There is no startup current limiting, it almost acts like a direct short from Vin to Vout at startup. Some even overshoots the target voltage by a considerable amount. I can think of one explanation that satisfies all the observations. The original HAT’s hottest point is between the HAT and the Pi itself. This is observable in the EEVBlog video linked above. I tested with the HATs installed on the Pis, making it essentially impossible to get a reading on the underside. Setting that explanation aside, my measurements indicated that the original HAT got very hot at higher power outputs, while the PoE+ HAT stayed quite stable. Above 7 watts of power output, the new HAT ran cooler as per my measurements. Eben 8:47: So this thing is a — I think there’ve been a lot of debates as to whether it’s a compliant, er — is it a compliant HAT? Is there a requirement that a HAT has PCB in all of the places that a HAT can have PCB? But yeah, so this thing is L-shaped; now what does that let us do?

Only power with PoE HAT works fine, RPI is working fine (no sudden restarts or anything like that), just eth0 link does not work. I would have loved to see an official solution to make the GPIO pins accessible with the HAT on, and not a inelegant solution like using those hokey pin extenders that were recommended for the original PoE HAT. Are we foiled, then? Nope. You see, there’s a good 1/4 inch of GPIO pin visible between the Pi and the HAT. It’s just enough room for a good old fashioned wire-wrapped connection, along with some solder for safety. Eben 3:45: And there are PoE modes where you put the power on the spare pairs that are spare in some standards. Nothing wrong with what you have done, and I'm not criticising, just trying to get a feel for how closely what you measure may be with reality.Pull evenly so that it detaches from all the pins at the same rate; do not pull one end of the connector off before the other

Unfortunately none of the published schematics from B+ onwards show any of the USB interfacing so you will likely need assistance from someone with access to the 'secret data' to pursue this further. Eben 10:15: That’s very neat! Good. And how much more efficient is it than the previous design? Is it more efficient than the PoE+ design? Or is it —

Running the HAT - Performance

our board does not have any EPROM id because we want everybody to put their proper HATs on top of our shield The HAT is a compact, single-sided board that sits within the footprint of the Raspberry Pi. It will fit comfortably inside an official Raspberry Pi case. A small (25mm) fan is pre-installed on the board. We see the product as a useful component for people building systems that may be in tougher environments, so the addition of the fan helps with cooling. The fan is controlled over I2C via a small ATMEL processor which allows for it to be temperature-controlled: when your Raspberry Pi processor hits certain temperatures, the fan will be turned on to cool it down. To enable this you will need to get the latest firmware (sudo rpi-update). What are you using to meausure voltages and currents ? Is it calibrated, professional equipment or just a cheapo voltage / current monitor ?