Abstract:
A rotating speed adjustment circuit for a heat dissipation fan includes a first node, a second node, a reception end for receiving a first control signal, a first resistor coupled to a voltage source and the first node, a second resistor coupled to the first node and the second node, a third resistor coupled to the second node and a ground end, a capacitor coupled to the first node and the ground end, a transistor coupled to the reception end, the second node and the ground end, an oscillator for generating an oscillating signal, and a comparator for comparing a signal of the first node and the oscillating signal, so as to output a second control signal to control a rotating speed of the heat dissipation fan.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a rotating speed adjustment circuit and related control system for a heat dissipation fan, and more particularly, to a rotating speed adjustment circuit and related control system capable of avoiding vibration and noise caused by current ripples on the fan coil, and preventing inaccuracy of passive devices from affecting the rotating speed. 
         [0003]    2. Description of the Prior Art 
         [0004]    In modern information society, a computer system has become a necessary tool in daily life. For any kind of computer system, an operating clock of a CPU is becoming higher and higher, causing heat generated more and more. Therefore, heat dissipation has become more important. In the prior art, a fan is a main way for heat dissipation. For saving energy and reducing noise, a lot of methods are developed to control a rotating speed of a CPU fan, one of which in particular is simultaneously utilizing two sets of signal sources to control the rotating speed. 
         [0005]    Please refer to  FIG. 1 , which is a schematic diagram of a fan control system  10  utilizing two sets of signal sources in the prior art. The fan control system  10  is utilized for controlling a rotating speed of a fan  108 , and comprises a temperature-based rotating speed control circuit  100 , a pulse width modulation (PWM) rotating speed control circuit  102 , an AND gate  104  and a logic circuit  106 . The temperature-based rotating speed control circuit  100  is utilized for sensing a temperature, e.g. the temperature of the air dissipated by the fan  108 , to generate a temperature control signal PWM TH . The PWM rotating speed control circuit  102  is utilized for receiving a system control signal V CTR  outputted by the outer control circuit, e.g. the CPU, to generate a temperature control signal PWM EXT . The AND gate  104  is utilized for performing an AND operation on the temperature control signal PWM TH  and the temperature control signal PWM EXT . The logic circuit  106  can output a temperature control signal PWM OUT  to perform pulse width modulation on the fan  108 , so as to control the rotating speed of the fan  108  by two signal sources. 
         [0006]    Please refer to  FIG. 2A ,  FIG. 2B  and  FIG. 2C .  FIG. 2A  is a schematic diagram of the temperature-based rotating speed control circuit  100  shown in  FIG. 1 .  FIG. 2B  is a schematic diagram of related waveforms of the temperature-based rotating speed control circuit  100 .  FIG. 2C  is a schematic diagram of the rotating speed corresponding to the temperature-based rotating speed control circuit  100 . The temperature-based rotating speed control circuit  100  comprises a thermistor R TH , resistors R 1 , R 2 , R 3 , a comparator  200  and an oscillator  202 . The thermistor R TH  and the resistor R 1  are coupled in a sequence between a reference voltage VREF and a ground, to generate a division voltage V TH . Since resistance of the thermistor R TH  is negatively proportional to the temperature, the voltage V TH  is lower when the temperature is higher. Besides, the resistors R 2 , R 3  are utilized for dividing voltage to generate a voltage V MIN , which is utilized for configuring a lowest rotating speed SP_min 1  to avoid the fan  108  stopped due to too low temperature. The oscillator  202  is utilized for generating an oscillating signal V OS . The comparator  200  is utilized for comparing the voltages V TH , V MIN  and the oscillating signal V OS , so as to output the temperature control signal PWM TH  to the AND gate  104 . 
         [0007]    As shown in  FIG. 2B  and  FIG. 2C , when the temperature sensed by the thermistor R TH  is higher than a threshold temperature T_th, that is, before time T 1 , the voltage V TH  will be lower than the voltage V MIN , causing the duty cycle of the temperature control signal PWM TH  increased, to increase the rotating speed of the fan  108 , and dissipate more heat. On the contrary, when the temperature sensed by the thermistor R TH  is lower than the threshold temperature T_th, that is, after time T 1 , the voltage V TH  will be higher than the voltage V MIN , causing the duty cycle of the temperature control signal PWM TH  to stay in the minimum duty cycle, so as to control the rotating speed of the fan  108  to be in the lowest rotating speed SP_min 1 , and avoid the fan  108  stopped due to too low temperature. 
         [0008]    On the other hand, please refer to  FIG. 3A  and  FIG. 3B .  FIG. 3A  is a schematic diagram of the PWM rotating speed control circuit  102  shown in  FIG. 1 .  FIG. 3B  is a schematic diagram of the rotating speed corresponding to the PWM rotating speed control circuit  102 . The PWM rotating speed control circuit  102  comprises a transistor N 1 , resistors R 4 , R 5 , a comparator  300 , an oscillator  302 . The transistor N 1  is a bipolar junction transistor, and can generate a system control signal B_V CTR  opposite to the system control signal V CTR . The system control signal B_V CTR  is compared with the oscillating signal V OS  generated by the oscillator  302 , so as to derive a temperature control signal PWM EXT . The resistors R 4 , R 5  are utilized for adjusting the high potential voltage of the signal B_V CTR , so as to determine the lowest rotating speed SP_min 2 . The rotating speed is thereof shown in  FIG. 3B . 
         [0009]    Therefore, via the temperature-based rotating speed control circuit  100  and the PWM rotating speed control circuit  102 , the fan control system  10  can control the rotating speed of the fan  108  by simultaneously utilizing two signal sources. However, the above-mentioned method has the following drawbacks. 
         [0010]    First, the duty cycle of the temperature control signal PWM OUT  derived by performing AND operation on the temperature control signal PWM TH  and PWM EXT  will not be a stable value. Instead, as shown in  FIG. 4A , it will generate current ripples on the fan coil, causing problems of vibration and noise. 
         [0011]    Second, in the temperature-based rotating speed control circuit  100 , since variation of the resistance of the thermistor R TH  related to the temperature is insufficient, the highest and lowest voltage ranges of the oscillator  202  are needed to be shrunk to compromise the characteristics of the thermistor R TH . However, if the highest and lowest voltage ranges are too small, the rotating speed will be affected dramatically by inaccuracy of the passive devices in the circuit. 
         [0012]    Third, a specification of the CPU fan requires that the CPU fan must be maintain a fixed rotating speed when the duty cycle of the system control signal V CTR  is less than 20%, to ensure the CPU fan to generate a basic amount of wind and reach the purposes of saving energy. However, the fan control system  10  in the prior art can not meet the specification, because the lowest rotating speed thereof changes with working temperature as shown in  FIG. 4B . 
         [0013]    Thus, for the heat dissipation fan controlled both by temperature-based and PWM signals, the industries are devoted to research a fan control system which can overcome the drawbacks mentioned above. 
       SUMMARY OF THE INVENTION  
       [0014]    It is therefore a primary objective of the claimed invention to provide a rotating speed adjustment circuit and related control system for a heat dissipation fan. 
         [0015]    The present invention discloses a rotating speed adjustment circuit for a heat dissipation fan. The rotating speed adjustment circuit comprises a first node, a second node, a reception end, a first resistor, a second resistor, a third resistor, a capacitor, a transistor, an oscillator and a comparator. The reception end is utilized for receiving a first control signal for controlling a rotating speed of the heat dissipation fan. The first resistor is coupled between a voltage source and the first node. The second resistor is coupled between the first node and the second node. The third resistor is coupled between the second node and a ground end. The capacitor is coupled between the first node and the ground end. The transistor comprises a first end coupled to the reception end, a second end coupled to the second node, and a third end coupled to the ground end, for controlling a signal connection from the second end to the third end according to a signal of the first end. The oscillator is utilized for generating an oscillating signal. The comparator comprises a first input end coupled to the first node, a second input end coupled to the oscillator and an output end, for comparing signals of the first node and the oscillating signal, so as to output a second control signal via the output end to control the rotating speed of the heat dissipation fan. 
         [0016]    The present invention further discloses a fan control system for a heat dissipation fan. The fan control system comprises a temperature-based rotating speed control circuit, a pulse width modulation (PWM) rotating speed control circuit, an AND gate, a rotating speed adjustment circuit, and a logic circuit. The temperature-based rotating speed control circuit is utilized for sensing a temperature of the heat dissipation fan to generate a first temperature control signal. The PWM rotating speed control circuit is utilized for receiving a system control signal to generate a second temperature control signal. The AND gate is coupled to the temperature-based rotating speed control circuit and the PWM rotating speed control circuit, for performing an AND operation on the first temperature control signal and the second temperature control signal, so as to generate a first control signal. The rotating speed adjustment circuit comprises a first node, a second node, a reception end, a first resistor, a second resistor, a third resistor, a capacitor, a transistor, an oscillator and a comparator. The reception end is coupled to the AND gate for receiving the first control signal. The first resistor is coupled between a voltage source and the first node. The second resistor is coupled between the first node and the second node. The third resistor is coupled between the second node and a ground end. The capacitor is coupled between the first node and the ground end. The transistor comprises a first end coupled to the reception end, a second end coupled to the second node, and a third end coupled to the ground end, for controlling a signal connection from the second end to the third end according to a signal of the first end. The oscillator is utilized for generating an oscillating signal. The comparator comprises a first input end coupled to the first node, a second input end coupled to the oscillator and an output end, for comparing signals of the first node and the oscillating signal, so as to output a second control signal via the output end. The logic circuit is coupled to the output end of the comparator and the heat dissipation fan, for driving the heat dissipation fan according to the second control signal. 
         [0017]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]      FIG. 1  is a schematic diagram of a fan control system utilizing two sets of signal sources in the prior art. 
           [0019]      FIG. 2A  is a schematic diagram of the temperature-based rotating speed control circuit shown in  FIG. 1 . 
           [0020]      FIG. 2B  is a schematic diagram of related waveforms of the temperature-based rotating speed control circuit. 
           [0021]      FIG. 2C  is a schematic diagram of the rotating speed corresponding to the temperature-based rotating speed control circuit. 
           [0022]      FIG. 3A  is a schematic diagram of the PWM rotating speed control circuit shown in  FIG. 1 . 
           [0023]      FIG. 3B  is a schematic diagram of the rotating speed corresponding to the PWM rotating speed control circuit. 
           [0024]      FIG. 4A  is a schematic diagram of the fan control system shown in  FIG. 1  performing an AND operation. 
           [0025]      FIG. 4B  is a schematic diagram of characteristics of the rotating speed of the fan control system shown in  FIG. 1 . 
           [0026]      FIG. 5A  is a schematic diagram of a fan control system according to an embodiment of the present invention. 
           [0027]      FIG. 5B  is a schematic diagram of the temperature-based rotating speed control circuit shown in  FIG. 5A . 
           [0028]      FIG. 5C  is a schematic diagram of the PWM rotating speed control circuit shown in  FIG. 5A . 
           [0029]      FIG. 5D  is a schematic diagram of the rotating speed adjustment circuit shown in  FIG. 5A . 
           [0030]      FIG. 6A  and  FIG. 6B  are schematic diagrams related to the PWM rotating speed control circuit shown in  FIG. 5C . 
           [0031]      FIG. 7A  and  FIG. 7B  are schematic diagrams of the duty cycle of signals related to the fan control system shown in  FIG. 5A . 
           [0032]      FIG. 8  is a schematic diagram of the rotating speed of the fan control system shown in  FIG. 5A . 
       
    
    
     DETAILED DESCRIPTION  
       [0033]    Please refer to  FIG. 5A , which is a schematic diagram of a fan control system  50  according to an embodiment of the present invention. The fan control system  50  is utilized for driving a heat dissipation fan  500 , and comprises a temperature-based rotating speed control circuit  502 , a pulse width modulation (PWM) rotating speed control circuit  504 , an AND gate  506 , a rotating speed adjustment circuit  508  and a logic circuit  510 . The temperature-based rotating speed control circuit  502  is utilized for sensing a temperature of the heat dissipation fan  500  to generate a temperature control signal PWM TH . The PWM rotating speed control circuit  504  is utilized for receiving a system control signal V CTR  to generate a temperature control signal PWM EXT . The AND gate  506  is coupled to the temperature-based rotating speed control circuit  502  and the PWM rotating speed control circuit  504 , and is utilized for performing an AND operation on the temperature control signal PWM TH  and the temperature control signal PWM EXT , so as to generate a first control signal PWM CTR1 . The rotating speed adjustment circuit  508  is coupled to the AND gate  506 , and is utilized for generating a second control signal PWM CTR2  to the logic circuit  510  according to the first control signal PWM CTR1 . The logic circuit  510  can output a temperature control signal PWM OUT  to perform pulse width modulation on the heat dissipation fan  500 . 
         [0034]    Please refer to  FIG. 5B ,  FIG. 5C  and  FIG. 5D , which are respectively schematic diagrams of the temperature-based rotating speed control circuit  502 , the PWM rotating speed control circuit  504  and the rotating speed adjustment circuit  508  shown in  FIG. 5A . In  FIG. 5B , the temperature-based rotating speed control circuit  502  comprises a thermistor R TH , resistors R 1 ˜R 6 , an oscillator  600  and a comparator  602 . Structures and operations of the temperature-based rotating speed control circuit  502  are similar to those of the temperature-based rotating speed control circuit  100  shown in  FIG. 2A . That is, the thermistor R TH  and the resistor R 1  are utilized for determining the voltage V TH , and the resistors R 2 , R 3  are utilized for determining the voltage V MIN . Moreover, the resistors R 4 , R 5 , R 6  are utilized for determining the high voltage OSCH and the low voltage OSCL of the oscillating signal V OS1  generated by the oscillator  600 , so as to conform to specifications of the fan. Please note that, resistance of the resistors R 4 , R 5 , R 6  can be adjusted by those skilled in the art according to different requirements. For example, at 38° C. when the thermistor R TH  is 6 Kohm, the highest rotating speed of the heat dissipation fan  500  is required to be 100%, and at 30° C. when the thermistor R TH  is 9 Kohm, the highest rotating speed of the heat dissipation fan  500  is required to be 50%. If resistance of the resistor R 1  is 7.5 Kohm and the voltage VREF is 5V, the following equations can be derived. 
         [0000]    
       
         
           
             
               
                 
                   V 
                   TH 
                 
                  
                 
                   ( 
                   
                     38 
                      
                     ° 
                      
                     
                         
                     
                      
                     
                       C 
                       . 
                     
                   
                   ) 
                 
               
               = 
               
                 
                   5 
                   × 
                   
                     6 
                     
                       6 
                       + 
                       7.5 
                     
                   
                 
                 = 
                 
                   2.222 
                    
                   
                       
                   
                    
                   V 
                 
               
             
             ; 
           
         
       
       
         
           
             
               
                 
                   V 
                   TH 
                 
                  
                 
                   ( 
                   
                     30 
                      
                     ° 
                      
                     
                         
                     
                      
                     
                       C 
                       . 
                     
                   
                   ) 
                 
               
               = 
               
                 
                   5 
                   × 
                   
                     9 
                     
                       9 
                       + 
                       7.5 
                     
                   
                 
                 = 
                 
                   2.727 
                    
                   
                       
                   
                    
                   V 
                 
               
             
             ; 
           
         
       
     
         [0035]    To meet the requirement that the highest rotating speed of the heat dissipation fan  500  is 100% at 38° C., the voltage OSCL can be set as 2.222V. To meet the requirement that the highest rotating speed of the heat dissipation fan  500  is 50% at 30° C., the voltage OSCH can be derived by the following equation. 
         [0000]    
       
         
           
             
               
                 
                   OSCH 
                   = 
                     
                    
                   
                     OSCL 
                     + 
                     
                        
                       
                         
                           
                             
                               V 
                               TH 
                             
                              
                             
                               ( 
                               
                                 38 
                                  
                                 ° 
                                  
                                 
                                     
                                 
                                  
                                 
                                   C 
                                   . 
                                 
                               
                               ) 
                             
                           
                           - 
                           
                             
                               V 
                               TH 
                             
                              
                             
                               ( 
                               
                                 30 
                                  
                                 ° 
                                  
                                 
                                     
                                 
                                  
                                 
                                   C 
                                   . 
                                 
                               
                               ) 
                             
                           
                         
                         
                           
                             100 
                              
                             % 
                           
                           - 
                           
                             50 
                              
                             % 
                           
                         
                       
                        
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     2.222 
                     + 
                     
                        
                       
                         
                           2.222 
                           - 
                           2.727 
                         
                         0.5 
                       
                        
                     
                   
                 
               
             
             
               
                 
                   
                     = 
                       
                      
                     
                       3.232 
                        
                       
                           
                       
                        
                       V 
                     
                   
                   ; 
                 
               
             
           
         
       
     
         [0036]    Then, resistance of the resistors R 4 , R 5 , R 6  can be adjusted properly to make the voltage OSCH and OSCL be the required values. Besides, the voltage V MIN  is utilized for determining the lowest working temperature of the temperature-based rotating speed control circuit  502 . If we hope the rotating speed of the heat dissipation fan  500  does not change with temperature when the temperature is below 30° C., we can set V MIN  to be 2.727V. As a result, when the temperature is below 30° C., the voltage V TH  is higher than the voltage V MIN . In such a condition, the duty cycle of the temperature control signal PWM TH  is determined by the voltages V MIN  and V OS1 . 
         [0037]    In  FIG. 5C , the PWM rotating speed control circuit  504  comprises a transforming unit  700 , an oscillator  702  and a comparator  704 . The transforming unit  700  is coupled to the system control signal V CTR  for transforming the system control signal V CTR  into a direct-current (DC) voltage signal V PWM . The oscillator  702  is utilized for generating an oscillating signal V OS2 . The comparator  704  is coupled to the transforming unit  700  and the oscillator  702 , and is utilized for comparing the DC voltage signal V PWM  and the oscillating signal V OS2 , so as to output the temperature control signal PWM EXT  to the AND gate  506 . 
         [0038]    Please note that, the PWM rotating speed control circuit  504  shown in  FIG. 5C  is for illustrative purposes of the present invention, and those skilled in the art can make alternations and modifications accordingly. For example, please refer to  FIG. 6A  and  FIG. 6B .  FIG. 6A  is a schematic diagram of the DC voltage signal V PWM  related to the duty cycle of system control signal V CTR .  FIG. 6B  is a schematic diagram of the duty cycle of the temperature control signal PWM EXT  related to the duty cycle of the system control signal V CTR . If amplitude of the oscillating signal V OS2  is between 1 volt and 3 volt, and a stable rotating speed is needed when the duty cycle of the system control signal V CTR  is less than 20%, the transforming unit  700  must be adjusted to make the DC voltage signal V PWM  outputted by the transforming unit  700  over 3 volt when the duty cycle of the system control signal V CTR  is less than 20%. In this way, the signal with duty cycle less than 20% cannot correspond to a duty cycle with the oscillating signal V OS2 , that is, the duty cycle of PWM EXT  is 0, so as to avoid effects on the rotating speed by variations thereof. 
         [0039]    In  FIG. 5D , the rotating speed adjustment circuit  508  comprises resistors R 7 , R 8 , R 9 , a capacitor C 1 , a transistor  800 , an oscillator  802  and a comparator  804 . The oscillator  802  is utilized for generating an oscillating signal V OS3 . Preferably, a frequency of the oscillating signal V OS3  is the same as that of the oscillating signal V OS2 , and is different from that of the oscillating signal V OS1 . The transistor  800  is preferably an n-type bipolar junction transistor, and is utilized for transforming the first control signal PWM CTR1  into a DC voltage V SET , which is compared with the oscillating signal V OS3  to generate the second control signal PWM CTR2 . In such a condition, since the second control signal PWM CTR2  is derived by comparing the DC voltage V SET  and the oscillating signal V OS3 , the duty cycle thereof can stay in stable to avoid current ripples on the fan coil. On the other hand, since the oscillating signal V OS3  of the rotating speed adjustment circuit  508  is different from the oscillating signal V OS1  of the temperature-based rotating speed control circuit  502 , the rotating speed can be less affected by inaccuracy of the passive devices. 
         [0040]    Therefore, the fan control system  50  can overcome the drawback of unstable rotating speed when the duty cycle of the system control signal V CTR  is less than 20%. Thus, the present invention can enhance efficiency of heat dissipation, save energy and reduce noise. Take the above-mentioned condition for example, that is, the highest rotating speed of the heat dissipation fan  500  is 100% at 38° C. and the highest rotating speed of the heat dissipation fan  500  is 50% at 30° C. Please refer to  FIG. 7A ,  FIG. 7B  and  FIG. 8 .  FIG. 7A  is a schematic diagram of the duty cycle of the system control signal V CTR  related to the DC voltage V SET  in different working temperature.  FIG. 7B  is a schematic diagram of the duty cycle of the system control signal V CTR  related to the temperature control signal PWM OUT  in different working temperature.  FIG. 8  is a schematic diagram of the rotating speed of the fan control system  50  related to the duty cycle of the system control signal V CTR . As shown in  FIG. 7A ,  FIG. 7B  and  FIG. 8 , when the duty cycle of the system control signal V CTR  is less than 20%, the DC voltage V SET  will stay in stable and not change with temperature, thereby stabilizing the duty cycle of the temperature control signal PWM OUT . In such a condition, when the duty cycle of the system control signal V CTR  is less than 20%, the rotating speed of the heat dissipation fan  500  can stay in stable to eliminate unnecessary ripples and noise. 
         [0041]    In conclusion, for the heat dissipation fan controlled by both temperature-based and PWM signals, when the duty cycle of the system control signal V CTR  is less than the default duty cycle, the present invention can stabilize the rotating speed of the heat dissipation fan to avoid vibration and noise caused by current ripples on the fan coil. Moreover, since the oscillating signal V OS3  of the rotating speed adjustment circuit  508  is different from the oscillating signal V OS1  of the temperature-based rotating speed control circuit  502 , the rotating speed can be less affected by inaccuracy of the passive devices. Therefore, the present invention can avoid vibration and noise caused by current ripples on the fan coil, and prevent inaccuracy of passive devices from affecting the rotating speed. 
         [0042]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.