Thermal throttling on the Xperia Z

A nice example how overpowered modern smartphones are.

For the past two years I have been using the Sony Xperia Z as my phone. Although it can actually be considered technically outdated by now, it still has a quad-core Snapdragon S4 Pro with up to 1.5GHz at its heart. With that high specs, the phone is a nice example of how overpowered modern smartphones can be and what problems arise in terms of cooling.

Of course modern processors now have tiny heat pipes built-in to distribute the heat more effectively, but in contrast there are actually new smartphones that have up to 10 cores.

Most recently I got a USB multimeter and whenever I do stuff on my phone while it charges, I check the actual current. During the first few tests nothing special happened. The phone draws about 1.3A to 1.8A when charging, even when you play around or listen to music. However, things got more interesting when using an app — in this case a game — that requires a lot of both, CPU and GPU power.

At first the temperature of the chipset increased more and more, while the charging circuit still drew about 1.4 amps. The magical point was when the CPU temperature hit somewhat around 60–70°C (measured with the CPU Z app). At this point the charging logic switched from loading the battery to directly powering the chipset. This change was visible via the USB multimeter, which showed 0.0A for a sec and then went up to around 500mA and even up to 800mA due to higher workloads. Further, my charging device is capable of supplying up to 2.4A, so the logic could charge the battery and power the CPU from the charging device at the same time, but it did not. Well, that values do not really tell us anything about thermal throttling, although the temperature is considerably high — then what does?

70°C is not an ideal operating temperature over a longer time.

Actually, CPU Z shows four active cores operating at about 1.2GHz when playing the abovementioned game. As the temperature rises the clock speed slowly decreases down to 1GHz, which is not that much, but you can really feel the impact on the game. The screen gets stuck from time to time and animations are not as fast as before. From there on I recognized that the CPU speed did not decrease any further, which could be a compromise — made by the hardware developers — between usable CPU power and thermal limits. Of course it is nice to keep thermal throttling at a minimum, but in my opinion 70°C is still to hot for a chipset that may operate at this temperature for a longer time. However, one thing that I found to be very interesting is, that not only the main processor itself is affected by the heat, but also the charging logic and/or battery. When leaving the app, the charging logic switched back to charging the phone battery. The temperature was still high and due to that the USB multimeter showed about 500mA less — namely 0.9A instead of 1.4A — than the current that is normally drawn at that battery percentage.

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