Patent Publication Number: US-2011057626-A1

Title: Power supply and charging circuit for high energy capacitors

Description:
The present invention relates to a power supply and charging circuit suitable for charging high energy storage capacitors. 
     High energy storage capacitors also known as ultra-capacitors or super capacitors are a viable alternative to storage batteries for powering a range of electrical devices such, as power tools including portable power tools. 
     Unlike a storage battery a super capacitor presents an extremely low impedance when it is initially being charged that is almost a short circuit. 
     Standard battery chargers and power supplies typically include protective or current limiting circuits that inhibit or shut down when trying to charge a low impedance load such as a super capacitor. Other types of chargers or power supplies operate in a constant current mode and may be capable of charging this type of load. However a disadvantage of a constant current mode charger is that with a given amount of power it may take such a charger relatively much longer to charge a load of this type. 
     Another problem with high energy storage capacitors is that they are typically available in limited sizes or voltages e.g. 2.7V 400F, 2.3V 220F, while electrical devices such as power tools that include readily available motors/gearboxes require higher values or voltages. The problem may be addressed in part by joining capacitors in series. However, a recognized problem when charging a series connected string of ultra-capacitors is that the voltages of the cells usually become different from one another. One prior art solution to this problem is to add balancing circuits to try to make every cell in an array have an equal voltage. However, the use of balancing circuits adds considerable cost and complexity to a power supply. The balancing circuits become larger and more expensive as the capacity and charging speed of the power supply increases. 
     The present invention may provide a charger that may minimize charging time by making maximum power available when it is needed most. Preferably power may be limited by available power, load and wiring impedance only. Available power generally is determined by fundamental capacity issues such as size and power rating of the power supply which for practical reasons should be kept as low as possible. 
     The present invention may provide a switched mode power supply (SMPS) including a flyback converter for charging a load such as a high energy capacitor. A typical flyback SMPS includes negative feedback sensing to maintain desired operating characteristics including voltage regulation at the output. Additionally, most flyback SMPSs employ secondary sensing to inhibit output runaway in case of loss of feedback sensing and/or if the output becomes short circuited. Secondary sensing is generally accomplished via an optical isolator that monitors voltage at the output and feeds back an appropriate control signal to the associated controller. In the case of a low impedance load such as a super capacitor a typical flyback SMPS will shut down or inhibit or limit its operation. 
     The present invention may provide a charger suitable for high energy storage capacitors that may avoid a need for a capacitor balancing circuit. 
     The present invention may provide a capacitor array, pack or module including terminals that allow each capacitor of the array to be charged in parallel. The capacitor array, pack or module may include multiple charging connections such that all cells in the array or pack are connected to a charger in parallel. The charging voltage may be selected such that none of the cells in the array or pack is overcharged. The charging connections may be arranged such that when the capacitor array or pack is inserted into a power tool at least some capacitors in the array or pack are connected in series to provide a higher voltage than is available from any one capacitor in the capacitor array or pack. 
     The present invention may provide a SMPS including a flyback converter that alleviates the disadvantages of the prior art. The present invention may also provide a capacitor array that avoids a need for a capacitor balancing circuit at least during a charging cycle. 
     According to one aspect of the present invention there is provided a switched mode power supply for charging a load such as a high energy capacitor and adapted to operate in a substantially constant power mode wherein said power supply includes a flyback converter having a primary circuit including a controller and a secondary circuit for providing an output to said load, wherein said power supply is arranged such that it utilizes substantially no current limiting in said secondary circuit, and wherein said load includes a plurality of high energy capacitors and a plurality of terminals, said terminals being connected to said capacitors such that said capacitors may be arranged in a first configuration for charging said load and may be arranged in a second configuration for discharging said load. 
     The first configuration may include each capacitor in the array connected in parallel. The second configuration may include at least some capacitors in the array connected in series. 
     The controller in the primary circuit may include pulse width modulation (PWM). A bias current associated with the PWM controller may be adjusted to avoid shut down of the power supply when the load is substantially a short circuit. 
     When the load begins to charge its voltage will generally increase. As voltage increases current may be reduced to maintain a constant power mode set for the charger (power=voltage×current). Constant power mode may be effected by adjusting current in the primary circuit. 
     An advantage of constant power mode is that it may require a relatively smaller amount of power to charge a load of a given size in a given time. A power supply embodying the principles of the present invention may therefore be more efficient in cost and size for a given power rating. 
     When the load becomes fully charged its terminal voltage may be regulated to avoid exceeding a design limit. At a predetermined terminal voltage the power supply of the present invention may switch to a “constant voltage” mode to maintain the load in a fully charged state. Constant voltage mode may be provided in any suitable manner and by any suitable means such as by means of a feedback control loop. 
     According to a further aspect of the present invention there is provided a switched mode power supply for charging a load such as a high energy capacitor and adapted to operate in a substantially constant power mode wherein said power supply includes a flyback converter having a primary circuit including a controller and a secondary circuit for providing an output to said load, wherein said secondary circuit includes means for isolating said load to substantially prevent its discharge at least when said power supply is turned off, and wherein said load includes a plurality of high energy capacitors and a plurality of terminals, said terminals being connected to said capacitors such that said capacitors may be arranged in a first configuration for charging said load and may be arranged in a second configuration for discharging said load. 
     The isolating means may include a diode. The isolating means may be located at an upstream point in the charging circuit to substantially prevent discharge of the load. The isolating means preferably is located in the charging circuit such that it is upstream of elements including resistors etc. that may cause current to flow and hence discharge the load. 
     Having regard to the relatively large number of charge cycles that the power supply of the present invention may be required to reliably deliver, component stress may be controlled or maintained within specified limits by incorporating temperature sensing of vital components in the circuit associated with the power supply. 
     According to a still further aspect of the present invention there is provided a method of charging an array of high energy capacitors having a plurality of terminals including arranging said terminals such that said capacitors are connectable in a first configuration for charging said array and are connectable in a second configuration for discharging said array. 
    
    
     Preferred embodiments of the present invention will now be described with reference to the accompanying drawing which shows one form of a fast charger according to the principles of the present invention. 
       FIG. 1  shows a fast charger  10  according to the present invention. The charger  10  includes a full wave rectifier stage  11  including a prefilter. DC power supplied by rectifier stage  11  is switched via mosfet transistor Q 1  at a specific frequency (eg. 100 KHz) through coupled storage inductor TR 1 . The gate of transistor Q 1  is controlled via PWM controller U 1 . Controller U 1  may include an integrated circuit controller such as a device type NCP1216 manufactured by ON Semiconductor. Controller U 1  utilizes pulse width modulation to provide desired charging characteristics including constant power and constant voltage control. When transistor Q 1  is on, energy is stored in the primary circuit of inductor TR 1  but is not transferred to the secondary circuit because diode D 2  is biased off. When transistor Q 1  is switched off the energy stored in the primary circuit of inductor TR 1  is transferred to the secondary circuit at which time diode D 2  is biased on due to Faraday&#39;s Law. Note that inductor TR 1  is not a transformer in the normal sense, rather it is an inductively coupled energy storage device that is tightly magnetically coupled. Inductor TR 1  also provides galvanic isolation between input and output of the charger. This process is repeated at the switching frequency whereby energy is transferred to the output load (such as a super capacitor) each time that diode D 2  is biased on, ie. transistor Q 1  is in its off state during the cycle “flyback”. Power from Inductor TR 1  is transferred to the load via isolating diode D 3 . Isolating diode D 3  is placed at a most upstream point in the circuit to prevent discharge of a load such as high energy capacitor array when the charger is turned off. 
     To protect the load (high energy capacitor array) from excess voltage, the charger includes an over voltage circuit  12 . Over voltage circuit  12  includes transistor Q 2 , controlled diode U 5  and resistor ladder R 16 , R 19 , R 20  to shunt (or cap) excess voltage to below a present level. This shunting of excess voltage will generate additional thermal energy which will be dissipated by transistor Q 2 . Over voltage circuit  12  is adapted to protect the load (high energy capacitor array) as well as the charger itself. 
     To provide constant power control, input current that is switched via transistor Q 1  is also monitored by input current sensing circuit  13 . Input current sensing circuit  13  includes resistor R 6  for converting the input current to a voltage which is supplied to PWM controller U 1  via resistor R 4 . Significantly the charger does not utilize current limiting on the output side of the circuit. An advantage of this is that it may avoid wasting power in the current limiting circuit including significantly higher I 2 R losses and overheating as well as degrading reliability and MTBF (mean time before failure). Given the significantly higher currents employed in the output compared to prior art designs a true constant power limited device may be provided without the associated power losses. 
     Output voltage is sensed via output voltage sensing circuit  14 . Output voltage sensing circuit  14  includes controlled diode U 3 , capacitor C 9  and resistor strings R 8 , R 9  and R 10 , R 11 , R 12 . 
     The sensed output voltage is supplied to PWM controller U 1  via optical coupling element U 2  (to maintain galvanic isolation between input and output) in the form of a negative voltage feedback signal. This enables PWM controller U 1  to adjust pulse width of the signal driving transistor Q 1  to provide constant voltage control after completion of the main charging cycle. Coupling element U 2  operates in combination with reference diode U 3  to cause a reduction of voltage across resistor R 5  (which then lowers PWM duty cycle) whenever output voltage increases above a pre-set reference determined by operating characteristics of the load (high energy capacitor array). 
     When the load is at or near full charge controller U 1  may switch to a burst mode of operation. During burst mode controller U 1  may “skip” certain switching cycles to increase conversion efficiency. This has the effect of reducing overall switching frequency of the controller during these periods while maintaining pulse width at minimum levels. 
     The charger includes an over temperature sensor T 1  for monitoring excess temperature. The over temperature sensor is adapted to shut down a primary circuit associated with PWM controller U 1  to prevent excess temperature. 
     The charger includes a normally closed pod switch (not shown) for detecting when a load (high energy capacitor array) is inserted into the charger. The pod switch is connected to controller U 1  via connector J 1 . Its main function is to keep the charger in a low power off state (when a load is NOT inserted) by forcing it to run a Hard Ship cycle burst mode. When a load is inserted such that full charge mode is initiated, this may also act as a safety feature which may cause a “soft-startup” when a discharged high energy capacitor array is first inserted into the charger to avoid a high instantaneous current and possibly arcing during this time. 
     The charger includes a display circuit  15  including resistor string R 13 , R 14  and R 18  and controlled diode UV and LED diode L 1 . Display circuit  15  is arranged such that LED diode L 1  comes on (at a specific voltage level) to indicate that the load is at or near full charge. 
       FIG. 2  shows a load comprising an array  20  of super capacitors  21 ,  22  connected to charger  10 . Super capacitors  21 ,  22  are assembled in array  20  including terminator A. Terminator A includes individual terminals  21   a,    21   b  associated with super capacitor  21  and individual terminals  22   a,    22   b  associated with super capacitor  22 . Array  20  is connected to charger  10  via terminator B associated with charger  10 . Terminator B includes terminals  23 - 26 . Terminals  23  and  25  are connected to the positive output terminal (POS) of charger  10  and terminals  24  and  26  are connected to the negative output terminal (NEG) of charger  10 . The arrangement of the terminals is such that when terminators A and B are coupled together capacitors  21  and  22  are connected to charger  10  in parallel. 
       FIG. 3  shows the array  20  of super capacitors  21 ,  22  connected to an electrical power tool  30 . Power tool  30  is connected to array  20  via terminator C. Terminator C includes individual terminals  31 - 34 . Terminals  31  and  34  are connected to respective input terminals of tool  30 . The arrangement of terminals is such that when terminators A and C are coupled together capacitors  21  and  22  are connected to tool  30  in series. 
       FIG. 4  shows a load comprising an array  40  of super capacitors  41 ,  42 ,  43  connected to charger  10 . Super capacitors  41 ,  42 ,  43  are assembled in array  40  including terminator A. Terminator A includes individual terminals  41   a,    41   b  associated with super capacitor  41 , individual terminals  42   a,    42   b  associated with super capacitor  42  and individual terminals  43   a,    43   b  associated with super capacitor  43 . Array  20  is connected to charger  10  via terminator B associated with charger  10 . Terminator B includes terminals  44 - 49 . Terminals  44 ,  46  and  48  are connected to the positive output terminal (POS) of charger  10  and terminals  45 ,  47  and  49  are connected to the negative output terminal (NEG) of charger  10 . The arrangement of the terminals is such that when terminators A and B are coupled together capacitors  41 ,  42  and  43  are connected to charger  10  in parallel. 
       FIG. 5  shows the array  20  of super capacitors  41 ,  42 ,  43  connected to an electrical power tool  50 . Power tool  50  is connected to array  40  via terminator C. Terminator C includes individual terminals  51 - 56 . Terminals  51  and  56  are connected to respective input terminals of tool  50 . Terminals  52  and  53  are connected together. Terminals  54  and  55  are also connected together. The arrangement of terminals is such that when terminators A and C are coupled together capacitors  41 ,  42  and  43  are connected to tool  50  in series. 
     Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.