Abstract:
A microcontroller generates a digital signal in response to input data, such as temperature and/or fan speed data. The digital signal is converted to an analog signal that is used to control a variable power supply. The variable power supply outputs a regulated substantially constant output voltage at a voltage level corresponding to the value of the digital signal. The analog signal is produced by a resistor divider and a feedback adjustment circuit that includes a plurality of resistive elements that are selectively coupled in parallel with a portion of the resistor divider in response to the digital signal. The resistive elements coupled in parallel with the portion of the resistor divider adjust the voltage generated at the resistor divider&#39;s center node. The voltage on the center node is the analog signal that is used to control the variable power supply.

Description:
The present invention relates generally to devices for controlling the speed of a fan, such as a fan for cooling heat generating equipment, by regulating a power supply voltage delivered to the fan, and in particular to a microcontroller based fan controller apparatus and control method. 
     BACKGROUND OF THE INVENTION 
     Many systems require a variable speed fan (or blower) to control circuitry temperature and/or heat output. Control of the fan can be accomplished using a series-pass circuit, however this solution generates too much heat and is energy wasteful. Alternatively, fan control can be accomplished using a standard pulse-width-modulated (PWM) power control circuit. Generally, a microcontroller generates a precisely-timed pulse train to drive the power switching circuitry, which outputs a desired voltage that drives the fan. However, the PWM arrangement has several shortcomings. First, there are a number of relatively large discrete components needed to generate, filter and regulate the PWM output voltage. When a power source is significantly above the maximum operating voltage of the fan, extra protection circuitry is needed to prevent a catastrophic failure in the event of a microcontroller crash. An inexpensive microcontroller&#39;s software-timing and clock-rate require the PWM circuitry to be designed to operate at a relatively low frequency, thus making the PWM arrangement less efficient and physically larger. Moreover, the microcontroller must communicate in real time with one or more host devices while performing the PWM power supply control as well as other functions; thus, resulting in a difficult and costly firmware programming challenge. 
     Accordingly, it is an object of the present invention to provide a fan controller system, and a PWM power supply control system, that removes the real-time power control functions from the microcontroller, thereby reducing the parts-count, energy waste and firmware complexity of such systems. 
     SUMMARY OF THE INVENTION 
     A microcontroller generates a digital signal in response to input data, such as temperature and/or fan speed data. The digital signal is converted to an analog signal that is used to control a variable power supply. The variable power supply outputs a regulated substantially constant output voltage at a voltage level corresponding to the value of the digital signal. The analog signal is produced by a resistor divider and a plurality of resistive elements that are selectively coupled in parallel with a portion of the resistor divider in response to the digital signal. An upper end of the resistor divider is coupled to the output voltage and a lower end is coupled to a reference voltage such as circuit ground. The resistive elements coupled in parallel with a portion of the resistor divider adjust the voltage generated at the resistor divider&#39;s center node. The voltage on the center node is the analog signal that is used to control the variable power supply. 
     The present invention relieves the microcontroller from having to perform real-time pulse-width modulation power regulation, thereby reducing the parts-count, energy waste and firmware complexity of the control system in which the microcontroller is used. Further, the present invention guarantees at least minimal operation, at a safe operating voltage, in the event of a microcontroller failure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the following drawings: 
     FIG. 1 is a circuit diagram for a fan control system in accordance with an embodiment of the present invention. 
     FIG. 2 is a circuit diagram for a fan control system in accordance with an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 there is shown a fan controller system  10  according to an embodiment of the present invention. System  10  includes microcontroller  12 , a variable-output power supply  16  (such as the single-chip power supply LT1074 produced by Linear Technology Corp.), LC filter  18 , resistor divider  20 , feedback adjustment circuit  14 , and fan  22 . The feedback adjustment circuit  14  includes a set of resistors  30 , each of which is controllably connected to the feedback adjustment circuit by a corresponding optocoupler  15  (such as a PS2501 from NEC). Resistor divider  20  is also called the feedback circuit. The effective resistance of the feedback circuit  20  is adjusted by the feedback adjustment circuit  14 . 
     LC filter  18  includes a Schottky diode  42  (to prevent the voltage output node of the power supply  16  from going below circuit ground), an inductor  44  and a capacitor  46 , connected as shown in FIG.  1 . The power supply  16  receives an input power supply voltage V IN  substantially in excess of the maximum operating voltage of the fan  22 . In a preferred embodiment, the input power supply voltage V IN  of power supply  16  is 48 volts, while the maximum operating voltage of fan  22  is 24 volts, and the V CC  power supply used by the microcontroller  12  is preferably 5 volts or less. 
     Each of the optocouplers  15  selectively, in response to a logic level control signal, connects a corresponding one of the resistors  30  in parallel with the upper portion (i.e., resistor  40 ) of the resistor divider  20 . The skilled artisan will recognize that each of the optocouplers  15  may be replaced with any coupler that is controllable by an appropriate electrical control signal, so long as the coupler controllably connects one node of a respective resistor  30  to the center node  29  of the feedback circuit  20  when the electrical control signal is in a first state and otherwise isolates the resistor  30  from the circuit. The resistors  30  may be replaced by various combinations of resistors coupled in parallel, series, or both without deviating from the scope of the present invention, so long as the resistance of the feedback circuit  20  is adjusted in a predictable manner as a function of a set of control signals generated by the microcontroller  12 . 
     The speed of fan  22  is controlled by varying the output voltage (V O ) from variable-output power supply  16  and LC filter  18 . Power supply  16  outputs a square wave signal V SW  having a peak voltage of V IN . The LC filter  18  converts the square wave signal into the DC output voltage (V O ). V O  is regulated by varying the effective resistance of resistor divider  20 , which provides a feedback voltage V FB  to power supply  16 . Power supply  16  adjusts the output voltage V O  until V FB  is approximately equal to an internal voltage reference (not shown) of power supply  16 . V FB  is preferably 2.2 1V in model LT1074 produced by Linear Technology Corp. 
     Resistor divider  20  includes resistor  40  coupled between the output voltage V O  and the center node  29  of the resistor divider, and resistor  36  coupled between the center node  29  and circuit ground node  35 . Each of the resistors  30  in the feedback adjustment circuit is coupled at one end to the output voltage V O  and at the other end to a terminal of an optocoupler  15 . If a respective optocoupler  15  is enabled, the second end of its corresponding resistor  30  is coupled to the center node  29  of the resistor divider, and thus the corresponding resistor is connected in parallel with resistor  40 . Otherwise, if the optocoupler  15  is disabled, its corresponding resistor is “floating.” 
     The effective resistance of resistor divider  20  is based upon the state of optocouplers  15 . The state of optocouplers  15  is set by a 3-bit binary digital signal  26 , which is determined by microcontroller  12  based on fan speed data from fan  22  and/or temperature data from maintenance network  28 . Each 3-bit value corresponds to a different fan speed, which in turn corresponds to a different effective resistance of resistor divider  20 . Each bit of the digital signal  26  is coupled to a control port of a respective optocoupler  15 , and causes that optocoupler  15  to either isolate or connect its corresponding resistor  30  to the center node  29  of the resistor divider  20 . More generally, the digital signal  26  includes a number of individual binary signals that is at least equal in number to the number of optocouplers  15 . 
     In other implementations, circuit ground node  35  may be replaced by a reference voltage node at a voltage less than that of the reference voltage of power supply  16  (preferably 2.21V). 
     Connecting any of resistors  30 , alone or in combination, reduces the net resistance of the upper portion of resistor divider  20 , thereby causing the feedback voltage V FB  to increase for any given output voltage V O  level. Power supply  16  adjusts V O  downward until the feedback voltage V FB  returns to the reference voltage of power supply  16 . Therefore, microcontroller  12  adjusts fan speed by changing digital signal  26 , which controls the optocouplers  15 , which varies the effective resistance of resistor divider  20 , which causes power supply  16  to adjust the output voltage V O  accordingly, thereby changing the fan speed accordingly. When resistors  30  are all isolated, also called floating, power supply  16  will output its maximum output voltage V O  (e.g., approximately 24 volts). When resistors  30  are all connected in parallel with resistor  40 , power supply  16  will output its minimum output voltage V O  (approximately 14.5 volts). 
     In the event that the microcontroller  12  suffers a software or other failure (e.g., a software crash or a hardware failure), the output of the microcontroller  12  is not predictable. Nevertheless, the operation of the resistor divider  20  and feedback adjustment circuit  14  assure that output voltage V O  generated by the power supply  16  and filter  18  will not exceed a well defined upper limit (e.g., 24 volts) and furthermore will not fall below a well defined lower limit (e.g., 14.5 volts), even when operation of the microcontroller  12  fails. So long as the fan controller system  10  continues to receive power and the power supply  16  does not suffer a circuit failure, the fan  22  will continue to be operated at a voltage within its allowed operating range. For instance, if microcontroller  12  crashes in a manner that leaves the digital signal at a value of 0 (i.e., with all resistors  30  floating), the speed of the fan will be the same as if the microcontroller were working properly and outputting a digital signal value of 0. Similarly, if the microcontroller  12  fails in a manner that leaves the digital signal at a value of 7 (i.e., with all resistors  30  coupled in parallel to resistor  40 ), the speed of the fan will be the same as if the microcontroller were working properly and outputting a digital signal value of 7. Thus, no matter what the state of optocouplers  15  at the time of a microcontroller failure, the fan will be protected from catastrophic power surges and will remain running. In contrast, if the microcontroller  12  were directly controlling the generation of a pulsed voltage signal, a failure of the microcontroller  12  could result in the generation of an output voltage that is above or below the operating range of the fan (e.g., at V IN  which is well above the operating voltage range of the fan, or at circuit ground, which is well below the operating voltage range of the fan). 
     Listed below in Table 1 are the specifications for a preferred embodiment of the present invention, along with Table 2, which provides output voltages corresponding to the values of the digital signal  26  for this preferred embodiment. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Specification For A Preferred Embodiment 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Feedback Adjustment 
                 Resistor 30-1 = 10K ohms 
               
               
                   
                 Circuit 14 
                 Resistor 30-2 = 20K ohms 
               
               
                   
                   
                 Resistor 30-3 = 47.5K ohms 
               
               
                   
                 Optocouplers 15 
                 PS2501 by NEC 
               
               
                   
                 Resistor Divider 20 
                 Resistor 36 = 442 ohms 
               
               
                   
                   
                 Resistor 40 = 4.32K ohms 
               
               
                   
                 LC Filter 18 
                 Schottky Diode 42 = MUR42O 
               
               
                   
                   
                 Inductor 44 = 330 μH 
               
               
                   
                   
                 Capacitor 46 = 150 μF 
               
               
                   
                 Variable-output Power 
                 LT1074 produced by Linear 
               
               
                   
                 supply 16 
                 Technology Corp. 
               
               
                   
                 Input Voltage 
                 V IN  = 48 volts 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Digital 
                 State of Feedback Adjustment Circuit 14 
               
             
          
           
               
                 Signal 
                 Resistor 30-1 
                 Resistor 30-2 
                 Resistor 30-3 
                 V o  (v) 
               
               
                   
               
             
          
           
               
                 0, 0, 0 
                 Float 
                 Float 
                 Float 
                 23.8 
               
               
                 0, 0, 1 
                 Float 
                 Float 
                 Connected 
                 22 
               
               
                 0, 1, 1 
                 Float 
                 Connected 
                 Connected 
                 18.7 
               
               
                 0, 1, 0 
                 Float 
                 Connected 
                 Float 
                 20 
               
               
                 1, 0, 1 
                 Connected 
                 Float 
                 Connected 
                 16.4 
               
               
                 1, 0, 0 
                 Connected 
                 Float 
                 Float 
                 17.3 
               
               
                 1, 1, 0 
                 Connected 
                 Connected 
                 Float 
                 15.3 
               
               
                 1, 1, 1 
                 Connected 
                 Connected 
                 Connected 
                 14.6 
               
               
                   
               
             
          
         
       
     
     The skilled artisan will recognize that other resistance values could be used for resistors  30  and that fewer or more than three resistors could be used in the feedback adjustment circuit. In the same vein, resistors  36  and  40  may have different resistance values from those shown in Table 1, depending on the desired maximum V O , the reference voltage of power supply  16  and/or the voltage on node  35 . The resistors used in the resistor divider  20 , and the number of resistors and the particular resistance values used in the feedback adjustment circuit  14 , can be selected to provide any selected minimum and maximum output voltage, more or fewer voltage steps between the minimum and maximum output voltages, and to set the voltages at each of those voltage steps. 
     The skilled artisan will also recognize that microcontroller  12  may serve other control and communications functions, such as controlling other devices and/or reporting the status of other devices for example. 
     FIG. 2 shows an alternative embodiment of a fan controller system  50 . Only the aspects of system  50  that differ from the controller system  10  in FIG. 1 will be described. All other aspects of systems  50  and  10  are the same. In particular, controller system  50  has a feedback adjustment circuit  32  that controllably connects resistors  34  in parallel with the lower portion (i.e., resistor  36 ) of the feedback circuit  20 , whereas the controller system  10  of FIG. 1 controllably connects resistors  30  in parallel with the upper portion (i.e., resistor  40 ) of the feedback circuit  20 . Thus, in the system  50  of FIG. 2, when the microcontroller  12  turns on one or more bits of the control signal  26 , the resistance of the lower portion of the feedback circuit decreases, which causes the voltage at the center node  29  of the feedback circuit  20  to decrease. This, in turn causes the power supply  16  to increase the output voltage until the voltage on center node  29  approximately equals its internal reference voltage. 
     While the present invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to the skilled artisan without departing from the true spirit or scope of the invention as defined by the appended claims.