UCC28951 Questions

[MrHerb]

Hi, I working on 48VV output, 60-200V input design based on UCC28951. Target output current is about 90A. My first attempt at this type of PSU design so excuse my lack of knowledge.
Following on from this thread: UCC28951 PSFB Controller - Sense Signal Filtering question

[Moderator action, added link]:
www.edaboard.com/threads/ucc28951-psfb-controller-sense-signal-filtering-question.401935/

Easy peasy[/USER]]
At 42V 5.5kW ( 135A ) you'll need some pretty impressive film/foil caps across the pir fet H bridge to soak up turn off current - else you'll get volt spikes that'll kill the fets - poof ...

you'll also need some pretty impressive snubbers across Qe, Qf to handle light loads - else poof again ...

Can you give me an example of the type of film/foil cap might be required, something like ECQ-E2106JF 10uf/250V ?

I've also been struggling a bit with the ringing across the output synchronous rectifier fets Qe and Qf at light loads and found there is a lot of power dissipation in the resistor of the RC snubber and RCD clamp when I tried that. I think I need to use fets with lower capacitance, I currently have IPW60R041P6 which I think is a bad choice Coss 310pf, Co eff 1200pf.

Also

cupoftea[/USER]]Also, please be sure to use SiC FETs......otherwise the PSFB's reverse recovery trick can be played out on you.

Sic Fets for the synchronous rectifier fets or the primary fets or both? I looked at something like the NTHL025N065SC1, Coss is similar 278pf (maybe Coss eff is better, it's not quoted in the datasheet).

Thanks.

​

 

[cupoftea]

OK thanks, but coupling means exactly that....and if 3 (pri) turns is perfectly coupled with 4 turns (sec), then there is no leakage inductance....if there is L(leak), then you cannot possibly have the full 3turns to 4 turns coupling....we simply cannot have it both ways.
And this is why soffware is needed for planars...because planars always use low turns numbers, and so they need the soffware to do the coupling and get the ns/np proper.

 

[Easy peasy]

your sentence is non sequitur,

how many planars have you designed and got into production above 100 watts ?

 

[cupoftea]

Ive never had access to the special software that they use to design the planar, as it costs a fortune as you know, and they always charge people so that they can use it for us. I went to Himag for an interview and the engineer showed me him using the software. It was sotware that was not available to be bought as i understood it.....unless you pay a fortune.
I also sent specs off to a place in Scotland. That was for 200W two tran fwd. DCDC
I think they charged £6000 up front...cost of their software used on our behalf, and the tooling. And that was just for a few peices...but they then had the tooling or whatever to allow many more to be made for us without that upfront cost for the further batches.
Just give them the Np/Ns spec and the L(pri) and the L(leak) max, and the size etc, and ask what they can come up with is the normal way with planars as i am lead to believe.

--- Updated Aug 16, 2024 ---

This stuff about full no.s of turns came up recently because we had to do flyback on PQ20/16 , but had to turn the bobbin "sideways" otherwise it couldnt fit in due to the "jutty out" termination bits....but then your final turn doesnt hug so closely around the 360 degs of the bobbin...and its a partial turn kind of thing.
This could affect OP as they have also low turns...and a partial turn of a low turn coil makes quite a big difference to the inductance thats coupled in there to the sec.

 

[Easy peasy]

All you need is KiCad and and excell spreadsheet ( or calculator ) to design a planar - they are no different to any transformer - you need to take care of the edges of the wdgs - and usually think about the thermals as they are normally run hard and rely on heat-sinking thru the core.