TPS2375: PoE PD solution with TPS2375 + LMR36015-Q1 (or similar, for either)?

apullin

Part Number: TPS2375
Other Parts Discussed in Thread: LMR36015-Q1, LMR36015, , LM5010, PMP10812, TPS2378, ,

Hi,

I need some input from a TI power supply FAE expert:
I need a PoE PD solution for a device where size and (most importantly) height are the major constraints.

I have found some available PoE modules and some reference designs, but all of them require a fairly large transformer as part of the DC/DC stage.

Would the combination of a PD-only device, like TPS2375, and a miniature, modern DC/DC converter with a high Vin_max, like LMR36015-Q1, work as a PoE-to-+5v supply?

The supply will only ever need to source < 5W into a +5V output.
The design already has an ethernet transformer in it, so that component is not an issue.

All of the combined PoE PD + DC/DC controller IC's that TI (and most others) offer appear to use a converter that requires a wound transformer, and the recommended parts are large ( >= 14mm height).
The LMR36015 reference design appears to only need an inductor that is 5mm high, so it looks promising.

I am not a power supply engineer by any means, so I might be glossing over something obvious here. Topologies, isolation, etc.

over 5 years ago

0 Thomas Lewis over 5 years ago

TI__Expert 5615 points

Hello apullin.

One of our PoE PD Application Engineers is assigned to this thread and may have additional comments later. However, I wanted to be sure you noticed a similar thread which came in recently (similar topic, height reduced transformer solution) located here --> e2e.ti.com/.../730155 .

I suggest you take a look to that thread - as the customer came in with a low profile transformer (but was seeking higher efficiency) from Wurth. It was part number 749 119 350; web page is katalog.we-online.de/.../749119350.pdf . They had partnered that with our TPS23753A.

Thank you for your interest in PoE solutions from Texas Instruments,

0 apullin over 5 years ago in reply to Thomas Lewis

Intellectual 575 points

Excellent, thanks.
Just to clarify: I don't specifically need the TPS2375 specifically, I just selected it becasue it seemed like the most minimal PoE PD controller that was avaialble.

0 Thomas Lewis over 5 years ago in reply to apullin

TI__Expert 5615 points

Understood apullin.

Hopefully, that other thread helps you.

If needing additional assistance, please reply with your prioritized lists of specs and performance goals in mind so that we can be more helpful. For example:

1) Height = 14mm
2) Total PCB footprint =
3) Efficiency =

Otherwise, if this gets you started in the right direction, please let us know by clicking the "Resolved" button.

Thanks,

0 apullin over 5 years ago in reply to Thomas Lewis

Intellectual 575 points

Sure ... although I don't have a strict height limit, just the need to minimize the height, more than anything else.

A target of 6mm would be great for height. 5mm (some inductors are that height) would be excellent, is a non-transformer solution is doable.
Board area < 500 mm^2 , since we have plenty of board space available.
Efficiency >= 60%, since this is not a battery-driven application.

0 Thomas Lewis over 5 years ago in reply to apullin

TI__Expert 5615 points

Apullin,

I am transitioning this thread to our PD Application Engineer, Tom Amlee, for further assistance. I think you have given us enough guidance so that he can recommend a design/concept for you now.

Thanks,

0 Thomas Amlee over 5 years ago in reply to Thomas Lewis

TI__Genius 9230 points

Hello Apullin,

Since solution height is your main concern, a buck topology, as you noticed, is a good choice. I have 3 suggestions:
1. PMP10812: Low cost 5V/1A Non-Sync Buck using TPS2378 (PD only) + LM5010
2. TPS23750EVM-107: 5V/2A Upside down buck using TPS23750 (PD+PWM)
3. Pair TPS2378 with a buck design from WEBENCH

The first 2 cases are good places to start from. These are low cost, but efficiency will be 80% (max). In the latter case, you can more easily spec for efficiency, BOM cost, or solution size as your design requirements become more solid. In all cases, you can find an off the shelf inductor that will meet your height requirement without having to increase switching frequency (small inductor, but more switching losses).

Lastly, if you are interested in using one of our PD +PWM controllers in an upside down buck topology, here is a design guide that will prove useful: www.ti.com/.../slva440.pdf

Regards,
Tom Amlee
PoE PD Apps Engineer

0 apullin over 5 years ago in reply to Thomas Amlee

Intellectual 575 points

Great, thanks.

Does TI have any app notes on the needs and implications of isolation for PoE supplies?

The integrated PD+PWM solution would be fine, but most of the reference designs appear to use large transformers, as it is doing isolation and a large step-down at the same time.

I would even be open to a 2-step DC/DC pathway, assuming that doing minimal voltage change in the isolation transformer would keep winding currents as low as possible, and then leaving doing the higher current conversion in a single-core inductor in a 2nd step.

0 Thomas Amlee over 5 years ago in reply to apullin

TI__Genius 9230 points

Hi Apullin,

Isolation provides the benefits of surge protection, noise immunity, and provides safety from ESD shocks (isolates path to earth ground) which is especially important if your system has USB ports or connections to computer systems.

Using a flyback converter with a point of load buck converter or module on its output is very common when 2 output voltages are needed. You can go that route, or save some BOM cost & choose a transformer for the single output voltage you need.

We do not have an app note specifically about isolation, but we have these 3 app notes that can help paint a picture:
www.ti.com/.../slva233a.pdf
www.ti.com/.../slua736.pdf
www.ti.com/.../slua469.pdf

Regards,
Tom Amlee
PoE PD Apps Engineer

0 Thomas Amlee over 5 years ago in reply to Thomas Amlee

TI__Genius 9230 points

Hi Apullin,

To be clear, the larger step down ratio does not result in a larger transformer. In fact, the larger input current required by a lower input voltage (for the same output) would require a transformer that has a higher saturation current (larger footprint).

For example, for a 5V/2A output, would require 48V/208mA at the input versus 12V/833mA.

For further details, take a look at our PD flyback design guide, slva305c. Section 2.5 goes into detail about DC/DC & transformer calculations.

Thanks,
Tom Amlee
PoE PD Apps Engineer

0 Thomas Amlee over 5 years ago in reply to Thomas Amlee

TI__Genius 9230 points

Hi apullin,

I am now closing this thread. If you are still working this issue, feel free to reply. If you have found resolution to your question, please click the green "resolved" button.

If you have additional questions along the design process, create another thread. I would be happy to assist.

Thanks,
Tom Amlee
PoE PD Apps Engineer

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TPS2375: PoE PD solution with TPS2375 + LMR36015-Q1 (or similar, for either)?