Flybuck converter at high power

Question

It is said that the flybuck topologies are cheaper than the flyback topologies. This is due to the fact that there is no need to have an optocoupler, the regulation is said to be simpler and probably that the compensation is simpler.

Nevertheless, the isolated buck converter is essentially used for low power.

Here are two extracts that I found of the previous purpose :

Does anyone know why the flybuck converter is not adapted for high power (I mean 50W to 100W). I would actually try to replace a flyback converter whose the input voltage is 85-265AC, the output has several outputs which are common voltage (~ 20V ~ 5V)

Thank you

Answer 1

Fly-Buck is not a converter topology. It's just an idea: The idea is to add extra winding(s) to a buck's choke to have isolated output(s) without disturbing the main/buck converter.

The main choke is designed and sized for the buck's output power requirements. As Andy stated in his answer, the extra windings have to steal some energy from the main winding to induce their output voltages (In other words, if the main output is unloaded there's no chance to get a stable output from the isolated side(s) without very large output capacitors). This means that to steal the required energy for an isolated output the buck's output should be significantly (or even almost fully) loaded.

This doesn't seem to be practical because you will end up with an unnecessarily big choke to get relatively low output power. For example, to get 50W isolated output the buck should be designed for, say, 500W.

Without disturbing the main buck converter, stealing some energy to generate voltage is not a big deal. This is like having an apple from an apple tree garden. If you steal one apple no one will notice. But if you try to steal a whole tree things will change.

Answer 2

I would actually try to replace a flyback converter whose the input voltage is 85-265AC, the output has several outputs which are common voltage (~ 20V ~ 5V)

Not a good idea; the flybuck still produces a non-isolated DC voltage as its main output and, this means that it is lethal when trying to replace a flyback converter connected to AC grid voltages: -

Image from AN-2292 Designing an Isolated Buck (Fly-Buck) Converter.

Does anyone know why the flybuck converter is not adapted for high power (I mean 50W to 100W).

The isolated output is meant to be low power because it steals energy from the main unisolated (and regular) buck output hence, unless your main output (unisolated) is several hundred watts, then it's not going to work in these circumstances.