Author Topic: Can I get -5V from an "isolated" DC-DC converter?  (Read 1151 times)

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Offline rea5245Topic starter

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Can I get -5V from an "isolated" DC-DC converter?
« on: December 28, 2019, 03:58:23 pm »
Hi. I have a +5V supply and I need to get -5V (low current, a few mA) for one part of my circuit. I'm looking at the PDME1-S5-S5-S DC-DC converter module (https://www.cui.com/product/resource/pdme1-s.pdf) which says it takes a +5V input and produces an isolated +5V output.

Am I correct that, since it's isolated, I can tie its +Voutput to my board's ground so that the 0V output is -5V relative to my ground?

Is there really a tiny transformer in that module, or is there another strategy for isolating power?

Thank you,
   Bob
 

Online Siwastaja

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Re: Can I get -5V from an "isolated" DC-DC converter?
« Reply #1 on: December 28, 2019, 04:16:16 pm »
Yes, you can do that, and yes, it has a tiny transformer and a crappy driver driving it.

Note they are unregulated and the ripple and EMC may not be that great.
 

Offline daqq

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Re: Can I get -5V from an "isolated" DC-DC converter?
« Reply #2 on: December 28, 2019, 09:04:13 pm »
Quote
Is there really a tiny transformer in that module, or is there another strategy for isolating power?
Yup, it's a tiny transformer. See:


Isolated modules are generally more expensive than a non isolated solution and may have various issues (lack of regulation and feedback, minimum load requirements...). Also, that module looks like a massive overkill for a few mA. Maybe you could look at non-isolated solutions such as:
https://www.analog.com/en/technical-articles/micropower-sot23-inverting-dcdc-converter-extends-battery-life.html
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Offline MagicSmoker

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Re: Can I get -5V from an "isolated" DC-DC converter?
« Reply #3 on: December 28, 2019, 11:18:19 pm »
Yes, you can do that, and yes, it has a tiny transformer and a crappy driver driving it.

Note they are unregulated and the ripple and EMC may not be that great.

Well, whaddya expect for $4?  :P

OP: If the output doesn't need to be isolated and the load only draws a few mA then the obvious solution is the classic '7660 inverting charge pump. The original part number is ICL7660 but it has been second sourced by pretty much every semi manufacturer on the planet: https://www.mouser.com/Semiconductors/Power-Management-ICs/Voltage-Regulators-Voltage-Controllers/Switching-Voltage-Regulators/_/N-668jtZ1yzvvqx?Keyword=7660&FS=True

 

Offline mariush

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Re: Can I get -5V from an "isolated" DC-DC converter?
« Reply #4 on: December 29, 2019, 12:10:28 am »
For non isolated output, look up  ratiometric / charge pump regulators.
Here's examples: https://www.digikey.com/short/zc8r13

Microchip makes one in DIP package, TC7660 : http://ww1.microchip.com/downloads/en/DeviceDoc/21465C.pdf
It's cheap and in the list above, for under 1$.
 

Offline rea5245Topic starter

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Re: Can I get -5V from an "isolated" DC-DC converter?
« Reply #5 on: December 29, 2019, 12:16:03 am »
This is a one-off hobby project, so I'm favoring a single-component, easy-to-solder solution.

- Bob
 

Offline MagicSmoker

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Re: Can I get -5V from an "isolated" DC-DC converter?
« Reply #6 on: December 29, 2019, 12:57:57 pm »
This is a one-off hobby project, so I'm favoring a single-component, easy-to-solder solution.


Check out Recom part number RFM-0505S - 5V in to 5V out at 1W for $1.41 in single quantities at Mouser.
 
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