How to select DC-DC converter topology?

Question

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.

Answer 1

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.

Answer 2

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 V_OUT and the range of V_IN (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.