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Comment by Phillipharryt | Hacker News Reader

From the article "The heat generation per unit area of an integrated circuit passed the surface of a 100-watt light bulb in the mid 1990s, and now is somewhere between the inside of a nuclear reactor and the surface of a star. "

I can't tell if this is hyperbole or not, it amazes me but no amount of googling is coming up with a useful answer. Is anyone able to confirm or deny it for me?

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After a quick napkin calculation+google it seems the sun has around 20kW of power per square centimeter. So not entirely unfeasible that a cooler star, or a nuclear reactor is closer to the typical 10-100W/cm2 of a modern cpu/gpu. Still some orders of magnitude off from our closest star. (Hope calculation is correct)

Looking at the wikipedia page on red dwarf stars[1], it appears a small star (M9V) might have 8% of the sun's radius and 0.015% of its luminosity. Thus, it would have about 156 times less area, and 2.3% of the output per area of the sun. So taking your figure as given, that means it would be less than 500W / cm^2.

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

> 20kW of power per square centimeter

I'm getting 6300 W/cm^2 (see: https://news.ycombinator.com/item?id=17589371)

The temperature of the sun at its surface is ~5772 Kelvin. To get power per unit area, use Stefan-Boltzman: σ * T^4, σ * (5772 K)^4 ≈ 6294 W/cm^2. Dividing the sun's luminosity (power) by its surface area will also give a similar value.

A 815 mm^2, 250 W GPU will be 250 W / 8.15 cm^2 ≈ 31 W / cm^2.

Doesn't that only include radiated heat? If I touched it, it would be hotter.

It wouldn't be hotter (the temperature wouldn't change), but it would transfer more heat to you.

From:

http://www.wolframalpha.com/input/?i=Sun+%7C+luminosity+%2F+...

The sun is: 6300 W/cm^2

> inside of a nuclear reactor

Inside means volume? That's not the same unit, so I'm not really sure how to calculate that.

Interestingly, on a per-volume bases, the sun's output is quite low. The sun is just very very large.

See: http://www.wolframalpha.com/input/?i=Sun+%7C+luminosity+%2F+...

And you find the sun is only: 1.383×10^-6 horsepowers per gallon :) Or if you insist: 0.2725 W/m^3, which as you can see is really really low.

Intel's latest CPU innovation, the i9 "Industrial Cooling Required" Edition with 28 cores, would come close to a nuclear reactor and does manage to replicate the heat output usually experienced during atmospheric reentry from orbit.

I doubt it comes close to the surface of the average star but certainly close to some of the cooler red dwarf stars and probably most brown dwarf stars.

(Intensity) = (Power) / (Unit Area)

(Sun's Radius) = 695 x 10^6 m

(Sun's Power) = 4 x 10^26 W

(CPU Output) = 75 W

(Die Size) = 37 mm x 37 mm

(Sun Intensity) = (4 x 10^26 W ) / (4 x pi x (695 x 10^6 m)^2) ~ 66 x 10^6

(CPU Intensity) = (75 W) / (0.0014 m^2) ~ 53 x 10^3

I'm getting a several orders of magnitude higher W/m output for the sun. Perhaps I made an algebra mistake?

Yes. You can handle 100W on a 300mm^2 chip. Or 300W on a large one, compare AMD Ryzen/ Nvidia Volta (MXM/V100).

this gives 3.46e+5, probably comparable to brown dwarf figures in the tread.

Sounds higher than a nuclear reactor, for which you can find data on nucleate boiling without forced flow to support about 10W/cm^2 without much risk of reaching critical heat flux (in the strict sense, as you then transition to a vapor isolation due to the surface rapidly heating beyond the temperature where nucleate boiling is sustained, and from what I remember/believe, you won't transition back without reducing the surface temperature to be below that at critical heat flux, possibly even lower due to the lack of agitation/uniformity of the vapor layer).

I did some research into what can be archived with suitable dielectric fluids and nucleate boiling to allow much higher heat flux on the surface of the silicon without requiring any energy input to actually separate hot and cold, but allowing the use of pumps and such to provide forced flow on the silicon. It seems that if you have a nozzle to provide sufficient flow speeds across the die you can get higher critical heat flux than can be reasonably handled through the power pins on an AMD EPYC socket, which has some limitations due to the LGA technology used for the contacts. A zero-insertion-force PGA socket should not be bound by this limit of temperature rise at the contact of the LGA spring/pin and the chip contact pad due to resistive heating resulting in the contact pressure declining (due to the spring weakening), which yields a feedback loop that can jump over to a domino effect on the other nearby power pins.

I remember the flow rate being proportional to the critical heat flux as well as the distance the flow from the nozzle has to cross and provide cooling for. There were speeds of iirc. about 20m/s if one were to cool a delidded AMD EPYC at maximum power draw @4GHz, or rather, extrapolating from what can be archived with a die surface temperature of 50 degree Celsius.

So, yeah, the heat flux is actually a problem, but I think one could integrate some sensors and sufficiently fast switches that shut the section of the die off if the temperature reaches dangerous levels (and do so fast enough to not damage anything, i.e. microseconds or what time there is), and use such a test chip to engineer a forced-flow direct-die (or maybe even with a heat spreader on top, but that costs you due to the nucleate boiling not going below 20 kelvin temperature loss, and additional system losses in the radiator/heat exchanger as well as pipes likely resulting in costs of about 5 additional kelvin). This could open the door to operating CPU dies at much, much higher power densities than currently normal. It's like a heat pipe on steroids. One would want circuity in the CPU to rapidly shut down in case the temperature rises quickly, as regardless of why this happens, not doing so will literally blow a hole in the chip before you can drain the inductors providing smooth power to the socket.

Your CPU is huge. For example, the highest-performance single-chip Pinnacle ridge (AMD Ryzen 2 2700X) is 213 mm² and has a TDP of 105W. That's roughly an order of magnitude difference.

keep in mind that 'the surface of a star', for various definitions of 'surface' is actually "cooler than you might think", because it's rather diffuse and more than made up for in total output by the sheer size of the star.

Temperature inside a nuclear reactor: 300 degrees Celsius. [1]

Temperature on the surface of the sun: 5,600 degrees Celsius. [2]

Temperature of a CPU: ~75 degrees Celsius. [3]

[1] http://academic.brooklyn.cuny.edu/physics/sobel/Nucphys/pile...

[2] http://coolcosmos.ipac.caltech.edu/ask/7-How-hot-is-the-Sun-

[3] https://www.computerhope.com/issues/ch000687.htm

not sure why you're being downvoted, but it's worth mentioning that the temperature is not what matters here but rather the heat generation per area. The temperature is a function of how quickly the heat can be pulled away as well as the heat generation.

Thanks for the clarification.

> per unit area

A CPU is a lot smaller than both a nuclear reactor and the sun.

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