Patent Publication Number: US-7583067-B2

Title: Variable power output regulator

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 11/094,983, filed Mar. 31, 2005, now U.S. Pat. No. 7,095,217, the teachings of which are fully incorporated herein by reference. 

   FIELD 
   This disclosure relates to direct current (DC) power sources and in particular to variable output DC power sources. 
   BACKGROUND 
   A variety of electronic devices such as cell phones, laptop computers, and personal digital assistants to name only a few, may be powered by one or more variable output DC power sources. A variable output DC power source may accept an unregulated input voltage and provide a variable output DC voltage and output current to a load of the electronic device. The unregulated input voltage may be an alternating current (AC) or DC input voltage. 
   Like other power supply sources, the variable output DC power source may be capable of providing a maximum output power to the load. At any time, the actual output power can be expressed as the product of the output voltage and output current. The instantaneous values of the output voltage/current of the variable output DC power source may be controlled by one or more control signals. These control signals may be provided according to a power management algorithm and may be the result of a set of sensing signal processing performed by power control circuitry. Other limitations may be imposed on the instantaneous output voltage/current of the variable output DC power source, but for clarity and simplicity, analysis herein is directed to the output power limiting features of the power control circuitry. Hence, if other limitations are not imposed, as the output voltage is reduced the output current can be increased as long as the product of the output voltage and output current is less than the maximum output power. Similarly, as the output current is reduced the output voltage can be increased as long as the product of the output current and output voltage is less than the maximum output power. 
   However, since power control circuits are relatively complicated and expensive, a conventional power control circuit limits the output current to a fixed maximum current level and limits the output voltage to a fixed maximum voltage level. The fixed maximum current and voltage levels are designed so that the product of each is at most equal to the maximum output power. Although a simple approach, this conventional power control circuit significantly reduces the safe operation region of the variable output DC power source. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features and advantages of embodiments of the claimed subject matter will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, where like numerals depict like parts, and in which: 
       FIG. 1  is a block diagram of an electronic system having a variable output DC power source; 
       FIG. 2  illustrates plots of both ideal and approximated output current versus output voltage of the variable output DC power source of  FIG. 1  for maximum output power; 
       FIG. 3  is a diagram of an embodiment of the power control circuitry of  FIG. 1  illustrating the circuitry performing a power limiting function; 
       FIG. 4  is circuit diagram of one embodiment of the threshold input circuitry of  FIG. 3 ; and 
       FIG. 5  is a flow chart illustrating operations that may be performed according to an embodiment. 
   

   Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the claimed subject matter be viewed broadly. 
   DETAILED DESCRIPTION 
     FIG. 1  illustrates an electronic system  100 . The electronic system may include a power source  110 , a variable output DC power source (VOPS)  102 , and an electronic device  103 . The electronic device  103  may include a load  108  and power control circuitry  104 . The power source  110  may be any variety of power sources capable of supplying an AC or DC input voltage to the VOPS  102 . The VOPS  102  may accept input power from the power source  110  and provide power to the load  108 . The electronic device  103  may be any variety of electronic devices, including, but not limited to, a server computer, a desk top computer, a laptop computer, a cell phone, a personal digital assistant, digital camera, etc. The load  108  may represent the load of the entire electronic device  103  or a part of the electronic device  103 . The load  108  may also represent a stand alone load which is not part of the electronic device  103 .  FIG. 1  illustrates only one of many possible topologies or systems since, for example, in other instances the VOPS  102  may be part of the electronic device  103 , or the power control circuitry  104  may be part of the VOPS  102 , etc. In one example, the power source  110  may be a common 120 volt/60 Hertz AC power line, the VOPS  102  may be a variable output ACDC adapter, and the electronic device  103  may be a laptop computer and the load  108  may represent the entire load of the laptop computer. 
   The variable output DC power source  102  may accept the unregulated input voltage and provide a variable output DC voltage (Vout) and output current (Iout) to the load  108 . The variable output DC power source  102  may provide varying Vout and Iout levels in response to one or more control signals (CS) from the power control circuitry  104 . As used herein, “circuitry” may comprise, for example, singly or in any combination, hardwired circuitry, programmable circuitry, state machine circuitry, and/or firmware that stores instructions executed by programmable circuitry. The power control circuitry  104  may accept one or more input signals via path  114 . The input signals may be representative of Iout and/or Vout provided by the variable output DC power source  102  to the load  108 . The power control circuitry  104  may provide one or more output control signals (CS) via path  106  to the VOPS  102 . 
     FIG. 2  illustrates a plot  200  of the maximum output power (Pm) of the variable output DC power source  102  of  FIG. 1  where the y-axis represents output current (Iout) and the x-axis represents output voltage (Vout) of the variable output DC power source  102 . Since the output power is the product of Vout and Iout, the plot  200  is the hyperbolic curve (Iout)(Vout)=Pm, where the permissible output current hyperbolically decreases with increasing output voltage levels. A particular point of a fixed current level (Io) and fixed voltage level (Vo) on the plot  200  is also illustrated. Conventional power control circuitry may limit the output voltage to Vo and the output current to Io thus limiting the safe operating region of the variable output DC power source. 
   The power control circuitry  104  consistent with an embodiment may monitor Iout and Vout and compare a signal representative of Iout to a particular threshold value depending on the value of Vout. The threshold value may be a fixed threshold value for an initial range of voltage levels, e.g., from about 0 volts to Vo, and the threshold value may be a variable threshold value for another range of voltage levels, e.g., from Vo to Vm. If the monitored output current is equal to or greater than the appropriate threshold level for an associated voltage level, the power control circuitry  104  may provide a control signal to the variable output DC power source  102 . 
   In response, the variable output DC power source  102  may drive the output current to the appropriate maximum current level for an associated output voltage. 
   Ideally, the maximum output current Im of the variable output DC power source  102  may be as detailed in equations (1) and (2):
 
 Im=Io , when Vout≦Vo  (1)
 
 Im=Pm/ Vout, when V o&lt; Vout≦V m   (2)
 
   where Io is a fixed current level and Vo is a fixed voltage level of a conventional system such that Vo×Io=Pm, where Vout is the output voltage level of the variable output DC power source  102 , and Pm is the maximum output power of the variable output DC power source  102 . Plot  202  represents the plot of Im values over the initial voltage range specified in equation (1) and plot  204  represents the plot of Im values over the first voltage range specified in equation (2). However, circuitry to limit the output current of the variable output DC power source  102  to the variable maximum output current Im as expressed by equation (2) may be complicated and expensive. 
   Accordingly, a method and circuitry consistent with an embodiment may establish another plurality of output current levels Ima in response to the current levels Im defined by equation (2). The plurality of output current levels Ima may approximate the plurality of output current levels Im as defined by equation (2) and may be given by equation (3):
 
 Ima=Io−k (Vout−V o ), for V o&lt; V out≦V m   (3)
 
where k is a constant representing the slope of the line  207  defined by equation (3). The constant k represents conductance and may be expressed in units of siemens. The constant k may also be expressed as the tangent(x) where the angle x is detailed in  FIG. 2 .
 
   A plot  207  defined by equation (3) for a selected k that provides a linear approximation for the plot  204  over the first voltage range, Vo&lt;Vout≦Vm is illustrated in  FIG. 2 . The difference between plots  207  and  204  has been exaggerated in  FIG. 2  for clarity of illustration. As detailed herein, the difference between plots  207  and  204  can be minimized to yield approximation errors of 1.0% or less. Error e 1  represents the maximum positive error between one of the output current levels defined by plot  204  and one of the output current levels defined by plot  207  which may occur at voltage V 1 . Error e 2  represents the maximum corresponding negative error over the same voltage range which occurs at the voltage Vm. Both errors e 1  and e 2  are dependent on the value of k and may be evaluated by analytical mathematical means. Since errors e 1  and e 2  are dependent on the value of k, k may be selected to result in errors e 1  and e 2  such that the absolute value of each error e 1  and e 2  divided by the respective ideal current limit at associated voltage levels V 1  and Vm are equal as detailed in equation (4). 
                          e   ⁢           ⁢   1              P   m       V   1         =            e   ⁢           ⁢   2              P   m     Vm               (   4   )               
Choosing k to result in errors e 1  and e 2  that satisfy equation (4) is one method of achieving a minimum overall relative approximation error for the linear plot  207  compared to the plot  204  over the same voltage range. Other approaches based on different conditions imposed to e 1 , e 2 , or both may be chosen to result in different values of k.
 
   In one example, the maximum output power Pm of the variable output DC power source  102  may be 64 watts. The voltage Vo may be 12 volts, the current Io may be 5.33 amps, and the maximum voltage Vm may be 16 volts. In this example, the value of k may be chosen to be 0.348 siemens to result in an error e 2  of only 0.04 A compared to ideal current of 4.0 A or only a 1.0% error at this voltage level. 
     FIG. 3  illustrates an embodiment  104   a  of the power limiting part of the power control circuitry  104  of  FIG. 1 . The power control circuitry  104   a  may include a current sense amplifier  302 , a current limit comparator  304 , a voltage limit comparator  306 , threshold input circuitry  410 , and power limiting control circuitry  308 . A sense resistor  303  having a resistance level RS may be utilized to sense the output current Iout of the variable output DC power source  102 . Other types of current sensors may also be utilized. The value of the voltage drop across the sense resistor  303  may provide a signal representative of the output current Iout. The current sense amplifier  302  may then amplify this signal and provide an output voltage signal Vs to the comparator  304 . 
   The output voltage signal Vs from the sense amplifier  302  may be defined by equation (5):
 
V s=RS×A×I out,  (5)
 
   where RS is the resistance value of the sense resistor  303 , A is the gain of the sense amplifier  302  and Iout is the output current of the variable output DC power source  102 . The comparator  304  may compare the signal (Vs) representative of the output current (Iout) to a threshold level. The threshold level (Vcl) may be a fixed threshold (Vcl=Vclo) or a variable threshold (Vcl=Vcl) depending on the value of Vout. The fixed threshold may be provided by the threshold input circuitry  310  to the comparator  304  if the output voltage Vout is less than or equal to the fixed voltage level Vo during the initial voltage range as illustrated in  FIG. 2 . The variable threshold may be provided by the threshold circuitry  310  to the comparator  304  if the output voltage Vout is Vo&lt;Vout≦Vm during the first voltage range as illustrated in  FIG. 2 . 
   The fixed threshold (Vclo) may be defined by equation (6):
 
V clo=RS×A×Io   (6)
 
   where RS is the resistance value of the sense resistor  303 , A is the gain of the sense amplifier  302  and Io is the selected fixed maximum current level over the initial range of output voltages less than or equal to Vo. Whenever the actual output current Iout equals Io, the voltage level Vs of equation (5) becomes equal to the voltage level Vclo of equation (6) and the comparator  304  provide an output voltage signal (CL) to the power limiting control circuitry  308  representative of this condition. In response, the power limiting control circuitry  308  may provide a control signal via path  106  to the variable output DC power source  102  to instruct the variable output DC power source  102  to drive its output current to Io. 
   The comparator  306  may receive a signal representative of the output voltage Vout. The comparator  306  may also receive a signal representative of a maximum voltage level Vm. The comparator  306  may compare such signals and output a voltage signal (VL) to the power limiting control circuitry  308  in response to this comparison. If the output voltage level is equal to or greater than Vm, the output voltage signal (VL) from the comparator  306  may be representative of this condition. In response, the power limiting control circuitry  308  may provide a control signal via path  106  to the variable output DC power source  102  to instruct the variable output DC power source  102  to drive its output voltage to Vm. 
     FIG. 4  illustrates an embodiment  310   a  of the threshold input circuitry  310  of  FIG. 3  that may provide the fixed threshold (Vcl=Vclo) to the comparator  304  if the output voltage Vout is less than or equal to Vo and may provide the variable threshold (Vcl=Vcl) to the comparator  304  if the output voltage Vout is greater than Vo and less than Vm. The variable current limit may be as detailed in equation (3) or Ima=Io−k×(Vout−Vo). The variable threshold Vcl may then be defined by equation (7):
 V cl=RS×A×Ima,   (7) 
   where Vcl is the variable voltage threshold input to comparator  304 , RS is the resistance value of sense resistor  303 , A is the gain of the sense amplifier  302 , and Ima is the maximum output current of the variable output DC power source  102  for a particular output voltage level in the first range of voltages where Vo&lt;Vout≦Vm. Given Ima as detailed in equation (3), equation (7) can be rewritten as detailed in equation (8).
 
V cl=RS×A×[Io−k× (Vout−V o )]  (8)
 
   Since RS×A×Io may be expressed as Vclo as detailed in equation (6), equation (8) may further be simplified to equation (9).
 
V cl= V clo−k 1(Vout−V o ), where  k 1 is a constant equal to  RS×A×k.   (9)
 
   The threshold input circuitry  310   a  may include operational amplifiers  402 ,  404 , transistors Q 1 , Q 2 , and resistors R 1 , R 2 , R 3 , and R 4 . Transistors Q 1  and Q 2  may be any variety of transistors. In one embodiment, transistor Q 1  may be a p-type metal oxide semiconductor field effect transistor (MOSFET) or PMOS MP 1 . Transistor Q 2  may be an n-type MOSFET or NMOS MN 1 . The first resistor R 1  may be disposed between a terminal  414  accepting the output voltage Vout and a source terminal of the transistor MP 1 . Node  406  may be connected to the inverting input of the operation amplifier  402 . The noninverting input of the operational amplifier  402  may be connected to the input terminal accepting the fixed voltage Vo. The transistor MP 1  may have its control or gate terminal coupled to the output of the operational amplifier  402 . 
   The second resistor R 2  may be connected between the drain of transistor MP 1 , the node  416 , and ground. The transistor MN 1  may have its control or gate terminal coupled to the output of the operational amplifier  404  to accept an output signal from the operational amplifier  404 . A third resistor R 3  may be coupled to an output node  420  and a terminal providing the fixed threshold level Vclo. The third resistor R 3  may also be coupled to the drain terminal of transistor MN 1 . The output node  420  may provide the output threshold level signal Vcl from the threshold input circuitry  310   a . The fourth transistor R 4  may be connected between the source terminal of transistor MN 1 , the node  418 , and ground. The inverting input terminal of the operational amplifier  404  may be coupled to node  418 , while its noninverting input may be coupled to node  416 . 
   In operation, operational amplifier  402  may drive the gate of MP 1  to conduct a current in order to permanently maintain the voltage level on its inverting input (node  406 ) at the same level with its noniverting input, the fixed voltage Vo. This is possible whenever the output voltage Vout is higher than Vo, the resulting current through both resistors R 1  and R 2  being I 1 =(Vout−Vo)/R 1 . When Vout&lt;Vo the current through transistor MP 1  cannot be further reduced, the gate of transistor MP 1  is driven to the maximum available voltage, transistor MP 1  is OFF and the current through resistors R 1  and R 2  becomes zero. Consequently the voltage on the resistor R 2 , i.e. between node  416  and the ground, is Vr 2 =0 when Vout&lt;Vo and Vr 2 =R 2  I 1 =(R 2 /R 1 )×(Vout−Vo) when Vout&gt;Vo. For reasons known to those skilled in the art through a feedback mechanism Vr 2  will be repeated on the resistor R 4 , namely between the node  418  and the ground, generating the current I 2 =Vr 2 /R 4  when Vout&gt;Vo and I 2 =0 when Vout&lt;Vo. Since the same current I 2  flows through the resistor R 3  it becomes evident that the output threshold voltage Vcl on the node  420  may be expressed as in equation (10) for Vout&gt;Vo and is constant Vcl=Vclo when the output voltage of the DC source Vout is less than Vo. 
   
     
       
         
           
             
               
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   In equation (10), Vcl is the variable threshold level provided at the output node  420 , Vclo is the fixed threshold level, R 1 , R 2 , R 3 , and R 4  are the resistance values of resistors R 1 , R 2 , R 3 , and R 4 , Vout is the output voltage, and Vo is the fixed voltage level defining the boundary between the initial and first range of output voltages as illustrated in  FIG. 2 . 
   By comparing equation (9) and (10), it becomes evident that the value of the resistors R 1 , R 2 , R 3 , and R 4  could be chosen such that equation (11) is true. 
   
     
       
         
           
             
               
                 
                   
                     
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     FIG. 5  illustrates a flow chart  500  of operations consistent with an embodiment. Operation  502  may include determining a first plurality of output current levels over a first range of output voltage levels for a variable output DC power source, each one of the first plurality of output current levels equal to a maximum output power level of the variable output DC power source divided by an output voltage level of the variable DC power source over the first range. For instance, in one embodiment the first plurality of output current levels (Im) may be those defined by plot  204  in  FIG. 2  over the range of output voltage levels where Vo&lt;Vout≦Vm. 
   Operation  504  may include establishing a second plurality of output current levels over the first range of output voltage levels in response to the first plurality of output current levels, the second plurality of output current levels decreasing with increasing voltage levels over the first range. For instance, in one embodiment the second plurality of output current levels (Ima) may be those defined by plot  207  in  FIG. 2 . Operation  506  may include monitoring an output current of the variable output DC power source. Finally, operation  508  may include driving the output current towards one of the second plurality of output current levels, e.g., Ima levels, if an output voltage of the variable output DC power source is within the first range and if the output current at the output voltage is greater than or equal to the one of the second plurality of output current levels (Ima) associated with the output voltage. 
   In summary, there is also provided power control circuitry for controlling a variable output DC power source. The power control circuitry may comprise a first comparator to compare a signal representative of an output current level of the variable output DC power source with a threshold level and provide a first output signal in response to the comparison. The power control circuitry may further comprise threshold input circuitry to provide the threshold level to the first comparator, the threshold level being a fixed threshold level if an output voltage of the variable output DC power source is less than or equal to a first fixed voltage level, the threshold level being a variable threshold level if the output voltage is greater than the first fixed voltage level. The power control circuitry may further comprise power limiting control circuitry to provide a control signal to the variable output DC power source in response to the first output signal from the first comparator. 
   In one embodiment the variable threshold may be representative of a second plurality of output current levels (Ima) of the variable output DC power source over the first range, the second plurality of output current levels (Ima) may approximate a first plurality of output current levels (Im) where each one of the first plurality of output current levels equals a maximum output power level of the variable output DC power source divided by an output voltage of the variable output DC power source over the first range. The first plurality of output current levels (In) hyperbolically decreases with increasing voltage levels over the first range and the second plurality of output current levels (Ina) may linearly decrease with increasing voltage levels over the first range. 
   There is also provided an electronic system. The system may comprise a variable output DC power source to provide power to a load, and power control circuitry to provide a control signal to the variable output DC power source. The variable output DC power source may be responsive to the control signal to adjust the output power level of the DC power source. The power control circuitry may comprise a first comparator to compare a signal representative of an output current level of the variable output DC power source with a threshold level and provide a first output signal in response to the comparison. The power control circuitry may further comprise threshold input circuitry to provide the threshold level to the first comparator, the threshold level being a fixed threshold level if an output voltage of the variable output DC power source is less than or equal to a first fixed voltage level, the threshold level being a variable threshold level if the output voltage is greater than the first fixed voltage level. The power control circuitry may further comprise power limiting control circuitry to provide a control signal to the variable output DC power source in response to the first output signal from the first comparator. 
   Advantageously, in these embodiments the output voltage of the variable output DC power source can be extended to operate in the Vo&lt;Vout≦Vm range. By approximating the hyperbolically decreasing plot of output current values, e.g., plot  204 , simplified power control circuitry can be more readily developed compared to other circuitry that may attempt to limit the output current to the hyperbolic plot. A linear plot of output current levels, e.g., plot  207 , may be developed to approximate the hyperbolically decreasing plot. Errors between the linear plot and hyperbolic plot can be minimized by mathematical and analytical means. 
   The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described (or portions thereof), and it is recognized that various modifications are possible within the scope of the claims. Other modifications, variations, and alternatives are also possible. Accordingly, the claims are intended to cover all such equivalents.