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November 26, 2014


Electronics & Systems
Fast Charging
Mobile Devices
Nadim Maluf

We see it in Qualcomm’s tweets. We see it in some published articles. We see it in fine prints from a few smartphone makers. It says “fast charging using Quick Charge 2.0.” What is it?

In my earlier posts, I spoke about two pieces that are essential for fast charging. The first piece is power delivery from the wall socket to the mobile device, and the second piece is battery management that ensures optimal operation of the battery itself under the stressful conditions of high charging power.

Well, Qualcomm’s Quick Charge 2.0 is about the first piece, and only the first piece, i.e., ensuring power delivery to the mobile device and the battery. It does not perform any meaningful battery management functions and certainly does not ensure the battery’s health.

What is its purpose and how does it work? Standard AC adapters and power delivery mechanisms into mobile devices have been limited to 5V and 1.6A. In other words, the AC adapter takes 120V/240V at the input and outputs a maximum of 8W of power at 5V. This has been the de facto standard for several years. This power output was plenty sufficient when mobile devices had batteries that were small, about 1,500 mAh or less. But as tablets first came on the market with batteries in excess of 4,000 mAh, and smartphones followed with batteries in excess of 2,000  mAh, it became clear that higher power output was needed from these AC adapters.

Naturally, the easy answer would be to just increase the current above 1.6A but still maintaining an output voltage of 5V. But this is highly impractical. First, the USB cable cannot be realistically changed — even though the new USB 3.0 standard aims at changing the cable dimensions, it will be years before such new standards make it through the industry. In other words, power delivery had to co-exist with present-day USB cables. Additionally, if you recall from your high school physics class, electric losses go as the square of the current. So increasing the current from 1A to 2A, for example, quadruples the losses, all going into heat. That’s not good! So if one cannot raise the current, then the next logical answer is to raise the voltage.

I will digress briefly here. Take a look at how power is transmitted from the power stations to your house. You will find, somewhere near your house, a large power switching station whose primary responsibility is to step down the voltage from the transmission lines, typically at 66 thousand volts, down to what ultimately becomes 240V at your house. Transmission lines use 66 kV precisely to limit the losses and use cables of reasonable dimensions and cost.

So QC 2.0 establishes a new methodology to deliver current at or near 1.6A, but raise the voltage to 9V or 12V, up from 5V. That effectively raises the maximum to nearly 20 Watts. Plenty for now. So you can see, there is nothing earth shattering about the concept. In fact, Apple had been doing precisely this on their Macs for many long years.

How does it work? The question specifically is: If the AC adapter can deliver 5, 9 or 12V at its output, how will the mobile device know which voltage to select? If you take a look at the terminus of a USB cable, you will see four electrical contacts. Two pins are for power and ground, and two pins are for data (called D+ and D-). The communication between the AC adapter and the mobile device happens over this pair of data wires. The combination of the D-/D+ voltages is the signal between the two devices on the choice of power voltage requested from the AC adapter (see picture below). If this voltage “wiggling” over the D+/D- is not present, then the mobile device defaults to the standard 5V. That’s really it!

So why is Qualcomm making such a big fuss of it? Of course, there is a marketing element, which is welcome. It’s great to see Qualcomm throw its weight behind the ecosystem of fast charging, even when QC 2.0 solves only one half of the fast charging problem. By now, you know that Qnovo solves the other half of this problem, the battery.

So to sum it up, if your phone is capable of QC 2.0, then it is great. You are capable of fast charging. But before you start fast charging, make sure that your battery will not get damaged in the process.

Happy Thanksgiving!

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