Sorting out USB-C power supplies: Specification deceptions and confusing implementations

Upfront in late November 2023’s most recent edition of the “Holiday Shopping Guide for Engineers” series was my recommendation to pick up a recently-introduced Raspberry Pi 5. But here we are, two months later as I write these words, and the Raspberry Pi 5 is still essentially sold out (echoing, ironically, my commentary introducing that shopping guide section, wherein I documented the longstanding supply constraints of its Raspberry Pi 4 precursor). I know. In my defense, however weak, I’ll note that I did write those words 1.5 months earlier, in mid-October (that excuse didn’t work, did it?). That said, the Raspberry Pi Foundation swears that production will ramp dramatically very soon, with supply improving shortly thereafter. Will it? I don’t know.

I bet at least some of you think that I get “special treatment” with the tech companies in constrained-supply situations like these, don’t you? Ha! Just two weeks ago, I finally gave up waiting on retailer supply and purchased a brand-new 8 GB Raspberry Pi 5 board plus an official case from a guy on eBay. He said he’d accidentally bought two of each and didn’t need the spare combo. Whatever. I didn’t get reseller-marked-up too badly, compared to most of the ridiculous pricing I’m seeing on eBay and elsewhere right now. The 8 GB board MSRP is $80, while that of the case is $10. I paid $123.39 plus tax for the combo, which probably left him with a little (but only a little) profit after covering his hardware costs plus the tax and shipping (or gas) he paid.

Don’t get me started on the Active Cooler shown in the first photo, which, if I wasn’t such a trusting fellow, I might think it doesn’t actually exist. Regardless, I still needed a power supply. A 5 V/3 A supply with a USB-C output such as the Raspberry Pi 15W USB-C Power Supply (standard “kit” for the Raspberry Pi 4, for example) might also work for the Raspberry Pi 5, especially if you only boot off a SD card and don’t have a lot of hooked-up, power-sucking peripherals:

That said, the Raspberry Pi 5’s bootup code will still grumble at you via displayed messages indicating that “current draw to peripherals will be restricted to 600mA.” And if you want to boot off a USB flash stick instead, you’ll need to tweak the config.txt prose first. Don’t even think about trying to boot off the m.2 NVMe SSD HAT (speaking of suspect vaporware) with only a 15 W PSU. And in general, you and I both know that the very first things I’ll likely do when I fire up my board are to run lengthy benchmarks on it, constrain its ventilation flow and see when clock throttling kicks in, try overclocking it, and otherwise abuse it. So yeah…27 W (or more).

The Raspberry Pi 27 W USB-C Power Supply shown above, in its white color option (black is also available) and UK plug option (among several others also available), in all cases matching the variants available with its 15 W sibling, was one obvious candidate. But…I know this is going to surprise you…it’s also near-impossible to track down right now. No problem, I thought. I have a bunch of 30 W USB-C wall warts lying around; I’ll just use one of them. Which, more than 500 words in, is where today’s story really begins.

Problem #1 centers on the term “wall wart”. More accurately, as the Wirecutter points out, I should probably be calling them “chargers” because fundamentally that’s all they are: power sources for recharging the batteries integrated within various otherwise-untethered devices (laptops, smartphones, tablets, smartwatches, etc.). Can you not only recharge a widget’s integrated battery but also simultaneously power that widget from the same charger? Sure, if the output power is high enough to handle this simultaneous-energy multitasking.

But trying to run a non-battery-powered device from a charger can be a recipe for disaster, specifically when that charger’s output power is close to what the device demands (such as my suggested 30 W charger for a Raspberry Pi 5 that wants to suck 27 W). Why? Chargers aren’t exactly known for being predictable in output as the power demands of whatever’s on the other end of the USB-C (which I’m using as an example here, although the concept’s equally relevant to USB-A and other standards) cable increase. As you near supposed “30 W”, for example, the output voltage might sag or, at minimum, exhibit notable ripple. The output current might also droop. Not a huge deal if all you’re doing is recharging a battery; it’ll just take a little longer than it might otherwise. But try to directly power a Raspberry Pi 5 with one? Iceberg dead ahead!

About that “30 W” (Problem #2)…if the wall wart has only one output, you can safely surmise that you’ll get a reasonable facsimile of that power metric out of it. But what if there are two outputs? Or more? And what if you only tap into one of the outputs? Will you get the full spec’d power, or not? The answer is “it depends”, and unfortunately the vendors don’t make it easy to get more precise than that. Here’s an example: remember the 30 W single-port USB-C GaN charger that I dissected around a year ago? Well, VOLTME also makes a two-output 35 W model:

Kudos to the company, as this graphic shows:

When either output is used standalone, it delivers the full 35 W. Use both outputs at the same time, on the other hand, and each is capable of 18 W max. Intuitive, yes? Unfortunately, as far as I can tell, VOLTME’s the exception here, not the norm. Take, for example, the two-output 70 W Spigen GaN charger that I take with me on trips:

It’s smaller and lighter than the single-output conventional-circuitry charger that came with my MacBook Pro. It’s also got enough “umph” (and outputs) to juice up both my laptop and my iPad Pro. Plus, its AC prongs are collapsable; love ‘em when jamming the adapter in my bag. All good so far. But one of the outputs is only 60 W max when used standalone and only 50 W max when used in tandem with the other (20 W max). The more powerful output is the bottom of the two in the above photo. And it’s not marked as such on the front panel for differentiation purposes. Inevitably, in the absence of visual cues to the contrary, I end up plugging my laptop into the upper, weaker output port instead.

Problem #3, particularly for 5 V devices on the other end of the cable, involves inconsistent output power at various output voltages. Let’s look back at that 30 W VOLTME teardown again:

I’ve written (more accurately, I suppose, ranted) before about USB-PD (Power Delivery), which supports upfront negotiation between the “source” and “sink” on their respective voltage and current capabilities-and-requirements, leading to the potential for higher output power. Programmable power supply (PPS), an enhancement to USB PD 3.0, supports periodic renegotiation as, for example, a battery nears full charge. Quoting from a Belkin white paper on the topic:

Programmable Power Supply (PPS) is a standard that refers to the advanced charging technology for USB-C devices. It can modify in real time the voltage and current by feeding maximum power based on a device’s charging status. The USB Implementers Forum (USB-IF), a nonprofit group that supports the marketing and promotion of the Universal Serial Bus (USB), added PPS Fast Charging to the USB PD 3.0 standard in 2017. This allows data to be exchanged every 10 seconds, making a dynamic adjustment to the output voltage and current based on the condition of the receiving device’s specifications. PPS’ main advantage over other standards is its capability to lower conversion loss during charging. This means that less heat is generated, which lengthens the device battery’s lifespan.

I mention this because the above photo indicates that this charger support PPS. But let’s backtrack and focus on its supported USB-PD options. It’s a 30 W charger, right? Well:

  • 20 V x 1.5 A = 30 W
  • 15 V x 2 A = 30 W
  • 12 V x 2.5 A = 30 W

The next one isn’t exactly 30 W, but I’d argue that close still counts not only in horseshoes and hand grenades but also with inexpensive-but-still-impressive chargers:

  • 9 V x 3 A = 27 W

But what’s the deal with that last one?

  • 5 V x 3 A = 15 W

Hmmm…mebbe just a quirk of this particular charger? How about this big bad boy from Anker?

Single output. 100 W. Surely, it’ll pump out more than 3 A at 5 V, right? Nope:

  • 5 V x 3 A = 15 W
  • 9 V x 3 A = 27 W
  • 12 V x 3 A = 36 W
  • 15 V x 3 A = 45 W
  • 20 V x 5 A = 100 W

And just determining this information necessitated tedious searching for a user manual online at a third-party site. I couldn’t even find mention of the product (via either its 317 product code or A2672 model number) on the manufacturer’s own website! And at this point, I’ll cut to the chase: they’re pretty much all like this.

That a charger will only output 100 W to a device that indicates it can handle 20 V is no shortage of smoke and mirrors in and of itself. But I’m actually willing to give the charger suppliers at least something of a “pass” here. Consumers value not only output power but also size, weight, and the all-important price tag, among other things. These factors likely constrain per-port (if not per-device) output current to 5 A or so. If I’m a portable computer manufacturer and I need 100vW of input power to support not only AC-connected operation but also in-parallel battery recharge at a reasonable rate, I’m going to make darn sure my device can handle a 20 V input!

But what about this seeming 3 A limitation for the 5 V output option? It’s not universal, obviously, since the Raspberry Pi 27 W USB-C power supply supports the following options:

  • 1 V x 5 A = 25.5 W
  • 9 V x 3 A = 27 W
  • 12 V x 2.25 A = 27 W
  • 15 V x 1.8 A = 27 W

In contrast, BTW, the official Raspberry Pi 15 W USB-C power supply only does this:

  • 1 V x 3.0 A = 15.3 W

My guess as to the root cause of this 5 V@3 A preponderance comes from a clue in a post on the Electrical Engineering Stack Exchange site that I stumbled across while researching this writeup:

The question is about USB Type-C connectivity.

The Type-C connectivity provides two methods of determining source capability.

The primary method is the value of pull-up on HOST side on CC pins. Type-C specifications define three levels of capability: 500/900 mA (56k pull-up to 5V), 1.5 A (22k pull-up), and 3A (10k pull-up). The connecting device pulls down this with 5.1k to ground, and the resulting voltage level tells the device how much current it can take over the particular connection. When the host sees the pull-down, it will turn on “+5Vsafe” VBUS. This is per Type-C protocol.

The secondary method is provided by nearly independent Power Delivery specification. If the consumer implements PD, it still need to follow Type-C specifications for CC pull-up-down protocol, and will receive “+5Vsafe” VBUS.

Only then the consumer will send serial PD-defined messages over CC pin to discover source capabilities. If provider responds, then negotiations for power contract will proceed.

If the consumer is not PD-agnostic, no messages will be generated and no responses will be returned, and no contract will be negotiated. The link power will stay at the default “Safe+5VBUS” power schema, per DC levels on CC pins.

Here’s the irony…my Raspberry Pi 4 board that I mentioned earlier? It’s the rare, early “Model A” variant, which contained an insufficient number and types of resistors to work correctly with some USB-C cables. But that’s not what’s going on here. As the above explanation elucidates, USB-C chargers must (ideally) at minimum support 5 V@3 A for broadest device compatibility. What I’m guessing mostly happens beyond this point is that charger manufacturers focus their development attention on other voltage/current combinations enabled by the secondary compatibility negotiation, leaving the 5 V circuitry implementation well enough alone as-is.

Agree or disagree, readers? Anything more to add here? I look forward to your thoughts in the comments! Meanwhile, I have a Raspberry Pi 27 W USB-C power supply on order from an overseas supplier…and I wait…

Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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