AN2687 Freescale Semiconductor / Motorola, AN2687 Datasheet - Page 3

• Manufacturer Part Number: AN2687
• Description: BLDC Fan Control using the MC68HC908QT2
• Manufacturer: Freescale Semiconductor / Motorola

Speed Control
Methods
Control by adjusting
off time at phase
switching
Control by closed-loop
pulse width
modulation (PWM)
PART 2: HARDWARE DESIGN
Introduction
MOTOROLA
The speed of a BLDC fan can be controlled by:
The rotation speed can be adjusted by the length of the off time. The off time is
inversely proportional to the energy provided to the fan. That is, the longer the
off time, the lesser the energy is supplied to the stator coils, which results in a
lower speed of the fan. But one condition should be satisfied is that it needs to
be synchronized to the feedback signal from Hall effect sensor (i.e. the
commutation).
The rotation speed can be adjusted by the duty cycle of the PWM. The principle
is the same as the off time adjustment method but the PWM frequency is much
higher than the commutation frequency and therefore it does not need to
synchronized to the feedback signal from the Hall effect sensor.
The advantages of using the PWM method are more accurate rotation speed
control and audio frequencies are avoided because of the higher frequency of
the PWM. The drawbacks are higher losses in the power element (switching
element) and the higher system cost.
In this application note, we will use the off time adjustment method to control
the fan speed. The advantages of the off time adjustment method are:
The following sections describe how to use MC68HC908QT2 MCU for variable
speed BLDC fan control. This BLDC fan reference design consists of two
demos:
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Freescale Semiconductor, Inc.
BLDC Fan Control using the MC68HC908QT2
For More Information On This Product,
adjusting the off time at phase switching, or
closed-loop pulse width modulation (PWM).
Circuit is relatively simple.
Very small losses in power element.
High reliability.
Additional circuit can be added to prevent the audio sound.
The evaluation demo consists of the trim-pot (VR1) and in-circuit
programming (ICP) feature for user evaluation and development
purposes.
The reference demo removes the ICP feature and uses the PWM to
drive the surface mount transducer instead of the buzzer in the
evaluation demo. The PCB is also designed to fit into the fan assembly.
Go to: www.freescale.com
PART 2: HARDWARE DESIGN
AN2687/D
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