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I would really appreciate some help on this one. I am doing my bachelor's thesis/project which requires me to design and construct a DC-DC converter. I have been working on it for a couple of weeks now.

My main issue is justifying the selection of a specific converter topology. Simply put, I have read a lot about different topologies, been through a lot of literature, but I don't really feel much better off than when I started.

How do I select the appropriate topology for my project? What are the relevant pros and cons?

The requirements are as follows:

  • Up to 21 V at max 3 A in voltage mode
  • Up to 5 A into minimum 3 ohms, max 21 V in current mode
  • Input voltage in the range 26-105 V
  • 210 mV output ripple
  • Can't use electrolytic capacitors

There are also some requirements for the control loop.

Here is my thought process:

I would prefer to avoid resonant converters as I don't want to open that can of worms. Furthermore, galvanic isolation would be good, but isn't necessarily a strict requirement.

For the flyback and forward topologies, I am a bit concerned about the ripple current in the input capacitor.

I am looking into the following topologies at the moment:

  • SEPIC
  • Flyback
  • Forward

I am open to considering other topologies.

Please ask if anything is unclear or you need any additional clarification.

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  • \$\begingroup\$ wait ... Vin > Vout, and buck isn't an option? \$\endgroup\$ – Brian Drummond Feb 7 at 17:55
  • \$\begingroup\$ +1 for expressing the problem well, having a good sense of question scope, and showing your work. Textbook good question. That said, I find the goals contradictory, is it constant-voltage or constant current, and what is the reason/context for wanting to switch one vs the other? \$\endgroup\$ – Harper - Reinstate Monica Feb 8 at 2:51
  • \$\begingroup\$ @BrianDrummond Buck isn't a preferable option as shorting the switching transistor would mean direct connection between input and output, which I see as a no-no for my use case \$\endgroup\$ – David Hansen Mar 19 at 12:14
  • \$\begingroup\$ @Harper-ReinstateMonica Thank you. To clarify, I need to be able to limit the current AND the voltage. This means that at any given time EITHER the voltage or the current will be the limiting factor. The use case for the converter is to either be current limited or voltage limited, for which there are two distinct areas of operation as described. \$\endgroup\$ – David Hansen Mar 19 at 12:14
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I recommend picking a topology, and running down the design path enough to get a feel for if you can meet your requirements.

If you can't, and can't see a way to easily fix it, move on to the next topology and repeat.

If you run out of topologies, edit your question to reflect the work you've done, where you're having trouble meeting the requirement, and how we can help.

Without sarcasm, the process seems central to your thesis. If it weren't, you'd be buying a converter.

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    \$\begingroup\$ I took your advice, and simply tried doing the math for the 3 topologies as well as making other selection criteria. As the flyback seemed the simplest and about as capable as the forward I chose to pursue this topology. \$\endgroup\$ – David Hansen Mar 19 at 12:09
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Picking a topology for a DC-DC converter starts with defining the requirements.

Personally, I'd first check the necessity of isolation. If it is not necessary then, depending on the relationship between VOUT and the range of VIN (e.g. check if output voltage will be lower than input voltage for the whole input range, as in your case) I'd go for buck (output voltage lower than input voltage), boost (output voltage is higher than input voltage) or SEPIC (output can be lower or higher than input voltage).

If isolation is a must then I'd check the output power and current.

  • For up to 150W output power and up to 5A output current flyback can be the simplest option.

  • I'd go for bridge converters for output powers higher than 400W.

  • Forward converter is almost the best topology for up to 400W output power and up to 50A output current.

Those are my opinions based on my experience.

NOTE: It's obvious that flyback, forward and bridge converters can be used as non-isolated converters but these are quite complex to design and build, and thus expensive compared to buck, boost or SEPIC.

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  • \$\begingroup\$ Thank you, I have ended up choosing a flyback converter, mainly for simplicity and because my choice of controller is very restricted, meaning a SEPIC converter could be problematic, though it was certainly an appealing choice for low input/output ripple. Honestly, it seemed a toss-up for forward vs. flyback. My main concern at the moment is handling the high output current (5A) with the flyback topology. \$\endgroup\$ – David Hansen Mar 19 at 12:05

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