Fan motor speed control circuit

A fan motor speed control circuit is disclosed. The circuit includes a digital/analog converting unit and a driving unit. The digital/analog converting unit takes charge of receiving a pulse width modulation (PWM) signal and converting it into a voltage signal. The driving unit is connected with the digital/analog converting unit in series for receiving the voltage signal, while the driving unit provides a first predetermined voltage level and a second predetermined voltage level as references. The fan motor is kept to run with a low constant rotation speed when the voltage signal level is higher than the first predetermined voltage level, with a full constant rotation speed when the voltage signal level is lower than the second predetermined voltage level, and with a variable rotation speed when the voltage signal level is lower than the first predetermined voltage level and higher than the second predetermined voltage level.

This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 092119947 filed in TAIWAN on Jul. 22, 2003, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to a rotation speed control circuit, and more particularly to a fan motor speed control circuit.

DESCRIPTION OF RELATED ART

With respect to fan motor speed control techniques, it is well known that people skilled in the art often uses a control circuit being composed of various type of transistors and driving integrated circuit (IC) chips, and an external driving signal for the control circuit, such as a pulse width modulation (PWM) signal, to control the rotation speed of a fan motor.

Referring toFIG. 1A, in a conventional fan motor rotation speed control technique, an external pulse width modulation (PWM) signal3is mainly used to drive a bipolar transistor2and a drive IC5, in order to control the terminal voltage of a stator coil I representing the fan motor (not shown). In addition, a Hall element4is used to detect the magnetic polarity of a rotor of a fan motor, in order that the current direction of the stator coil1matches with the magnetic pole position of the rotor. Referring toFIG. 1B, in another conventional fan motor speed control technique, an external voltage VAand an external pulse width modulation (PWM) signal3are used to drive two bipolar transistors12and13, and a drive IC14.

In the above-mentioned conventional techniques, however, when the frequency of the pulse width modulation (PWM) signal is low, the switch between ON and OFF state of the transistors2,12,13directly driven by the pulse width modulation (PWM) signal often leads to vibrations during the operation of the fan motor, and thus produces undesired noises easily.

SUMMARY OF THE INVENTION

A conventional fan motor rotation speed control circuit cannot avoid the noise problem caused by the ON/OFF switch of the fan motor when the frequency of a pulse width modulation (PWM) signal is low.

In order to solve this problem, this invention discloses a fan motor speed control circuit that reduces the above-mentioned noises and smoothens the speed change of the fan motor.

One object of this invention is to provide a fan motor speed control circuit to control the rotation speed of a fan motor by means of a digital/analog converting processing and a low to high frequency signals switching processing.

According to one embodiment of the invention, the fan motor speed control circuit comprises a digital/analog converting unit and a driving unit. In this embodiment, the digital/analog converting unit takes charge of receiving a pulse width modulation (PWM) signal and converting it into a voltage signal, while the driving unit is connected with the digital/analog converting unit in series to receive the voltage signal and provides a predetermined high voltage and a predetermined low voltage as references.

When the above-mentioned voltage signal level is higher than the above-mentioned predetermined high voltage level, the fan motor is kept to run with a low constant rotation speed. When the voltage signal level is lower than the predetermined high voltage level but higher than the predetermined low voltage level, the fan motor is allowed to have a variable rotation speed. When the voltage signal level is lower than the predetermined low voltage level, the fan motor is kept to run with a high constant rotation speed.

The advantage of this invention is to reduce the vibration degree and noise problem occurred in a conventional fan motor control circuit directly driven by an external low frequency PWM signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 2, a fan motor speed control circuit30in accordance with the invention comprises a digital/analog converting unit301and a driving unit302. The digital/analog converting unit301is used for converting an inputted PWM digital signal3into an analog signal such as a voltage signal. The driving unit302takes the analog signal and compares it with two predetermined voltages Vref1and Vref2set by the driving unit. After that, a digital signal representing the comparison result is used to control the rotation speed of the fan motor40.

The fan motor speed control circuit according to one embodiment of the invention is shown inFIG. 3A. In this embodiment, a digital/analog converting circuit22is employed as a digital/analog converting unit, and a drive IC23is employed as a driving unit. In this embodiment, the digital/analog converting circuit22is mainly composed of a transistor221, a diode222, a capacitor223, and a plurality of resistors224a˜224d.On the other hand, the drive IC23comprises at least an oscillating element231, a control element232and a plurality of terminals233a˜233h.These terminals are electrically coupled with or connected to a voltage source Vcc, an external capacitor24, a Hall element25and the stator coil21of the fan motor, respectively. Moreover, the oscillating element231is activated by an external capacitor24and has a high frequency outputted digital signal, such as 25 KHz. Therefore, the drive IC23in this embodiment is different from the conventional drive IC5and drive IC14.

Hereinafter, the working principle of the fan motor speed control circuit according to this embodiment is described. First, an input pulse width modulation signal3is converted into an analog voltage signal by the digital/analog converting circuit22, and received by the drive IC23. The drive IC23takes the level VTHof the analog voltage signal as a threshold voltage. After that, the control element232compares the threshold voltage level VTHwith the upper voltage level VHand the lower voltage level VLof a high frequency signal (as shown inFIG. 3B) output by the oscillating element231. When VTHis higher than VH, the control element232controls the terminal voltage of the stator coil21to make the fan motor have a zero rotation speed. When VTHis lower than VHbut is higher than VL, the control element232controls the two terminal voltages of the stator coil21to make the fan have a variable rotation speed. When VTHis lower than VL, the fan motor has a high constant rotation speed. In this embodiment, the relation between the duty cycle of a PWM signal3and the rotation speed of a fan motor is shown inFIG. 3C. In this diagram, the left side and the right side of the speed changing point respectively shows the cases of VH>VTH>VLand VTH<VLas described above.

A fan motor speed control circuit according to another embodiment of the invention is shown inFIG. 4A. In this embodiment, a digital/analog converting circuit22is employed as a digital/analog converting unit, and a drive IC23is employed as a driving unit. In this embodiment, the digital/analog converting circuit22comprises the same elements as those described earlier. On the other hand, the drive IC23further comprises a terminal233ibesides an oscillating element231, a control element232and a plurality of terminals233a˜233h.These terminals are electrically coupled with or connected to a voltage source Vcc, an external capacitor24, a Hall element25, the stator coil21of the fan motor, and a voltage-dividing circuit26, respectively. This voltage-dividing circuit26is composed of resistors26aand26b,the objective of which is to provide a reference voltage Vreftaken from the voltage level of the resistor26bby the voltage-dividing circuit26.

Noteworthily, the oscillating element231is activated by an external capacitor24and has an output digital signal of high frequency, such as 25 KHz. This is how the drive IC23in this embodiment differs from the conventional drive IC5and drive IC14.

The working principle of the fan motor speed control circuit of this embodiment is similar to those described above. The same part will not be described again. The differential part is that, after the drive IC23obtained the level VTHof a threshold voltage, the control element232compares the threshold voltage level VTHwith a low voltage level VLof a high frequency signal output by the oscillating element231(as shown inFIG. 3B), and a reference voltage Vref. Herein, the reference voltage Vrefis lower than the high voltage level VHof the high frequency signal, which is shown inFIG. 3B.

When VTHis higher than Vref, the control element232controls the terminal voltage of the stator coil21to make the fan motor have a low constant rotation speed. When VTHis lower than Vrefbut higher than VL, the control element232controls the terminal voltage of the stator coil21to make the fan motor have a variable rotation speed. Otherwise, when VTHis lower than VL, the fan motor keeps a high constant rotation speed. In this embodiment, the relation between the duty cycle of a PWM signal3and the rotation speed of a fan motor is shown inFIG. 4B.

In summary, this invention has been described in details with the above embodiments and variations. Any person skilled in the art should understand that all embodiments are described herein for exemplary purposes rather for restriction. That is, any modification or changes to the fan motor speed control circuit as described above without departing from the spirit and scope of the invention should be embraced in the appended claim.