AN2680

BLDC fan

5/15

2

BLDC fan

The basic DC brushless fan is a 2-wire device over which a DC voltage is applied. Brushless

DC fans are called "brushless" because the electric motor is commutated electronically.

The basic brushless DC motor consists of two main parts:

a)

the rotor

b)

the stator

a)

Rotor: As the name implies, the rotor is the part that rotates. The rotor houses the

permanent magnets, and in the case of the fan, the fan blades are also attached to

the rotor. The number of poles in the permanent magnet varies according to the

characteristics of the motor.

b)

Stator: The stator is the stationary part of the motor. It consists of the motor coil

number which varies according to the characteristics of the motor. The stator for

the 2-phase motor consists of four coils.

2.1

Principle used for fan speed control using PWM

The speed of the DC fan can be modified by varying the DC voltage across the two

terminals of the fan motor. However if we take a DC fan and switch on the DC supply across

it, the fan motor takes some time to speed up.This is because the fan motor has an inductive

coil so it does not respond immediately to the applied voltage. If we switch the power off

before the motor reaches full speed, the motor starts to slow down. If we switch the power

on and off quickly enough the fan motor and hence the fan run at some speed between the

zero and full speed. This is what is achieved through the PWM signal. The fan speed can be

modified with the variation in the duty cycle of the PWM signal.

2.2

Method of fan speed control using a power MOSFET

To control the speed of the DC fan using the PWM signal, we need to use a switch which

can be switched on and off at PWM frequency and hence control the supply voltage across

the motor of the DC fan. This switch can be made by using a high switching speed power

MOSFET. The action of switch is to connect and disconnect the power across the fan at

PWM frequency.

2.2.1

PWM control using transistor at low side drive

In low side drive connection, the fan positive terminal is kept at constant DC voltage (5 V for

5 V fan) while the negative terminal of the fan is connected to the drain of power MOSFET.

The source of the power MOSFET is connected to ground. The PWM signal is applied to the

gate terminal of the power MOSFET, thus the power MOSFET switches between on and off

condition at the rate of the PWM signal. When the power MOSFET is ON, the current builds

up in the coil of the fan motor and the fan starts to attain speed, whereas when the power

MOSFET is off, the fan starts losing speed. Figure 2 illustrates the low side drive transistor.