Abstract:
The present invention relates to a system and method controlling motor rotation speed and provides a cooling system and method configured to control a temperature associated with an integrated circuit. The cooling system includes a brushless motor, a temperature monitoring input, a clock input, and a motor controller. The motor controller is configured to control the rotational speed of the motor using at least a speed control method by comparing the environmental temperature signal to a predetermined threshold: if the environmental temperature signal is less than the predetermined threshold T 1  or higher than T 2 , controlling the rotational speed of the motor uses the speed control method and only one of the environmental temperature signal and the clock signal; and if the environmental temperature signal is greater than the predetermined threshold T 1  and less than T 2 , controlling the rotational speed of the motor uses the speed control method and both of the environmental temperature signal and the clock signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Chinese patent application No. 201010195275.7, filed on Jun. 8, 2010, and entitled “A control pulse generating circuit used to regulate the rotational speed of a brushless direct current motor”. 
     FIELD OF THE INVENTION 
     The present invention generally relates to systems and methods controlling motor rotation speed and, more particularly, to systems and methods for controlling brushless direct current motor rotation speed. 
     BACKGROUND OF THE INVENTION 
     Single-phase speed regulating brushless direct current motor has advantages on energy efficiency and noise control, and has a wide range of application and, particularly, applications and environments including electronics, where energy efficiency and noise control are often of paramount consideration. In particular, these motors are often employed with cooling fans in consumer electronics equipment, such as for computer hard drives, data storage devices, video game consoles, CD/DVD players, and the like, which generally dissipate a lot power and generate a lot of heat while in operation. 
     Among the existing technologies regularly utilized in controlling the rotation speed of brushless direct current motor for cooling fans, there are three main methods for regulating the motor rotation speed: fixed speed; variable speed controlled by a clock signal, such as, a pulse width modulation (“PWM”) signal; and variable speed controlled by a temperature feedback. 
     Fixed speed fans are not able to vary the rotation speed, which is energy inefficient and cause more noise. For example, in a typical computer housing, there is a fixed speed cooling fan that operates continuously once the computer is turned on. The fixed speed cooling fan operates at its maximum power and does not meet the trend towards low-power consuming devices. 
     Variable speed fans are controlled by a clock signal, for example, a PWM signal and, generally have a fan speed that is near linear with the duty cycle of the PWM signal. For example, a device can use a PWM signal with specific duty cycle setup according to different operation mode to control the cooling fan. Therefore the cooling fan can work at low speed with respect to low work-load operation mode. Such designs are more energy efficiency and reduce noise; however, they may not offer as much efficiency and flexibility as may be desired. 
     Variable speed fans are controlled by a temperature feedback such as, a detected temperature of a component such as a power supply of an electric device, a processor, a memory, a hard disk drive, and the like. This temperature measurement is used to determine the rotational speed of the motor coupled to a cooling fan. This method provides more flexibility and reduce power consumed by the fan and motor and significantly reduces the noise generated by operation of the fan and motor when the detected temperature is relatively low. Fan and motor control based on temperature alone, however, presents a number of drawbacks because the fan speed control is reactive, rather than proactive. In other words, adjusting fan speed based on temperature only adjusts the fan speed after the temperature has already increased. When power dissipation is increased in a component, it takes time for the thermal information being transferred from the heat generating device to reach a temperature sensor. In this situation, desired cooling function may not be provided before the device reaches a still higher temperature or even reaches an undesirable temperature. In addition, other components in a system may also be heating up and the total heating of the system may not be linear with the temperature of the component where temperature is detected. 
     Therefore, it would be desirable to have a system and method for regulating the motor rotation speeds with a more robust, energy-efficient, flexible, yet not overly-complex, method of control. 
     SUMMARY OF INVENTION 
     The present invention overcomes the aforementioned drawbacks by providing a motor rotation speed control system that regulates the motor rotation speed by using both the environmental temperature and a clock signal, such that the motor rotation speed can be regulated under various functions corresponding to different circumstance. 
     In accordance with one aspect of the present invention, a cooling system configured to control a temperature associated with an integrated circuit is provided. The cooling system includes a brushless motor, a temperature monitoring input, a clock input, and a motor controller. The brushless motor is configured to drive a cooling device based on a rotational speed of the motor. The temperature monitoring input is configured to receive an environmental temperature indicating a temperature associated with the integrated circuit. The clock input is configured to receive a clock signal having a predetermined duty cycle. The motor controller is configured to receive the environmental temperature signal from the temperature monitoring input and the clock signal input and coupled to the brushless motor to control the rotational speed of the motor. The motor controller compares the environmental temperature signal to a predetermined threshold T 1  and T 2 : if the environmental temperature signal is less than the predetermined threshold T 1  or higher than T 2 , controlling the rotational speed of the motor uses the speed control method and only one of the environmental temperature signal and the clock signal; and if the environmental temperature signal is between the predetermined threshold T 1  and T 2 , controlling the rotational speed of the motor uses the speed control method and both of the environmental temperature signal and the clock signal. 
     More specifically, the motor controller includes a pulse generating circuit, which includes a temperature signal processing circuit, a clock signal processing circuit, a triangular wave generating circuit, and a control pulse output circuit. The temperature signal processing circuit is configured to receive the environmental temperature signal, and output an electric current signal, which has a relationship with the environmental temperature signal. The clock signal processing circuit is configured to receive the clock signal and the electric current signal (and an input pulse signal), and generate a direct current voltage signal. The triangular wave generating circuit is configured to generate a triangular wave signal. The control pulse output circuit configured to process the triangular wave signal and the direct current voltage signal, and deliver a control pulse signal to control the rotational speed of the brushless motor. 
     The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a motor control system  100  that is configured to regulate a rotation speed of a brushless direct current motor, in accordance with the present invention. 
         FIG. 2  is a circuit diagram showing the temperature signal processing circuit and the PWM signal processing circuit of  FIG. 1   
         FIG. 3  is a circuit diagram showing a another configuration of the temperature signal processing circuit and the PWM signal processing circuit of  FIG. 1 . 
         FIG. 4  is a circuit diagram showing a triangular wave generating circuit of  FIG. 1 . 
         FIG. 5  is a circuit diagram showing a brushless motor control system that is configured to regulate rotation speed of a brushless direct current motor, in accordance with one configuration of the present invention. 
         FIG. 6  is a circuit diagram showing a minimum duty cycle setup circuit of  FIG. 5 . 
         FIG. 7  is a circuit diagram showing a control pulse output circuit of  FIG. 5 . 
         FIG. 8  is a graph illustrating relationships between the environmental temperature and the currentI 22 , the currentI 23 , and the current I 20  of  FIG. 2 . 
         FIG. 9  is a graph illustrating the relationship between the direct current voltage signal Vcf, the duty cycle d of the PWM input signal and the environmental temperature based on the circuit of  FIG. 3 . 
         FIG. 10  is a graph illustrating the relationship between the direct current voltage signal Vcf, the triangular input signal, the duty cycle d of the PWM input signal and the environmental temperature based on the circuit of  FIG. 5 . 
         FIG. 11  is a graph illustrating relationships between the environmental temperature, rotation speed of the brushless direct current motor, and the duty cycle d of the PWM input signal, in accordance with the present invention. 
         FIG. 12  is a flow chart setting forth the steps of a process for regulating the rotation speed of a brushless direct current motor, in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a circuit diagram showing a brushless motor control system  100  that is configured to regulate a rotation speed of a brushless direct current motor, in accordance with the present invention. The system  100  includes a control pulse generating circuit  10 , a hall signal generating circuit  20 , a logic control circuit  30 , and a drive circuit  40 . 
     In  FIG. 1 , the control pulse generating circuit  10  can be configured to generate a control pulse signal  172 . The duty cycle of the control pulse signal  172  has a linear relationship with both the duty cycle of a PWM input signal  154  and an environmental temperature signal  162 . The Hall signal generating circuit  20  can be configured to generate a direction reversal control signal  182  when a generator rotor passes through a Hall component. The logic control circuit  30  can be configured to receive the direction reversal control signal output from the Hall signal generating circuit  20  and the control pulse signal output from the control pulse generating circuit  10 , process and output these signals to the drive circuit  40 . The drive circuit  40  is configured to control and regulate rotation speed of the brushless direct current motor by using the received the direction reversal control signal and the control pulse signal from circuit  30 . 
     More specifically, as shown in  FIG. 1 , the control pulse generating circuit  10  includes a temperature signal processing circuit  110 , a PWM signal processing circuit  120 , a triangular wave generating circuit  130 , and a control pulse output circuit  140 . The temperature signal processing circuit  110  can be configured to convert a detected environmental temperature signal  162  into an electric current signal  152 . This electric current signal  152  has a linear relationship with the environmental temperature and can be output to the PWM signal processing circuit  120 . The PWM signal processing circuit  120  can be configured to receive a PWM input signal  154  and the electric current signal  152  output by the temperature signal processing circuit  110 , and convert the PWM input signal  154  into a direct current voltage signal Vcf  156 , which has a linear relationship with both the duty cycle of the PWM input signal  154  and the environmental temperature signal  162 . The direct current voltage signal Vcf  156  can be output to a negative input terminal of the control pulse output circuit  140 . The triangular wave generating circuit  130  can be configured to generate a triangular wave signal  158 , which can be output to a positive input terminal of the control pulse output circuit  140 . The control pulse output circuit  140  can be configured to generate a control pulse signal  172  by using the direct voltage signal Vcf  156  received on the negative input terminal and the triangular wave signal  158  received on the positive input terminal. The duty cycle of the control pulse signal  172  has a linear relationship with both the duty cycle of the PWM input signal  154  and the environmental temperature signal  162 . 
       FIG. 2  is a circuit diagram showing a first configuration of the temperature signal processing circuit  110  and the PWM signal processing circuit  120  of  FIG. 1 . As shown in  FIG. 2 , the temperature signal processing circuit  110  includes a first transistor  212 , a second transistor  222 , a third transistor  232 , a fourth transistor  242 , a fifth transistor  252 , a sixth transistor  262 , a seventh transistor  272 , a first resistor  281 , a comparator  282 , a temperature sensitive resistor  283 , and a first electric current source  291 . 
     As shown in  FIG. 2 , the first resistor  281  is connected to a first reference voltage Vref. a negative input terminal  284  of the first comparator  282  is connected to a shared terminal  285  of the first resistor  281  and the temperature sensitive resistor  283 , a positive input terminal  286  of the first comparator  282  is connected to a second reference voltage Vref 1 , and an output terminal  288  of the first comparator  282  is connected to a collector electrode  276  of the seventh transistor  272 . A base electrode  274  of the seventh transistor  272  is connected to a collector electrode  276  of the seventh transistor  272 . A base electrode  274  of the seventh transistor  272  is connected to a base electrode  264  of the sixth transistor  262 . An emitter electrode  278  of the seventh transistor  272  and an emitter electrode  268  of the sixth transistor  262  are both connected to a ground  209 . A collector electrode  266  of the sixth transistor  262  is connected to a negative terminal  290  of the first electric current source  291 , a collector electrode  216 , and a base electrode  214  of the first transistor  212 . A positive terminal  292  of the first electric current source  291  is connected to a power supply VCC. Respective base electrodes  214 ,  224 ,  234  of the first transistor  212 , the second transistor  222 , the third transistor  232  are connected to each other. Respective emitter electrodes  218 ,  228 ,  238  of the first transistor  212 , the second transistor  222 , the third transistor  232  are connected to the ground  209 . A collector electrode  226  of the second transistor  222  is connected to a collector electrode  256  of the fifth transistor  252 . The collector electrode  256  and a base electrode  254  of the fifth transistor  252  are short circuited and connected to a base electrode  244  of the fourth transistor  242 . An emitter electrode  258  of the fifth transistor  252  and an emitter electrode  248  of the fourth emitter  242  are both connected to the power supply Vcc. A collector electrode  246  of the fourth transistor  242  is a first output terminal  201  of the temperature signal processing circuit  110 , which outputs an electric current I 22  to the PWM signal processing circuit  120 . A collector electrode  236  of the third transistor  232  is a second output terminal  202  of the temperature signal processing circuit  110 , which outputs an electric current I 23  to the PWM signal processing unit  120 . 
     A voltage VT of the terminal  285  which is the input voltage of the negative terminal  284  of the comparator  282  can be calculated by: 
                     VT   =     Vref   ×     RT       R   ⁢           ⁢   1     +   RT           ;           Eqn   .           ⁢     (   1   )                 
where RT is the resistance value of the temperature sensitive resistor  283  and R 1  is the resistance value of the first resistor  281 . The temperature sensitive resistor  283  is used to detect the environmental temperature. The resistance value RT of the temperature sensitive resistor  283  can change following the environmental temperature changes, which results in the voltage VT changes, as indicated by Eqn. (1). Further, as indicated by Eqn. (1), the voltage VT and the resistance value RT of the temperature sensitive resistor  283  are linearly related to each other. Since the resistance value RT of the temperature sensitive resistor  283  and the environmental temperature have a linear relationship, the voltage VT is linearly related to the environmental temperature.
 
     As shown in  FIG. 2 , the first comparator  282  can compare the second reference voltage Vref 1  with the voltage VT and output an electric current I 20 . The electric current I 20  can be calculated by: 
                       I   ⁢           ⁢   20     =         (       Vref   ⁢           ⁢   1     -   VT     )     ×   gm     =       (       Vref   ⁢           ⁢   1     -     Vref   ×     RT       R   ⁢           ⁢   1     +   RT           )     ×   gm         ;           Eqn   .           ⁢     (   2   )                 
where gm is a gm transconductor value of the first comparator  282 . As indicated by Eqn. (2), the electric current I 20  is linearly related to RT, and therefore is linearly related to the environmental temperature.
 
     As shown in  FIG. 2 , an electric current that passes through the first transistor  212  is directly proportional to the current difference between the first electric current source  291  and electric current I 20 . Further, the first transistor  212  and the second transistor  222  constitute a current mirror, and the fourth transistor  242  and the fifth transistor  252  constitute a current mirror. The electric current at the emitter electrode  258  of the fifth transistor  252  is proportional to the electric current of the first electric current source  291 , hence the electric current I 22  is linearly related to the environmental temperature. The electric current I 22  is also the output electric current signal of the temperature signal processing circuit  110 . The collector electrode  236  of the third transistor  232  can output the electric current I 23 , and the direction of the electric current I 23  is the opposite to the direction of the electric current I 22 . The electric current I 22  and  123  can be calculated by: 
                       I   ⁢           ⁢   22     =         I   ⁢           ⁢   21     -     I   ⁢           ⁢   20       =       I   ⁢           ⁢   21     -       (       Vref   ⁢           ⁢   1     -     Vref   ×     RT     R   ⁢           ⁢   1   ×   RT           )     ×   gm           ;           Eqn   .           ⁢     (   3   )               and                             I   ⁢           ⁢   23     =     M   ×     [       I   ⁢           ⁢   21     -       (       Vref   ⁢           ⁢   1     -     Vref   ×     RT     R   ⁢           ⁢   1   ×   RT           )     ×   gm       ]         ;           Eqn   .           ⁢     (   4   )                 
where M is the area ratio between the respective emitter electrode  238  and  218  of the third transistor  232  and the first transistor  212 .
 
     Therefore, the temperature signal processing circuit  110  is used to convert a detected environmental temperature signal into a current signal  122  and  123 . The current signal  122 ,  123  has a near linear relationship with the environmental temperature and is output to the PWM signal processing circuit  120 . 
     In  FIG. 2 , the PWM signal processing circuit  120  includes a second current source  293 , a controlled switch  295 , a second resistor  287 , and a capacitor  289 . 
     The positive terminal  296  of the second current source  293  is connected to the power supply Vcc, and the negative terminal  294  of the second current source  293  is connected to the normally closed terminal  297  of the controlled switch  295  and the collector electrode  236  of the third transistor  232  of the temperature signal processing circuit  110 , such that the negative terminal  294  can receive the current I 23  from the second output terminal  202  of the temperature signal processing circuit  110 . The control terminal  298  of the controlled switch  295  can receive the PWM signal, and the normally open terminal  299  of the controlled switch  295  is connected to the ground  209  through the second resistor  287 . The capacitor  289  is parallel connected to the second resistor  287 . The normally open terminal  299  of the controlled switch  295  is connected to the collector electrode  246  of the fourth transistor  242  of the temperature signal processing circuit  110 , such that the terminal  299  can receive the current I 22  from the first output terminal  201  of the temperature signal processing circuit  110 . Further the normally open terminal  299  is also an output terminal  204  of the PWM signal processing circuit  120 , which can output a direct current voltage signal Vcf to the control pulse output circuit  140 , as shown in  FIG. 1 . 
     As shown in  FIG. 2 , the direct current voltage signal Vcf from the PWM signal processing circuit  120  is the voltage between the two terminals of the capacitor  289 . When the PWM input signal has a low electric level, the controlled switch  295  is turn on, where the terminal  297  and the terminal  299  are connected. In this situation, the current difference between the second current source  293  and the current I 23 , the current I 22  charge the capacitor  289 . When the PWM input signal has a high electric level, the controlled switch  295  is turn off, where the terminal  297  and the terminal  299  are disconnected, and the capacitor  289  can discharge through the second resistor  287 . Therefore, the voltage at the two terminals of the capacitor  289  can change following the duty cycle of the PWM input signal. Further, the current I 22  and I 23  from the temperature signal processing circuit  110  can flow into the capacitor  289 , such that the voltage at the two terminals of the first capacitor  289  can also change following the changes of the currents I 22  and I 23 . Hence, the direct current voltage signal Vcf output from the PWM signal processing circuit  120  has a near linear relationship with both the duty cycle of the PWM input signal and the environmental temperature. 
       FIG. 3  is a circuit diagram showing another configuration of the temperature signal processing circuit  110  and the PWM signal processing circuit  120  of  FIG. 1 , in accordance with the first configuration of the present invention. The overall structure, functions and operations of the temperature signal processing circuit  110  in  FIG. 3  are essentially the same as those in  FIG. 2 , and a description thereof similar components and function will be omitted. 
     As shown in  FIG. 3 , the PWM signal processing circuit  120  includes: an eighth transistor  312 , a ninth transistor  322 , a third resistor  332 , a fourth resistor  342 , a fifth resistor  352 , a sixth resistor  362 , a seventh resistor  372 , an eighth resistor  382 , and a second capacitor  392 . In  FIG. 3 , the third resistor  332  and the fourth resistor  342  are series connected, the fifth resistor  352  and the sixth resistor  362  are series connected, the seventh resistor  372  and the eighth resistor  382  are series connected, and these three pairs of series connected resistors are connected between the power supply Vcc and the ground  209 . The shared terminal  340  of the third resistor  332  and the fourth resistor  342  can receive PWM input signals, and the shared terminal is connected to a base electrode  314  of the eighth transistor  312 . A collector electrode  316  of the eighth transistor  312  is connected to a base electrode  324  of the ninth transistor  322 , and an emitter electrode  318  of the eighth transistor  312  is connected to the ground  209 . A collector electrode  326  and a base electrode  324  of the ninth transistor  322  are short circuited to each other and both are connected to a shared terminal  350  of the fifth resistor  352  and the sixth resistor  362 . An emitter electrode  328  of the ninth transistor  322  is connected to a shared terminal  370  of the seventh resistor  372  and the eighth resistor  382 . The second capacitor  392  is parallel connected to the two terminals of the eighth resistor  382 . The shared terminal  370  of the seventh resistor  372  and the eighth resistor  382  is also an output terminal  304  which can output the voltage signal Vcf from the PWM signal processing circuit  120  to the control pulse output circuit  140 . 
     The difference between the PWM signal processing circuit  120  shown in  FIG. 3  and  FIG. 2  lies in: the fifth resistor  352  and sixth resistor  362  in  FIG. 3  replace the second current source  293  in  FIG. 2 ; the third resistor  332  and the fourth resistor  342 , eighth transistor  312 , and the ninth transistor  322  in  FIG. 3  replace the controlled switch  295  in  FIG. 2 ; the seventh resistor  372  and the eighth resistor  382  in  FIG. 3  replace the second resistor  287 ; and the collector electrode  326  of the ninth transistor  322  is connected to the shared terminal of the fifth resistor  352  and the sixth resistor  362 , and the collector electrode  316  of the eighth transistor  312  and the base electrode  324  of the ninth transistor  322  are connected to each other. 
     With regard to the temperature signal processing circuit  110  and the PWM signal processing circuit  120  in  FIG. 3 , when the PWM signal has a low electric level, the eighth transistor  312  is turned off such that the voltage at the base electrode  324  of the ninth transistor  322  can become high which consequently can turn on the ninth transistor  322 . In this situation, the current difference between the fifth transistor  252  and  123  can flow through the ninth transistor  322  to the second capacitor  392  and charge the second capacitor  392 . When the PWM signal has a high electric level, the eighth transistor  312  is turned on, such that the voltage at the base electrode  324  of the ninth transistor  322  can become low which can turn off the ninth transistor  322 . In this situation, the second capacitor  392  discharges electricity via the eighth resistor  382 , and as a result, the voltage at the two terminals of the second capacitor  392  can change following the PWM input signal changes. In the meantime, the current I 22  from the temperature signal processing circuit  110  can flow to the second capacitor  392 , which can cause the voltage at the two terminals of the second capacitor  392  to change following the currents I 22  changes. 
     Under the assumption that the duty cycle of the PWM input signal is d, the resistance value of the seventh resistor  372  is much larger than the resistance value of the eighth resistor  382 , and the current that passes through the sixth resistor  362  is much smaller than the current that passes through the fifth resistor  352 , the voltage at the second capacitor  392  can be approximately expressed as: 
                     Vcf   =         Vcc   ×       R   ⁢           ⁢   8         R   ⁢           ⁢   7     +     R   ⁢           ⁢   8           +     I   ⁢           ⁢   22   ×   R   ⁢           ⁢   8     +       (       Vcc     R   ⁢           ⁢   5       -     I   ⁢           ⁢   23       )     ×       1   -   d       C   ⁢           ⁢   2           =       Vcc   ×       R   ⁢           ⁢   8         R   ⁢           ⁢   7     +     R   ⁢           ⁢   8           +       [           ⁢       I   ⁢           ⁢   21     -       (       Vref   ⁢           ⁢   1     -     Vref   ×     RT       R   ⁢           ⁢   1     +   RT           )     ×   gm       ]     ×   R   ⁢           ⁢   8     +       [           ⁢       Vcc     R   ⁢           ⁢   5       -     M   ⁡     (       I   ⁢           ⁢   21     -       (       Vref   ⁢           ⁢   1     -     Vref   ⁢           ⁢     RT       R   ⁢           ⁢   1     +   RT           )     ×   gm       )         ]     ×       1   -   d       C   ⁢           ⁢   2               ;           Eqn   .           ⁢     (   5   )                 
where R 5 , R 7 , and R 8  are the resistance values of the fifth resistor  352 , the seventh resistor  372  and the eighth resistor  382 , respectively. As indicated by Eqn. (5), since the resistance value RT of the temperature sensitive resistor  283  is linearly related to the environmental temperature, the direct current voltage signal Vcf from the PWM signal processing circuit  120  is related to both the duty cycle of the PWM input signal d and the environmental temperature.
 
       FIG. 4  is a circuit diagram showing a triangular wave generating circuit  130  of  FIG. 1 , in accordance with the present invention. 
     As shown in  FIG. 4 , the triangular wave generating circuit  130  includes a ninth resistor  409 , a tenth resistor  410 , an eleventh resistor  411 , a second comparator  452 , a third current source  462 , a fourth current source  472 , a tenth transistor  422 , an eleventh transistor  432 , and a third capacitor  442 . 
     In  FIG. 4 , one terminal of the ninth resistor  409  is connected to the power supply Vcc, and the other terminal is connected to the ground  209  through the tenth resistor  410  and the eleventh resistor  411 . A shared terminal  412  of the ninth resistor  409  and the tenth resistor  410  is connected to a negative input terminal  454  of the second comparator  452 . A shared terminal  413  of the tenth resistor  410  and the eleventh resistor  411  is connected to a collector electrode  426  of the tenth transistor  422 . A base electrode  424  of the tenth transistor  422  and a base electrode  434  of the eleventh transistor  432  are short circuited. An emitter electrode  428  of the tenth transistor  422  and an emitter electrode  438  of the eleventh transistor  432  are connected to the ground  209 . A positive terminal  466  of the third current source  462  is connected to the power supply Vcc. A negative terminal  464  of the third current source  462  is connected to both a positive terminal  476  the fourth current source  472  and a positive input terminal  456  of the second comparator  452 . A negative terminal  474  of the fourth current source  472  is connected to a collector electrode  436  of the eleventh transistor  432 . An output terminal of the second comparator  452  is connected to both a base electrode  424  of the tenth transistor  422  and a base electrode  434  of the eleventh transistor  432 . The negative terminal  464  of the third current source  462  and a connector terminal  444  of the third capacitor  442  are an output terminal  450  of the triangular wave generating circuit  130  which can output a triangular signal. 
     It is important to note that the output current from the fourth current source  462  is twice as big as the output current from the third current source  472 . A high voltage level and a low voltage level of the triangular signal output from the third capacitor  442  can be expressed as: 
                       Vh   =     Vcc   ×         R   ⁢           ⁢   10     +     R   ⁢           ⁢   11           R   ⁢           ⁢   9     +     R   ⁢           ⁢   10     +     R   ⁢           ⁢   11             ⁢     
     ⁢   Vl   =     Vcc   ×       R   ⁢           ⁢   10         R   ⁢           ⁢   9     +     R   ⁢           ⁢   10             ;           Eqn   .           ⁢     (   6   )                 
where Vh is the high voltage level, Vl is the low voltage level, and R 9 , R 10 , R 11  are the resistance values of the ninth resistor  409 , the tenth resistor  410 , and the eleventh resistor  411 , respectively.
 
       FIG. 5  is a circuit diagram showing a brushless motor control system  500  which is configured to regulate rotation speed of a brushless direct current motor, in accordance with a second configuration of the present invention. The system  500  includes a control pulse generating circuit  510 , a hall signal generating circuit  520 , a logic control circuit  530 , and a drive circuit  540 . The control pulse generating circuit  510  includes a temperature signal processing circuit  110 , a PWM signal processing circuit  120 , a triangular wave generating circuit  130 , a control pulse output circuit  560  and a minimum duty cycle setup circuit  550 . 
     It is important to note that the difference between the system  500  and the system  100  shown in  FIG. 1  lies in that the control pulse generating circuit  510  includes the minimum duty cycle setup circuit  550 . The overall structure, functions and operation of the system  500  in  FIG. 5  and the system  100  in  FIG. 1  are essentially the same, and a description thereof similar components and function will be omitted. 
     In  FIG. 5 , the minimum duty cycle setup circuit  550  is used to generate a current voltage signal Vmin with minimum duty cycle setup, which can be output to a second negative input terminal of the control pulse output circuit  560 . The control pulse output circuit  560  is used to compare the direct current signal Vcf, the current voltage signal with minimum duty cycle setup Vmin, and the triangular wave signal to generate a control pulse signal whose duty cycle has a linear relationship with both the duty cycle of the PWM input signal and the environmental temperature signal. 
     In the system  500 , compared with they system  100 , the minimum duty cycle setup circuit  550  is added to the control pulse generating circuit  510 , which can provide a minimum duty cycle setup function in a cooling system driven by the brushless direct current motor. Assume the duty cycle of the PWM signal is d and the minimum duty cycle setup by the minimum duty cycle setup circuit  550  is Dm %, when d is smaller than Dm %, the rotation speed of the fan in the cooling system will be a constant value regardless of the environmental temperature. 
       FIG. 6  is a circuit diagram showing a minimum duty cycle setup circuit  550  of  FIG. 5 , in accordance with a second configuration of the present invention. As shown in  FIG. 6 , the minimum duty cycle setup circuit  550  includes a twelfth resistor  612  and a thirteenth resistor  613 . Two terminals of the twelfth resistor  612  are connected to the power supply Vcc and the ground through the thirteenth resistor  613 , respectively. A shared terminal  620  of the twelfth resistor  612  and the thirteenth resistor  613  is an output terminal of the minimum duty cycle setup circuit  550 , which can output the current voltage signal Vmin with minimum duty cycle setup. 
     In  FIG. 6 , the relationship between the current voltage signal with minimum duty cycle setup Vmin and the power supply Vcc can be expressed as: 
                       V   ⁢           ⁢   min     =     Vcc   ×       R   ⁢           ⁢   13         R   ⁢           ⁢   12     +     R   ⁢           ⁢   13             ;           Eqn   .           ⁢     (   7   )                 
where R 12  and R 13  are the transistor values of the twelfth resistor  612  and the thirteenth resistor  613 , respectively. The following condition has to be satisfied: Vl&lt;Vmin&lt;Vh, where Vh and Vl are the high electric level and low electric level of the triangular wave signal from the triangular wave generating circuit  130  respectively.
 
       FIG. 7  is a circuit diagram showing a control pulse output circuit  560  of  FIG. 5 , in accordance with the second configuration of the present invention. The control pulse output circuit  140  includes a twelfth transistor  702 , a thirteenth transistor  712 , a fourteenth transistor  722 , a fifteenth transistor  732 , a sixteenth transistor  742 , a seventeenth transistor  752 , an eighteenth transistor  762 , a nineteenth transistor  772 , a fifth current source  785 , a sixth current source  786 , a seventh current source  787 , and an eighth current source  788 . 
     As shown in  FIG. 7 , a positive terminal of the fifth current source  785 , a positive terminal of the sixth current source  786 , a positive terminal of the seventh current source  787 , and a positive terminal of the eighth current source  788  are all connected to the power supply Vcc. A negative terminal of the fifth current source  785  is connected to an emitter electrode  708  of the twelfth transistor  702  and the base electrode  758  of the seventeenth transistor  752 . A negative terminal of the sixth current source  786  is connected to an emitter electrode  718  of the thirteenth transistor  712 , an emitter electrode  728  of the fourteenth transistor  722 , and a base electrode  734  of the fifteenth transistor  732 . A negative terminal of the seventh current source  787  is connected to an emitter electrode  738  of the fifteenth transistor  732  and the emitter electrode  758  of the seventeenth transistor  752 . A negative terminal of the eighth current source  788  is connected to a collector electrode  776  of the nineteenth transistor  772 . A collector electrode  736  of the fifteenth transistor  732  is connected to a collector electrode  746  of the sixteenth transistor  742 . The collector electrode  746  of the sixteenth transistor  742  is short circuited to its base electrode  744 , which is short circuited to a base electrode  764  of the eighteenth transistor  762 . A collector electrode  766  of the eighteenth transistor  762  is connected to a collector electrode  756  of the seventeenth transistor  752  and a base electrode  774  of the nineteenth transistor  772 . A emitter electrode  706  of the twelfth transistor  702 , a emitter electrode  716  of the thirteenth transistor  712 , a emitter electrode  726  of the fourteenth transistor  722 , an emitter electrode  748  of the sixteenth transistor  742 , an emitter electrode  768  of the eighteenth transistor  762 , and an emitter electrode  778  of the nineteenth transistor  772  are all connected to the ground  209 . A base electrode  704  of the twelfth transistor  702  is connected to a triangular wave signal which is output from the triangular wave generating circuit  130  as shown in  FIG. 5 . A base electrode  714  of the thirteenth transistor  712  is connected to the current voltage signal Vmin with minimum duty cycle setup from the minimum duty cycle setup circuit  550  as shown in  FIG. 5 . A base electrode  724  of the fourteenth transistor  722  is connected to the direct current voltage signal Vcf which is output from the PWM signal processing circuit  120  as shown in  FIG. 5 . A collector electrode  776  of the nineteenth transistor  772  is the output terminal of the control pulse processing circuit  560 , which can generate a control pulse signal whose duty cycle has a near linear relation with both the duty cycle of the PWM input signal and the environmental temperature signal. 
     When the direct current voltage signal Vcf is higher than the voltage signal of minimum duty cycle setup Vmin, Vcf&gt;Vmin, the duty cycle of control pulse signal is determined by the voltage signal of minimum duty cycle setup Vmin and the triangular wave signal. In this situation, the minimum duty cycle Dm can be expressed as: 
                   Dm   =         Vh   -     V   ⁢           ⁢   min         Vh   -   Vl       =               R   ⁢           ⁢   10     +     R   ⁢           ⁢   11           R   ⁢           ⁢   9     +     R   ⁢           ⁢   10     +     R   ⁢           ⁢   11         -       R   ⁢           ⁢   13         R   ⁢           ⁢   12     +     R   ⁢           ⁢   13                   R   ⁢           ⁢   10     +     R   ⁢           ⁢   11           R   ⁢           ⁢   9     +     R   ⁢           ⁢   10     +     R   ⁢           ⁢   11         -       R   ⁢           ⁢   10         R   ⁢           ⁢   9     +     R   ⁢           ⁢   10             .               Eqn   .           ⁢     (   8   )                 
When the direct current voltage signal Vcf is smaller than the voltage signal of minimum duty cycle setup Vmin, Vcf&lt;Vmin, the duty cycle of control pulse signal is determined by the direct current voltage signal Vcf and the triangular wave signal. In this situation, the duty cycle D of the control pulse signal is:
 
     
       
         
           
             
               
                 
                   D 
                   = 
                   
                     
                       
                         Vh 
                         - 
                         Vcf 
                       
                       
                         Vh 
                         - 
                         Vl 
                       
                     
                     = 
                     
                       
                         
                           Vh 
                           - 
                           
                             { 
                             
                               
                                 
                                   
                                       
                                     
                                       
                                         Vcc 
                                         × 
                                         
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             8 
                                           
                                           
                                             
                                               R 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               7 
                                             
                                             + 
                                             
                                               R 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               8 
                                             
                                           
                                         
                                       
                                       + 
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       [ 
                                       
                                         
                                           I 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           21 
                                         
                                         - 
                                         
                                           
                                             ( 
                                             
                                               
                                                 Vref 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 1 
                                               
                                               - 
                                               
                                                 Vref 
                                                 × 
                                                 
                                                   RT 
                                                   
                                                     
                                                       R 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       1 
                                                     
                                                     + 
                                                     RT 
                                                   
                                                 
                                               
                                             
                                             ) 
                                           
                                           × 
                                           gm 
                                         
                                       
                                       ] 
                                     
                                     × 
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       R 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       8 
                                     
                                     + 
                                     
                                       [ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         
                                           Vcc 
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             5 
                                           
                                         
                                         - 
                                         
                                             
                                           
                                             M 
                                             ( 
                                             
                                               
                                                 I 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 21 
                                               
                                               - 
                                             
                                           
                                         
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           Vref 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                         - 
                                         
                                             
                                           
                                             Vref 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             
                                                 
                                               
                                                 RT 
                                                 
                                                   
                                                     R 
                                                     ⁢ 
                                                     
                                                         
                                                     
                                                     ⁢ 
                                                     1 
                                                   
                                                   + 
                                                   RT 
                                                 
                                               
                                               ) 
                                             
                                             × 
                                             gm 
                                           
                                           ) 
                                         
                                       
                                       ] 
                                     
                                     × 
                                     
                                       
                                         1 
                                         - 
                                         d 
                                       
                                       
                                         C 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                       
                                     
                                   
                                 
                               
                             
                             } 
                           
                         
                         
                           ( 
                           
                             Vh 
                             - 
                             Vl 
                           
                           ) 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
     Therefore it can be seen that the duty cycle of the control pulse signal from the control pulse output circuit  560  is related to both the duty cycle of the PWM input signal and the environmental temperature. 
     In the following description, two temperature thresholds will be introduced: a first temperature threshold T 1 , a second temperature threshold T 2 , T 1 &lt;T 2 . Assume, when environmental temperature T is equal or lower than the first temperature threshold T 1 , T≦T 1 , the first reference voltage Vref and the second reference voltage Vref 1  can be set as 
                   Vref   ⁢           ⁢   1     -     Vref   ×     RT       R   ⁢           ⁢   1     +   RT           =   0     ;         
and when the environmental temperature T is equal or higher than the second temperature threshold T 2 , T≧T 2 , the first reference voltage Vref and the second reference voltage Vref 1  can be set as
 
     
       
         
           
             
               
                 I 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 21 
               
               - 
               
                 
                   ( 
                   
                     
                       Vref 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     - 
                     
                       Vref 
                       × 
                       
                         RT 
                         
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           + 
                           RT 
                         
                       
                     
                   
                   ) 
                 
                 × 
                 gm 
               
             
             = 
             0. 
           
         
       
     
     As shown in  FIG. 5 , a triangular voltage wave signal at the positive input terminal of the control pulse output circuit  560 , which is the output signal from the triangular wave generating circuit  130 , has a maximum voltage Vh and a minimum voltage Vl; Vcf is a direct current voltage signal at a negative input terminal of the control pulse output circuit  560 , which is the output signal from the PWM signal processing circuit  120 ; and a direct current voltage signal with minimum duty cycle setup Vmin at another negative input terminal of the control pulse output circuit  560 , is the output signal from the minimum duty cycle setup circuit  550 . 
     When the environmental temperature T is lower than the first temperature threshold T 1 , T≦T 1 , the first reference voltage Vref and the second voltage Vref 1  are set as 
                 Vref   ⁢           ⁢   1     -     Vref   ×     RT       R   ⁢           ⁢   1     +   RT           =   0.         
In this situation, the duty cycle of the control pulse signal is:
 
                       D          T   ≤     T   ⁢           ⁢   1         =               Vh   -     [       Vcc   ×       R   ⁢           ⁢   8         R   ⁢           ⁢   7     +     R   ⁢           ⁢   8           +                       I   ⁢           ⁢   21   ×   R   ⁢           ⁢   8     +       (       Vcc     R   ⁢           ⁢   5       -     M   ×   I   ⁢           ⁢   21       )     ×       1   -   d       C   ⁢           ⁢   2           ]             Vh   -   Vl       .             Eqn   .           ⁢     (   10   )                 
Therefore, when the environmental temperature T is lower than or equals T 1 , the duty cycle D of the control pulse signal is related to the duty cycle d of the PWM input signal, and is not related to the environmental temperature T.
 
     When the environmental temperature T is higher than the second temperature threshold T 2 , T≧T 2 , the first reference voltage Vref and the second voltage Vref 1  are set as 
                 I   ⁢           ⁢   21     -       (       Vref   ⁢           ⁢   1     -     Vref   ×     RT       R   ⁢           ⁢   1     +   RT           )     ×   gm       =   0.         
In this situation, the duty cycle of the control pulse signal D is:
 
                       D          T   ≤     T   ⁢           ⁢   2         =         Vh   -     [       Vcc   ×       R   ⁢           ⁢   8         R   ⁢           ⁢   7     +     R   ⁢           ⁢   8           +       Vcc     R   ⁢           ⁢   5       ×       1   -   d       C   ⁢           ⁢   2           ]         Vh   -   Vl       .             Eqn   .           ⁢     (   11   )                 
Therefore, when the environmental temperature T is higher than T 2 , the duty cycle D of the control pulse signal is related to the duty cycle d of the PWM input signal, and is not related to the environmental temperature T.
 
     When the environmental temperature T is higher than T 1  and lower than T 2 , T 1 &lt;T&lt;T 2 , the duty cycle D of the control pulse signal can be calculated by Eqn. (9), which is 
                       D            T   1     &lt;   T   &lt;     T   2         =         Vh   -     {                   Vcc   ×       R   ⁢           ⁢   8         R   ⁢           ⁢   7     +     R   ⁢           ⁢   8           +                   [       I   ⁢           ⁢   21     -       (       Vref   ⁢           ⁢   1     -     Vref   ×     RT       R   ⁢           ⁢   1     +   RT           )     ×   gm       ]     ×                 R   ⁢           ⁢   8     +     [           ⁢       Vcc     R   ⁢           ⁢   5       -           M   (       I   ⁢           ⁢   21     -                           (       Vref   ⁢           ⁢   1     -           Vref   ⁢           ⁢           RT       R   ⁢           ⁢   1     +   RT       )     ×   gm     )       ]     ×       1   -   d       C   ⁢           ⁢   2               }         Vh   -   Vl       .             Eqn   .           ⁢     (   12   )                 
Therefore, when the environmental temperature T is higher than T 1  and lower than T 2 , the duty cycle D of the control pulse signal has a near linear relationship with both the environmental temperature and the duty cycle d of the PWM input signal.
 
       FIG. 8  is a graphic chart showing relationships between the environmental temperature and the current I 20 , the current I 22 , and the current I 23  of  FIG. 2 .  FIG. 8  can be drawn according to Eqn. (2), Eqn. (3) and Eqn. (4). 
       FIG. 9  shows a graphic chart of the relationship between the direct current voltage signal Vcf, the duty cycle d of the PWM input signal and the environmental temperature of  FIG. 3 .  FIG. 9  can be drawn according to Eqn. (5). 
       FIG. 10  shows a graphic chart of the relationship between the direct current voltage signal Vcf, the triangular input signal, the duty cycle d of the PWM input signal and the environmental temperature of  FIG. 5 .  FIG. 9  can be drawn according to Eqn. (5). Vmin can be calculated by Eqn. (7). 
       FIG. 11  is a graphic chart showing relationships between the environmental temperature, rotation speed of the brushless direct current motor, and t the duty cycle d of the PWM input signal, in accordance with the present invention. 
     More specifically, for example, as shown in  FIG. 1 , the control pulse signal generated by the control pulse output circuit  140  in  FIG. 1  is used to control the rotation speed of the motor. The logic control circuit  30  in  FIG. 1  can output the control pulse signal to various bridge circuits of the driver circuit  40 , based on the directional control signal generated by the Hall signal generation circuit  10  for controlling the direction of the coil current, and can control the direction of the current in the coils of the motor and the average load of the current based on the duty cycle of the control pulse signal so as to control the rotation speed of the motor. 
     It is important to note that the higher the duty cycle D of the control pulse signal, the faster the rotation speed of the motor; the lower the duty cycle of the control pulse signal, the slower the rotation speed of the motor. 
     When the environmental temperature is lower than T 1 , and the duty cycle d of the PWM input signal is higher than the set minimum duty cycle D m , the duty cycle D of the control pulse signal only has a linear relationship with the duty cycle of the PWM input signal, as in Eqn. (10) Therefore, the rotation speed of the motor is also only linearly related to the duty cycle of the PWM input signal and is not related with the environmental temperature. As shown in  FIG. 11(   a ), the rotational speed of the motor increases from Rm to Rm 1  regardless of the temperature. 
     When the environmental temperature is higher than T 1  but lower than T 2 , and the duty cycle d of the PWM input signal is higher than the set minimum duty cycle D m , the duty cycle D of the control pulse signal has a near linear relationship with both the environmental temperature and the duty cycle of the PWM input signal, as in Eqn. (12). Therefore, the rotation speed of the motor has a near linear relationship with both the environmental temperature and the duty cycle of the PWM input signal. As shown in  FIG. 11(   a ), when the input duty cycle is higher than Dm %, the rotational speed of the motor can change as temperature and the input duty cycle change. More specifically, in  FIG. 11(   a ), when the input duty cycle is constant, the rotational speed of the motor will increase as the temperature increases; and vice versa. As shown in  FIG. 11(   b ), when the temperature is constant, the rotational speed of the motor can increase as the input duty cycle increases. 
     When the environmental temperature is higher than T 2 , and the duty cycle d of the PWM input signal is higher than the set minimum duty cycle D m , the duty cycle D of the control pulse signal only has a linear relationship with the duty cycle of the PWM input signal, as in Eqn. (11). Therefore, the rotation speed of the motor is also only linearly related to the duty cycle of the PWM input signal and is not related with the environmental temperature. As shown in  FIG. 11  ( a ), the rotational speed can increase when the input duty cycle increases and it is not related with the temperature. When the input duty cycle is 100%, the rotational speed reaches its maximum rate of Rm 2 . 
     As shown in  FIG. 11(   b ), when the duty cycle d of the PWM input signal is lower than the set minimum duty cycle D m , the duty cycle of the control pulse signal is determined by the minimum duty cycle setup circuit  550 . It does not change as the environmental temperature and the duty cycle of the PWM input signal change. The rotation speed of the motor keeps the same. This indicate that system  500  has the ability to set up a minimal duty cycle D m . When the duty cycle d of the PWM input signal is smaller than Dm, regardless the level of the temperature, the rotational speed of the cooling fan is a constant Rm. 
       FIG. 12  is a flow chart showing the process of the method  1200  of regulating the rotation speed of a brushless direct current motor, in accordance with present invention. In block  1210 , the method  1200  can generate a direction reversal control signal when the north and south poles of the generator rotor pass through a hall component. In block  1220 , the method  1200  can generate a control pulse signal whose duty cycle has a linear relationship with both the duty cycle of a PWM input signal and an environmental temperature signal. In block  1230 , the method  1200  can regulate and control the rotation speed of a brushless direct current motor by using the direction reversal control signal from the block  1210  and the control pulse signal from the block  1220 . Further, using the control pulse signal in block  1230  enables the method  1200  to regulate and control the rotation speed of the brushless direct current motor by using both environmental temperature and PWM signals simultaneously. 
     More specifically, block  1220  include the following steps. In block  1222 , method  1220  can convert the environmental temperature signal that has been detected into a current signal that has a near linear relation with the environmental temperature signal. In block  1224 , method  1220  can, based on the current signal, convert the PWN signal into a direct current voltage signal that has a near linear relationship with the duty cycle of the PWM input signal and the environmental temperature signal. In block  1226 , method  1220  can compare the direct current voltage signal and a triangular wave signal to generate the control pulse signal whose duty cycle has a linear relationship with both the duty cycle of the PWM input signal and the environmental temperature signal. 
     More specifically, in this example, a temperature sensitive device is used to convert the environmental temperature that has been detected to current signals which will affect the magnitude of the direct current voltage signal within a certain range of temperatures. Then the direct current voltage signal is compared with the triangular wave signal to generate a control pulse generating circuit whose duty cycle has a linear relationship with both the duty cycle of the PWM input signal and the environmental temperature signal. The control pulse signal is used to regulate the rotation speed of the motor so that the environmental temperature and the PWM input signal are used at the same time to control the rotation speed of the motor. 
     Detailed description is provided above for a motor control system provided by the present invention. Embodiments are used herein to describe the principles and modes of carrying out the present invention, the above description of embodiments is only to help understand the methods and core thinking of the present invention; at the same time, those skilled in the art may modify modes of carrying out and application scope of the present invention according to the spirit thereof. In summary, the contents of the specification may not be construed as restrictive to the present invention. 
     The present invention provides a motor control system configured to regulate a motor rotation speed. When the duty cycle of a PWM signal is smaller than a minimum duty cycle, regardless the level of the temperature, the motor rotation speed is a constant; when the temperature is below a first temperature threshold and duty cycle of the PWM signal is higher than the minimum duty cycle, the motor rotation speed increases as the duty cycle of the PWM signal increases and vice versa; when the temperature is above the first temperature threshold and below a second temperature threshold, and the duty cycle of the PWM signal is higher than the minimum duty cycle, the motor rotation speed changes following the both the temperature change and the PWM signal duty cycle change; and when the temperature is above the second temperature threshold, the motor rotation speed changes only following the PWM signal duty cycle and the temperature does not affect the rotation speed.