Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

  • State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 65 subscribers
  • Views 368 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS23756: Forward Flyback Monster

eljeffo
Intellectual 560 points
Part Number: TPS23756
Other Parts Discussed in Thread: TPS23758, PMP40514, , TPS23730EVM-093

Today I learned that there is a difference between forward and flyback transformer and inductor requirements.  Would would have anticipated such a thing?!  

I have another TPS23758 in the works so hopefully that's "so simple" that this thread isn't urgent.  But I would still like to understand what I'm doing since I have a pile of parts which would be good to use at some point.

I started off with the schematic from the PMP40514 (A secondary side forward "looking" converter).  I picked the Coilcraft POE13P-50L (A flyback transformer)... at that time it said transformer for the TPS23756, so I figured I was good :)

https://www.coilcraft.com/en-us/products/transformers/power-transformers/power-over-ethernet/poexxp/

I noticed in another post the following quote:

For your reference, I would highly encourage the customer to follow the PMP405414 design. It uses the TPS23756 and is an active clamp forward with 12V output. Since it is a proven design, I see no reason not to directly copy it. 

(https://e2e.ti.com/support/power-management/f/196/p/869697/3217677?tisearch=e2e-sitesearch&keymatch=tps23756%2525252525252520forward#3217677)

But at the bottom of the PMP40514 Schematic it is titled:

Project Title: Class 3 Isolated High-Efficiency Active Clamp Flyback Converter for PoE PD Reference Design

So me being completely clueless that I was clueless... suddenly started thinking... hey my PMP40514 output has a single diode and needed a big inductor (330nH)... certainly looks like a forward converter.

  (I even tried the 10uH SER1360-103KL for the TPS23756-EVM Flyback and it DOES NOT WORK! (as you know... and I didn't))

So... why does the flyback transformer work for my forward circuit?  I read that the flyback uses the transformer like an inductor to hold energy, but a forward doesn't use the transformer that way (which is why the big inductor value is needed later). 

Is my crazy flyback transformer storing energy and delivering it out of phase or something insane like that?  

Is my circuit going to suddenly stop working (I'm drawing at least 10W out of it with no issue.. my electric load is on order!  bear with me while I order the world!.. I just got my bode meter today!)

Is my circuit perhaps just a little inefficient? Or am I in for a big EMI trap when I try to get certified?

Being that the PDF says PMP40514 is a flyback, but... the secondary looks like a forward... are we in some middle ground that... I don't even know how to interpret where I am.

The crazy thing is I am not smart enough to pick this transformer myself, and I swear that coilcraft just a few months ago was promoting the POE13P-50L for the TPS23756... but there's no sign any more.. am I losing my mind?  Now that I look, it's clearly not listed in any reference design. 

Anyways, if this monster I've created is no good... it's fine.  I just want to know that if this POE13-50L flyback transformer shouldn't be used in the PMP40514 forward secondary, to know now... rather than thousands gobs of money later.

If you know, please share :)  If not, no big deal. :)  I'm just trying to determine the "right thing to do" with this pile of TPS23756 and POE13-50L parts next to me :) -- it's a long story

Thanks!

  • 0
    Intellectual 560 points

    Whoa what did I write... I thought forward converters need more secondary inductance... why did the decrease from 10uH -> 330nH fix my forward converter? 

  • 0 in reply to eljeffo
    Intellectual 560 points

    Ah well.. I did reverse the dots on the pri:sec for the transformer (Like PMP40514) so maybe I did make a flyback configuration on the transformer... 

    But with just a single diode on the secondary side... is it still a flyback? There must be some basic concept I am totally missing! :)

  • 0 in reply to eljeffo
    Intellectual 560 points

    Okay here's my guess.  I've unknowingly created a Flyback Converter Continuous Conduction Mode (CCM) Single-Ended Topology device.

    So... if that's the case... how does the TPS23756 know if it's a flyback or forward controller? does it matter?

  • 0 in reply to eljeffo
    Intellectual 560 points

    Aha, the single diode pretty much forces current to stop when the transformer is going the other way... unlike a forward converter where there's always at least one diode conducting.  So we're pretty much forced into a flyback topology.   So says my brain as far as I can think of.

    So.... how does the TPS23756 know forward vs flyback... and why are other controllers not able to figure it out?  hahaha thanks so much as always for your thoughts.

  • 0 in reply to eljeffo
    TI__Mastermind 21320 points

    Hello,

    Although there were a lot of questions here, I feel that this was a stream of conscious, so I will try to answer some core concepts here. 

    For the record,  PMP40514 is an Active Clamp flyback.  Most of our flybacks are not active clamps. However, most (if not all) of our forward reference designs are active clamp forward. An example of a true active clamp forward is the TPS23730EVM-093 https://www.ti.com/tool/TPS23730EVM-093

    or https://www.ti.com/tool/PMP6584

    So that is what is probably causing the confusion. 

    Unfortunately there is no way for the controller to know what type of topology it is in, but it would not necessarily act any different. The TPS23756 has two GATE drivers for FETs. The IC has additional pins to control the frequency, dead time, blanking period, etc. so the designer can control how the IC acts in whichever topology it is placed. In that way, the IC's behavior is controlled so it "knows" how to act but truly it is just being programmed by the input pins. 

    I hope this helps.

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Intellectual 560 points

    Thanks Michael.  Your confirmation that I built a flyback makes me happy as a clam!