Diode D11 prevents fast charging with 30W chargers

MNT Pocket Reform
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I have an Apple 30W USB-PD charger, which supports only either 9V or 5V. At 9V, it gives about 8.8V “real”.
That turns out to be quite the problem for the charger IC in the pocket, in combination with the diode D11. While it’s a Schottky diode, at ~9V and 1A it has about 0.3V voltage drop.

The MP2762A charger IC switches between boost and buck charging “when the input voltage exceeds 8.5V”.

8.8V - 0.3V = 8.5V

And indeed I can see the charger trying to switch to buck charging (fast), but then the voltage drops below 8.5V, and it goes back to boost charging again (very slow). And then the cycle repeats.

For testing, I’ve bridged D11 with a clamp, and immediately the charger switches to buck charging and stays there.

The MP2762A datasheet shows a “typical application circuit” where the 10mOhm R1 (fronting IN) is immediately connected to the USB port. From the results above, it would seem like that is not just typical but actually necessary :frowning:

Ping @minute :slight_smile:

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Now documented here Charger Mod/Fix for Next Rev Hardware: Use OTG function of MP2650 (#3) · Issues · Reform / MNT Pocket Reform · GitLab

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I know this hasn’t been validated / tested thoroughly, but does this imply that it may be possible to charge from 30W chargers with a small hardware modification?

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Yes. There are however open questions regarding charging speed. But it does work!

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Have you tried it out this way very much? I’m tempted to bridge it and see how it works unless there’s a compelling reason not to!

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I am not familiar with the details of this charging IC.

When operating within the USB standard you do not need the diode for reverse polarity protection. Doubly so if the IC happens have one of its own.

There might be something leaking out of IC when not powered but connected to some device that the diode would prevent saving you battery - unless the IC prevents that on its own.

Having vs not having the diode shifts this point at which the IC flips between boosting/not boosting to a slightly different voltage. What is more concerning is that this behavior happens at all. How much can the chip behavior be changed in software?

I just copied the USB design including this kind of diode from some board that had both the diode and a solder jumper for bridging over it in case you were sure you do not need it. I am certainly in the USB A-A cable user club and don’t trust that all cables follow standards closely all that much so I can understand that many designs include protective elements that you would not need under normal operation.

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My pocket has the diode removed and replaced with a wire, since when I posted this!

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