ASUS VivoBook S14/S15

Battery life. For us all in the current generation of mobility, battery life is one of the most crucial factor in deciding a purchase. Want a larger battery? Be prepared to make some trade-offs. However, we can get away with less trade-offs these days, thanks to modern technology.

The technology I’m referring to here is of course FinFET. We talked about transistor design in detail here in this editorial, but we didn’t focus much on its power consumption. Let’s expand upon this topic a little further.

A little introduction on 2D and 3D transistors

Firstly, let’s have a brief rundown on transistor design is all about again. I believe that this image by Intel is able to show the difference between 2D and 3D transistors pretty clearly.

transistor-comparison-for-battery

The main takeaway point is that 2D transistors reached a bottleneck where 2D transistors can no longer be shorter than a certain gate length. This is because the fundamental problem with the design of a 2D transistor. As the gate length becomes tinier, the distance between the source and drain channel is also tinier. Because of this, the transistor can turn on when it’s supposed to be off. It’s no longer reliable if the transistor actually does that. On another side effect, larger amounts of leakage current means higher parasitic power loss for doing absolutely nothing. It’s wasted entirely.

How do we counter all of this? By re-imagining the entire structure of a transistor – this time to make sure of the vertical axis too. That’s the origin of the name “3D transistors”, actually.

A 3D transistor has its gate wrapped all around the substrate, drastically increasing the effectiveness of the gate channel to control the transistor’s operation.

How FinFET affects us all

Ever since the transition from the traditional 2D planar transistor to the advanced 3D transistor design – also known as tri-gate transistor, there are a lot of other major positive side effects that came along with it. One of it is power consumption.

According to Intel, FinFET shows more than 50% power reduction at constant performance. That’s a huge benefit! Now, let’s propagate this information to the mobile side of things.

Qualcomm has stated in their press release that the Snapdragon 625 “yields up to 35% reduction in power consumption compared to the previous generation”. In their press release regarding the Snapdragon 820 however, it shows a fantastic 30% drop in power consumption compared to the previous generation’s Snapdragon 810. Of course, we’re neglecting the changes in the architecture for now.

Image by Qualcomm
Image by Qualcomm

Effect on overall battery life

For the sake of simplicity, let’s take the Snapdragon 820 and Snapdragon 810 for this comparison. Also, we need a disclaimer on the math we’re doing here too.

Numbers shown below are mostly based on assumptions. In this case, we purely look at the chipset’s power consumption alone and not the storage, memory, chips, etc. Take these calculations as a rough estimate instead of something solid. It’s just an idea.

We have some base values here, but we need to know the specs of a battery. Batteries in smartphones vary greatly in its voltages and amperage. After going through multiple battery ratings thanks to iFixIt teardown pictures [linked here (1), (2), (3), (4), (5)], we found out that the voltage varies between 3.8V to 3.85V. Let’s just assume that our battery here gives out 3.8V at 3000mAh, or 3Ah. Also, the “h” here means hours of the device on battery. It’s an arbitrary number of hours when it comes to maths, but for us – this is what we call “hours of battery life

Calculating wattage on Snapdragon 810 chipset

Doing a little math – which is very easy – should tell us how much power it consumes.

\huge 3.8V\times3Ah=11.4Wh

So we get 11.4Wh – which means there’s a total of 11.4 watts of energy stored in the battery, and it will be distributed throughout the arbitrary time known as “h” that we described earlier. Let’s just say that the Snapdragon 810-based smartphone can last for 15 hours. It’s an assumption for our baseline calculation.

\huge \frac{11.4Wh}{15}=0.76W

Okay, so now we get the number 0.76 watts on a Snapdragon 810 chipset. It means that if the Snapdragon 810-based smartphone with a 3,000mAh battery rated at 3.8V, it will be fully depleted after 15 hours if the constant power consumption is at 0.76W for 15 hours.

Calculation wattage on Snapdragon 820 chipset

How about the Snapdragon 820-based smartphone? We can roughly take the figure by Qualcomm and apply it here. We know that the Snapdragon 820 consumes about 23% less than the Snapdragon 810, so:

\huge 0.76W\times\frac{77}{100}=0.5852W

Now we know the wattage of the Snapdragon 810 chipset, we can reverse the formula and find out what battery capacity can deliver the same amount of hours of usage.

\huge 0.5852W\times15=8.778W

\huge 8.778W\times\frac{1000m}{3.8V}=2310mAh

As you can see, the math involved here is directly linear. 23% decrease from 3000mAh is 2310mAh. But why did we calculate other things and not directly scale it down by 23%?

The answer lies within the wattage.

Addressing heat woes

This simple value can actually tell us about TDP, or thermal design power. In Wikipedia, TDP is briefly described as “the maximum amount of heat generated by a computer chip or component (often the CPU or GPU) that the cooling system in a computer is designed to dissipate in typical operation“.

To understand this part, we all have to understand the first law of conservation of energy. It states that no energy can be created or destroy, and it can only be converted from one form to another. Our main focus here is electrical energy and also heat energy.

Image from XDA Developers, as they prove Snapdragon 810 does indeed overheat
Image from XDA Developers, as they prove Snapdragon 810 does indeed overheat

Traditional 2D transistors are super-inefficient compared to 3D transistors. That means given a specific amount of electrical energy, 2D transistors will end up losing more electrical energy as heat energy, hence the term “dissipated energy”.

3D transistors are indeed efficient – having much less energy to be wasted as heat. With less heat means less powerful cooling systems can be used to maintain the temperature. But then it leads back to the main topic of today – the battery life.

The “longer battery life” paradox

So far we’ve been talking about one thing – the transition from 2D transistors to 3D transistors results in a more efficient system and can save you a lot more battery. That is true, but earlier, I mentioned about the battery heat issues too.

Generally, we all know that phones get hot when we use it heavily. What most of us don’t realize is how heat temperature in general can affect battery life. In general, hotter smartphones tend to have worse battery life. It might be due to background apps (happens in Windows, iOS, Android, Linux, etc.) or overall inefficient chipset, like the Snapdragon 810 chipset that was infamous for overheating.

Illustration of the paradox. This cycle feeds on itself until it thermal throttles or shuts down due to heat.
Illustration of the paradox. This cycle feeds on itself until it thermal throttles or shuts down due to heat.

With the brand new FinFET on Snapdragon 820, Kirin 950, or Samsung’s latest chipset on the Galaxy series of smartphones, it addresses the heat paradox to a certain degree. Overall, smartphones that uses 3D transistor designs have much better battery life compared to those that use 2D transistors, even though they’re powered by the same battery.

Wrap up

Intel has stated that moving from the traditional 2D transistors to 3D transistors only costs about 2-3% more than before. It’s a no-brainer to use 3D transistors in all computing devices like CPU, GPU, and potentially in memory chips for RAMs and SSDs too.

The key takeaway points are these:

So, in summary, what have we talked about and learned today?

  • 3D transistors rule
  • 3D transistors consume less power
    • Much more efficient than 2D transistors
      • Results in longer battery life
  • Heat and battery life is paradoxical
    • Dissipates less heat
      • Phone does not overheat
        • Battery doesn’t heat up
          • Even longer battery life

We’ve seen laptops gaining significantly longer battery lives when upgraded from Sandy Bridge (traditional 2D transistor) to Intel’s 3rd generation Ivy Bridge architecture – the first ever FinFET-based processor in the market. On EveryMac.com, Apple’s 11-inch MacBook Air listed the mid-2012 Sandy Bridge variant to have 5 hours of battery life, whereas the mid-2013 Ivy Bridge version to have 8-9 hours of battery life. Of course, the only difference here is the additional 3Wh on the battery. That’s a huge improvement by just one upgrade!

We can already see current generation of smartphones like the ASUS ZenFone 3/Ultra/Deluxe, Honor 8, Huawei P9, Mate 9, P10, and even the Samsung Galaxy A(2017) smartphones and their Galaxy S flagships are benefitting from this. Even Mediatek Helio X30 is joining in the fun soon. It’s only natural for all smartphones to use chips that are based on the FinFET process.


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