# How does multi-voltage power supply work?

So I was looking at this USB power supply by Xiami, and I noticed in the specs, it says:

 Input voltage 12V/9V/5V Output voltage 12V/9V/5V 

So how is it able to achieve that? I understand there might be some circuitry that would convert between the voltages, but how does it know which voltage is needed?

• Have you looked at the USB Power Delivery protocols at all? – ThreePhaseEel Nov 22 '17 at 4:00

It looks like it supports USB-C which has power negotiation. I would assume that the power bank has the circuitry to negotiate with a connected peripheral on how much voltage it wants.

• Ah.. that clears things up.. So that means the device would need to implement the standard as well.. thx. – hndr Nov 22 '17 at 3:24

This is a power bank that supports multiple power levels. Apparently this device can take a charge from variable power source, and can deliver variable negotiable power later to a USB device. Unfortunately the producer completely fails to indicate which negotiation standard is supported. Without knowing this and having corresponding charger and consumer devices, the higher voltages are useless, and the power bank will deliver only default 5 V and 2A, a fairly ordinary parameters.

Currently the advanced charger technology offers two major methods of power negotiations, Power Delivery (promoted by USB alliance), and QuickCharge, a proprietary method introduced by Qualcomm.

The Power Delivery offers four "normative profiles", 5V at 3-5A, 9V at 3A, 15V at 3A, and 20V.

Qualcomm QC method offers variable profiles, but the set of 5 - 9 - 12 was the original one.

Therefore, from listed specifications (5/9/12V), I would guess that this powerbank supports an early version of QuickCharge, so to use it at its full potential you would need a QC charger to charge it up, and your tablet/phones should support QC as well.

• Nice summary (+1). Re: "the producer completely fails to indicate which negotiation standard is supported" FYI on this different page for the product, they mention QC2.0 several times, so your conclusion fits with that. – SamGibson Nov 22 '17 at 16:19
• @SamGibson, I guess I should investigate the product over all angles, and over all related web pages. But the link (specification !!!) posted by OP doesn't have this information, which sparks my nasty remark. Thanks for pointing this out. – Ale..chenski Nov 22 '17 at 16:27
• I agree it's strange (and not helpful by the manufacturer) that they don't mention QC2.0 on the page labelled as "Specs", given by the OP, and instead mention it elsewhere - surely it being QC2.0 compatible would be part of the specifications?! Anyway, my comment was just to point out that I found confirmation of your deduction about it using QC, even though the page originally linked didn't say that. :-) – SamGibson Nov 22 '17 at 16:58