16-03-2012, 02:39 PM
Intelligent Thermal Management Using Brushless DC Fans
[attachment=18453]
INTRODUCTION
The Brushless DC fan (BDC Fan) has become the air mover
of choice in computing equipment, office automation products,
home entertainment systems, and the like. Unlike conventional
DC fans, the BDC fan is mechanically robust because it contains
no rotating commutator/brush assembly to shed dust particles,
wear out, or act as an ignition source. In addition, its magnetic coils
are stationary and are usually mounted within a rigid frame for
superior structural integrity and thermal dissipation. BDC fans are
electrically quiet: they lack the rotating magnetic fields of AC
motors and the arcing of conventional DC motors that broadcast
electronic noise.
SYSTEM CONSIDERATIONS
BDC fan management circuitry is tasked with making judicious
use of the fan, while at the same time monitoring its
operation. Speed control is employed to minimize fan wear, save
power and reduce acoustic noise. This is accomplished by either
running the fan only when measured temperature is above a
prescribed limit (off/on control), or by modulating fan speed with
measured temperature (temperature proportional control).
SYSTEM DESIGN
The primary design considerations are
the choice of temperature sensor and the
method of fan drive and speed control. The
choice of sensor dictates the nature and
complexity of the analog signal processing
needed on the front end of the BDC Fan
Manager. Output circuit size, cost, component
count, and heat dissipation determine
the selection of speed control and drive
methodology.
Fan Control Methodology:
As previously stated, temperature proportional
speed control is superior to off/
on control. Fan speed control can be accomplished
using linear voltage speed control
(voltage regulation), or pulse-width
modulation (PWM). Linear voltage speed
control is the classic method; its popularity
stems from low cost and low component
count. The limitations become apparent,
however, when driving larger fans at higher
power levels or when attempting to idle
fans at low speeds in order to reduce acoustic
noise and/or energy usage.
Pulse-Width Modulation (PWM):
PWM fan speed control methodology
(Figure 3B) has decided advantages over
linear voltage speed control:
1. The output drive transistor (Q1) is
either on or off. The average power
dissipation of Q1 is therefore collector-
to-emitter saturation voltage,
(VCESAT) times the fan current (IFAN),
times the duty cycle. Assuming a
VCESAT of 0.3V and a fan operating
current of 250mA, the maximum
power dissipation of Q1 is only 75mW,
eliminating the need for high power
output transistors and their
associated size, weight, and cost.
2. Unlike linear voltage speed control,
there is negligible voltage loss in the
PWM circuit, eliminating the need for
special voltages or multiple power
supply schemes.
[attachment=18453]
INTRODUCTION
The Brushless DC fan (BDC Fan) has become the air mover
of choice in computing equipment, office automation products,
home entertainment systems, and the like. Unlike conventional
DC fans, the BDC fan is mechanically robust because it contains
no rotating commutator/brush assembly to shed dust particles,
wear out, or act as an ignition source. In addition, its magnetic coils
are stationary and are usually mounted within a rigid frame for
superior structural integrity and thermal dissipation. BDC fans are
electrically quiet: they lack the rotating magnetic fields of AC
motors and the arcing of conventional DC motors that broadcast
electronic noise.
SYSTEM CONSIDERATIONS
BDC fan management circuitry is tasked with making judicious
use of the fan, while at the same time monitoring its
operation. Speed control is employed to minimize fan wear, save
power and reduce acoustic noise. This is accomplished by either
running the fan only when measured temperature is above a
prescribed limit (off/on control), or by modulating fan speed with
measured temperature (temperature proportional control).
SYSTEM DESIGN
The primary design considerations are
the choice of temperature sensor and the
method of fan drive and speed control. The
choice of sensor dictates the nature and
complexity of the analog signal processing
needed on the front end of the BDC Fan
Manager. Output circuit size, cost, component
count, and heat dissipation determine
the selection of speed control and drive
methodology.
Fan Control Methodology:
As previously stated, temperature proportional
speed control is superior to off/
on control. Fan speed control can be accomplished
using linear voltage speed control
(voltage regulation), or pulse-width
modulation (PWM). Linear voltage speed
control is the classic method; its popularity
stems from low cost and low component
count. The limitations become apparent,
however, when driving larger fans at higher
power levels or when attempting to idle
fans at low speeds in order to reduce acoustic
noise and/or energy usage.
Pulse-Width Modulation (PWM):
PWM fan speed control methodology
(Figure 3B) has decided advantages over
linear voltage speed control:
1. The output drive transistor (Q1) is
either on or off. The average power
dissipation of Q1 is therefore collector-
to-emitter saturation voltage,
(VCESAT) times the fan current (IFAN),
times the duty cycle. Assuming a
VCESAT of 0.3V and a fan operating
current of 250mA, the maximum
power dissipation of Q1 is only 75mW,
eliminating the need for high power
output transistors and their
associated size, weight, and cost.
2. Unlike linear voltage speed control,
there is negligible voltage loss in the
PWM circuit, eliminating the need for
special voltages or multiple power
supply schemes.