Blowers; Fan Speed Control And Cpu Throttling - Intel Thin Mini-ITX System Design Manual

Thermals & Acoustics
4.1.1

Blowers

 When using a 4-wire system blower (one that is PWM controlled), use a blower that
defaults to maximum speed when no PWM signal is supplied to the blower; for
example, when the blower is plugged into the system fan header on the
motherboard
 A blower that has both top and bottom side inlets can be leveraged to pull air
underneath the motherboard to enhance voltage regulator cooling. Alternatively,
the bottom side air inlet could be coupled to a hole in the chassis pan to pull air
through the gap between the flat panel display and the chassis component
mounting pan to help cool the flat panel display.
 Maximize the flow cross sectional area to the heat exchanger to minimize pressure
drop through the heat exchanger, and to minimize internal air temperature rise at
the blower exit.
 Larger diameter blowers are more efficient at moving air than smaller diameter
blowers.
 Larger diameter blowers tend to spin at lower RPM than smaller diameter blowers.
The lower rotation speed results in better sound quality; i.e. lower tonality level.
4.1.2

Fan Speed Control and CPU Throttling

A system integrator should choose a motherboard that provides CPU and System fan
speed control that responds to CPU and PCH and motherboard temperature. It is
important to understand that there are two mechanisms that can cause CPU throttling.
The voltage regulator can have a temperature sensor circuit that is present to detect
whether the voltage regulator is operating above its intended maximum temperature
limits. The Intel microprocessor is designed to receive a signal from the voltage
regulator (VR_HOT#) that will cause the CPU to start reducing its operating frequency
to reduce the power output of the voltage regulator in order to reduce the thermal
heat dissipation occurring in the voltage regulator. This type of throttling will not be
obvious when looking at DTS values of the CPU die. It will be evident when looking at
the current processor operating frequency or CPU operating power consumption. The
second, and more commonly known, source of CPU throttling is when the CPU die
itself exceeds its intended maximum temperature limit. In this case, the DTS values
will reach -1 resulting in induced CPU throttling.
The following figure shows an example of voltage regulator induced throttling. It is
evident when looking at the CPU power consumption plot over time. In this example,
the voltage regulator is causing its power output to no longer be a constant value of
59W, but to follow a continuous square wave. The power output is cycling between
59W and 35W.
Thin Mini-ITX Based PC System Design Guide 37