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USB PD for older device (piefed.social)

submitted 1 month ago by SorryImLate@piefed.social to c/askelectronics@discuss.tchncs.de

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I have an older electrical device which I would like to switch to charging with a USB C cable and a standard USB power supply. The device's power supply unit supplies 14V and 400 mA, so 5.6 W. The device documentation certifies it for 12V with 2 Ah.

I found a useful answer on stackexchange about the Source Power Rule for USB 3.1. This implies to me that my device is not compatible with 3.1 because at a power draw below 15W, PD 3.1 defaults to 5V.

According to Wikipedia, a USB4 DFP is required to supply at least 7.5W Type-C current, which is above the power requirements for my device.

Based on this, it seems to me that the power supply of my device is too non-standard to be supplied by a standard USB power supply. Have I understood this correctly?

Thank you for your help.

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[–] SorryImLate@piefed.social 1 points 1 month ago (1 children)

So, if I added this board to my device, I must be sure to only charge it with a power supply that supports PPS in the 12v range - is that correct?

[–] cmnybo@discuss.tchncs.de 5 points 1 month ago

There are plenty of chargers that support a fixed 12V output without needing PPS. Just check the specs before buying one.

If you use a CH224K USB PD trigger chip, it has a power good pin. It will go low if the power supply is providing the requested voltage. You can use it with a MOSFET and driver circuit to switch the output on only when it gets the correct voltage.