PMP8896: High efficiency, isolated POE design with short (<13 mm) flyback transformer

Will Tidd

Part Number: PMP8896

I want to implement the PMP8896 reference design: http://www.ti.com/tool/PMP8896, in my own design; however, I have strict constraints on my flyback transformer height, it must be < 13 mm. Can you recommend a replacement that is shorter than the transformer spec'd, pn: NA6223-AL, which is 13.7 mm?

The reason I like this design is b/c of high efficiency, i.e. ~90%, at my typical power consumption. I am OK with other recommended designs, and my power spec is only 7W. So, my requirements are: 7W max and efficiency optimized (+90% desired) at 4W as an isolated design.

Regards,
will

over 5 years ago

0 Robert Taylor1 over 5 years ago

TI__Expert 6540 points

Hi Will,

It is definitely possible to find a lower profile transformer. I am going to loop in our PoE expert to find you the right part number.

Thanks,

Robert

0 David Strasser over 5 years ago in reply to Robert Taylor1

TI__Intellectual 2530 points

Yes I can help. The CA1174-AL transformer from Coilcraft should work in this circuit. It has a larger footprint, but the max height is 8.51mm. Also, take a look at the PMP9175 design. It is an update to PMP8896 and uses an improved gate drive circuit for the sync FET. The CA1174-AL has a lower primary to secondary turns ratio which may require the improved gate drive circuit to clamp the reverse voltage.

0 Will Tidd over 5 years ago in reply to David Strasser

Prodigy 80 points

Thanks. I have a few questions then:

What would the pin mapping be from the PA6340-AL on the PMP9175 to the C1174-AL. Is the drive and sync interchangable, then just connect pins 8/9/10 ont the C1174?
What about this design? http://www.ti.com/lit/ug/slvu315a/slvu315a.pdf?keyMatch=POE70P&tisearch=Search-EN-everything

Regards,

-will

0 David Strasser over 5 years ago in reply to Will Tidd

TI__Intellectual 2530 points

Will,

Yes the pin mapping from the PA6340 to C1174 is: 1 to 5, 2 to 3, 3 to 2, 4 to 1, 6 to 7, 7 to 8/9/10, 10 to 11/12.

The slvu315a design will meet your needs for 5V/7W. The efficiency will be less with the diode rectifier. The transformer has a smaller footprint and meets your height requirements.

Thanks,

David

0 Will Tidd over 5 years ago in reply to David Strasser

Prodigy 80 points

David,

Thanks for all the info, plenty of options to choose from.

I was thinking I would just implement the active rectifier solution from the PMP9175 with the slvu315a design, instead of the diode rectifier, to gain back some efficiency. No issues with this, correct?

-will

0 David Strasser over 5 years ago in reply to Will Tidd

TI__Intellectual 2530 points

Will,

The transformer used on slvu315a does not have a winding to drive the sync FET, so you will still need to use the C1174-AL transformer. I have looked for a smaller transformer with a sync FET gate drive winding and could not find one.

Thanks,

David

0 Will Tidd over 5 years ago in reply to David Strasser

Prodigy 80 points

David,

One last thing, do you think one of the EP7 or EP10 could work for the PMP9175: https://www.mouser.com/datasheet/2/336/-180276.pdf

Regards,

will

0 David Strasser over 5 years ago in reply to Will Tidd

TI__Intellectual 2530 points

Will,

There is no winding on the secondary for the sync FET gate drive. The EP7 is limited to 3W. The limitation of the EP10 is it only has 4 pins per side. It takes all the pins just to terminate the main secondary. Some manufacturers have a new "plus" size EP10 with a 10 pin bobbin, but I could not find any catalog transformers using these new cores/bobbins.

The winding configuration also makes a difference in how well the sync FET works. There needs to be a split primary for low leakage and the sync FET gate winding needs to be second in the stack up, next to the first half primary, to enable fast turn off of the sync FET.

Thanks,

David

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PMP8896: High efficiency, isolated POE design with short (<13 mm) flyback transformer