Abstract:
A novel a circuit for driving a fan includes an output terminal for supplying the fan with drive power, a pulse width modulation driver, and a limiter. A first power terminal of the fan is held at a first voltage (e.g., 0V), and a second power terminal of the fan is coupled to the output terminal of the driver circuit. The PWM driver provides a series of fan drive pulses on the output terminal, and the limiter prevents the voltage on the output terminal from falling below a predetermined voltage. The predetermined voltage is greater than the first voltage at which the fan&#39;s first power terminal is held, and is sufficient to keep the fan in motion even when the duty cycle of the PWM signal is 0%. In a particular embodiment the limiter includes a voltage clamp. In a more particular embodiment, the voltage clamp is a diode. In another particular embodiment, the limiter includes a switch for combining a PWM signal with a DC voltage at an output.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation of co-pending U.S. patent application Ser. No. 10/214,414, filed Aug. 6, 2002 by the same inventor, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates generally to electronic control of cooling fans, and more particularly to a system and method for reducing undesirable noise resulting from driving a fan with pulse width modulation.  
         [0004]     2. Description of the Background Art  
         [0005]     It is common practice to regulate the speed of fans used to cool electronic components such as computers. Regulating the speed of fans increases fan life, decreases noise caused by unnecessarily high airflow, and conserves electrical energy.  
         [0006]     One common method of controlling fan speed is to drive the fan using low frequency pulse width modulation (PWM). According to PWM, electrical power is supplied to the fan in a series of pulses. The fan&#39;s speed is controlled by controlling the width (duration) of the pulses. The percentage of time that a pulse is being applied to the fan is referred to as the “duty cycle.” When the pulses are wide (high duty cycle), the fan speed is correspondingly high. When the duty cycle decreases (relatively narrower pulses), the fan speed correspondingly decreases.  
         [0007]     Although PWM has proven to be one of the most efficient methods of controlling fan speed, objectionable noise, commonly referred to as “growling”, is generated when the duty cycle is reduced below approximately 30 percent. Typically, the noise occurs at a frequency equal to the PWM, and is especially noticeable when using high operating current fans, and low operating speeds. Accordingly, fan control circuits commonly require that the fan be driven with at least 30% PWM duty cycle, which may be faster than necessary or desirable for quiet operation.  
         [0008]     What is needed, therefore, is a system and method for using PWM to drive a fan at low duty cycles, without causing growling noise. What is also needed is a system and method for using PWM to drive a fan at speeds lower than speeds corresponding to a 30 percent duty cycle.  
       SUMMARY  
       [0009]     The present invention overcomes the problems associated with the prior art by providing a system and method for generating a fan drive voltage that pulses between a non-zero base voltage and a peak drive voltage. The invention facilitates driving a fan with pulse width modulation (PWM) at low duty cycles, without producing undesirable “growling” noise.  
         [0010]     In one embodiment, a circuit for driving a fan includes an output terminal for supplying the fan with drive power, a pulse width modulation driver, and a limiter. A first power terminal of the fan is held at a first voltage (e.g., 0V), and a second power terminal of the fan is coupled to the output terminal of the driver circuit. The PWM driver provides a series of fan drive pulses on the output terminal, and the limiter prevents the voltage on the output terminal from falling below a predetermined voltage. The predetermined voltage is greater than the first voltage at which the fan&#39;s first power terminal is held.  
         [0011]     In a particular embodiment, the limiter is a voltage clamp coupled to the output terminal. In a more particular embodiment, the limiter includes a diode coupled between the a DC voltage source and the output terminal. Optionally, the limiter includes a plurality of diodes, and a plurality of bypass elements, each bypass element coupled in parallel with an associated one of said diodes. The bypass elements facilitate the selective activation of each diode, which in turn facilitates the selection of one of a plurality of DC voltages for the predetermined voltage.  
         [0012]     In another particular embodiment, the limiter includes a switch, and is operative to combine a DC voltage with a PWM drive by selectively asserting either the DC voltage or the PWM signal on the output terminal of the fan driver circuit.  
         [0013]     A method of quietly driving a fan is also described. The method includes the steps of providing a PWM drive output, combining the PWM drive output with a DC voltage, and providing the combined drive signal at an output. The value of the DC voltage is selected to be sufficient to keep the fan in motion even when the duty cycle of the PWM drive output is 0%.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The present invention is described with reference to the following drawings, wherein like reference numbers denote substantially similar elements:  
         [0015]      FIG. 1  is a block diagram of a computer system using quiet fan speed control according to the present invention;  
         [0016]      FIG. 2  is a block diagram showing the constrained PWM driver circuit of the computer system of  FIG. 1  in greater detail;  
         [0017]      FIG. 3  is a graph showing the output of the PWM controller shown in  FIG. 2 ;  
         [0018]      FIG. 4  is a graph showing the voltage at one node of the circuit of  FIG. 2 ;  
         [0019]      FIG. 5  is a graph showing the output of the constrained PWM driver circuit of  FIG. 2 ;  
         [0020]      FIG. 6  is a diagram showing an alternate voltage combiner;  
         [0021]      FIG. 7  is a diagram showing another alternate voltage combiner; and  
         [0022]      FIG. 8  is a flowchart summarizing one method of using pulse width modulation to drive a fan according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]     The present invention overcomes the problems associated with the prior art, by providing a system and method for driving a cooling fan using a constrained pulse width modulation (PWM) drive output. In the following description, numerous specific details are set forth (e.g., particular voltages, polarities, circuit configurations, etc.) in order to provide a thorough understanding of the invention. Those skilled in the art will recognize, however, that the invention may be practiced apart from these specific details. In other instances, details of well known electronics practices (e.g., controlling a conventional PWM output) and components have been omitted, so as not to unnecessarily obscure the present invention.  
         [0024]      FIG. 1  is a block diagram of a computer  100 , including a fan  102 , a constrained PWM driver circuit  104 , miscellaneous computer components  106 , speed control logic  108 , and sensor  110 . Fan  102  provides cooling for computer  100 , by moving air out of computer  100 . The moving air carries away heat generated by miscellaneous computer components  106 , which include, for example, a motherboard, hard drives, removable media drives, a monitor, etc.  
         [0025]     The speed of fan  102  is controlled by constrained PWM driver circuit  104 . Fan  102  has a first power terminal  112  coupled to a first voltage source  114 , and a second power terminal  116  coupled to an output terminal  118  of driver circuit  104 . In this example, voltage source  114  is ground, but this is not a requirement of the invention.  
         [0026]     Driver circuit  104  provides an advantage over conventional PWM driver circuits, because driver circuit  104  provides a pulsed voltage at its output  118  that has a base voltage that is higher than that provided by first voltage source  114 . In contrast, conventional PWM drivers typically operate at a base voltage equal to the voltage being asserted on first power terminal  112  by first voltage source  114 . In other words, a conventional PWM driver would pulse fan  102  between an “off” state and a “on” state, whereas driver circuit  104  pulses fan  102  between a “partially on” state and an “on” state. Because fan  102  is always at least partially on, undesirable “growling” noise is eliminated even when fan  102  is driven at a low duty cycle. Indeed, the duty cycle of the output of driver circuit  104  can be 0%, because the base voltage keeps fan  102  running at a minimum speed. Driver circuit  104  can then add voltage pulses from 1% to 100% duty cycle, to increase the speed of fan  102  from the minimum speed provided by the base voltage to a maximum speed.  
         [0027]     The width (duration) of the pulses provided by driver circuit  104  control the speed of fan  102 . Driver circuit  104  determines the appropriate width (duty cycle) of the pulses based on input from temperature sensor  110  and/or control logic  108 . For example, in response to sensor  110  providing a signal, via line  120 , indicating a relatively high temperature, driver circuit  104  will increase the duty cycle, and thus the speed of fan  102 . If, however, the signal from sensor  110  indicates a relatively low temperature, then driver circuit  104  will reduce the duty cycle, thereby lowering the speed of fan  102 , eliminating unnecessary airflow noise, and conserving energy. Optionally, speed control logic  108  provides a speed control signal, via line  122 , to driver circuit  104  based on some other criteria, for example computer  100  being placed in an inactive state.  
         [0028]      FIG. 2  is a block diagram showing PWM driver circuit  104  in greater detail to include a PWM control circuit  202 , a first resistor  204 , a first transistor  206 , a second resistor  208 , a second transistor  210 , a base voltage limiter  212 , a first voltage source  214  (gnd), a second voltage source V A , a third voltage source V B , a fourth voltage source V C , and an output terminal  216 . Base voltage limiter  212  includes a diode  218  coupled between third voltage source V B  and output terminal  216 . The output of PWM controller  202  is coupled to the control terminal (base) of transistor  206 , and is coupled to voltage source VC via resistor  204 . One conduction terminal (collector) of transistor  206  is coupled to the control terminal (gate) of transistor  210 , and is coupled via resistor  208  to voltage source VA. The other conduction terminal (emitter) of transistor  206  is tied to ground  214 . One conduction terminal (drain) of transistor  210  is coupled directly to voltage source VA, and the other conduction terminal (source) is coupled to output terminal  216 . Output terminal  216  is also coupled to voltage source VB via diode  218 .  
         [0029]     In this particular embodiment, PWM control circuit  202  is a conventional PWM integrated circuit chip, resistor  204  is a 10K ohm resistor, transistor  206  is an NPN transistor, resistor  208  is a ???K ohm resistor, transistor  210  is a P channel FET, diode  218  is a shottky diode, VA=12V, VB=5V, and VC=3.3V. These voltages were selected, at least in part, because 3.3V is a typical integrated circuit operating voltage, and typical fans operate in the range of 4V-12V. It should be understood, however, that the invention is not limited to the use these particular components and voltages. In fact, it is expected that the invention may be practiced with a wide range of components and voltages depending on the particular type of fan and other application specifics.  
         [0030]     The operation of driver circuit  104  will be explained with reference to  FIGS. 2-5 .  FIG. 3  is a graph showing the voltage at the output of PWM controller  202  and the base of transistor  206  as a function of time. PWM controller  202  provides conventional PWM output, varying the pulse width of the output based on control input received via lines  120  and  122 . When the output of PWM controller  202  rises to 0.7 volts, the base of transistor  206  conducts, thus limiting the voltage on the base to 0.7 volts. PWM controller  202  maintains the 0.7 voltage output for a time period corresponding to the desired duty cycle, after which PWM controller pulls its output to ground (0V). Thus, the output of PWM controller  202  is pulsed between a base voltage of 0V and a peak voltage of 0.7V. The frequency of the pulsed output provided by PWM controller remains constant. The duty cycle is changed by modulating the duration (width) of the pulses. The duration of the pulses shown in  FIG. 3  correspond to approximately a 25% duty cycle.  
         [0031]      FIG. 4  is a graph showing the voltage on the gate of transistor  210 . When the output of PWM controller  202  is 0V, transistor  206  is nonconducting, and resistor  208  pulls the voltage on the gate of transistor  210  up to the voltage of voltage source VA. When the output of PWM controller  202  transitions to 0.7 volts, transistor  206  goes into conduction and pulls the gate of transistor  210  to ground  214 . Thus, the voltage on the gate of transistor  210  pulses between a base voltage of 0V and a peak voltage of VA. Note that the pulsed voltage on the gate of transistor  210  is inverted as compared to the pulsed output of PWM controller  202 . In particular, when the output of PWM controller  202  is at 0V, the gate of transistor  210  is at VA. When the output of PWM controller  202  is at 0.7V, the gate of transistor  210  is at 0V.  
         [0032]      FIG. 5  is a graph showing the voltage on the output of PWM driver circuit  104 . When the voltage on the gate of transistor  210  is at V A , transistor  210  is nonconductive, and the voltage on output terminal  216  is pulled low, because output terminal  216  is coupled to ground through fan  102 . However, when the voltage on output terminal  216  goes low enough, diode  218  conducts, clamping the voltage on output terminal  216  at a voltage one diode drop (0.35V) below VB. When the voltage on the gate of transistor  210  is at 0V, transistor  210  is conductive, and the voltage on output terminal  216  increases to V A . Thus, driver circuit  104  provides a pulsed modulation voltage on output terminal  216  that pulses between a base voltage (VB-0.35V) and the maximum driving voltage V A . Note that the base voltage is sufficiently greater than the voltage (0V) tied to first power terminal  112  of fan  102  to keep fan  102  operating.  
         [0033]     Without limiter  212 , the source terminal  220  of transistor  210  would produce an ordinary PWM drive output that pulses between a base voltage of 0V and a peak voltage of VA, and growling noise would be produced when fan  102  is driven at a low duty cycle. However, because limiter  212  combines a DC voltage with the ordinary PWM drive output of transistor  210 , fan  102  can be driven at a low duty cycle without producing objectionable growling noise. For example, in this example, limiter  212  clamps the voltage on output terminal  216  so that it cannot fall below a base voltage of VB-0.35V, or approximately 4.65V. This minimum DC base voltage is sufficient to quietly spin fan  102  at a minimum speed, thereby allowing the addition of low duty cycle PWM pulses without causing growling noise. Note that the minimum fan speed produced by the base voltage is far lower (approximately a factor of three) than the speed resulting from a 30% duty cycle used in prior art PWM drivers to eliminate the growling noise.  
         [0034]      FIG. 6  is a diagram of an alternate limiter  212 A that provides for the adjustment/selection of the base voltage added to the PWM output, and therefore provides for the adjustment/selection of the minimum operating speed of fan  102 . Alternate limiter  212 A includes a plurality (3 in this example) of diodes  602 ,  604 , and  606  coupled in series between voltage source VB and output terminal  216 . Diodes  602 ,  604 , and  606  are shottky diodes, each producing a voltage drop of about 0.35V when conducting. Limiter  212 A further includes a first fused link  608  coupled in parallel with diode  604 , and a second fused link  610  coupled in parallel with diode  606 .  
         [0035]     Fused links  608  and  610  function as bypass elements which prevent conduction by diodes  604  and  606 , respectively. When fused links  608  and  610  are intact, no voltage drop is produced by diodes  604  and  606 , and diode  602  prevents the voltage on output terminal  216  from falling below VB-0.35V, the same as in limiter  212  of  FIG. 2 . However, fused links  608  and  610  are selectively interruptible. When one of fused links  608  and  610  is interrupted, the respective one of diodes  604  and  606  conduct, producing a corresponding voltage drop. If both of fused links  608  and  610  are interrupted, then both of diodes  604  and  606  conduct, and produce corresponding voltage drops.  
         [0036]     A user can therefore select between three different DC voltages to combine with the PWM output being provided by source terminal  220 . If neither of links  608  and  610  are interrupted, then the voltage on source terminal  220  and output terminal  216  is clamped at VB-0.35V. If one of links  608  and  610  are interrupted, then the voltage on source terminal  220  and output terminal  216  is clamped at VB-0.7V. If both of links  608  and  610  are interrupted, then the voltage on source terminal  220  and output terminal  216  is clamped at VB-1.05V. If VB is at 5V, then limiter  212 A provides for the selection of 4.65V, 4.3V, or 3.95V for a base output voltage. It should be noted that a greater number of diodes can be connected in series to provide a wider selection of base voltages. Further adjustment of the base voltage can also be provided by regulating the voltage provided by voltage source VB.  
         [0037]     Links  608  and  610  can be interrupted in various ways. For example, they can be traces on a printed circuit board which can be selectively scratched by an assembler. Another example would be to use fused links that can be blown with laser light, electrical current, etc. Yet another example would be to use 0 ohm resistors or jumper wires that can be selectively removed. In yet another example, the number of diodes to use can be determined at the assemble stage, and unrequired diodes can be omitted and replaced with 0 ohm resistors.  
         [0038]      FIG. 7  is a diagram of another alternative limiter  212 B. Limiter  212 B uses switching to combine the PWM output provided on source terminal  220  with the DC voltage provided by voltage source VB. Alternate limiter  212 B includes a multiplexer  702  and a comparator  704 . Multiplexer  702  includes a first input terminal  706  coupled to source terminal  220 , a second input terminal  708  coupled to DC voltage source VB, and a control terminal  710 . Comparator  704  also includes a first input terminal  712  coupled to source terminal  220 , a second input terminal  714  coupled to DC voltage source VB. The output of comparator  704  is asserted on control terminal  710  of multiplexer  702 .  
         [0039]     Responsive to the output of comparator  704  being asserted on its control terminal, multiplexer  702  selectively couples either the PWM signal on terminal  220  or the DC voltage from source VB with output terminal  216 . Comparator  704  compares the PWM signal with the voltage provided by voltage source VB. If the voltage of the PWM signal falls below VB, then comparator  212 B asserts a signal on control terminal  710  causing multiplexer to coupled the second input terminal  708  with output terminal  216 , thereby preventing the voltage on output terminal  216  from falling below VB. When the PWM signal is above VB, however, comparator  704  asserts another signal on control terminal  710 , causing multiplexer  702  to couple source terminal  220  with output terminal  216 . Thus, alternate limiter  212 B produces an output similar to that shown in  FIG. 5 , except that the base voltage of the pulsed output is VB instead of VB-0.35V.  
         [0040]     Alternate limiter  212 B is shown to illustrate that a base DC voltage can be combined with the PWM drive signal via switching. Virtually any switch (e.g., switching transistors) capable of switching between the PWM signal and the DC base voltage, when the PWM signal goes lower than the base voltage, can be employed.  
         [0041]      FIG. 8  is a flow chart summarizing one method  800  for controlling a fan according to the present invention. In a first step  802 , a PWM output is provided. In a second step  804 , the PWM output is combined with a DC voltage. Then, in a third step  806 , the combined PWM/DC drive output is provided to a fan, to quietly drive the fan at a speed dependent on the PWM component of the output, but not slower than a minimum speed determined by the DC voltage component. This method facilitates driving a fan with low duty cycle PWM output, without generating undesirable growling noise from the fan.  
         [0042]     The description of particular embodiments of the present invention is now complete. Many of the described features may be substituted, altered or omitted without departing from the scope of the invention. For example, the circuits described herein may operate at voltages and polarities other than those set forth herein. As another example, (referring to  FIG. 2 ) the gate of transistor  210  could be precisely driven to provide a drive signal similar to that shown in  FIG. 5 , thus eliminating the need for limiter  212 . However, this approach requires that the transistors be driven in the linear region, and results in greater power consumption and heat generation. These and other deviations from the particular embodiments shown will be apparent to those skilled in the art, particularly in view of the foregoing disclosure.