Hardware Reference
In-Depth Information
R
total
= T
case
- T
inlet
/P
power
T
case
is the maximum allowable CPU case temperature, T
inlet
is the maximum allowable
inlet temperature to the CPU heatsink, and P
power
is the maximum power dissipation of
the processor. For example, the Pentium 4 3.4E (Prescott) processor is rated for a maxim-
umcasetemperatureof73°CandhasamaximumTDP(ThermalDesignPower)outputof
103watts.Intelrecommendsamaximumheatsinkinlettemperatureof38°C,whichmeans
the heatsink required to properly cool this chip needs to be rated 0.34°C/W, or (73°C -
38°C) / 103W.
A more extreme example is the Core 2 Extreme QX6800 quad-core processor, which is
rated for a maximum 54.8°C at 130W TDP (Thermal Design Power). This requires an ex-
ceptionally high performance heatsink rated 0.13°C/W, or (54.8°C - 38°C) / 130. A res-
istance rating this low usually requires some form of liquid cooling.
Another useful formula can describe processor power:
P
power
= C × V
2
× F
P
power
is the maximum power output of the processor, C is the capacitance, V
2
is the
voltage squared, and F is the frequency. From this you can see that if you double the fre-
quency of a processor, it consumes twice as much power, and if you double the voltage,
it consumes four times as much power. Consequently, if you lower the voltage by half, it
consumes only one-fourth the power. These relationships are important to consider if you
areoverclockingyourprocessorbecauseasmallincreaseinvoltagehasamuchmoredra-
matic effect than a similar increase in speed.
Ingeneral,increasingthespeedofaprocessorby5%increasesthepowerconsumptionby
onlythesameamount.Usingthepreviousheatsinkcalculation,iftheprocessorspeedwas
increasedby5%,the103Wprocessorwouldnowdraw108.15Wandtherequiredheatsink
ratingwouldgofrom0.34°C/Wto0.32°C/W,aproportionalchange.Inmostcases,unless
youareoverclockingtotheextreme,theexistingheatsinkshouldwork.Asacompromise,
you can try setting the voltage on manual and dropping it a small amount to compensate,
thereby reducing the power consumption. Of course, when you drop the voltage, the CPU
might become unstable, so you need to test it. As you can see, changing all these settings
in the interest of overclocking can take a lot of time when you consider all the testing re-
quired to ensure everything is working properly. You have to decide whether the rewards
are worth the time and energy spent on setting it up and verifying the functionality.
Note that most professional heatsink manufacturers publish their °C/W ratings, whereas
many of what I call the “boutique” heatsink vendors do not. In many cases the manufac-
turers of many of the more extreme heatsinks don't do the testing that the professional
manufacturers do and are more interested in the looks than the actual performance.
