Active-Clamp Forward Converter IC Selection

[kkeeley]

Hi,

I'm looking at designing an Active-Clamp Forward SMPS of around 100W max output. It would be used on 230V AC only and would need to output around 5V@10A, 3.3V@10A and also supply 2.7V for the active termination used on the backplane.
I'm thinking of using an NCP1654 fot the PFC stage, a NCP1562 for the main converter, I'm not sure if I should step down directly to a 5V rail then use a post regulator to generate the 3.3V rail and 2.7V rail or step down to a higher voltage rail and then use a post regulator for the 5V rail aswell. I'm thinking of using an NCP1587 as the post regulators.

My first question is what Secondary Side Controller can I use? Can I use two NCP4306's, if so how, as I haven't found an example used on a forward converter.

Question two, which output configuration from th he main converter would be better, a single 5V, a single 12/20V one, or multiple outputs, say 5V and 3.3V?

Thanks for any suggestions and happy to provide more details if required.

 

[BradtheRad]

1.
Consider the effect when you add a sudden load to any of your 3 supplies. Will it cause the other supplies to drop? Do you plan to power tolerant equipment on those supplies? Etc.

2.
Also consider the amplitude of waveforms coming from the transformer. Is it easier (cheaper, lighter, cooler, more compact, more stable, etc.) to have one secondary producing pulses of 40A (assuming 50% duty cycle), or two secondaries producing 20A pulses?

Or is it efficient to split the burden to two transformers (one producing 5V, one for 3.3v)? Which is easier to construct or purchase or design or cool or enclose?

 

[Easy peasy]

Active clamp - can suffer from bad behaviour for maximal load transients - so be sure this is addressed in any app notes from a chip supplier

 

[kkeeley]

Thank you both for your feedback, I will consider all your suggestions as I work through the design. The design is by no means completed, it is still in the early stages of gathering information so as to try and make the best choices based on the needs of the project and trying to keep it affordable.

 

[treez]

National semi (now texas) do ACF chips.
Some have a p channel reset in the low side...and some have a high side nfet reset.....for the hi side they often have a bootstrap hi side driver........christophe basso in his book (Switch mode power supplies) writes about the effect that Easy Peasy just mentioned.........they do have this mode that you have to look out for.
Maybe its just better to do a 2 tran forward..........if you arrange the leakage in the ACF you can sometimes get zvs switch on, but you get a hard switch off......but in truth, i dont think its worth it......just do a 2TF converter instead.

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I would make a 2TF output say 12V or so, and then use two separate sync bucks for the 5v and the 3v3.....say some of the sync bucks from linear.com are good.

Alternatively, use one of the vicor modules for your 5v and 3v3....they are really good and very efficient.

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for control of the 2TF , just use a tl431/opto....you can drive the 2TF with a ucc28c45 or similar...in current mode

 

[kkeeley]

Thanks for the suggestion, I had look at the ucc28c family before, I'll take another look at them.

 

[treez]

you could always make the 2TF output , say 30V...then your high current stages are just the 3v3 and 5v converters...in other words.....you leave the high current till last...so you dont have to ship the high current around very much.
The vicor modules can also go from 48v to 3v3 very easily.

 

[kkeeley]

Thanks again for the suggestion, the idea of having a higher volts lower current first stage is worth considering as well.