19Jul 2016

This post includes contributions from Robert Nalesnik. I discussed in the past how fast charging requires two components: i) power delivery – that means getting extra electrical power from the wall socket to the battery and ii) battery management – that means making sure you don’t destroy the battery’s lifespan with all the extra power.

How much more power do you need? Quite a bit more if you want to charge considerably faster. It’s like your car engine: if you want to go faster then you will consume more gas. For a typical smartphone, power levels go up from the conventional 5 Watts to 15 or even close to 20 Watts in some cases.

Delivering higher levels of power is a very active area. Qualcomm has Quick Charge, Mediatek has Pump Express, and there is the USB Power Delivery standard with support from Intel, and the Chinese manufacturer Oppo has VOOC. Not surprisingly, with so many parties trying to influence or even define the standards of power delivery, there is plenty of confusion to go around.

First, let’s refresh some basic high-school science:     Electrical power = current x voltage.

So if we want to deliver more power, we can either increase the current, the voltage or both. Increasing current is relatively easy but more current means a lot more heat…that is until something begins to melt. Not good! That usually puts an upper limit somewhere between 3 and 5 A on the charging current.

The other approach is to increase voltage, from the conventional 5 V up to 9 V, or even 12 V, and in some limited cases even more.

High current charging

High-current charging leverages the fact that modern single-cell lithium ion batteries can be charged using an inexpensive 5 V AC adapter that can be manufactured for about one dollar.

Increasing the charging current is limited by i) the maximum current rating of the USB cable between the AC adapter and the mobile device, as well as of the tiny connector in your device where the USB cable plugs in; ii) cost and iii) heat and safety.

Let’s do some math. A typical USB cable assembly can support a maximum current of 1.8 A. So 5 x 1.8 = 9 Watts max. That’s fine for standard charging but not sufficient for fast charging a smartphone. The new USB type-C cables (with symmetrical connectors that can be used in any orientation you like) can support up to 3 A, in other words, a maximum of 15 Watts. Much better! Under some very limited cases and using special cables, one can push USB type-C to 5 A, or 25 Watts. But at 5 A cost begins to skyrocket, so instead, we see designs gravitating towards 3 A, or equivalently 15 Watts.

To put this in perspective, 15 Watts can charge your typical 3,000 mAh battery at a rate of 1 C, meaning you will get 50% of your battery charge in 30 minutes, and a full charge in  over an hour.

A quick word on heat: if you remember Ohm’s law from your high-school physics, heat increases as the square of the current. That means as the charging current increases from 1.8 A to 3 A, or 1.66X, heat inside your device will increase as 1.66 x 1.66 = 2.8X. Ouch! That’s a lot of heat to remove from the device….and a great topic for a future post.

Power1

High voltage charging

Let’s pause for a moment and think about the high-voltage transmission lines that we frequently see from highways outside of urban areas. Electric utility companies transport electrical power from power-generating stations (e.g., dams) that can be hundreds of miles from a city. If they use the 120 V that you get at your outlet, then the overhead transmission lines will have to carry millions of amperes…this is not only physically impossible, but also economically just prohibitive. So the transmission lines run at a much higher voltage, anywhere up to 800,000 volts. These transmission lines naturally don’t come straight to your house. Instead, they terminate into smaller substations (hidden off main roads near your neighborhood) where the voltage is then gradually “stepped down.”

That’s the same concept used in mobile devices. The voltage from the AC adapter is now raised above 5 V. But what voltage should it be? 9 V, 12 V? more? This is decided by a “handshake” protocol between a specialized chip (usually the power management IC, also known as PMIC) inside your smartphone and the AC adapter when the USB cable is plugged in. This is the approach taken with Qualcomm’s Quick Charge and the USB Power Delivery standard, each using a different signaling mechanism. There is a saying among power supply engineers that “voltage is cheaper than current”, and indeed lower cost components and cables are a primary benefit of high voltage charging.

The USB Power Delivery (USB PD) standard allows voltages of 5, 9, 15 and 20 V and currents up to 3 A. This gives power levels of 15, 27, 45 and 60 W, respectively. Additionally, currents up to 5 A are allowed at 20V, enabling up to 100W. Qualcomm has similar predefined power levels at 5, 9, 12, and 20 V. High-voltage charging has a clear advantage of attaining power levels above 25 W, which makes it the preferred choice for laptops, ultrabooks and 2 in 1s tablets.

power2

How will this abundance of approaches settle out in the market? From a historical perspective, Qualcomm was early to see an opportunity to define high voltage charging in a simpler and cheaper way than the USB committee. They launched Quick Charge 2.0 in 2013 and followed up with the latest 3.0 version in 2015. Qualcomm has been quite successful establishing Quick Charge as a defacto charging standard for smartphones. More recently, Intel and others are successfully driving USB PD and the Type-C connector into PC markets and Type-C is well on its way to become the standard connector across all classes of mobile devices.

In smartphones, the next few years will likely still see multiple power delivery approaches, with chipset and adapter vendors evolving to multi-standard support to bridge compatibility gaps – meaning a smartphone can support multiple protocols such Qualcomm, USB PD, Pump Express…etc. From a Qnovo perspective, we are agnostic and complementary to whatever power delivery approach our customers choose. The higher power makes greater the need for the second component of fast charging: battery management.