Designing the PoE+ HAT for Raspberry Pi 5: compact, efficient power and networking

This is, once again, a machine transcription that has been edited by a human; if you spot anything that’s obviously off, drop us a note in the comments.

Eben 0:09: We’ve got another PoE HAT.

Dominic 0:10: We have, yes!

Eben 0:11: How many PoE HATs is this now?

Dominic 0:12: I think this is the third one, isn’t it? Yes.

Eben 0:14: Yes. So we had PoE, and then we had PoE+, and then we have —

Dominic 0:19: This one!

Eben 0:19: This new thing, which is not called PoE++; it’s called PoE+ for Raspberry Pi 5.

Dominic 0:25: Yes. Because PoE++ is a higher power standard.

Eben 0:28: — is another thing. Okay, fine. So this is a — to be clear, this is a PoE+, so the standard it implements is PoE+, which is the 25-ish watt?

Dominic 0:39: Yes.

Eben 0:39: — standard. And it’s intended for use with Raspberry Pi 5. Now, why did we need to do another PoE HAT?

Dominic 0:48: Well, Pi 5 introduces a lot of new things, and one of the things, to make board space in the right area, was the PoE connector — the little four-pin connector — got moved to the other edge of the board.

Eben 1:01: This was part of James’s plan to put the Ethernet jack in all possible positions on different versions of Raspberry Pi.

Dominic 1:10: I can’t say if that’s James’s plan or not. But that appears to be the starting point.

Eben 1:14: So we’ve had almost all of the four combinations, now, of MagJack at the top and bottom and PoE connector at the top and bottom; we’ve had three — across 3B+, 4 and 5, we’ve had three of the four possible options.

Dominic 1:29: We have. He’s not gonna go to the other edge of the board yet.

Eben 1:33: So there’s a simple mechanical reason, which is that that four-pin connector — what does the four-pin connector do?

Dominic 1:39: Aha! That takes the volts from the PoE — from your Ethernet cable, so it’s around 48V, and brings it out to those pins there then for the PoE HAT to take that, and convert it to the 5V, right?

Eben 1:54: So we have a PoE-capable MagJack?

Dominic 1:57: Yes.

Eben 1:57: What does that mean? That’s extra taps on the windings, is it?

Dominic 2:00: That is indeed extra taps on the primary windings. So this is non-isolated volts direct from the cable through the transformer — well not through the transformer, but it’s this tap on the transformer. And then those four pins go up to the PoE HAT, which then gives us the isolation required to generate then the 5V to power the Pi 5.

Eben 2:21: Right. So it provides isolation, it provides voltage conversion regulation, and it feeds 5V back into the GPIO connector in the top left-hand corner.

Dominic 2:32: Indeed so. Yes. And there’s one other thing it does, is all the PoE negotiation as well. So it will negotiate with the switch.

Eben 2:39: Okay. So you have to advertise, you have a PoE-capable switch or a midspan injector, and you have to advertise to that that you are PoE-capable before it will give you the volts.

Dominic 2:48: Yes. And then which power range you want as well.

Eben 2:51: In theory, provided it’s not a poor-quality injector that just gives you some volts anyway.

Dominic 2:53: Yes. But there are also the two power ranges, the 13W and the 25W power range, right, so then the Pi can detect which power range is available.

Eben 3:04: How does that signalling work?

Dominic 3:06: On the on the Ethernet cable?

Eben 3:08: Yes.

Dominic 3:09: It does it by — first of all, it looks for some capacitance on the cable to know that there is something there. And then —

Eben 3:16: The switch, this is the switch —

Dominic 3:18: Through the switch. Yes. And then it applies a voltage to the cable, to then try and detect a resistor of a particular value. And if it can detect that, then it knows which power range available. There are more advanced methods for the higher, for PoE++ that we were referring to, but we don’t do that, so no need to describe how all that magic works.

Eben 3:45: And there are PoE modes where you put the power on the spare pairs that are spare in some standards.

Dominic 3:53: Yes.

Eben 3:53: And then there are modes where you put it on the single pairs, and you have to support both of those.

Dominic 4:00: Yes, yep. So that’s why there are four pins. So each pair ends up with a pin. And because of crossover cables, you end up with each pin could be positive or negative. Officially, it should only be in pairs, but if things go wrong, it could be anywhere. So there’s a very fancy bridge rectifier at the front. It’s an active bridge we use —

Eben 4:24: So it’s not just four diodes in a little circle there.

Dominic 4:26: Well it would be eight diodes, because you’ve got four wires coming in.

Eben 4:30: Yes indeed.

Dominic 4:30: So it’d be eight diodes. And so we’ve got effectively —

Eben 4:33: And what would be the problem with them being diodes, actual diodes?

Dominic 4:36: It’s just power drop. And so you just — a diode, because these — you’ve got to withstand at about 100V, worst case peak energy, if there’s unplug events and things, overshoots and undershoots. So the diodes have got to be quite chunky diodes, and so you end up with .7 to 1V drop across each of the diodes, and you’ve got two of those, and you’re you’re taking over half an amp: that’s power. And power is heat. And so what we have is an active bridge, which is made up of little FETs in there, and that then makes the — they have Rds(on). So that’s I squared R losses at that point, but the R —

Eben 5:17: The R is tiny —

Dominic 5:18: It’s tiny compared —

Eben 5:19: Milliohms.

Dominic 5:20: So we’ve dropped — it becomes 90% efficient instead of… a lot less percent efficient.

Eben 5:28: Cool. Okay, so that’s PoE HATs, what they do. Are there any other — so let’s talk a little bit about the evolution from PoE HAT, to PoE+ HAT, to PoE+ HAT for Raspberry Pi 5: what were the technologies that were introduced in each of these — cause this isn’t just a mechanical thing, right? There’s also some new technology in this design. And there was some new technology in the PoE+ HAT originally. So what’s been introduced at each stage?

Dominic 5:55: So the first PoE HAT was only a 13W product, and that had a much more conventional transformer on there.

Eben 6:07: When I think about transformers, I think about a sort of a ferrite core with some windings —

Dominic 6:14: Indeed so.

Eben 6:15: — some wire wrapped round it by a machine and then clamped together, and then some wires coming out.

Dominic 6:19: Yep. And you can see that on the original one, and that was what’s known as a flyback converter. And then we moved, on the next one, the PoE+ HAT, we did two things: we changed to a forward converter, because forward converters are typically more efficient when you want the top end of the power range —

Eben 6:41: And that’s very much what PoE+ HAT was engineered for, right? PoE+ HAT is engineered to give you another, kind of, double the peak power, and to be most efficient at the top end of that range.

Dominic 6:52: Indeed so. So at just below 25 watts, well, the 20 to 20-odd watts range, it’s much more efficient than a flyback converter is. But we also changed the transformer technology. So we went from having a wire around the ferrite to using a PCB, which has turns in tracks on the PCB with a ferrite clamped over it. And that was a separate little board. And you can see that — that gets soldered in.

Eben 7:25: I remember the first time I saw one of those: I mean, it’s such a beautiful little space-age solid-state bit of stuff, right? It really does look beautiful.

Dominic 7:32: So… the winding density is less because you’ve got to have the PCB substrate, but you get other advantages. And so your skin effect — which is an effect when you have high frequencies — virtually disappears [inaudible] because you’ve got a lot of air, you’ve got a lot of surface area for your copper. And you can just thicken up the copper on the PCB. So if you need —

Eben 7:56: How many layers of PCB is that?

Dominic 7:58: I can’t remember —

Eben 7:59: It’s a lot.

Dominic 7:59: It’s probably eight, ten layers PCB. And what we’ve done now for the latest one is taken that, instead of having a separate PCB, we’ve now got one PCB, which then has the ferrite clamped on. And so that one PCB has the 40-pin connector on it, it has the four-pins power connector, and then also has the turns built in —

Eben 8:21: So we’ve built, rather than buying a transformer from somebody else, we’ve actually built our own transformer on our own PCB, and we just put a ferrite around it.

Dominic 8:21: And because of the various bits of optimisations we’ve been able to do, we’ve gone back to a flyback converter, but we’ve been able to keep a very good power efficiency on there. And that’s allowed us to reduce the board area of the amount of components on the board. So it’s now L-shaped.

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?

Dominic 9:05: Well, that now lets us — previous PoE HATs had a fan on board.

Eben 9:13: And that wasn’t for cooling the PoE HAT, right?

Dominic 9:14: No, that was for cooling the SoC.

Eben 9:17: Right. So it was a sort of bonus feature of the PoE HAT, was it also had a thermal solution for the CPU.

Dominic 9:23: Yes. And so it had this big hole in the middle of the PCB, so there was a lot of wastage of the PCB, because it just gets thrown away —

Eben 9:31: We were already almost an L-shaped PCB.

Dominic 9:34: Yes, but we needed the area. If you have a look at it, it is quite a dense board. But because we’ve been able to simplify some of the electronics now and integrate things more, especially around the area of the transformer, we’ve got this L-shaped board. And that then allows us to put it in the new Pi 5 case as well.

Eben 9:59: Which itself has a fan.

Dominic 10:01: And that has the fan. Yes. And so there’s been a lot of 3D work to make sure it all fits, and it’s all very compact, but you can now have a case with a built-in fan and a PoE solution, all nicely together.

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 —

Dominic 10:26: It’s more efficient at the lower power levels.

Eben 10:29: So we now have efficiency across the power consumption range, rather than concentrating that efficiency up at the top.

Dominic 10:35: Yes. So with Pi 5 being able to go more into sleep modes and things like that, and that’s then — is better for your green credentials, because you’re not then wasting so much power in the conversion. And that’s where a flyback converter is more efficient than the forward converter.

Eben 10:52: So we’ve gone back to a flyback converter, gets us to the low-end efficiency, we’ve done clever stuff to mitigate the nominal inefficiency of the flyback converter at high load, and then you get nice high efficiency all the way across the range.

Dominic 11:07: Yes.

Eben 11:08: Excellent. Well, that’s good news.

Dominic 11:10: Excellent. I hope so!

Eben 11:11: Yes. I’m looking forward to having a play with it.

Dominic 11:13: So am I.