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Design ideas for a low-count part, low power isolated DC-DC power supply?
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SiliconWizard:
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.
mariush:
Adum5000 : https://www.analog.com/media/en/technical-documentation/data-sheets/ADuM5000.pdf

5v to 5v 100mA digital isolator .. around 33% efficient. So you'd need 150mA at the input to get 50mA at output.

Higher dc input voltage? Any switching regulator capable of 150mA output would do, go for higher frequency to get smaller inductors, ceramic capacitors etc

for example AOZ1282 (max 36v in, max 30.6v out , 600mA, 1 mhz) .. that's around 85-95% efficient.


Want AC to DC or something like that?

there's regulators like LM5008 which can accept up 95v input voltage.  Assuming around 70% efficiency when doing 90v in, 5v @150mA out, you're looking at around 1100mW or  1100 / 90v = ~ 12mA

To make sure you don't go over 90v, you could always use a basic linear regulator like  LR8K4 for example - takes up to 440v , outputs desired voltage (maximum Vin - 12v) at up to 20mA
So you could have AC in - > diode for half wave rectification -> small capacitor to have min 100v at ~ 15mA all the time -> LR8K4 to do 100v+ to 90v 12mA -> LM5008 to do 90v-> 5v 150mA -> adm5000 to isolate.
Marco:

--- Quote from: mariush on November 06, 2018, 09:29:12 pm ---Adum5000

--- End quote ---
Technically still using a transformer, just a tiny integrated one, but then that's hard to avoid.
schmitt trigger:
Your post title includes the word "isolated".
That on its own, suggests a transformer being employed somewhere.

The reason flybacks are so extremely popular for low-power, isolated supplies is precisely because of its low parts count, and low cost.
SiliconWizard:

--- Quote from: Marco on November 06, 2018, 09:41:49 pm ---
--- Quote from: mariush on November 06, 2018, 09:29:12 pm ---Adum5000

--- End quote ---
Technically still using a transformer, just a tiny integrated one, but then that's hard to avoid.

--- End quote ---

Yes, I've already used some of those "isoPower" isolators from AD. They work fine and pretty compact.
And yes, they use integrated micro-transformers as far as I know.
From experience, they tend to have rather high inrush currents at power on and they are pretty noisy (can make EMC testing fail even when you think you have routed them properly - they need extra care). But they do work fine.

I'm looking for ideas for a discrete solution. I'd like to avoid transformers and although that may seem pretty difficult at first sight, the challenge is to find other approaches. Maybe capacitive solutions for instance.

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