USB current capability clusterf**** - Page 1
Forum section: electronics / projects

author: poorchava
author_rank: Super Contributor
timestamp: November 12, 2020, 09:55:39 pm
content: Hi, I'm working on a gizmo, which needs about 11...12W of power. I need this to charge via some form of USB without using additional chip to do the power negotiation. I've done some research and digging through tons of different specs and this is what I came up with:USB-C source may be either 1.5A or 3A, depending on pullup on CC lines. In this case sink must have 5k1 pulldowns on CC linesand some way of checking if the source is capable of sourcing 3A. Requires real USB-C, as opposed to ubiquitous chargers that have USB-A -> USB-C cables. Those won't work since USB-A doen't have CC pins.USB-B connectors have BC1.2, which means that if charger has D+ and D- shorted, it can source up to 1.5A or up to 2.4A. I do have a chip, which can test for this and recognize a DCP from a normal USB that provides 500mA.There are multitude of powerbanks, car chargers, cables, etc, all marketed as 3A or 3.1A. They are predominantly usb-micro. This would fit the bill, but I've failed to find the actual spec, which mandates 15W over micro-usb.I have a hard time believing, that a micro-usb can handle 3A of current... Most connectors from brand name suppliers quote only 1A as rated current.Can anyone shed some light onto this? I essentially need my gizmo that consumes up to 10W to work from USB cable, when the built-in 10Ah 1S LiIon pack runs dry. I want to base the solution on MP2632B ASSP, which also seems to fit the bill of drawing 3A from a 5V USB port.

author: ve7xen
author_rank: Super Contributor
timestamp: November 12, 2020, 11:20:09 pm
content: I don't think new devices should be using USB-micro. Everything new is USB-C, as it should be. And it's the only way to do what you want in a standards-compliant manner.The simplest standards compliant way would be to use a USB-C connector with the 5.1k resistors. You should also measure the voltage across the resistors to determine if the power source is capable of supplying 3A or not. This is simple and futureproof, you won't need any special connectors, and it doesn't require any more chips or software.Otherwise I think you will need something like Qualcomm QuickCharge to negotiate a higher Vbus with the power source and maybe a new DC-DC converter to handle that. IMO it is less likely your users have a 3A USB-A power source than a USB-C one; I think all the high current ones I have ever seen say 2.4A. Like you, I can't find any connectors quoting more than 1.8A on the power contacts, so I don't know where they're even sourcing connectors for this. I wouldn't design a device around the assumption that the user is able to supply a 3A USB-A power supply, especially if you then need to support it.

author: poorchava
author_rank: Super Contributor
timestamp: November 13, 2020, 07:19:54 am
content: The chargers are actually everywhere. First example:https://www.amazon.com/AUKEY-Charger-Compatible-Qualcomm-Certified/dp/B018RR30TKThe QC thing is irrelevant, but the connector does have to support 3A of current. Some "reputable" (in the mobile market...) companies like Aukey, Anker and such also have 3A Micro-USB cables.QC is impossible to do. The source sice ICs can be had easily, but the device side negotiation is embedded in Snapdragon SoCs. Some 1 year ago I've looked into it and it was not possible to buy a standalone sink controller for QC.

author: ve7xen
author_rank: Super Contributor
timestamp: November 16, 2020, 11:52:26 pm
content: Quote from: poorchava on November 13, 2020, 07:19:54 amMy point is more the assumption that your user will own one is flawed. If they have to buy something, they should buy a USB-C device, it will be more useful to them in the future and it makes your life easier. I wouldn't want to be supporting a micro-B device that requires 15W of power, you're going to end up with a ton of users plugging them in to 1-2.4A chargers and wondering why it's not reliable, or using shitty cables with 28AWG wire. With the USB-C solution you can detect if there isn't enough power from the source and flash a red LED or something, and at least for now the cables tend to be much higher quality.I figured QC was probably a no-go due to it not being standards based, but IMO it's really the only option other than USB-C I'd consider here. USB-C neatly solves the problem in a standard way, there aren't any other standards-compliant solutions, so just do that.I did find a Wurth micro-B part that is rated to 3A on the power pins:https://www.we-online.de/katalog/en/COM_MICRO_SMT_TYPE_B_HORIZONTAL_HIGH_CURRENT#629105150521

author: ejeffrey
author_rank: Super Contributor
timestamp: November 17, 2020, 12:10:58 am
content: Quote from: poorchava on November 13, 2020, 07:19:54 amIt isn't so much that the chargers aren't there as that they are all over the map in capacity and the way the capacity is indicated to the device. Detecting that you have a 3A capable charger is essentially impossible.If you are going to ship a known-good power adapter with your product this is only somewhat annoying. You will have to explain to your users to only use the provided adapter and deal with the tech support when they inevitably assume they can use any USB charger and it doesn't work. If you are counting on your users to already have an adapter, it is going to be incredibly annoying.USB-C solves this neatly. The only problem is that it won't work with compliant USB-C to USB-A charge cables and an existing USB-A charge port. But as established, you can't really expect a users existing USB-A charger to supply 3A. If you are going to supply a new charger or require your users to buy one, why not go with type C?

author: julian1
author_rank: Frequent Contributor
timestamp: November 17, 2020, 12:30:25 am
content: For usb-A to usb-C - is it the cable that signals the source current capability? - using a resistor fitted in the plug?I just tested one, and the alternate CC pin that is not pulled to gnd to indicate plug polarity reads 0.4-0.45V.This voltage, signals that the unit can supply is 500 mA and 900 mA for the USB 2.0 and USB 3.0 according tohttps://www.allaboutcircuits.com/technical-articles/introduction-to-usb-type-c-which-pins-power-delivery-data-transfer/.However my charger (expensive brand model) is rated at 2.4A.That means that if one mixes up the cables and chargers, the cable can signal less current than available (the charger/cable combination I tested), or possibly also more current than the upstream device can supply.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 17, 2020, 05:07:23 am
content: Quote from: poorchava on November 13, 2020, 07:19:54 amIt has been reverse engineered and there's a way to get a microcontroller to bit bang it.https://github.com/vdeconinck/QC3Control

author: TimNJ
author_rank: Super Contributor
timestamp: November 17, 2020, 10:10:34 pm
content: Quote from: julian1 on November 17, 2020, 12:30:25 amThe standard says that all USB Type-A to Type-C cables (adapting "legacy" Type-A chargers to "modern" Type-C systems) should be fitted with a 56K pullup, which signifies 500mA charging. Rationale being that you can't guarantee that older Type-A chargers will be able to provide the increased current. Some shoddier designs, of which there are many, may not have proper current limiting to prevent them from overheating/failing.