Based on my current experience with PoE, the following are some concerns I need to tackle in order to create a good but low cost PoE system. I would love to get input from experts in the field and potential users as to what they would like to see!
In my recent customer-specific design, the most costly components from high to low were:
- ESP-WROOM-32 module. I'm sure I could get the cost down if I went from module to bare chip on board, but at this point, I really don't want to deal with the intentional radiator concerns involved in case I end up selling this, so I'm sticking with a module.
- The MagJack Ethernet connector. I was surprised how expensive this was. To be compatible with PoE, you can't just use any random connector, it needs to have specific signals routed out which are usually omitted in low cost Ethernet jacks. Another possibility is to have separate connector and magnetics, but this tends to take more space and I'm not sure at this point if it would save any money. More research is needed.
- Flyback transformer. Ethernet provides magnetic coupling of signals to ensure galvanic isolation, which is needed when connecting devices across long distances. To preserve galvanic isolation of the complete system, the power system requires galvanic isolation as well. This increases cost compared to a non-isolated system. I have read that if a system has no user-touchable metal parts and doesn't connect to anything locally, isolation can be omitted. This would be fine for simple sensor nodes for instance. But I'm reluctant to do this for a generic development board that will be used in who knows which way. I would love to get feedback on this though.
- PoE-PD controller. A specific chip to correctly interface with the 802.3af source equipment. Since PoE uses a nominal voltage of 48V (up to 57V), these chips and the surrounding components should be rated to handle up to 100V, so they tend to be spendy. Unavoidable, but choosing a good part that integrates a lot of the surrounding devices like diode bridges and TVS plus integrates the PWM power controller can reduce the overall system cost and size.
- Ethernet Phy. The ESP32 integrates the Ethernet MAC but an external Phy is required. Unfortunately there is some fun interference going on between pin uses because GPIO0 is used both for boot selection and during programming, and as clock input for the MAC. I'm using the Olimex ESP32 Gateway as a reference for how to deal with this.
Questions to be answered
I need to figure out the following and would love to get input from the community (yes that's YOU :)):
- Which is more important: low cost or isolation? I'll try to reduce cost either way, but an isolated supply is going to cost more and be larger than non-isolated.
- Anyone know a good source for low cost flyback transformers? Low cost means < $1.50 in quantities of 100+ in this case.
- Anyone know a good source for low cost Ethernet Magjacks with PoE connections? Low cost means < $2.50 in quantities of 100+ in this case.
- Availability of higher power output will increase cost. How much power is reasonable? I don't think it makes sense to burden most users with extra cost because someone wants a lot of power, but having some idea of what others find reasonable would be nice.
- The board will likely have 5V and 3.3V power available. 12V and 3.3V is another option. Any preference either way?
- I like using the CH340G as USB UART bridge because it works well for me and has an easy leaded package, but it has been brought to my attention that there are driver issues with it on Macs. Can anyone comment on this? How problematic is it? Is there a solution that works on the most recent OS?
- In general, I'd like to hear: If a thing like this existed, what would you use it for? Answering this question will go a long way in helping me answer the rest. :)
- Related to the above: does something like this exist already and have I somehow missed it? Can you point me to it? Thanks. :)