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According to the USB Battery Charging v1.2 Spec a Dedicated Charging Port (DCP) shall output an average voltage of 4.75V to 5.25V at a current of 0.5A to 5A. So if I'm reading this correctly, supplying 5V at 5A should be perfectly fine for a DCP.

However I have trouble understanding how to signal a portable device it is connected to a DCP and it may draw up to 5A.

Section 3.2.4.1 of the aforementioned document suggests, that all I really have to do, is short D+ and D- (or ensure a maximum impedance of 200Ω). The portable device shall then simply assume it's fine to draw up to 5A and needs no further communication. Is the portable device going to simply ramp up the current until it detects the 5A max current?

I'm confused, because places like Wikipedia suggest, that 5V 5A being the available max power. However the USB spec suggests that a portable device should not draw more than 1.5A at 5V (Allowed PD Current Draw from Charging Port in Table 5-2 of the spec).

So which one is it?

I specifically don't want to implement a smart USB supply at this point, able to supply several different voltages between 5V and 20V. I'm talking about a simple 5V of \$V_{USB}\$ power supply, but one that delivers up to 5A.

The goal is to build a 5V 5A power supply that will be able to supply the maximum of 25W to a device. However I want to make sure the portable device is aware of the available current, and doesn't limit current to below 5A by itself. Hence me asking.

Thanks in advance!

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    \$\begingroup\$ Realize that 5 A through a single USB connector is really "pushing it to the maximum". I simply would not do that. Yes, the specification says 5 A but you will have issues with contact resistance. USB isn't designed for that. If you insist on USB, use two connectors in parallel (so 2.5 A each). Better solution: use a 5.5 mm jack like you find on many power adapters. Even better solution for 25 W: Use 20 V, 1.5 A. \$\endgroup\$ May 27 '21 at 12:01
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However I have trouble understanding how to signal a portable device it is connected to a DCP and it may draw up to 5A.

A PD (Portable Device) may not draw more than 1.5 A from DCP:

enter image description here

A legacy DCP can source up to 5A, true, but the voltage is not guaranteed. The 5 A limit is set for "safety reasons". If the draw is above 1.5A, a DCP is allowed to shut down. Or it may have a continuous load curve as shown in "blue":

enter image description here

You can make a charger that keeps VBUS=5V and source up to 5A, but no legacy (and legal) devices will make use of this power.

The goal is to build a 5V 5A power supply that will be able to supply the maximum of 25W to a device.

This goal can be achieved only by using Power Delivery mechanisms using Type-C connector, and with electronic marker inside the Type-C overmold indicating the cable capability of 5A. To use this power, the PD must also have implemented the proper PD protocol. :-)

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  • \$\begingroup\$ I've seen plenty of devices drawing 3A from a DCP, so I guess it was kinda-sorta beyond the spec, but it seems it's pretty common to draw more current than the 1.5A. I guess I'll just build one of those and see what devices do. \$\endgroup\$
    – polemon
    May 29 '21 at 7:49

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