TPS23730: Non-isolated PoE Pd

Hey David, 

Yes it is possible to use the TPS23730 in a non-isolated design, but it is not typically used here. At this time we don't have any designs, but it can used in a buck. The second gate drive will have to be disabled, and use the feedback directly. You can look at some of our buck designs with the TPS23755 or TPS23753A. 

More typically customers use a stand alone PD (TPS2372), and then pair it with a buck controller (LMxxx). Check out the TPS2372-3EVM-757 for an example. 

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Regards, 

Michael P.

Applications Engineer

Texas Instruments 

Hello, 

1. Yes the TPS23730 has a DCDC controller integrated into it, so there are a number of features associated with it. Comparing the TPS23730 and TPS2372 is not a fair comparison. 

Yes I know you cannot use these parts, I mentioned them as examples of a PD + PWM device used in a buck. 

2. It is hard to speculate because efficiency is dependent on every component choice. A synchronous buck controller requires a PWM square wave to control the operation --- the LM3650 does this as its only function, and the TPS23730 does this as well as the PoE negotiation. 

3. Yes, DCDC isolation is not part of the IEEE standard. 

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

Regards, 

Michael P.

Applications Engineer

Texas Instruments 

Hey David, 

For TPS2372 vs TPS2373, I would recommend the TPS2372 for a buck. The TPS2372 does not have advanced startup -- this feature is optimized for flybacks and ACF's, and would need to be disabled if used with a buck. So the TPS2373 is extra cost and extra parts for a feature you're not using. So we recommend TPS2372. 

The TPS23730 has a programmable UVLO for shutdown, and not all DCDC controllers have that. The TPS23730 also has SSFD (dithering), which is part of the DCDC but not all controllers have it. 

The TPS23730 can also present the TPH/TPL signal in series or parallel, and the TPS2373/TPS2372 can only send it in parallel. 

The advanced startup and hiccup features are for the DCDC converter portion. The TPS2373 /TPS2372 does have overcurrent feature, but it is for total power through the input. The TPS23730 is monitoring both this current and the primary DCDC current, so it just depends on the buck controller you choose. 

But as I go through our reference designs, most of them are standalone PD's and then buck controllers for non-isolated designs.  I think the nice thing here is you can basically pick the TPS2372-4EVM-006 and copy it directly, and that choice is independent of the buck. You can work with our buck controller team to find the best buck solution, and just stick it on the output of the TPS2372-4EVM-006. So hopefully it is more plug & play. I think that is more a typical trend in PoE, because as I said, bucks are typically low cost designs, so the attention to DCDC design details is usually not the focus vs cost down + size reduction. 

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

Regards, 

Michael P.

Applications Engineer

Texas Instruments 

Hey David, 

Our guidance is that if the end user has access to the ground plane or if there is any exposed metal, the design should be isolated. For example, a USB port. 

You can have a non-isolated DCDC and still isolate your USB port, so the standard is not forcing you to use a flyback. That's what I was trying to say, is that the standard does not force the DCDC topology to be isolated. However, if you need isolation because of other factors, might as well make the DCDC isolated since that will probably deliver the overall best solution. Most designs are isolated. 

If I were designing the product, with the given grounding scheme, I would isolate it. To comply with the standard, the metal casing would need to be isolated from the PoE input, and if the DCDC isn't isolating it then you have to come up with another way, and personally I can't think of anything cost effective. The only real benefit of the buck here is board space / height. but an isolated DCDC will give you better thermals and better efficiency, so to me the trade off is worth it. 

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

Regards, 

Michael P.

Applications Engineer

Texas Instruments 

David, 

1.The standard you posted is the IEEE802.3.bt standard. 

d_zero said:

You stated  previously " Yes, DCDC isolation is not part of the IEEE standard. " while you collage provided this from the standard

So

2a.) For 5V/50W output, our TIDA-050045 gets 94% efficiency on the converter. I don't know of a 50W buck close to this. Bucks are typically in the mid to high 80% range. 

I am not sure why you couldn't do 48V-5V with an isolated solution, and 48V to 120V with a non-isolated solution?

b.) for smaller space transformers you could look at planar transformers. They are more expensive but smaller. 

3.) yes seems fine. If you are wanting to ensure your design is IEEE802.3.bt compliant you can get it Ethernet Alliance Certified: 

https://training.ti.com/zh-tw/introduction-poe-certification-program 

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

Regards, 

Michael P.

Applications Engineer

Texas Instruments 

Hey David, 

1. if you only need 5W, then maybe a flyback would be better. I assumed you needed this power level based on the quote below. 

d_zero said:

We can't use TPS23755 or TPS23753A because we need PoE PD  802.3bt Type 3 Class4 40W

the 120V could be isolated / floating from the 5V plane. Ultimately you know the grounding scheme better than I do, so it is Up to you on how you want to proceed with cost and the isolation requirement. 

3. Appears acceptable but I cannot speak on their behalf. 

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

Regards, 

Michael P.

Applications Engineer

Texas Instruments 

Hey David, 

I think the best way to get into contact is through their website: https://ethernetalliance.org/poecert/ 

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

Regards, 

Michael P.

Applications Engineer

Texas Instruments