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Design ideas for a low-count part, low power isolated DC-DC power supply?
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Marco:
If you want to keep the coupling capacitance in the pF range you just can't switch fast enough with capacitive coupling, since you get picojoule range energy transfer per cycle.
SiliconWizard:

--- Quote from: mikeselectricstuff on November 06, 2018, 10:14:48 pm ---Why not just buy an off-the-shelf DC-DC module ? That easily satisfies "as few parts as possible"

--- End quote ---

Challenge and cost, mostly. Also, most modules I have seen so far are usually for higher output power and a bit too bulky, but if you know of miniature ones and reasonably priced, that could be an option.
SiliconWizard:

--- Quote from: Marco on November 06, 2018, 10:41:30 pm ---If you want to keep the coupling capacitance in the pF range you just can't switch fast enough with capacitive coupling, since you get picojoule range energy transfer per cycle.

--- End quote ---

True, I'm fine with nF-range values or even around 1 µF, although the question is the isolation voltage you can get while keeping the caps reasonably small.
Wolfgang:

--- Quote from: SiliconWizard on November 06, 2018, 08:46:15 pm ---As the title suggest, I'm looking for ideas to implement an isolated DC-DC power supply with as few parts as possible. Targetting low input and output voltage (5V max) and low max output power (50 mA max). Efficency would not matter much as long as it's not ridiculously low (thus > 20-30% would be great).

Obviously, flyback topologies are popular, but I would like to find alternate ways, avoiding the need of a transformer if possible.

Any ideas welcome.

Note: a 1:1 converter is fine and even preferred.

--- End quote ---

You of course know that you can buy stuff like this in potted modules for just a few €.

I tried to make something like this because I needed a version with extra low ripple running from USB.
Commercial modules are fairly noisy.

https://electronicprojectsforfun.wordpress.com/power-supplies/a-usb-powered-15v-supply/

This has 15V output, but the circuit can be easily adapted to 5V if needed.


SiliconWizard:
I should definitely have added that I was preferably not looking for an off-the-shelf solution. ;D

Anyway, as far as transformerless goes, the capacitive approach seems the best fit so far, although it's not an ideal means of isolation. (Reminds me of Dave's 'Does current flow through a capacitor' topic ;D )
I'm currently simulating a simple solution which bears similarities with this: https://www.maximintegrated.com/en/app-notes/index.mvp/id/4553 , only I just use 2 diodes instead of a full bridge, which is fine for what I need (and has lower loss), and no specific h-bridge IC. This should do the trick although again it's not ideal isolation-wise, but my main goal here is not about safety, it's mainly for getting a floating ground.

This will certainly fit my requirements but obviously the safer approach still seems to use some kind of flyback DC/DC converter. The ADuM isoPower series is nice, although it's not perfect either.

In fact it sort of all comes down to transmitting power wirelessly, and there aren't that many options for this. Inductive, capacitive, photovoltaic... sure I could also consider a purely mechanical solution but I don't think that would fit my volume, cost and efficiency requirements - not to mention being mostly silent. ;D

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