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Hi Heath, all,
The goal of 10BASE-T1L + PoDL should be to maximize interoperability while offering the performance needed for as many applications as possible.
We need to answer two questions to come to a conclusion on your proposal.
1. Do we need a multi-voltage power standard ?
2. What is/are the right voltage(s) ?
With regard to (1), we can learn from other standards that multi-voltage power schemes tend to be far more complicated and fail to achieve wide interoperability.
Take for example USB-PD. This standard supports 3 voltage levels. Despite being released already years ago, the market traction is low.
In fact, this USB standard is eclipsed by a proprietary fast charging standard (you can all guess which one), that offers a simpler mechanism to achieve high charging speeds.
For a PD design, supporting multiple voltage levels creates quite a bit of extra complication and testing. The PD can choose to only support one of the voltage, but this then goes at the cost
of not working with all PSEs.
Similarly on the PSE side, a multi-voltage PSE is much more complicated than a single-volt one. Also power budgeting becomes more complicated.
I believe that interoperability and product simplicity is best served by choosing a single voltage to be supported.
That takes us to question (2)... which one ?
Higher voltages are (far) more efficient at transferring power.
For example, we need to deliver 8W over 100m of single-pair cable with AWG24.
One supply is 24V, the other is 53V.
The 24V supply will source 13.1W, with 5W dissipated in the cable.
The 53V supply will source 8.4W with 0.4W dissipated in the cable.
Both deliver 8W to the respective PD.
Additionally, 8W is about the limit that a 24V supply can deliver in a stable manner (for this cable), the 53V supply can go above 40W.
Very few electronic designs operate from 50V directly. We know this problem from PoE.
But converting from 50-60V down to 24, 12, or 5V is not a complicated or costly issue, many efficient switch mode designs exist that can do this.
Conclusion:
- Operating from a single voltage will greatly simplify PD and PSE design and support interoperability
- Picking the highest voltage range allows us to deliver the most power and deliver power FAR more efficiently than a lower supply voltage can
- It is not complicated to build PDs that need 24V internally to support a PoDL voltage of 50+V
Kind regards,
Lennart
On Wed, 2019-05-15 at 23:45 +0000, Stewart, Heath wrote:
All,
I wanted to open a discussion on the power class proposal referenced in i-321.
http://www.ieee802.org/3/cg/comments/Comment_i-321_Stewart_3cg_clause_104_modifications_v1.pdf
The proposal assigns class power voltage, current and power values to make required corrections to cable resistance and to accommodate the following use cases:
- 24V nom
- 50V max
- SELV max
All other changes derive from the above introduced values. The proposal attempts to cover all suggestions offered during a large number of discussion during and between cg sessions in the Vancouver meeting.
In the time since the proposal was submitted a couple of weeks ago I have collected a number of additional inputs from the cg task force. My goal in sending this to the reflector is to kickstart public and private discussions with the goal
of streamlining our efforts in cg next week.
It is worth pointing out the there are a number of possible end markets: eg building automation and industrial automation. We have gotten really good input from these end markets and strive to enable relevant cabling resistances while remaining
aware of economic and technical feasibility.
While process automation is an exciting market as well, efforts to marry PoDL to process automation Intrinsic Safety (IS) requirements stalled early on. Therefore process automation requirements are not directly considered in this PoDL
contribution.
The vast majority of the feedback from colleagues has regarded the 3 proposed voltage ranges. Mainly the comments address the appropriateness of supporting 1 vs 2 vs 3 voltage ranges.
Please feel free to reply on this thread or offline. I will attempt to come prepared to modify proposed text to reflect the will of the group.
Cheers,
Heath Stewart
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