Generating +15/-15VDC with a single boost converter

[ratatax]

Hello

For powering the opamps on my projects i need +15/-15 V from a wide DC input range (let's say 8V...16V).
I found those modules on ebay : https://www.ebay.co.uk/itm/15V-Positive-Negative-Dual-DC-DC-Step-up-Boost-Converter-Module-3-3V-5V-12V-/172763714588


I'd like to design this thing on my circuit, but they sanded the chip (duh!). Does anyone knows how they create the double voltage from a single buck converter ? I guess it's a single-rail stepdown/boost converter since it uses only one inductor.
It uses three big output diodes, which may give some informations for experienced people designing power supplies, but unfortunately i'm not in this domain at all... Searching for dual converter schematics on the net doesn't give much results.. any idea ?

[iMo]

I did negative voltage such I wired 2 diodes and 2 capacitors (a voltage doubler) to the node where the inductor is wired to the original diode.
It seems The above module is doing that the same way.

[Rerouter]

Not as cheap, but at a starting point there are drop in modules that will work, and give you an actually specification you can trust in how much current and noise you can expect,

https://au.mouser.com/Power/DC-DC-Converters/_/N-brvxe?P=1yx5k7uZ1yxt6orZ1yxt6peZ1yxt7erZ1yxt78xZ1yxt6phZ1yo7fn8Z1yxt7cyZ1yxt74mZ1yxt78nZ1y9oph1Z1yxt6pfZ1yxt7dyZ1yxt793Z1z0k0eqZ1yzxopsZ1z0k0enZ1z0k0epZ1yztqpbZ1yqwpv0Z1z0k0eiZ1z0wavnZ1yqwpgk&Ns=Pricing%7c0

They start at about $14 AUD from where I have found them,

As for the exact chip they are using in that module, hard to say, most regulator IC websites have some level of parametric search, Texas Instruments atleast the last time I went looking even had a wizard to spit out the exact schematic for a given problem with a BOM and everything. you would be looking for a chip that does +- output, most of them will not be rated at exactly +-15V, that just happens to be the feedback divider they chose.

[ratatax]

Thanks for this schematic that's quite an elegant solution !

For an integrated DC-DC module I found cheaper on LCSC : https://lcsc.com/product-detail/Power-Modules_Guangzhou-Aipu-Elec-Tech-KW3-05D15E3_C232043.html
3W regulated, 4.5/9Vin, +/-15V out, $5.
But I need to think a bit, designing it myself would be cheaper, with more readily available parts and you know exactly which components you use.

[iMo]

It is maybe an elegant solution but you have to understand the limits too.
The -V is not "regulated" and usually "weak".
It means it never will be exactly -15V, and with larger loads than a couple of mAmps the voltage will drop down.

As I've written above, the module in your first post is doing that -V the same way, therefore if you happy with that module you may use the "elegant" solution, indeed

[T3sl4co1l]

How much current do you need?

Attaching a negative voltage doubler will give the wrong voltage, and not play well with most switching regulators.

Have you considered a multi-winding inductor?  A pair of dual inductors, primaries wired in parallel, secondaries wired for complementary outputs, will do.  (This is a SEPIC configuration, with an extra output that happens to be negative.  Coupling caps can be used to improve performance, but mind how the negative output is wired!)

Hmm, I did this a few months ago actually, a board that needed voltages for a traditional analog front end, meanwhile everything else was 5V supply.  So there was a 500mA boost regulator and dual inductors wired in this way, to give about 30mA available at the 15V outputs.

Note that, since the outputs are coupled through inductors, the cross-regulation isn't great.  This was wired for +15V regulation, and the -15V does whatever; I got down to -16.5V with full load on +15 and none on -15, and the opposite (about -13V) with full load on -15 and none on +15.  That's perfectly adequate for analog purposes, but if you were hoping for something more accurate, you may consider making extra (18-20V?) and regulating it down, or using two independent (switching) regulators period.

Tim

[ratatax]

The outputs will be used to power a line-level preamp and a headphones amplifier, all made of smd opamps and transistors.
With a big margin I estimated it would be safe to have a 2W power supply, so it's 66mA for +15 and 66mA for -15.

I didn't expected the doubled-negative voltage would drop so easily -- i haven't this module, i'm just looking at what is available for my needs--.

I'll check for the multi-wired inductors, haven't looked at this solution yet.

[schmitt trigger]

In that instance, then you should consider a small flyback, with a transformer with dual secondaries.

It is customary to have the feedback on the + output. Therefore if it is operated in discontinuous current mode, the cross regulation between the + and the - supplies is actually quite good.

An example of such a transformer is this one:
https://www.digikey.com/product-detail/en/wurth-electronics-midcom/750313457/1297-1136-6-ND/4959544

[Bassman59]

How about using the LT1372 with a transformer? Read the data sheet, it's all there.