending on your application. The output resistance with a 15v input is about 50 ohms, so a 20 mA load would drop a volt. This is the extreme, its much more likely that you'll have a few mA and have drops of 150 mV. Look at your op-amp for its common mode voltage range and output swing. Most op-amps work fine on +-12v and you'll exceed this. One thing to watch is that you're close to the abs max input for this part (16.5v). If your +15v is regulated, you're fine. These charge pumps are nice problem solvers for little bits of negative low currents. Unfortunately, few high voltage devices have been made since this venerable part was made in the 70's. There are a few from Microchip and others that exceed 50 mA from an 18v input- do a search on Mouser.You could supply the negative rail for several OP-07's with such a charge pump. The only limitation is heavy output loads (1k) or sinking lots of negative current. If its instrumentation, usually no problem. If its a signal generator - trouble.Good Luck.

author: TimFox
author_rank: Super Contributor
timestamp: March 25, 2025, 04:56:02 pm
content: Quote from: pcprogrammer on March 25, 2025, 07:40:02 amIn that large list, some of the units are not “isolated”: they have a common connection between input and output, so check the individual description.

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 25, 2025, 05:46:32 pm
content: Quote from: TimFox on March 25, 2025, 04:56:02 pmCorrect, but it is not clear from the original post that isolation is actually needed. The charge pump the OP found is not isolated either.I have some modules from Aliexpress based on the XL6007 that make +/- voltages from a single input. The links to the ones I bought now give me a 404, so no longer available for me. But here is a link to some that look the same:https://www.aliexpress.com/item/33034097806.html

author: TimFox
author_rank: Super Contributor
timestamp: March 25, 2025, 07:05:15 pm
content: A non-isolated converter where the output voltage has the same polarity as the input (e.g. a buck or boost converter that changes the voltage) will not work for his application.An isolated converter, with no internal connection from input to output, can be wired appropriately.A non-isolated converter that explicitly inverts the polarity (like an appropriate charge pump) can be used.Be careful to read the individual description, since all of these types exist.

author: pcprogrammer
author_rank: Super Contributor
timestamp: March 25, 2025, 07:54:53 pm
content: Quote from: TimFox on March 25, 2025, 07:05:15 pmYou can tap the switch output from a boost converter via a capacitor to create a negative voltage and get a positive voltage from the switch output via a diode. I have seen this in multiple devices, and the modules I revered to in my previous post use the same principle.Using the +15V as input for such a setup might not be useful, but if there is also, lets say, 5V in the design, that voltage can be used as a source. It does depend on how much current you need if it will do the trick.

author: Phil Smith
author_rank: Regular Contributor
timestamp: April 12, 2025, 09:02:59 pm
content: Thank yo very much guys for your input.I've ended up using a Chinese +/-15vdc module for a breadboard prototype and the following module for the final PCB design:https://www.mouser.co.uk/ProductDetail/709-DPAN02A-15

author: lakis70
author_rank: Contributor
timestamp: April 14, 2025, 09:02:20 pm
content: Quote from: jwet on March 25, 2025, 04:41:19 pmHow do you filter the noise of the charge pump? Do you have a circuit in mind?