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Design ideas for a low-count part, low power isolated DC-DC power supply?
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nctnico:
Another wild idea: how about using coils for wireless charging? These are small and self adhesive. Stick one on the top and one on the bottom of your board and you'll have your transformer with a hefty isolation barrier. Using resonant mode for energy transfer is probably a good idea.
SiliconWizard:

--- Quote from: nctnico on November 07, 2018, 01:55:17 am ---Another wild idea: how about using coils for wireless charging? These are small and self adhesive. Stick one on the top and one on the bottom of your board and you'll have your transformer with a hefty isolation barrier. Using resonant mode for energy transfer is probably a good idea.

--- End quote ---

Obviously a bit larger than I would like as far as area goes, although for a few tens of mW we could probably get away with pretty small coils. Regarding height, it's hard to beat though!
I like the idea. Will also work on that.

Edit: given the very short distance (a board's thickness) between the coupled coils and the resonant mode, we may even get enough power with coils routed on the PCB directly with a limited number of turns. That gets interesting! Noise coupled to surrounding parts of the PCB may be a concern, though.

SiliconWizard:

--- Quote from: Marco on November 07, 2018, 01:30:31 am ---It doesn't really short it ... it just makes the MOSFET burn all the energy put into one of the coupling the capacitor each cycle, which creates a limited amount of power, but still too high for comfort.

--- End quote ---

Right... they use a full diode bridge so that wouldn't be a short. I was thinking of my approach which uses an half bridge basically, and thus you'd get a short-circuit. With an H-bridge driver, a full diode bridge prevents that problem, so it's preferable. The H-bridge approach gets you a x2 factor (minus the Vf and losses), so I took an half-bridge approach which was simpler and would do the trick in my case.
not1xor1:

--- 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 ---

what about just an high frequency oscillator and a transformator with small toroid core?
a  555 or even a buffered cmos logic oscillator or (I do not know if there is a low voltage part) just a self oscillating mosfet driver
as far as I remember there are various ANs on the subject on the net
capt bullshot:
Lowest part count on such DC/DC usually is kind of a self-oscillating Royer converter.
Many of the small (1W) DC/DC modules are built internally this way. One needs a transformer with a center tapped primary, another feedback primary and a secondary winding, two transistors and two resistors on the primary side, a rectifier and a capacitor on the secondary. With a bit more effort, the converter can also be done with only one center tapped primary.

Often, if there's a buck regulator in your circuit anyway, one can couple a small transformer to that regulator and have an isolated output with very few components involved. Anyway, any half-decent solution with low part count involves a transformer.

Edit: There are smallish driver chips (e.g. the TI SN6505B) that ease the job of building such converters. Beware, this chip might cost more than an optimized converter built from discrete components.
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