CPU clock and heat
 

Hya,

I am simply curious and have not found answer.

Question is:
Why does CPU generate more heat as clock goes up?

Happy Penguins!

The faster the clock speed; the more electricity it draws in. The electrons moving at a faster pace into the conduits cause more friction thereby generating more heat.

This was a gross over-simplified explanation but that is the main idea.

Think of it as a resistor. When more current is sent through the resistor (clock gets faster, needs more juice), the power (watts) increases. As the resistor uses more watts, it generates more heat.

Another thought would be a light bulb. The brighter the bulb, the hotter it gets (more wattage used).

Hya,

Thanks for replies.

Why "speed" of electron gets faster as clock goes up? Also, as friction goes up, (under same voltage), current goes down.

Ohm's law (E =IR) then the wattage (defined as EI), is R(I^2) or (E^2)/R. Friction goes up means R gets bigger; so wattage goes down.

I also think R is intrinsic to circuit, (so clock does not affect..)

Another point I do not understand is: How electron gets higher speed? (under same voltage)

I still need to find missing link.

Happy Penguins!

Electrons always move at the same speed......If the current increases, it simply means that more electrons are moving.

Current is defined as the rate of movement of charge: 1 ampere = 1 coulomb/second, and 1 coulomb = 6.241E18 electrons. In a computer chip, something like nano-amps might be more meaningful.....1 nano-amp ~ 6 billion electrons/second.

In re the orginal question---another perspective:
Think of each operation in a CPU as needing a specific amount of energy. Do the operations faster and the rate of energy consumption (AKA power) goes up.

Hya,

PMorph: Thanks for the link.

The second (Pstat) and third (Psc) equations are somewhat intuitive. (Ohm's law)

The last sentence sounds like a fact, whether or not I understand.

So, I need to figure out the first equation (Pdyn)

I need to figure out if I understand correct: Following is my interpretation.

1. "due to the diabatic charging and discharging of the circuit's internal and external capacitances when the circuit is operating." To make life simple, there is only one capacitor, (charged = 1, discharged =0; its capacity is 1pF) then this state can be changed as clock speed.
2. If clock is 1 Hz, this means that this capacitor stays either 1 or 0 for one second.
3. Capacitor can store energy (but not consume.)
4.1 If this state stays on, no heat generation.
4.2 If this state changes, energy has to be discharged, -> this will generate heat, if there is any resistor around.

So, this capacitor can generate 1 unit of heat per 1 second at most. Now, if clock is 2Hz, 2 unit at most.
This sounds like the first equation (Pdyn), but Fig 3.3 does not show any resistor.

Where is the resistor?

Happy Penguins!

Hya

Thanks, I miss your post while I was trying to figure out the Pdyn.

I think your re-phrasing is well describing.

I guess my confusion started when I understand CPU chip as a resistor load.

Cheers

Happy Penguins!

Back to your comments on capacitors and resistors. Unless you are dealing with superconductors, every circuit has resistance---ie a current flow always results in a voltage drop (no matter how small)

If you like puzzles, try this:

2 identical capacitors--each 1 farad, one of which is charged to 10 volts, and the other is discharged. Connect them thru a resistor.
Before connecting, the energy in the first one is 0.5*CV^2 = 0.5*1*100 = 50 units
After the connection, charge is conserved (Q=CV). With twice the capacitance, the voltage is now 5 volts, and the stored energy is now 0.5*1*25 = 12.5 units. The missing energy has been dissipated in the resistor (as heat).

Now, let the value of the resistor be zero. Charge must still be conserved, so the voltage after connecting will be the same as before. Where does the energy go?

When you increment the CLOCK, the CLOCK increases. Now, because the processor is CLOCKIN' so much, it gets very HOT...

They call this the Gallo effect, as it was discovered by Professor Deven Gallo of harvard university.

Hya

Yes, it is well known, but question is why "clockin" results in heat.

It was a while ago since I attended to physics class. I still have not figured out, but I do not think capacitor has to be "fully" charged, -- it can be partially charged. As soon as I solve this puzzle, I will post again.

By the way, I do not think the logic I followed at post #7 works when logic circuit is made of flip flop (two transistors). They are resistor load (not capacitor load), so to keep either 1 or 0, some sort of power is necessary,
and I do not think that changing state consumes power.

1 Hz (1 second of load) and 2Hz (0.5second load times 2) which is same amount.

Happy Penguins!