Schlagwort: power

  • Anatomy of a product quality issue: PoE HAT

    Anatomy of a product quality issue: PoE HAT

    Reading Time: 5 minutes

    One of the neat new features of the Raspberry Pi 3 Model B+ is its support for IEEE 802.3af Power-over-Ethernet (PoE). This standard allows up to 13W of power to be delivered over the twisted pairs in an Ethernet cable without interfering with the transmission of data. The Raspberry Pi board itself provides a PoE-capable Ethernet jack and circuit protection components; the power regulation electronics, which would be too costly and bulky to include on the main board, live on a separate HAT.

    Raspberry Pi PoE HAT Power over ethernet

    The Raspberry Pi 3B+ wearing a PoE HAT

    When we announced the 3B+, we revealed that an official Raspberry Pi PoE HAT was in the works and, after a few unforeseen production delays, we we released this HAT at the end of August. Feedback was, and remains, generally very positive; but fairly quickly, we started to see some reports from users who were experiencing issues.

    The problem

    The problem they reported was this: when powering certain Raspberry Pi units via the PoE HAT, it was not possible to draw the full rated current from the USB ports.

    Our 5V USB output, denoted VBUS, is fed by the main 5V rail via a current-limiting switch. This switch is designed to protect the system by detecting short-circuit, over-current, or reverse-voltage events, and disconnecting the USB ports in response. Our current-limiting switch is set to a limit of just over 1A.

    Despite the PoE HAT’s ability to supply up to 2.5A, the experiments we ran in response to the reports suggested that, when it was used to supply some boards, the USB supply would trip out at a much lower current. Mice and keyboards worked fine, but higher-current devices such as wireless dongles and hard disks would fail.

    Our initial theory was that the PoE HAT was injecting noise into the Pi via the 5V rail, and that this was somehow upsetting the switch. However, we were able to rule this out, since we found no evidence of high-frequency noise at the input to the switch. Another theory was that the flyback transformer’s close physical proximity to the switch was somehow coupling noise in. But we were able to rule this out as well: we showed that the behaviour persisted when the HAT was connected using a right-angle header, which moves the power electronics away from the Raspberry Pi.

    What was happening?

    The PoE HAT works by converting the incoming 48V from the Ethernet lines to 5V using a flyback transformer. In simple terms, the primary side of the transformer is switched across the 48V, and energy is stored in the transformer in the form of a magnetic field. The primary is then disconnected and the magnetic field collapses. This changing magnetic field induces a voltage (scaled based on the transformer turns ratio) in the secondary, which is rectified by a schottky diode and output capacitance. This output capacitance is formed from the output capacitors on the PoE HAT itself, the capacitors on the Raspberry Pi 5V rail, and, when the switch is on, the VBUS reservoir capacitors.

    The switching frequency of the flyback transformer is relatively low (~100 kHz). This means that when the system is under load, each switching cycle must transfer a relatively large amount of energy. During each cycle, the 5V rail is discharged according to the load on the system, and charged up again by the flyback’s secondary, dumping more energy into the caps. In each cycle, a spike of high current is pushed through the output diode into the capacitors.

    To cut a long story short, putting a current probe on the input to switch showed large current spikes, as energy from the flyback made its way into the VBUS reservoir capacitors. This was expected. However, it turned out that the switch was erroneously registering these spikes as true over-current events. The switch is supposed to have a filter that allows it to ignore brief spikes, but we discovered that only one of the two approved versions of the switch did this correctly.

    Current into switch (yellow) and VBUS voltage (blue)

    If it’s not been tested, it’s broken

    It’s a truism that if you don’t test an aspect of a design, it will certainly be broken. Those of us with a Broadcom background sometimes refer to this as Alan Morgan’s rule, after its most enthusiastic proponent.

    Extensive testing over all configurations, operating parameters, and use cases is the only way to minimise the likelihood of releasing a product with a hardware issue. Even relatively simple hardware can end up catching you out by throwing up some unexpected bug or issue. And even the big guys with huge development teams and test labs occasionally mess things up — anyone remember the Pentium FDIV bug?

    We made several mistakes with the first version of the PoE HAT:

    • USB load testing was performed using boards that had the working switch
    • Our field testing programme was abbreviated because the product was late
    • We didn’t inquire as to whether our field testers were using high-current peripherals (they weren’t)

    It’s embarrassing to have released a product with a bug like this, but it’s a lesson well-learned, and we will be improving our internal processes to prevent a recurrence.

    The solution

    Fortunately, this bug turned out to be easy to fix. We designed an L-C filter to apply further smoothing to the output current from the HAT. The filter consists of a little extra input and output capacitance and a 4.7µH inductor (chosen to have a suitable current rating and DC resistance), as well as 330mR resistor in parallel to provide damping. We were even able to wrap the mod up in a little mezzanine PCB that fits neatly underneath the board.

    The original, un-modded board

    Hand-modded board with L-C filter

    Final board with mezzanine

    Once we had confirmed that there was a problem with the PoE HAT, we took the product off sale, and recalled and reworked the outstanding units. We are now happy to announce that most Approved Resellers should now have the revised boards in stock. We believe that most people who have been affected by this issue have already returned their PoE HATs for a refund; if you’re experiencing issues and haven’t yet returned your product, you can get in touch with your reseller to arrange a replacement.

    I’d like to thank the members of the Raspberry Pi engineering team, our contract manufacturing partners Taijie, our licensee partners and Approved Resellers, and also the community members who kindly tested prototypes of the fixed board design. This hasn’t been the easiest product launch in our history, but hopefully the lessons learned have set us up well for the future.

    Website: LINK

  • More power to your Pi

    More power to your Pi

    Reading Time: 9 minutes

    It’s been just over three weeks since we launched the new Raspberry Pi 3 Model B+. Although the product is branded Raspberry Pi 3B+ and not Raspberry Pi 4, a serious amount of engineering was involved in creating it. The wireless networking, USB/Ethernet hub, on-board power supplies, and BCM2837 chip were all upgraded: together these represent almost all the circuitry on the board! Today, I’d like to tell you about the work that has gone into creating a custom power supply chip for our newest computer.

    Raspberry Pi 3 Model B+, with custome power supply chip

    The new Raspberry Pi 3B+, sporting a new, custom power supply chip (bottom left-hand corner)

    Successful launch

    The Raspberry Pi 3B+ has been well received, and we’ve enjoyed hearing feedback from the community as well as reading the various reviews and articles highlighting the solid improvements in wireless networking, Ethernet, CPU, and thermal performance of the new board. Gareth Halfacree’s post here has some particularly nice graphs showing the increased performance as well as how the Pi 3B+ keeps cool under load due to the new CPU package that incorporates a metal heat spreader. The Raspberry Pi production lines at the Sony UK Technology Centre are running at full speed, and it seems most people who want to get hold of the new board are able to find one in stock.

    Powering your Pi

    One of the most critical but often under-appreciated elements of any electronic product, particularly one such as Raspberry Pi with lots of complex on-board silicon (processor, networking, high-speed memory), is the power supply. In fact, the Raspberry Pi 3B+ has no fewer than six different voltage rails: two at 3.3V — one special ‘quiet’ one for audio, and one for everything else; 1.8V; 1.2V for the LPDDR2 memory; and 1.2V nominal for the CPU core. Note that the CPU voltage is actually raised and lowered on the fly as the speed of the CPU is increased and decreased depending on how hard the it is working. The sixth rail is 5V, which is the master supply that all the others are created from, and the output voltage for the four downstream USB ports; this is what the mains power adaptor is supplying through the micro USB power connector.

    Power supply primer

    There are two common classes of power supply circuits: linear regulators and switching regulators. Linear regulators work by creating a lower, regulated voltage from a higher one. In simple terms, they monitor the output voltage against an internally generated reference and continually change their own resistance to keep the output voltage constant. Switching regulators work in a different way: they ‘pump’ energy by first storing the energy coming from the source supply in a reactive component (usually an inductor, sometimes a capacitor) and then releasing it to the regulated output supply. The switches in switching regulators effect this energy transfer by first connecting the inductor (or capacitor) to store the source energy, and then switching the circuit so the energy is released to its destination.

    Linear regulators produce smoother, less noisy output voltages, but they can only convert to a lower voltage, and have to dissipate energy to do so. The higher the output current and the voltage difference across them is, the more energy is lost as heat. On the other hand, switching supplies can, depending on their design, convert any voltage to any other voltage and can be much more efficient (efficiencies of 90% and above are not uncommon). However, they are more complex and generate noisier output voltages.

    Designers use both types of regulators depending on the needs of the downstream circuit: for low-voltage drops, low current, or low noise, linear regulators are usually the right choice, while switching regulators are used for higher power or when efficiency of conversion is required. One of the simplest switching-mode power supply circuits is the buck converter, used to create a lower voltage from a higher one, and this is what we use on the Pi.

    A history lesson

    The BCM2835 processor chip (found on the original Raspberry Pi Model B and B+, as well as on the Zero products) has on-chip power supplies: one switch-mode regulator for the core voltage, as well as a linear one for the LPDDR2 memory supply. This meant that in addition to 5V, we only had to provide 3.3V and 1.8V on the board, which was relatively simple to do using cheap, off-the-shelf parts.

    Pi Zero sporting a BCM2835 processor which only needs 2 external switchers (the components clustered behind the camera port)

    When we moved to the BCM2836 for Raspberry Pi Model 2 (and subsequently to the BCM2837A1 and B0 for Raspberry Pi 3B and 3B+), the core supply and the on-chip LPDDR2 memory supply were not up to the job of supplying the extra processor cores and larger memory, so we removed them. (We also used the recovered chip area to help fit in the new quad-core ARM processors.) The upshot of this was that we had to supply these power rails externally for the Raspberry Pi 2 and models thereafter. Moreover, we also had to provide circuitry to sequence them correctly in order to control exactly when they power up compared to the other supplies on the board.

    Power supply design is tricky (but critical)

    Raspberry Pi boards take in 5V from the micro USB socket and have to generate the other required supplies from this. When 5V is first connected, each of these other supplies must ‘start up’, meaning go from ‘off’, or 0V, to their correct voltage in some short period of time. The order of the supplies starting up is often important: commonly, there are structures inside a chip that form diodes between supply rails, and bringing supplies up in the wrong order can sometimes ‘turn on’ these diodes, causing them to conduct, with undesirable consequences. Silicon chips come with a data sheet specifying what supplies (voltages and currents) are needed and whether they need to be low-noise, in what order they must power up (and in some cases down), and sometimes even the rate at which the voltages must power up and down.

    A Pi3. Power supply components are clustered bottom left next to the micro USB, middle (above LPDDR2 chip which is on the bottom of the PCB) and above the A/V jack.

    In designing the power chain for the Pi 2 and 3, the sequencing was fairly straightforward: power rails power up in order of voltage (5V, 3.3V, 1.8V, 1.2V). However, the supplies were all generated with individual, discrete devices. Therefore, I spent quite a lot of time designing circuitry to control the sequencing — even with some design tricks to reduce component count, quite a few sequencing components are required. More complex systems generally use a Power Management Integrated Circuit (PMIC) with multiple supplies on a single chip, and many different PMIC variants are made by various manufacturers. Since Raspberry Pi 2 days, I was looking for a suitable PMIC to simplify the Pi design, but invariably (and somewhat counter-intuitively) these were always too expensive compared to my discrete solution, usually because they came with more features than needed.

    One device to rule them all

    It was way back in May 2015 when I first chatted to Peter Coyle of Exar (Exar were bought by MaxLinear in 2017) about power supply products for Raspberry Pi. We didn’t find a product match then, but in June 2016 Peter, along with Tuomas Hollman and Trevor Latham, visited to pitch the possibility of building a custom power management solution for us.

    I was initially sceptical that it could be made cheap enough. However, our discussion indicated that if we could tailor the solution to just what we needed, it could be cost-effective. Over the coming weeks and months, we honed a specification we agreed on from the initial sketches we’d made, and Exar thought they could build it for us at the target price.

    The chip we designed would contain all the key supplies required for the Pi on one small device in a cheap QFN package, and it would also perform the required sequencing and voltage monitoring. Moreover, the chip would be flexible to allow adjustment of supply voltages from their default values via I2C; the largest supply would be capable of being adjusted quickly to perform the dynamic core voltage changes needed in order to reduce voltage to the processor when it is idling (to save power), and to boost voltage to the processor when running at maximum speed (1.4 GHz). The supplies on the chip would all be generously specified and could deliver significantly more power than those used on the Raspberry Pi 3. All in all, the chip would contain four switching-mode converters and one low-current linear regulator, this last one being low-noise for the audio circuitry.

    The MXL7704 chip

    The project was a great success: MaxLinear delivered working samples of first silicon at the end of May 2017 (almost exactly a year after we had kicked off the project), and followed through with production quantities in December 2017 in time for the Raspberry Pi 3B+ production ramp.

    The team behind the power supply chip on the Raspberry Pi 3 Model B+ (group of six men, two of whom are holding Raspberry Pi boards)

    Front row: Roger with the very first Pi 3B+ prototypes and James with a MXL7704 development board hacked to power a Pi 3. Back row left to right: Will Torgerson, Trevor Latham, Peter Coyle, Tuomas Hollman.

    The MXL7704 device has been key to reducing Pi board complexity and therefore overall bill of materials cost. Furthermore, by being able to deliver more power when needed, it has also been essential to increasing the speed of the (newly packaged) BCM2837B0 processor on the 3B+ to 1.4GHz. The result is improvements to both the continuous output current to the CPU (from 3A to 4A) and to the transient performance (i.e. the chip has helped to reduce the ‘transient response’, which is the change in supply voltage due to a sudden current spike that occurs when the processor suddenly demands a large current in a few nanoseconds, as modern CPUs tend to do).

    With the MXL7704, the power supply circuitry on the 3B+ is now a lot simpler than the Pi 3B design. This new supply also provides the LPDDR2 memory voltage directly from a switching regulator rather than using linear regulators like the Pi 3, thereby improving energy efficiency. This helps to somewhat offset the extra power that the faster Ethernet, wireless networking, and processor consume. A pleasing side effect of using the new chip is the symmetric board layout of the regulators — it’s easy to see the four switching-mode supplies, given away by four similar-looking blobs (three grey and one brownish), which are the inductors.

    Close-up of the power supply chip on the Raspberry Pi 3 Model B+

    The Pi 3B+ PMIC MXL7704 — pleasingly symmetric

    Kudos

    It takes a lot of effort to design a new chip from scratch and get it all the way through to production — we are very grateful to the team at MaxLinear for their hard work, dedication, and enthusiasm. We’re also proud to have created something that will not only power Raspberry Pis, but will also be useful for other product designs: it turns out when you have a low-cost and flexible device, it can be used for many things — something we’re fairly familiar with here at Raspberry Pi! For the curious, the product page (including the data sheet) for the MXL7704 chip is here. Particular thanks go to Peter Coyle, Tuomas Hollman, and Trevor Latham, and also to Jon Cronk, who has been our contact in the US and has had to get up early to attend all our conference calls!

    The MXL7704 design team celebrating on Pi Day — it takes a lot of people to design a chip!

    I hope you liked reading about some of the effort that has gone into creating the new Pi. It’s nice to finally have a chance to tell people about some of the (increasingly complex) technical work that makes building a $35 computer possible — we’re very pleased with the Raspberry Pi 3B+, and we hope you enjoy using it as much as we’ve enjoyed creating it!

    Website: LINK