power supply isolation options

[AlienCircuits]

Hello,

I would like to design my first PFC bus power supply, and I am finding it difficult to pick the right topology to work with this approach while still having isolation.

It seems like most of the PFC pre regulators that I've seen online are not isolated. I think they might be isolated later on with a flyback converter if isolation is necessary.

When I go to make a PFC boost converter with good power capability, I find that including a transformer makes the price go up considerably.

How do PC PSU companies sell 500W+ power supplies for not much more than it would cost me to just buy the transformer? Do they go without isolation?

 

[barry]

I have to believe that any commercial 500W power supply has some kind of transformer or inductor in it. So, why are they cheaper than your transformer alone? How many transformers are you buying at a time, one? Then there's your answer-the economics of scale. Just going from 1 to 250 units will drop the price by about half.

 

[mtwieg]

When both PFC and isolation are needed, they are usually handled using completely separate stages. First is a nonisolated boost PFC, then is an isolate DC-DC converter. It turns out that this approach gives better results than trying to build a single stage isolated PFC (using a flyback/boostbuck stage), especially at higher power levels.

 

[AlienCircuits]

What about about low power levels, in the range of 30W or less? I found a reference design from ON semiconductor that uses a single stage flyback PFC for laptop adapters. I want to step my line voltage down to about 16-18V, and using a boost PFC stage means the input to my step down stage will be more like 200-400V, and then that seems to really lessen my transformer options with the large input voltage requirement.

 

[mtwieg]

Using a flyback is an option at lower power levels. Usually the problem with flybacks is the much larger input filters required to meet EMI restrictions. Also a flyback input stage requires transistors with much higher voltage ratings (same for the transformer).

Also, if you try to do everything in one stage, you must take into account the 100/120Hz ripple which will show up on your output voltages. Getting that ripple to be low may take a lot of capacitance, which is another reason that two stage solutions are often preferred.

 

[AlienCircuits]

Thanks for the advice. I think that I will be looking at the standard PFC boost stage then.

- - - Updated - - -

Ok, I have one more question that I might as well ask in here while its there.

A ~400V output from my boost PFC regulator seems pretty standard according to what I've read. If this is the case, why are there not more 400V rated transformers used to step the voltage down to something usable by most electronics? I was thinking I could use a forward topology to get down from 400V to 16-18V, but I find that my transformer selection is pretty poor for how common this need should be.

Am I going about this in a bad way, or is there a proper place to find transformers for this type of application?