Author Topic: USB-C PD with fluctuating power supply?  (Read 802 times)

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Offline rs20Topic starter

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USB-C PD with fluctuating power supply?
« on: February 18, 2020, 02:46:19 am »
So I'm familiar with the idea that if you have a practically limitless supply of power ( > 100W at all times in this context, e.g. mains power or a well-charged lead acid battery), then you can use a chip like STUSB4700, optionally configure it for whatever limits you want to impose, and you're good to go -- you get a USB-C 'power supply' that can provide up to 100W to a phone or laptop, depending on how much the laptop/phone wants to pull. This is all fine.

What I'm wondering about is what the best approach to take is when your power supply is highly variable in terms of how much power is available -- e.g., a solar panel, human-powered hand crank, etc. Ideally I could just dump the energy into the Vbus line, clamping the voltage to 19.5V or whatever the present limit is, and the phone would detect a sag in voltage well below 19.5V as a signal that it should reduce current consumption. But that would be a "breach of contract" on the part of the power supply, and I believe the STUSB4700 would detect the undervoltage and trigger an error. So that would probably end up with the phone being charged in some sort of horrible hiccup mode or something undesirable like that. But maybe it would be fine?

I could update the configuration in the STUSB4700 multiple times per second, have enough storage just for a few seconds of power, and the device could retrieve the new settings and modulate its input accordingly. But I highly doubt that would work; I assume the device would assume that the power supply won't regularly "change its mind" on how much power it can supply.

I could probably regularly reset the device's USB or PD data lines to force a re-retrieval of the up-to-date limits, but would the charging process get interrupted?

In short, I'm really not sure what the best way to modulate the consumption of the phone/laptop is, since the whole arrangement seems to be optimized for allowing the load to determine how much power it consumes. Any suggestions would be most greatly appreciated! The dream is an arrangement where the generated power is going straight into the phone/laptop's battery, even though it's varying greatly over time.

 


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