ESP32 Passive PoE (Power over Ethernet) design with basic schematic

An ESP32 passive PoE (Power over Ethernet) powered circuit board can be very useful for industrial and commercial use. The power of passive PoE is apparent in industrial installations, especially those that are low power such as sensor and control nodes. Given that the usual DC voltage level used with PLC type systems and other standard industrial controllers is almost always less than 50V, it becomes easy to design very simple, custom ESP32 PoE systems that save you the hassle of routing power cables all over the place.

Passive PoE is very simple to work with and is not to be confused with active PoE, which involves smart power controllers and negotiation before powering up a PoE device.

Passive PoE vs Active PoE

A passive ESP32 PoE system basically runs a power cable inside the ethernet cable. The power cable can run any voltage or current that you feed in provided that the cable can sustain it electrically.

Therefore, Passive PoE = Usual ethernet cable + DC adapter cable running inside.

Active PoE designs involve power management ICs on both ends that only work when they have successfully negotiated the power delivery. The power cable that is physically present inside the ethernet cable is still the same as that with passive PoE systems.

Active ESP32 PoE systems would use the popular PoE standards 802.3af or 802.3at. The power delivery and negotiation is managed by dedicated PMIC chips and not by the ESP32.

Ethernet Connector and Cabling for Passive PoE

As mentioned before, ethernet cables contain physical conductors inside when they support PoE. Power delivery happens over specific pins of the ethernet connector and these pins are not usually exposed when a regular ethernet cable or connector is used.

Here is an example of what a PoE ethernet connector from Hanrun looks like when it has support for PoE power delivery twisted pairs.

Make sure you pick an appropriate connector! Using a non-PoE ethernet connector will not work. Also, a PoE connector can accept a non-PoE cable for ethernet-only use.

If you compare this connector to another ethernet connector or jack with built-in magnetics and LEDs, you will notice that the regular ethernet connectors will lack the PoE pins highlighted in red above. Those connectors do not expose the power conductors, nor do they contain rectifier diodes within the connector that help with polarity protection in a PoE system.

Passive PoE System - How It Works

An ESP32 passive PoE system design would contain the following main circuit components:

A PoE compatible ethernet jack or connector, preferably with built-in rectifiers
Ethernet PHY with LAN8720A connected to the ESP32
Input filtering circuits with spike protection and for PoE DC power smoothing
A buck converter for stepping down the PoE cable voltage to 5V or 3.3V as needed
ESP32 itself, and any other peripherals that you want on the system

Passive PoE has no safeguards! Passive PoE systems must be able to operate using the voltage that the injector supplies (usually 48V or lower). If not, a good design will try to shut down gracefully rather than explode. Based on the budget and design requirements, it is up to the designer to figure this out.

A simple buck converter is usually more than enough for a passive PoE system. For example, here are the absolute maximum ratings for buck converters like SY8401 and SY8291 from Sylergy. These buck converters operate from very high input voltage of over 40 volts and are suitable for using in robust passive PoE applications.

The peak current output of buck converters like this is enough to sustain the spikes that ESP32 causes during WiFi transmission (we have tested this).

Passive PoE Power Block Schematics

Finally, here is the schematic for a very simple DC-DC converter regulator block that steps down the PoE DC voltage to 5V. The 5V can then be fed into the ESP32 via another higher frequency buck converter or a simple LDO.
If you have no use for 5V in your design, you might as well step the PoE input down to 3.3V directly.

Transient response matters! It is always a good idea to pick a high frequency buck converter for stepping down the PoE DC voltage to ESP32 power supply levels. A faster buck converter means smaller size and faster response to load transients caused by ESP32!

Note that the above circuit only displays a simple view of the passive PoE step-down converter that can handle ESP32 consumption requirements.

To make the whole ESP32 passive PoE system work, you will need to add:

ESP32 ethernet PHY interface using LAN8720 or similar
More smoothing, if high noise or ripple is expected on the power line (optional)

Impact on ESP32 Ethernet Firmware

Adding PoE, either active or passive, does not involve the ESP32 in any way. The ESP32 will not have to control anything additional and thus no firmware change is necessary.
That being said, some active PoE power management chips might need configuration via IOs or I2C. However, that is not very likely unless you are building very complex or high power systems.

Practical Power Delivery Limitations

Unlike IEEE 802.1af, there is no negotiation mechanism involved in passive PoE systems. Therefore, the passive PoE system is simply limited by the sourcing limit of the PoE power source and the voltage drop or cable resistance that connects the PoE device to the PoE power source.

However, when designing PoE hardware for the ESP32, I noticed a bottleneck that is important to pay attention to. In the PoE jack internal schematics shown above, there is a pair of rectifiers on the power lines.

This pair of rectifiers may not be rated for high current even though your ethernet cable and electronics can handle high current. The internal magjack rectifiers are always rated for 802.3af compliant levels.

Therefore, for “non-standard” applications where the PoE voltage and current draw are higher than active PoE would have, it is best to use magjacks with no internal rectification. One such jack is shown below.

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Change Log

01 April 2021
– Initial Release
06 May 2021
– Added section “Practical Power Delivery Limitations”

References

Reference 1: SY8401 Datasheet
Reference 2: Adafruit Passive PoE Injector
Reference 3: Adafruit Active PoE Injector