USB Type C PD questions - Page 1
Forum section: electronics / projects

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: May 20, 2019, 04:48:34 pm
content: Hello,I'm working on an existing design for a portable LED studio light, that at the moment charges from a 20V Laptop adapter (but in theory could charge from anything between 5 and 24V). And since I want to include a usb connection (for data) in the next design anyway, i was wondering how easy it would be to use USB Type C PD.I haven't worked with it yet, and kind of failed to explain it to myself.As far as I understood there are 'contracts' that the device can support and then communicates those to the charger, and i would probably need a controller IC for that. But looking at the websites of TI and cypress just confused me more with all the different chips available, and their features.The controller would need to support usb data (not sure if that even goes through the controller), be capable of the full 100W (so the light could be used with an external supply if desired) and have internal switches, if possible, as space is very limited.A Bonus would be if it supported source and sink contracts, as the charger has a 20V @ 6A capable OTG function, so i could in theory use it as a power bank (but only if it didn't need any extra components).Can anyone recommend a controller for such a situation?

author: Bratster
author_rank: Frequent Contributor
timestamp: May 20, 2019, 04:56:02 pm
content: This is for sink only, but can be completely stand alone and very simple integrate into your product.https://blog.st.com/stusb4500-usb-c-controller-charging/amp/That's what I'm looking at for possibly using Type-C ports on stuff I tinker with.Sent from my Pixel 2 XL using Tapatalk

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: May 20, 2019, 05:14:00 pm
content: thanks.Do you know if i can leave out the VBus mosfet? The charger can be turned off by default and only start drawing current, once the controller got the contract. Or is that against the USB C spec?Also, what happens if the charger can't support the requested contracts (say the requested one is 20V @ 3A, but it can only do 20V @ 1A)? Can the controller read that and tell my device to charge at that 1A or would it fall back to the next lower power contract?

author: MaTkEOxjC
author_rank: Contributor
timestamp: May 20, 2019, 07:10:05 pm
content: If you stick to sink only it will become easier. You're already complaining about limited spaceI build three of these:https://hackaday.io/project/20424-pd-buddy-sinkQuote from: TmaxElectronics on May 20, 2019, 05:14:00 pmThe source will sent a "ready" signal after the sink requested a specific voltage and the source adjusted. So I guess you cannot omit the VBus mosfet. Why would you even? Don't make it more complicated.Isn't 20V 6A outside of the USB C PD spec?With the PD buddy sync you need to adjust the load appropriately (should be the same with any other system).So at first you (sink) request the capabilities from the source and source sends it to the sink, then you(sink) send back the capability you want from the source. Now you can adjust the load in the sink and after the source gives you a ready signal you can switch on the load.

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: May 20, 2019, 07:55:49 pm
content: QuoteI think so, I just wanted to say that it could at least meet the power requirements.I wanted to omit the VBus mosfet, because the charger circuit (buck-boost) can already disconnect all the high power circuitry and draw effectively no power. I thought that would make that mosfet kinda unnecessaryIf I understood the datasheet correctly, the protocol is handled by the MCU, which then turns on the VBus mosfets right? And if the software already handles all of that it should be easy to start the charger in idle mode (input disconnected) and only switch it on once the the source is ready, if the spec allows for something like that.

author: jbb
author_rank: Super Contributor
timestamp: May 21, 2019, 08:11:48 am
content: Your buck-boost charger might need quite a bit of input bulk capacitance. So you might need a VBus MOSFET to prevent big inrush currents. Also, keep an eye out for cable ringing on plug-in (cable inductance plus input capacitance equals LC resonant tank).On the power front, don’t rely on getting all of that 100W. Here are some thoughts:- USB power source has voltage tolerance. Let’s assume 5%, so we get 20 * 0.95 = 19V- USB cable has some resistance. I think I read a spec of 0.25 Ohms round-trip somewhere. With 5A load we lose another 1.25V- we can only count on 17.75V at the device- we can’t go over 5A, so we need to set the charger with some headroom. Let’s say it’s good to 5%. We set it to 5A/(1+5%) = 4.75A (rounding...). But one of the production units is 5% low, so the current limit might just be 4.51A.- now we can only count on 80.1W going into the charger- with care, your charger could be, say, 92% efficienct. (Note, efficiency will change with battery voltage.)- thus we get 73.7W out of the charger.The above assumes a ‘perfect storm’ where every tolerance is against you. Which will sometimes happen in production. If you design your equipment to survive that, any condition which is less bad is gravy.We see that improved charger efficiency directly improves power availability. And also that improved current limitaccuracyindirectly improves power too, which is a good trick to keep in your back pocket.

author: MaTkEOxjC
author_rank: Contributor
timestamp: May 21, 2019, 09:35:25 am
content: Quote from: TmaxElectronics on May 20, 2019, 07:55:49 pmIt might be possible without VBus mosfet, but not sure how the caps in front of the buck-boost influence it. I'd use a VBus mosfet.Yep, the µC with the fusb302bmpx handels the PD communication (separate from USB 2.0/3.x data lines). You can set the software to a specific V/I setting and if the charger has this profile and is ready the pd-buddy sink will enable the output.Quote from: jbb on May 21, 2019, 08:11:48 amSeems like a good thoughts, generally talking, but what does USB-C PD specifications say?Depending on the version, afaik the sink can request voltage increase in small steps to cover voltage drop. I'd also assume if a device announces 5A capability that it can provide 5A (and has a little headroom itself, manufacturer has to have for the tolerance on his side).I would not expect that the specifications allow for a 100W charger to deliver only 80W.

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: May 21, 2019, 03:50:12 pm
content: Quoteyeah i did think about that, but since the laptop charger I'm using at the moment can only do 60W (of which only about 50W actually make it to the lamp) it would be at least as good, even with a longer cable. It couldn't run the LED at full power even at 100W.The Idea is just to make the Lamp usable, even if the battery is dead (charge with ~25W and run the LED at whatever is left). Charging at 100W would probably lead to an early demise of the cells anywayQuotecompletely forgot about that. The Datasheet has 60uF in the example schematic (I've used 100uF). I'm guessing its like the old USB spec where a max. of 10uF without inrush limiting is allowed?

author: jbb
author_rank: Super Contributor
timestamp: May 21, 2019, 07:45:56 pm
content: Well, if you’re looking at 25W of LEDs, and don’t want to waste your batteries, how about using the 30V 3A class (60W). This should allow say 45W after the charger. Also, if you read the standard you’ll find that going to 5A requires a special Electronically Marked Cable whereas 3A does not.An unfortunate downside of USB C is that there are alotof options which can make it hard for people to understand why this cable and that charger deliver poor results with a particular product :-(.You can get the complete standards from the USB website.Also, be aware that some chargers are simply out of spec and provide 20V all the time. Given the CC pins are really close to the VBus pins and could touch during cable insertion, you need to protect your CC pins to survive connection to like 25V DC.