Abstract:
A method for charging a capacitor charging circuit comprises producing a digital pulse train, converting the digital pulse train to an AC signal, amplifying the AC signal to produce a high voltage AC signal, rectifying the high voltage AC signal to produce a capacitor charging signal, sampling characteristic data from the capacitor charging circuit, optimizing the digital pulse train based on the characteristic data, and charging the capacitor using the capacitor charging signal. The digital pulse train may be continually optimized based on the characteristic data.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a circuit and method for charging and storing a high voltage used with a camera flash.  
       BACKGROUND  
       [0002]     As cell phones and other portable electronic devices grow in complexity, manufacturers strive to include ever greater functionality in these devices to attract customers. Recently, small digital cameras have been included with some cellular telephones. However, these cameras are not always used in situations where a sufficient amount of natural light is present to ensure a well exposed picture is taken. Electronic flashes are a simple and cheap method of providing proper lighting for photographic applications where the amount of natural light is limited. However, the inclusion of electronic flashes in portable electronic devices has been hampered by the bulk and complexity of these flashes. As such, there is a need for smaller, more compact flash systems for use with portable devices.  
         [0003]     As is known to one skilled in the art, conventional flash circuits are made using a number of discrete components including multiple resistors, capacitors and inductors, among others. These analog components may be used for the charging of a storage capacitor. In addition to these components, flyback transformer circuits may be used to charge a storage capacitor for a flash circuit using a series of pulses of primary current to a flyback transformer. However, due to incomplete energy depletion of the secondary windings of the flyback transformer during discharge, as well as the transient amount of current necessary to charge empty discrete components in the circuit upon start-up, the phenomenon known as inrush current arises. Those skilled in the art will be familiar with the phenomenon and know that it is undesirable from a performance standpoint. Therefore, in addition to the need for smaller, more compact flash systems, there is an additional need for a system which can be charged quickly and efficiently while experiencing a minimum of inrush current.  
       SUMMARY OF THE INVENTION  
       [0004]     In one embodiment, a method for charging a capacitor charging circuit comprises producing a digital pulse train, converting the digital pulse train to an AC signal, amplifying the AC signal to produce a high voltage AC signal, rectifying the high voltage AC signal to produce a capacitor charging signal, sampling characteristic data from the capacitor charging circuit, optimizing the digital pulse train based on the characteristic data, and charging the capacitor using the capacitor charging signal. The digital pulse train may be continually optimized based on the characteristic data.  
         [0005]     In another embodiment, a portable electronic device comprises a cellular telephone, a primary power source connected to the cellular telephone, a transformer connected to the primary power source for boosting voltage provided by the primary power source, a diode connected to the transformer for rectifying fluctuating current provided by the transformer, a capacitor connected to the diode for storing charge, a microcontroller providing an output according to a stored program, and an electronic switch coupled to the microcontroller for drawing power through the transformer.  
         [0006]     In an alternative embodiment, the flash converter circuit further comprises a feedback loop coupling the output of the diode to the microcontroller. The stored program varies the output provided by the microcontroller in response to changes in the signal on the feedback loop.  
         [0007]     In another alternative embodiment, the flash converter circuit further comprises a feedback loop coupling the output of the diode to the microcontroller. The stored program operates to dynamically vary at least one of the charging frequency and duty cycle of the output provided by the microcontroller according to a received signal to optimize a performance characteristic of the flash converter circuit. The charging speed of the flash converter circuit, as well as the incidence of inrush current in the flash converter circuit are both performance characteristics which may be optimized in various embodiments of the present invention.  
         [0008]     In yet another embodiment of the present invention, a microcontroller included as a processor for a consumer device may be used to generate a series of digital pulses with which to drive a transformer, which in turn is used to store a high voltage charge on a capacitor. The microcontroller may be used to produce all manner of dynamic signals from a basic square wave to more complex methods of adaptive pulse shaping. It will be known to one skilled in the art to select the optimum waveform based on the requirements of the application at hand.  
         [0009]     By using a microcontroller already present in a device, such as the microcontroller native to a cellphone or the like rather than using discrete components to generate a series of digital pulses, the size of the circuit for charging a capacitor including the printed circuit board and the number of components used thereon will therefore be reduced in one embodiment of the present invention while a fast charging time is maintained.  
         [0010]     It is estimated that the exemplary embodiment of the present invention could be at least 10% smaller in size and concomitantly cheaper than conventional flash module converter circuits while maintaining its performance (e.g. speed of charging). This is especially desirable where a miniature flash module is packed into compact devices such as with a mobile phone camera and other such portable devices. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a known camera flash charging circuit;  
         [0012]      FIG. 2  shows a camera flash charging circuit according to an exemplary embodiment of the present invention;  
         [0013]      FIG. 3  shows a flowchart according to which a digital pulse signal may be provided by a microcontroller in one embodiment of the present invention;  
         [0014]      FIGS. 4   a ,  4   b  show a pair of oscilloscope readouts for the voltage level and the current drain characteristics of one embodiment of the present invention;  
         [0015]      FIGS. 5   a ,  5   b  show a pair of oscilloscope readouts for the voltage level and the current drain characteristics of another embodiment of the present invention;  
         [0016]      FIGS. 6   a ,  6   b  show a pair of oscilloscope readouts for the voltage level and the current drain characteristics of another embodiment of the present invention;  
         [0017]      FIGS. 7   a ,  7   b  show a pair of oscilloscope readouts for the voltage level and the current drain characteristics of yet another embodiment of the present invention;  
         [0018]      FIG. 8  shows a top view of the camera flash charging circuit of  FIG. 2 ;  
         [0019]      FIG. 9  shows a bottom view of the camera flash charging circuit of  FIG. 2 ;  
         [0020]      FIG. 10  shows a oblique view of the camera flash charging circuit of  FIG. 2 ; and  
         [0021]      FIG. 11  shows a side view of the camera flash charging circuit of  FIG. 2  being tested. 
     
    
       [0022]     Before any embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangements of components set forth in the following description, or illustrated in the drawings. The invention is capable of alternative embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the terminology used herein is for the purpose of illustrative description and should not be regarded as limiting.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     A basic camera flash system has three major parts: a small battery, which serves as the power supply, a gas discharge tube which actually produces the flash, and a circuit normally made up of a number of discrete components. The discharge tube usually consists of a tube filled with xenon gas, with electrodes on either end and a trigger electrode which can be metal or an electrically conductive layer at the body of the tube. Because of the high voltage needed to ionize the gas of the discharge tube and create a flash, the flash circuit needs to boost the voltage from the battery substantially before it may be successfully applied to the discharge tube.  
         [0024]     Traditionally, forward-type converters have been used for booster circuits in conventional electronic flash apparatuses because they are simple in structure and are little affected by variations of oscillating transformers. However, as cameras have been made more compact, low-capacity batteries have increasingly been used to provide power for the camera flash. In contrast, high guide numbers are required of the camera flash, necessitating a high intensity flash to properly expose the image. Accordingly, flyback-type converters which are more efficient than forward-type converters are gaining in popularity over traditional forward-type converters. Efficient charging of a capacitor by a low current may be achieved using a flyback converter.  
         [0025]     As is known to one skilled in the art, to boost voltage using a flyback converter a fluctuating current is needed, which may be provided in a flash circuit by continually interrupting the DC current flow. Rapid, short pulses of DC current are passed to the flyback transformer from a simple oscillator, continually fluctuating its magnetic field. The oscillator&#39;s main elements are the primary and secondary coils of a transformer, another inductor (the feedback coil), and a transistor acting as an electrically controlled switch. The switched DC power is converted to an AC signal at the transformer and as such can be stepped up or down in voltage by the transformer.  
         [0026]     Accordingly,  FIG. 1  shows a known camera flash charging circuit using a switch mode power supply for charging a capacitor  190 . Stored charge on the capacitor  190  may be later used to activate a photo flash tube (not shown). Generally, switch mode power supplies are used in applications where low DC input must be converted to high DC voltage output to charge a capacitor. Because this requires a transformer, and because the transformer needs a fluctuating power supply to operate, with a switch mode power supply the primary current to the transformer is controlled by a series of on and off switched pulses, thus giving rise to the name. Various means can be employed to control the series of pulses in the primary current feeding the transformer.  
         [0027]     The degree to which the transformer steps up or down the voltage between the primary  160  and secondary  170  coils of the transformer depends on the number of loops in each coil and/or the space and materials between the loops (for example, one coil might be wound around the other, or both might be wound around an iron core). In a step-up transformer like the one shown in  FIG. 1 , the secondary coil  170  will have many more loops than the primary coil  160 . As a result, the voltage generated on the secondary coil  170  will be much greater than that present on the primary coil  160 .  
         [0028]     The charging circuit of  FIG. 1  is activated by closure of the charging switch  155 , which sends a short burst of current from the power supply  100  through the feedback coil  165  to the base of the transistor  150 . Applying this current to the base of the transistor  150  allows current to flow from the collector to the emitter of the transistor  150 . When the transistor is “switched on” in this way, a second burst of current can then flow from the power supply  100  to the primary coil  160  of the transformer. This burst causes a change in voltage in the secondary coil  170 , which in turn causes a change in voltage in the feedback coil  165 . This voltage in the feedback coil  165  conducts current to the transistor base, making the transistor  150  conductive yet again, and the process repeats. As the circuit continually interrupts and repeats itself in this way, voltage is gradually boosted on the secondary coil  170  of the transformer.  
         [0029]     The high-voltage output from the transformer is rectified by the rectifier diode  180  from a fluctuating current back into a steady direct current. This high-voltage charge is then used to charge a flash electrolytic capacitor  190 . A second transformer (not shown) may be used to further boost the voltage from the capacitor  190  before applying this voltage to a discharge tube (not shown) to produce the flash.  
         [0030]     In a novel alternative, an exemplary embodiment according to the present invention is shown in  FIG. 2 . At the heart of this embodiment lies the microcontroller  210 , which is provided to output a dynamic, programmable digital control signal to the flyback converter in the embodiment shown in  FIG. 2 . Rather than the charging method used in the prior art, this embodiment of the present invention features dynamic pulse charging as a control signal wherein the pulses need not be fixed duty cycle or fixed pulse width. Either of these phenomenon can be independently varied to optimize the charging characteristics of the circuit. In the embodiment shown in  FIG. 2 , the dynamic pulse charging is provided by the microcontroller  210 .  
         [0031]     The microcontroller  210  may in one embodiment be a microprocessor also serving other functions in the device with which the capacitor charging circuit is included. For example, were the present circuit included with a cellphone having an onboard digital camera, a microprocessor ordinarily included with the phone to handle telephony and other applications may be made to serve as the microcontroller  210  shown in  FIG. 2 .  
         [0032]     Borrowing the functionality of an already present component in the form of the microcontroller  210  helps reduce the total space needed for the present digital flash converter by eliminating some of the discrete components that would otherwise be necessary in the circuit. Prior art charging circuits featured bulky discrete or analog components.  
         [0033]     However, the present capacitor charging circuit uses fewer discrete components, focusing instead on using digital components to produce a pulse train to reduce the total number of components needed overall when compared with a conventional flash circuit. Specifically the need for an LC oscillator circuit is eliminated by harnessing the power of the microcontroller  210  and as such, the size and especially cost for the capacitor charging circuit can be reduced, a factor especially important for portable electronic devices where the size and cost of the devices as a whole are critical constraints.  
         [0034]     Furthermore, a greater flexibility is provided for the capacitor charging circuit. Rather than having a control signal the frequency and profile of which is fixed based on the inherent capacitance and inductance values of the discrete components used to produce it (such as the LC oscillator shown by the feedback coil  165  and the capacitor  166  in  FIG. 1 ), a wide range of control signals may be programmed into the microcontroller  210  and activated at will.  
         [0035]     These control signals may be dynamically varied during the operation of the charging circuit based on data received by the microcontroller  210  in a feedback loop. The pulse train must be modified based on this feedback received from the circuit to optimize performance characteristics of the charging circuit such as the charging speed and the incidence of inrush current.  
         [0036]     In one embodiment, the microcontroller  210  is provided by a BASIC Stamp II microprocessor from Parallax Inc. The BASIC Stamp II microprocessor is an embedded processor having on-board power regulation, program storage, and a BASIC interpreter. The BASIC Stamp II microprocessor has fully programmable I/O pins that can be used to directly interface to a variety of components. The microcontroller  210  is connected to a power supply  220 . In one embodiment, this power supply  220  provides a voltage of 5V to the microcontroller  210 .  
         [0037]     An electronic switch  250  receives control signals from the microcontroller  210  to activate and deactivate the flow of current from the power supply  200  to the primary coil  260 . In one embodiment, this electronic switch  250  may comprise a FET, Part No. ZXM61N02F manufactured by Zetex Semiconductors.  
         [0038]     The primary coil  260  and secondary coil  270 , taken together, comprise a flyback transformer for use with the circuit shown in  FIG. 2 . In one embodiment, this flyback transformer may be a T-15-063 Tokyo Coil transformer. A power supply  200  is provided for energizing the flyback transformer which in turn charges the capacitor. In one embodiment, this power supply  200  provides a voltage of 3.6V.  
         [0039]     A rectifier diode  280 , also known as a flyback diode when used in the arrangement shown in  FIG. 2 , is provided attached to the secondary coil  270  of the transformer to rectify the output of the transformer. In one embodiment, this rectifier diode  280  may comprise a surface mount fast recovery rectifier, Part No. SRA9 manufactured by EIC Discrete Semiconductors.  
         [0040]     After passing through the rectifier diode  280 , the output of the flyback converter is collected by the capacitor  290 . This capacitor  290  will ultimately be used to discharge a large voltage to the flash tube of the camera during picture taking by a user of the camera. In one embodiment, the capacitor  290  has a value of 15 μF.  
         [0041]      FIG. 2  shows a resistor bridge formed by the resistors  230  and  235 , which are connected to the input of the capacitor  290  in the manner shown in  FIG. 2 . These resistors are used to form a voltage divider which converts and detects the charge level of the capacitor  290 , and provides a distinct signal to the input pin P 0  of the microcontroller  210  when the voltage at the capacitor  290  reaches 290V. In one embodiment, the resistor  230  has a value of 1 MΩ and the resistor  235  has a value of 4.7 kΩ.  
         [0042]     In one embodiment, The microcontroller  210  is configured to use an output provided by pin P 1 , and an input provided by pin P 0 . Pin P 1  is programmed to provide a pulse train of a certain frequency which can be activated and de-activated. Activation is dependent on the input at pin P 0 , provided by a voltage divider formed by resistors  230  and  235  which detects the charge level of the capacitor  290 . A resistor  251  is used to tie the gate input of the FET  250  to ensure the ground voltage at the gate will not float when there is no signal from pin P 1  of the microcontroller  210 . In one embodiment, the resistor  251  has a value of 1 kΩ.  
         [0043]     Starting with an initially uncharged capacitor  290 , the capacitor charging circuit is powered up. The microcontroller  210  determines from the input on pin P 0  that the charge level of the capacitor  290  is insufficient, and activates its pulse train via pin P 1  to provide an alternating sequence of pulses to the FET  250  which cyclically energizes the flyback transformer and charges the capacitor  290  via the secondary coil  270  of the flyback transformer.  
         [0044]     When the capacitor is charged to the desired level, based on the signal present at pin P 0 , the microcontroller  210  de-activates the pulse train to the FET  250 . As long as the circuit is powered, the microcontroller  210  continues to monitor the charge level of the capacitor and activates the pulse train when necessary.  
         [0045]     It will be understood by one skilled in the art that various alternatives may be used in place of the components and values specified above. For example, it will be understood that multiple sources are available to generate the pulse train used to charge the capacitor circuit. By way of illustration, any of the following could be used in lieu of the microcontroller  210 : a function generator, pulse forming circuit, microprocessor, and a digital signal processor. An ASIC could also be used in lieu of a program run by the microcontroller. However, when a microcontroller is used to generate the pulse train such as with the microcontroller  210  shown in  FIG. 2 , the microcontroller could be used to control other, distinct functions of the capacitor charging system, such as the synchronization of the flash module and the camera module, resulting in a still more compact circuit needing fewer discrete components.  
         [0046]     In addition, the principles of the present invention are not limited to an invention having the values listed above as exemplary embodiments for discrete components. For example, a number of electronics switches will suffice in place of the FET  250  described above from Zetex Semiconductors. For example, a transistor as well as an IGBT can also be used for a similar effect. Likewise the resistances of the resistors  230  and  235  forming the bridge circuit may be altered in the event that it is desirable to charge the capacitor to a level other than 290V.  
         [0047]     Furthermore, it will also be understood that the present invention is not limited to the type of converter discussed above. With any flash circuit having a capacitor charged by a transformer energized by an alternating signal, a microcontroller performing other tasks as well in the device may be used to provide the alternating signal. This alternative signal may or may not be amplified between the microcontroller and the transformer. In short, the principles of the present capacitor charging circuit are applicable to any flash charging circuit using a switch mode power supply. One skilled in the art will understand what substitutions and changes may be made to the exemplary embodiment above without straying from the inherent principles of the capacitor charging circuit described herein.  
         [0048]      FIG. 3  shows a flowchart  300  according to which a digital pulse signal may be provided by the microcontroller  210  in one embodiment of the present invention. Step  310  begins the process which signifies the powering up of the capacitor charging circuit. In step  320  a determination is made as to whether the capacitor  290  has yet reached the level of 290 volts. This particular value is discussed here in the context of the exemplary embodiments of the values of the discrete components listed above; however, it will be understood by one skilled in the art that 290V is only an exemplary embodiment and other embodiments are possible. This determination is made by the microcontroller  210  receiving an input on pin P 0  taken from the voltage divider formed by the resistors  230  and  235  shown in  FIG. 2 .  
         [0049]     If in step  320  the determination is made that the capacitor  290  has reached a level of 290 volts, the process proceeds to step  330  and pauses for a short period of time before returning to step  310  and starting over. If, however, it is determined that the capacitor  290  has not reached this level, then the process proceeds to step  340  wherein the microcontroller  210  proceeds to oscillate the FET  250  for a short period of time to charge the capacitor  290 .  
         [0050]     This process may be accomplished using code stored on the microcontroller  210 . As discussed above, the microcontroller  210  is in one embodiment a Stamp II microprocessor capable of running programs written in the BASIC language. One such program designed to execute the process diagrammed in  FIG. 3  is reproduced below:  
                                                                           ′{$STAMP BS2}                btnWk   VAR Byte                btnWk = 0 ′Button Workspace Initialization′           DIR0 = 0 ′pin 0 is input           DIR1 = 1 ′pin 1 is OUTPUT           DIR2 = 1 ′pin 2 is OUTPUT           DIR3 = 1 ′pin 3 is OUTPUT           DIR4 = 1 ′pin 4 is OUTPUT           DIR5 = 1 ′pin 5 is OUTPUT           DIR6 = 1 ′pin 6 is OUTPUT           DIR7 = 1 ′pin 7 is OUTPUT           DIRH = %11111111 ′set pin 8-15 as outputs           Loop:           BUTTON 0, 1, 0, 0, btnWk, 1, Charged           FREQOUT 1, 1000, 32767           GOTO loop           Charged:           PAUSE 1 ′pause 1 milli sec           GOTO loop                      
 
         [0051]      FIGS. 4 through 7  show oscilloscope readouts capturing the performances of various embodiments of the present capacitor charging circuit. Each of the  FIGS. 4 through 7  include a pair of readouts A and B. Readout A highlights the voltage level of the capacitor  290  shown in  FIG. 2 . This voltage level is shown highlighted in channel  2  of the figures. Readout B on the other hand, from each of these pairs of figures, highlights the current drain characteristics from the power supply  200  shown in  FIG. 2 . These current drain characteristics are shown highlighted in channel  1  of the figures. In order to monitor the current characteristics, a small series “sense” resistor was used. The value of this resistor was 0.1 Ω. Therefore, as an example, an observed voltage of 100 mV would imply 1A.  
         [0052]     Both the frequency and duty cycle of the fluctuating signal to the FET  250  may be used as initial sets of input parameters for the circuit. Likewise, the magnitude, duration and RMS value of the peak current during charging of the capacitor  290  may be used as key characteristics defining performance of the charging circuit. According to an exemplary embodiment of the present invention, the charging speed and inrush current of the circuit may be optimized by varying the charging frequency and duty cycle of the output produced by the microcontroller  210 .  
         [0053]     In a preferred embodiment, this optimization is directed to maximizing the charging speed, shown in  FIGS. 4 through 7  as the slope of the voltage level shown in the second channel, while minimizing the incidence of inrush current, shown in  FIGS. 4 through 7  as the maximum height of the spike shown in the current drain characteristics highlighted in the first channel. While there is some inherent tradeoff between a fast charging speed and the magnitude of the inrush current phenomenon, use of a microcontroller  210  to produce the pulse waveform driving the present capacitor charging circuit allows the input parameters which effect these performance characteristics to be easily and effectively modified to produce the optimum performance characteristics for the circuit. Furthermore, this optimization may be carried out dynamically by the microcontroller  210  during the charging operation based on feedback received from the circuit.  
         [0054]     Taken together,  FIGS. 4 through 7  demonstrate the relationship between input parameters for the capacitor charging circuit and their resulting impact on the characteristics of the circuit for various embodiments of the present capacitor charging circuit.  FIG. 4  for example, shows the results obtained using the exemplary embodiments of the values listed above for the discrete components of the present circuit and the program listed above with the microcontroller  210 .  FIGS. 5, 6  and  7  show the range of results which may be obtained by modifying these parameters, namely the charging frequency and duty cycle of the output produced by the microcontroller  210 .  
         [0055]     Lastly, as to the remaining figures,  FIGS. 8, 9  and  10  show a top, bottom and oblique view of the camera flash charging circuit of  FIG. 2  respectively.  FIG. 11  shows a side view of the camera flash charging circuit of  FIG. 2  being tested.