Abstract:
According to aspects of the present invention, systems and methods are provided for faster charging of electrical energy storage components such as supercapacitors while maintaining the safety limits. In one or more exemplary embodiments, a flyback transformer is used to provide constant energy charging to the supercapacitor several times faster than in conventional systems or methods, due to the high frequency output of the flyback transformer, while not exceeding the power output rating of the power supply. According to one embodiment, a cycle-by-cycle energy transfer limit is used to charge one or more supercapacitors.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claims priority to and the benefit of, pursuant to 35 U.S.C. §119(e), U.S. Provisional Patent Application Ser. No. 61/303,416, filed Mar. 4, 2010, entitled “Inductive Charging,” by Zafarullah Khan, the disclosure of which is herein incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    Aspects of the present disclosure relate generally to the charging of electrical energy storage components. More particularly, aspects of the present disclosure relate to fast charging of supercapacitors. 
       BACKGROUND 
       [0003]    Batteries are useful for the purpose of storing electrical energy. The use of batteries is not particularly convenient and causes a number of problems for the users, such as environmental hazards, safety problems, maintenance costs, charge/discharge rate limitations, finite number of possible charge cycles, narrow operating temperature range, battery life, and need for continuous replacement. Growing demands of portable systems, which can overcome the above-mentioned problems including the large size for a given power output, lead to introduction of supercapacitors as a replacement of or supplement to batteries. Further, use of supercapacitors minimizes charge time and overall system size for a given power output. 
         [0004]    Conventional supercapacitor charging methods involve some form of current limiter to limit the charging current applied to the supercapacitor. The current limiter controls the current between a supercapacitor and battery or any other power source, thus preventing flow of excess current, because a completely discharged supercapacitor appears like a short circuit to the charging circuitry due to its very low Equivalent Series Resistance (ESR). 
         [0005]    The rate of energy transfer into the supercapacitor is given by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                          
                         e 
                       
                       
                          
                         t 
                       
                     
                     = 
                     
                       V 
                       + 
                       I 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0006]    where de/dt is the rate of energy transfer in Joules/Sec, V is the instantaneous voltage across the super capacitor in Volts, and I is the value of the constant current limit in Amperes. 
         [0007]    As can be seen from (1), the rate of energy transfer is very slow at low voltages when the current limit is applied, irrespective of the power output capability of the power supply that is providing the charging current. As a result, the charging time is very long, as can be seen from (2): 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                     = 
                     
                       
                         C 
                         × 
                         V 
                          
                         
                             
                         
                          
                         max 
                       
                       I 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where T is the time in seconds, V max is the final steady state voltage in Volts, C is the capacitance in Farads, and I is the constant current limit in Amperes. 
         [0008]    The current limit I is set so that the maximum rate of energy transfer (at Vmax) does not exceed the power output capability of the power source  210 . Thus, 
         [0000]    
       
         
           
             I 
             = 
             
               P 
               
                 V 
                  
                 
                     
                 
                  
                 max 
               
             
           
         
       
     
         [0000]    where I is the current limit, P is the power output capability of the power source, and V max is the final steady state voltage attained by the supercapacitor. Thus, in terms of power output capability of the power source  210  P, equation (2) can be written as: 
         [0000]    
       
         
           
             
               
                 
                   T 
                   = 
                   
                     C 
                     × 
                     V 
                      
                     
                         
                     
                      
                     max 
                     × 
                     
                       
                         
                           V 
                            
                           
                               
                           
                            
                           max 
                         
                         P 
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   
                     2 
                      
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
         [0009]    Another disadvantage of using the current limited charging method is that it can only charge the Supercapacitor to a voltage lower than or equal to the voltage of the DC power source. 
       SUMMARY 
       [0010]    In one or more aspects, the present invention provides a solution to the above mentioned problems by employing a flyback transformer to provide constant energy charging at a rate equal to the power output capability of the power source, irrespective of the instantaneous voltage of the supercapacitor, using a cycle-by-cycle energy transfer limit to charge the supercapacitors at a constant rate of energy transfer, instead of a constant current limit as used by conventional systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0012]      FIG. 1  illustrates a first system for inductive charging of an electrical energy storage component, according to one embodiment of the present invention. 
           [0013]      FIG. 2  illustrates a second system for inductive charging of an electrical energy storage component, in accordance with one embodiment of the present invention. 
           [0014]      FIG. 3  is a flow chart illustrating operational steps of a first method for inductive charging of an electrical energy storage component, in accordance with one embodiment of the present invention. 
           [0015]      FIG. 4  is a flow chart illustrating operational steps of a second method for inductive charging of an electrical energy storage component, in accordance with one embodiment of the present invention. 
           [0016]      FIG. 5  is a flow chart illustrating operational steps of a third method for inductive charging of an electrical energy storage component, in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Reference is now made in detail to the description of the embodiments of systems and methods for inductive charging of electrical storage components, as illustrated in the drawings. However, the present invention should not be construed as limited to the embodiments set forth herein; rather, these embodiments are intended to convey the scope of the aspects of the present invention to those skilled in the art. Furthermore, all “examples” given herein are intended to be non-limiting. 
         [0018]    Referring now to the drawings,  FIG. 1  illustrates a first system  100  for inductive charging of an electrical energy storage component, according to one embodiment of the present invention. As shown, a power source  210  provides a DC voltage and is operatively connected to a transistor switch  220 . A flyback transformer  230  has a primary winding that is operatively coupled to the transistor switch  220  and the power source  210 , with the secondary winding being operatively coupled to a fast switching diode  240 . A supercapacitor  140  is operatively connected to the secondary winding and the switching diode  240 . The switching diode  240  is operative to rectify a charging current flowing from the secondary winding to match the polarity of the supercapacitor  140 . An analog-to-digital converter (ADC)  245  is operatively connected to the supercapacitor  140  and the switching diode  240  at an input. The ADC  245  is operative to measure the voltage of the supercapacitor  140  and provide a corresponding charge level signal. A programmable controller  250  has a DC voltage input  251  that is operatively coupled to an output of the ADC  245 . The programmable controller  250  also has a pulse enable output  252  and a pulse width control output  253 . A pulse generating circuit  265  has a pulse enable input  267  that is operatively coupled to the pulse enable output  252  of the programmable controller  250 , a pulse output  266  that is operatively coupled to the transistor switch  220 , and a pulse width control input  268  that is operatively coupled to the pulse width control output  253  of the programmable controller  250 . The pulse generating circuit  265  is responsive to the charge level signal provided by the ADC  245 , and includes a pulse generator that is operative to modulate the transistor switch  220  such that when the switch  220  is closed, current in the primary winding of the flyback transformer  230  ramps up, and when the switch  220  is open, energy stored in the primary winding is transferred to the secondary winding and the charging current flows into the supercapacitor  140 . 
         [0019]      FIG. 2  illustrates a second system  200  for inductive charging of an electrical energy storage component, such as a supercapacitor  140 , according to one embodiment of the present invention. As shown, the system  200  includes a power source  210 , a transistor switch  220 , a flyback transformer  230 , and a switching diode  240 . The system  200  includes an ADC  245  to measure the voltage of the supercapacitor  140 , a pulse generator  265  to generate pulses used to modulate the switch  220 , an ADC  260  to read the voltage across the current sensing resistor  270 , and a programmable controller  250 . 
         [0020]    The charging time for the supercapacitor  140 , using the flyback transformer  230  is given by the equation below: 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                     = 
                     
                       C 
                       × 
                       V 
                       × 
                       
                         V 
                         
                           L 
                           × 
                           
                             I 
                             2 
                           
                           × 
                           N 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where T is the time in seconds, V is the final steady state supercapacitor voltage in Volts, C is the capacitance in Farads, I is the peak current in the primary winding of the flyback transformer in Amperes, L is the inductance of the primary winding of the flyback transformer in Henrys, and N is the frequency of operation of the flyback transformer. Again, in terms of power output P of the power source  210 , equation (3) can be written as (assuming 100% efficiency): 
         [0000]    
       
         
           
             
               
                 
                   T 
                   = 
                   
                     C 
                     × 
                     V 
                     × 
                     
                       
                         V 
                         
                           2 
                            
                           
                               
                           
                            
                           P 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   
                     3 
                      
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
         [0021]    As can be seen from (3a) and (2a), the charging time for the supercapacitor  140  drops to half in comparison to when the Current limiter is used. 
         [0022]    According to one or more embodiments, a flyback transformer  230  is used that has primary and secondary windings, with a suitable primary inductance of L Henrys, depending on the application. The primary winding is connected to a source of DC voltage  210  with a transistor switch  220  in series. When the switch  220  is closed, the current in the primary winding starts ramping up. 
         [0023]    The energy stored in the primary winding is given by: 
         [0000]    
       
         
           
             
               
                 
                   E 
                   = 
                   
                     
                       1 
                       2 
                     
                     × 
                     L 
                     × 
                     
                       I 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0024]    The secondary winding is connected across the supercapacitor  140  with a fast switching diode  240  in series to rectify the current to match the polarity of the supercapacitor  140 . When the primary switch  220  is opened, the energy stored in the primary winding is transferred to the secondary winding and causes a brief charging current to flow into the supercapacitor  140 . As a consequence, the energy stored in the primary winding is transferred to the supercapacitor  140 . This cycle is continuously repeated at a rapid rate, due to the high frequency output of the flyback transformer  230 , until the supercapacitor  140  is charged to the desired voltage. Further, using the flyback transformer  230  enables charging the supercapacitor  140  to a voltage higher than the voltage of the DC power source as well, apart from the lower voltage charging only using the conventional method. The amount of energy transferred to the supercapacitor  140  per cycle can be controlled by controlling the peak value attained by the current in the primary winding of the flyback transformer  230 . This is done by controlling the “on” period of the switch  220  according to the following equation: 
         [0000]        T=LI/V   (5),
 
         [0000]    where T is the “ON” time in seconds of the switch  220 , L is the inductance in Henries of the primary winding of the transformer  230 , I is the desired peak current in Amperes through the primary winding of the transformer  230  and V is the voltage in volts of the power source  210 . 
         [0025]    The “ON” time of the switch  220  is determined by the pulse width output  266  of the pulse generator  265 . The width of the pulse outputted at  266  can be controlled by the control circuitry  250  via the pulse width control output  253  that connects to the pulse width control input  268  of the pulse generator. 
         [0026]    The number of pulses N needed to charge the supercapacitor  140  is determined by: 
         [0000]        N=C ( V 1 2   −V 2 2 )/ LI   2   (6),
 
         [0000]    where C is the capacitance in Farads of the supercapacitor  140 , V 1  is the actual voltage in volts of the supercapacitor  140 , V 2  is the desired voltage in volts of the supercapacitor  140 , L is the inductance in Henrys of the primary winding of the transformer  230 , and I is the peak current, in Amperes, through the primary winding of the transformer  230 . 
         [0027]    In accordance with one or more embodiments of the present invention, the control circuitry  250  can be programmed to perform steps for inductive charging of an electrical energy storage component. In accordance with one embodiment, in a first step, the control circuit  250  measures the voltage across the supercapacitor  140  with the help of the ADC  245 , and then in a second step, the control circuit determines the number of pulses needed to charge the supercapacitor  140  to the desired voltage, using equation (6). In a third step, the pulse generator  265  is enabled with the help of the pulse enable input  267 , and the pulse generator  265  starts outputting the pulses to the switch  220 , through the pulse output  266 . The switch  220  turns on for the duration of the pulse and then turns off. In a fourth step, after the required number of pulses have been outputted, the control circuit  250  again measures the voltage of the supercapacitor  140 . If the voltage is found to be less than the desired voltage (due to leakage or due to load current being drawn from the supercapacitor  140 ), then the control circuit  250  again computes the number of pulses needed to charge the supercapacitor  140  to the desired voltage and the second step and the fourth steps are repeated. 
         [0028]      FIG. 3  is a flow chart illustrating operational steps of a first method  500  for inductive charging of an electrical energy storage component, in accordance with one embodiment of the present invention. As described above, control circuitry  250  is programmable to perform the operational steps of the method. As shown, in the first operational step of the method, step  301 , a desired voltage for the supercapacitor  140  is determined, and next, at step  303 , a desired peak current for the primary winding of the flyback transformer  230  is determined. From step  303 , operational flow proceeds to step  305 , where a pulse width is set to obtain the desired peak current. Next, at step  307 , the voltage V 2  across supercapacitor  340  is measured, and then, as shown at step  309 , it is determined if the measured voltage V 2  is less than the desired voltage V 1 . If V 2  is less than V 1 , then flow proceeds from step  309  to step  311 , where the pulse generator  265  pulses the switch  220 , and then after a brief delay to account for decay, operational flow returns to step  307 . If it is determined at step  309  that V 2  is not less than V 1 , then the operational flow ends, at step  315 . 
         [0029]      FIG. 4  is a flow chart that illustrates operational steps of a second method  400  for inductive charging of an electrical energy storage component, in accordance with one embodiment of the present invention. As described above, the control circuitry  250  is programmable to perform the operational steps of the method. As shown, in the first operational step of the method, step  401 , a desired voltage for the supercapacitor  140  is determined, and next, at step  403 , a desired peak current for the primary winding of the flyback transformer  230  is determined. From step  403 , operational flow proceeds to step  405 , where a pulse width is set to obtain the desired peak current. Next, at step  407 , the voltage V 2  across supercapacitor  140  is measured, and then, as shown at step  409 , the number of pulses N needed to charge the supercapacitor to the desired voltage V 1  is determined. Next, the number of pulses that have been given is counted, and if the count is less than the number of pulses needed, N, then the pulse generator  265  pulses the switch  220 , then increments the pulse count by one, at step  413 , and returns to determine if the pulse count is less than the number of pulses needed, after the switch has been pulsed. If at step  409  it is determined that the pulse count is not less than the number of pulses needed, then operational flow ends, at step  415 . 
         [0030]      FIG. 5  is a flow chart illustrating operational steps of a third method  500  for inductive charging of an electrical energy storage component, in accordance with one embodiment of the present invention. As described above, the control circuitry  250  is programmable to perform the operational steps of the method. As shown, in the first operational step of the method, step  501 , a desired voltage for the supercapacitor  140  is determined, and next, at step  503 , a desired peak current for the primary winding of the flyback transformer  230  is determined. From step  503 , operational flow proceeds to step  505 , where a pulse width is set to obtain the desired peak current. Next, at step  507 , the voltage V 2  across supercapacitor  140  is measured, and then, as shown at step  509 , it is determined if the measured voltage V 2  is less than the desired voltage V 1 . If V 2  is less than V 1 , then flow proceeds from step  509  to step  511 , where the number of pulses N needed to charge the supercapacitor  140  to the desired voltage V 1  is determined, and then at step  513 , the pulse generator  265  pulses the switch  220 , and then after a brief delay to account for decay, operational flow returns to step  507 . If it is determined at step  509  that V 2  is not less than V 1 , then operational flow ends, at step  515 . 
         [0031]    Most devices depending on supercapacitors for backup power need some voltage headroom to remain operational, the reason being that a supercapacitor&#39;s voltage decays down very slowly in the event of power failure. As a non-limiting example, if a device needs 3.3V to operate, it may use a 6V supercapacitor-based system as backup so that the device remains operational for a long period of time, as the supercapacitor&#39;s voltage will decay very slowly from 6V to 3.3V. Further, when the supercapacitor-based system is first powered up and the supercapacitor starts charging, the device does not become operational until the supercapacitor has attained 3.3V. In such systems too, aspects of the present invention provide significant improvement in the time that the system takes to become operational from the moment it is turned on, because of the use of a flyback transformer to charge the supercapacitor at a constant rate of energy transfer, irrespective of the supercapacitor voltage, whereas the rate of energy transfer is very low when the supercapacitor voltage is close to zero for a conventional current limited charger. This phenomenon can be better explained using a non-limiting example that uses a 100 F supercapacitor. If a device becomes operational at 4V and the maximum voltage reached by the supercapacitor is 6V, then using the conventional constant current limited charging method with power output limited to 6 W maximum, the current has to be limited to 1 A. Accordingly, the time required to reach 4V using equation (2) is: 
         [0000]    
       
         
           
             
               100 
               × 
               
                 4 
                 1 
               
             
             = 
             
               400 
                
               
                   
               
                
               
                 seconds 
                 . 
               
             
           
         
       
     
         [0000]    Using a method according to an aspects of the present invention, with power output limited to 6 W as above we have from equation (3a), the time required is: 
         [0000]    
       
         
           
             
               100 
               × 
               4 
               × 
               4 
               × 
               
                 2 
                 6 
               
             
             = 
             
               133.33 
                
               
                   
               
                
               
                 seconds 
                 . 
               
             
           
         
       
     
         [0032]    As can be seen from this comparison, the device becomes operational in nearly one third of the time that it would take using conventional current limited charging method. 
         [0033]    The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching. 
         [0034]    The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope.