inverting a dc dc boost circuit

[Gibson486]

Is it possible to take a boost dc dc switching converter (with an external switch) and simply invert it to get negative voltage? I swear I saw a TI app note on this very subject.  I remember it being as simple as adding a cap and just switching polarity at the output, but that seems way to easy. I cannot seem to find the app note, and asking TI a question on their forum is kind of a nightmare (keeps saying that I am not logged in when I am).

Thanks!

[PartialDischarge]

You have to connect the output of the dc-dc to ground with the inductor. So now the ground of the device turns into the negative voltage

https://www.ti.com/lit/an/slva469c/slva469c.pdf?ts=1613760830893&ref_url=https%253A%252F%252Fwww.google.es%252F

[Gibson486]

Thanks! I could have sworn there was a much more generic version of this datasheet that did not have a specific part number tied to it, but this works well too!

[Gibson486]

Hmmm...maybe this is a little more difficult with an external mosfet? I am trying to do this with an LT3757. I only have so many parts on hand....so I am limited to that boost converter for now. On the datasheet, looks like they suggest using a SEPIC topology to do an inverting controller.  Never really used that topolgy before, but I do not have the correct transformers to do that, so I am trying to do this with the high current inductors I have in house...

I am just trying to generate -90V from a 24V supply.

[PartialDischarge]

Normally the inverting is done with buck converters (https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3844.html) .

Also you'd need a part that can handle at least 120V between VCC and GND if you wan to generate -90V from 24V

[Siwastaja]

Note, while widely apparent in appnotes, forums and practical engineering coffee tables, "inverting buck" is a misnomer. This topology is more formally called "buck-boost", using this term you'll find it in literature: https://en.wikipedia.org/wiki/Buck%E2%80%93boost_converter

It can decrease or increase the magnitude of voltage.

Yes, buck becomes buck-boost if you just change the ground reference label. Yes, it's really the same topology.

One reason for confusion is that you can simply use a controller/regulator IC that is usually used for buck, to implement this inverting buck-boost circuit.

Using the same principal components - two switches (one of which can be a diode if bidirectional/regenerative (swapping in/out) operation is not required), one inductor, and filter capacitors - can't be arranged in too many different functional topologies. You'll see buck, boost and buck-boost are all very closely related.