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Comment by tails4e | Hacker News Reader

What is the cold temp helping with? AFAIK transistors no longer get faster at cold, at least since 16nm the perforamce at sub 1V thin-ox transistors is pretty flat with temperature. Cold helps a lot with power though, leakage at hot (100C) could be 20W, so maybe going cold allowed more dynamic power dissapation? I'd be intersted to hear what the overclock experts like yourself think.

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Transistors have a maximum operating temperature[1] (Tj_max), and the package has a fixed thermal resistance[2].

If the junction-to-case thermal resistance is 0.5 deg C/W and CPU dissipates 100W, then the junction will operate at 50 deg C above the case temperature, whatever that is.

If the case is kept at room temperature (say 25 deg C), which is the best possible a simple air or water cooled system can do, then the junction will operate at 75 deg C.

For simple discrete transistors and diodes, Tj_max is typically 150-175 deg C. For modern CPUs I believe it's closer to 100-125 deg C.

Thus, lowering the case temperature means the chip can consume more power before reaching the maximum junction temperature.

A greater power budget means you could raise operating voltages to help with overclocking for example. Keep in mind power dissipation scales with voltage squared.

There might be other effects too, I'm no overclocking expert, but this is one aspect.

[1]: https://en.wikipedia.org/wiki/Junction_temperature

[2]: https://en.wikipedia.org/wiki/Thermal_resistance

Leakage increases with temperature, and the speed at which FET switches also increases with temperature.

Speed, is that due to Vgs threshold voltage decreasing as temp increases? On the other hand you have Rds(on) rising as temp increases, which would negate some of that effect I would imagine (lower gate drive current).

But not my field so no idea how that interplay works out.

Carrier injection speed is a function of temperature.

Think of FET as a solid state thermionic valve, it's not that far away from it, if you dig into physics.

Thanks. You definitely don't want the junction temp anywhere near 125c, the leakage would be very high. I thought most processors in due temp was 75-85c max, and will throttle severely if it goes above that.

Tj_max is listed under "absolute maximum ratings", so indeed operating conditions should be well below that. I see I was a bit sloppy in my wording.

The cold temperature increases the clocks that AMD's boosting algorithm will request. See here for a demonstration using LN2: https://www.youtube.com/watch?v=g94rNe4XSGU

Unless anything has changed the silicon/die itself isn't staying sub ambient at all it's just that there's a limit to hot steep temperature gradient you can build when the ambient/sink temperature is fixed.

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