CPU amps and watts question

[shaneshane1]

I have a simple question that i think i know the answer to but i am not 100% sure

A computer CPU (mine for example!) is 1.4V and has a TDP (thermal design power) of 67Watts! there is a little example here: Thermal design power - Wikipedia, the free encyclopedia now at that TDP the CPU would be pretty maxed out without some extra cooling options needed to bring the heat down!

So my question is that the CPU uses 1.4Volts to function and it can run at 67Watts before extra cooling is needed...etc basic equation is: 67W/1.4V = ~47Amps! Now that's a lot Amps! or is it more like 12v(the supply rail)-1.4v(the CPU) = 10.6v 67Watts/10.6=6.3Amps i don't know!

Now how does a power supply cope with these amps, does the power supply use the power from the 12V from the 24pin connection as well as other connections like the 12V atx connection, because most power supplies do not produce that many amps per connection! or does it NOT draw that much current? thanks...

 

[DemonicSandwich]

The main PSU of a PC is way too noisy to power the CPU directly. Instead the motherboard has it's own (usually multiphase) switching regulator to power the CPU.

While that may 47A at 1.4v, that's only like 7-8A from the 12v rail of the PSU.

 

[audioguru]

The CPU uses 47A only when it is number crunching very hard. Most of the time it is idling.
Doesn't it have a variable speed fan that you can hear? Then you can hear when it is working hard.

The 12V has a fairly low current used for the hard drive motor, CD, DVD and a few fans.

 

[shaneshane1]

The main PSU of a PC is way too noisy to power the CPU directly. Instead the motherboard has it's own (usually multiphase) switching regulator to power the CPU.

While that may 47A at 1.4v, that's only like 7-8A from the 12v rail of the PSU.

i suppose my question is more theory than practice! so regardless of the PSU 12v rail 7-8A, is the actual CPU using really high amps under FULL number crunching load(say 67Watts)? 47Amps!? because people keep telling me that the CPU (not the power supply) only uses ~5.5A!? well not by this formula it doesn't!

67Watts/1.4V = ~47A!?

would someone be able to give me a little insight into how(if i am right) the CPU can use ~47A when the 12v supply cant produce that!? thanks...

 

[audioguru]

47A at 1.4V is 66W.
5.5A at 12V from a switching regulator that has a 1.4V/47A output is also 66W.

In "the good old days" the CPU used a high current from the 5V supply.

 

[shaneshane1]

47A at 1.4V is 66W.
5.5A at 12V from a switching regulator that has a 1.4V/47A output is also 66W.

In "the good old days" the CPU used a high current from the 5V supply.

so its true? the CPU is using 47A and the 12v rail is using 5.5A?

 

[MikeMl]

A decent switch mode power supply is ~90% efficient, so to get 67W to the CPU (1.4*47.9A), the switcher would draw 67/0.9 = 74.4W, so it would draw 74.4/12 = 6.20A off the 12V rail.

 

[shaneshane1]

A decent switch mode power supply is ~90% efficient, so to get 67W to the CPU (1.4*47.9A), the switcher would draw 67/0.9 = 74.4W, so it would draw 74.4/12 = 6.20A off the 12V rail.

Ok thank you very much! i just HAD to clear this up in my head! got sick of everyone telling me that the CPU only consumes ~5.5A! they were not looking at the big picture... the CPU itself does in fact consume ~47A @ 1.4V so now i know how it works!(well enough to except that it does in fact work this way even though the PSU is at ~5.5A, 67W)thanks everyone!

 

[MrAl]

Hello there,


Yes it is true that the CPU uses more current than the 12v line delivers because the motherboard has multiple buck converters on it that can produce more amps at the cost of more volts. A 12v line rated for 10 amps can deliver almost 40 amps at 3v if a good buck converter is used. In other words, it acts almost like a transformer because in theory the buck converter is a true power converter. 12v at 10 amps is 120 watts, and 120 watts divided by 3v is 40 amps, so you can see how this works. Since the CPU uses 1.4v if we approximate this as 1.5v we can divide that 120 watts by 1.5v and we get 80 amps. That is very similar to how a transformer works (with ac of course) but the buck works with DC power.
12v at 5 amps would be 60 watts, and 60 divided by 1.5 would be 40 amps.

Another interesting question related to this is just how can we get 40 amps into the CPU with such small pins on the CPU package. If you look at a drawing of a CPU pinout you'll see multiple pins used for the power supplies, so the current can be divided up among the many pins.

The other nice thing about doing it this way is the main power from the power supply only has to pass 5 to 10 amps of current to the mother board rather than 40 to 80 amps of current which would cause a much greater voltage drop in the wires and connectors. The motherboard buck converters can use much shorter trace runs and doesnt need to use any connectors other than the CPU socket.

CPU's were using more and more power as time went on and that was creating a power density problem so manufacturers starting going to multi-core designs rather than trying to get the higher power heat losses out of the package somehow.

 

[shaneshane1]

Hello there,


Yes it is true that the CPU uses more current than the 12v line delivers because the motherboard has multiple buck converters on it that can produce more amps at the cost of more volts. A 12v line rated for 10 amps can deliver almost 40 amps at 3v if a good buck converter is used. In other words, it acts almost like a transformer because in theory the buck converter is a true power converter. 12v at 10 amps is 120 watts, and 120 watts divided by 3v is 40 amps, so you can see how this works. Since the CPU uses 1.4v if we approximate this as 1.5v we can divide that 120 watts by 1.5v and we get 80 amps. That is very similar to how a transformer works (with ac of course) but the buck works with DC power.
12v at 5 amps would be 60 watts, and 60 divided by 1.5 would be 40 amps.

Another interesting question related to this is just how can we get 40 amps into the CPU with such small pins on the CPU package. If you look at a drawing of a CPU pinout you'll see multiple pins used for the power supplies, so the current can be divided up among the many pins.

The other nice thing about doing it this way is the main power from the power supply only has to pass 5 to 10 amps of current to the mother board rather than 40 to 80 amps of current which would cause a much greater voltage drop in the wires and connectors. The motherboard buck converters can use much shorter trace runs and doesnt need to use any connectors other than the CPU socket.

CPU's were using more and more power as time went on and that was creating a power density problem so manufacturers starting going to multi-core designs rather than trying to get the higher power heat losses out of the package somehow.

These are the answers i am looking for! thanks...