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I am working on a project where I need an isolated power supply. My input is 10-20V and I am looking to get an isolated 3.6V out @ 5A or more. My priorities are easy of implementation, voltage stability, and cost, in that order. There are no specific size limitations. I am new to both types of circuits, what are the pros and cons of both in this application?

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  • \$\begingroup\$ Device dependant pros and cons. Choose a device for each. \$\endgroup\$
    – Andy aka
    Aug 28, 2020 at 16:11
  • \$\begingroup\$ Please show how you plan to provide isolation from a buck converter. \$\endgroup\$
    – winny
    Aug 28, 2020 at 17:47
  • \$\begingroup\$ Linear Technologies used to offer some nice Flyback IC's which work by regulating on the BackEMF(right term?) and such do not need a seperate isolated feedback path. That way you get a very compact and reliable circuit. \$\endgroup\$
    – schnedan
    Aug 28, 2020 at 20:29
  • \$\begingroup\$ @winny, it's Flybuck vs. Flyback topology. I am trying to figure out what the advantage of Flyback over a Flybuck is, or vice versa. \$\endgroup\$
    – Bogdan
    Aug 28, 2020 at 20:31
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    \$\begingroup\$ If you need isolation, you can’t use the buck output and you are left with just the flyback output part, which won’t work without a load on the buck part. As your question stands, it’s unclear what you are asking. \$\endgroup\$
    – winny
    Aug 28, 2020 at 20:56

2 Answers 2

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I've generally only seen people use fly-bucks where they don't care too much about output regulation, but care more about cost.

Flybucks are smaller and simpler because they do regulation on the primary side since the secondary tracks the primary side relatively closely.

Flybacks actually regulate the output and require control to cross the isolation barrier. They also have large voltage spikes that are intrinsic to the design and require snubbing. This increase in complication gives you more precise regulation, meaning you can use it at higher power levels.

More reading here: https://www.ti.com/lit/an/snva674b/snva674b.pdf

For your specific case, if ease of implementation is most important, then you want fly-buck, but at near 20W of power, you are probably going to get not great regulation (if that's what you mean by stability), but flybuck will be cheaper as it doesn't require isolated control nor snubbing.

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  • \$\begingroup\$ Given that the output will be charging a battery, super close regulation isn't essential. Do you know by any chance what the reasonable current limit on the fly-buck design would be? I am shooting for 5-6A @ 3.6-3.7V. \$\endgroup\$
    – Bogdan
    Aug 28, 2020 at 21:52
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    \$\begingroup\$ The problem with charging a battery without precise regulation would be over charging. Is there a specific reason you are isolating the single cell battery? \$\endgroup\$
    – Nino
    Aug 28, 2020 at 21:57
  • \$\begingroup\$ it's a charger for a complicated battery pack. I can't say more as I am working under NDA, but it's basically voltage floating between say 3.6v and 3.7v wouldn't be an issue. Much outside that would be an issue. I suspect that I'll end up having to build both and test both, and see which one works better. \$\endgroup\$
    – Bogdan
    Aug 28, 2020 at 23:16
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    \$\begingroup\$ Still not sure why it would need to be isolated. There are battery charger buck ICs that operate from 10-20V. A 3.7V battery doesn't need to be isolated. If you are talking about doing active cell balancing in a larger stack, then flyback will be more appealing as you can do a multi winding transformer for the whole pack. \$\endgroup\$
    – Nino
    Aug 28, 2020 at 23:47
  • \$\begingroup\$ it's complicated, and again, can't say more due to NDA, it's not my project, I am only helping with a small part. In any case, that linked helped. I did more digging on TI's page and they don't recommend Fly-Bucks for applications that are 12W or more in power. We need more, so it solves that problem. \$\endgroup\$
    – Bogdan
    Aug 30, 2020 at 16:48
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Here is a good rundown of when and where Fly-Buck design is appropriate:

https://e2e.ti.com/blogs_/b/powerhouse/archive/2015/01/05/fly-buck-frequently-asked-questions-faqs

In general, based on my research, flybuck design is less complicated, less costly, but is limited to about 12W or so. By contrast flyback design is more precise at higher power levels, but requires feedback from across the isolation barrier via an optocoupler. This is according to TI.

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