Patent Publication Number: US-9420675-B2

Title: Driver circuit for a flash tube

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase under 35. U.S.C. §371 of International Application PCT/SE2014/050166, filed Feb. 11, 2014, which claims priority to Swedish Patent Application No. 1350168-9, filed Feb. 13, 2013. The disclosures of the above-described applications are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The invention relates in general to a driver circuit for a flash tube. 
     BACKGROUND 
     Generally, in driver circuits for flash tubes, it is desirable to control the amount of energy provided to a flash tube connected to the driver circuit as well as the color temperature of the resulting emitted light from the flash tube. 
     A driver circuit typically comprises at capacitor C configured to feed energy to a flash tube for a flash. The flash tube discharge by igniting ignition circuits inside the flash tube and thus drains the capacitor C. A first method of controlling the amount of energy provided to a flash tube and the color temperature of the emitted light from the flash tube is illustrated in  FIGS. 1A-1B . In  FIG. 1A , by charging the capacitor C up to a particular charging voltage, an amount of energy corresponding to the energy level E C  is stored in the capacitor C. When said amount of energy E C  is provided to the flash tube, the resulting emitted light from the flash tube will have the desired color temperature T des . If the capacitor C is instead charged up to a lower charging voltage, a lower amount of energy corresponding to the energy level E des  is stored in the capacitor C. Thus, when said lower amount of energy E des  is provided to the flash device, the resulting emitted light from the flash device will instead have the color temperature T B . However, it may often be desirable to achieve the desired color temperature T des  of the resulting emitted light from the flash device, but while only providing the amount of energy E des  to the flash device. 
     In  FIG. 1B , the capacitor C is charged to a particular charging voltage V corresponding to an amount of energy E des +E′. As the amount of energy in the capacitor C is drained by the flash device, the discharge of energy is interrupted at time t 1  when the amount of already discharged energy by the flash device corresponds to the desired amount of energy E des . This will result in that the remaining amount of energy E′ is cut off and not discharged by the flash device. Consequently, the emitted light from the flash tube will have the color temperature T 1 . According to the inherent relationships shown in  FIG. 1B , a particular charging voltage V and a discharge interruption timing t 1  can be found such that the amount of energy provided to the flash tube is E des  and the color temperature T 1  is approximately the same as T des , i.e. T 1 ≈T des . Thus, in case of using a flash tube, it is in this manner possible to provide a desired amount of energy E des  to the flash tube and still achieve the desired color temperature T des  of the resulting emitted light, as shown by the arrow in  FIG. 1A . 
     A second method of controlling the amount of energy provided to a flash tube and the color temperature of the emitted light from the flash tube is to have a set or bank of different capacitors, e.g. C 1 -C 3 , which are configured to provide energy to the flash tube for the flash. This is illustrated in  FIGS. 2A-2B . A given capacitor, e.g. C 3 , of a particular capacitance being charged to a particular charging voltage V 3  corresponding to an energy level E 3  will generate a particular color temperature T des  of the emitted light when provided to a flash device at a flash tube. Here, if a different amount of energy is desired to be provided to the flash tube for the flash, while keeping the color temperature T des  of the emitted light, any one of the different capacitors C 1 -C 3  may be used separately or be combined to provide the desired amount of energy. However, since the number of capacitors sources C 1 -C 3  in the set is finite due to the inherent implementation and economic considerations of having a large amount of capacitors, only finite number of discrete energy levels, e.g. E 1 , E 2 , E 3 , E 1 +E 2 , E 1 +E 3 , E 2 +E 3 , E 1 +E 2 +E 3 , will be possible for the desired color temperature T des . 
     However, both of the methods described above suffer from disadvantages. For example, by using the first method described above in reference to  FIGS. 1A-1B , the amount of energy E C  has to be lowered in order for the flash tube to get a desired color temperature. Another disadvantage with the first method is that the circuits used to interrupt the current have difficulties handle high currents. 
     Furthermore, achieving according to the second method a desired color temperature T des  for a continuous, non-discrete range of energy levels E for even a flash device is not a scalable or cost efficient solution. 
     There is therefore a need for an improved solution for achieving a desired color temperature T des , which solution solves or at least mitigates at least one of the above mentioned problems. 
     SUMMARY 
     It is understood by the inventor that it is highly desirable to provide a driver circuit for a flash tube capable of providing a desired energy to a flash tube and that the flash tube also emits a desired color temperature during the flash time. 
     This issue is addressed by a driver circuit for a flash tube. The driver circuit comprises a first and a second output for a flash tube, a capacitor, an inductor and a switch. The inductor and the switch being connected in series with the first and a second output across the capacitor. A component which only allows current flow in one direction connected across the first and the second output and the inductor, with a polarity opposite to a direction of energy supply from the capacitor to the first output. The driver circuit further comprises a controller for controlling the switch. The controller comprises receiving means for receiving parameters related to desired flash characteristics. The controller being configured to control said switch based on said parameters to obtain said desired flash characteristics. 
     Since the driver circuit comprises receiving means for receiving parameters related to desired flash characteristics and the controller controls the switch based on the received parameters it is possible to obtain the desired flash characteristics from a flash tube connected to the driver circuit. This is a highly desirable feature of a flash device from a photographer&#39;s point of view since it enables a more predictable and reliable flash when taking a photograph. 
     Another advantage of the driver circuit is that it provides the option to individually control different parameters related to the desired flash characteristics. In an exemplary embodiment of the driver circuit it is therefore possible to individually control the color temperature, the flash energy or the flash time. This is an advantage if the photographer wants to only change one characteristic of the flash and keep another characteristic constant. 
     A further advantage of the driver circuit is that it provides more options, since it allows a photographer to control characteristics of the flash individually. 
     Yet another advantage of the driver circuit is that for different capacitor voltages, the colour temperature and flash energy can be kept constant, controlled by the duty cycle. Therefore several flashes with the same colour temperature can be fired independent of capacitor charging in between, as long as sufficient energy is stored in the capacitors. 
     Yet a further advantage of the driver circuit is that when the flash energy is changed, the voltage of the flash capacitors need not be changed before the flash is fired to get a desired colour temperature, as long as sufficient energy is stored in the capacitors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, advantages and effects as well as features of the invention will be more readily understood from the following detailed description of exemplary embodiments of the invention when read together with the accompanying drawings, in which: 
         FIGS. 1A and 1B  shows schematic graphs illustrating a first method of controlling the amount of energy provided to and the color temperature of the emitted light from a single flash device according to a prior art example. 
         FIGS. 2A and 2B  shows schematic graphs illustrating a second method of controlling the amount of energy provided to and the color temperature of the emitted light from a single flash device according to a prior art example. 
         FIG. 3  illustrates a schematic block diagram of a driver circuit according to an exemplary embodiment of the invention. 
         FIG. 4  illustrates several diagrams  41 - 44  of different currents and voltages in the driver circuit  10  when the switch  15  is switched on and off in repetitive duty cycles when the duty cycle in  FIG. 4  is 50%. 
         FIG. 5  illustrates several diagrams  51 - 54  of different currents and voltages in the driver circuit  10  when the switch  15  is switched on and off in repetitive duty cycles when the duty cycle in  FIG. 5  is 80%. 
         FIG. 6  illustrates a schematic block diagram of a flash generator/flash device according to the embodiment of the invention 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like reference signs refer to like elements. 
       FIG. 3  illustrates a driver circuit  10  for a flash tube  19  according to an exemplary embodiment of the present invention. The driver circuit  10  may be used in a flash generator (not shown) or in a flash device (not shown). Other types of devices with a flash tube in the device or connected to the device can also use the driver circuit  10  according to the exemplary embodiments of the present invention. An example of another device is a camera with a built in flash tube. The driver circuit  10  comprises a capacitor  13 , an inductor  14  and a switch  15 . The inductor  14  and the switch  15  being connected in series with the first  11  and the second output  12  across the capacitor  13 . Further, a component  16  which only allows current flow in one direction is connected across the first  11  and the second output  12  and the inductor  14 , with a polarity opposite to a direction of energy supply from the capacitor  13  to the first output  11 . 
     The capacitor  13  can also be of different types. The capacitor  13  can be a foil type capacitor or an electrolytic type capacitor  13 . Different types of capacitors  13  have different internal resistant. Foil type capacitors have low internal resistance compared to electrolytic type capacitors. Therefore it is possible to discharge a foil type capacitor  13  faster and thus generate a higher current density and a higher color temperature compared with an electrolytic type capacitor  13 . 
     In the exemplary embodiment illustrated in  FIG. 3  only one capacitor  13 , one inductor  14 , one switch  15  and one diode  16  are illustrated. Other exemplary embodiments of the driver circuit  10  according to the present invention the driver comprise several capacitors  13 , inductors  15 , diodes  16  and switches  15 . In theses exemplary embodiments are the capacitors  13  connected in parallel with each other. Having several capacitors  13  connected in parallel give the capacitors  13  a higher capacitance which make is possible to store more energy compared to using only one capacitor  13 . Capacitors  13  connected in parallel in other exemplary embodiments can also be of different types. A first capacitor  13  can be a foil type capacitor and the second type of capacitor  13  can be an electrolytic type capacitor  13 . Different types of capacitors  13  have different internal resistant. Foil type capacitors have low internal resistance compared to electrolytic type capacitors. Therefore the discharge of a foil type capacitor will go faster and generate a higher current density and a higher color temperature compared with an electrolytic type capacitors. By mixing capacitors of different types, another flash energy and another color temperature can be achieved from a flash tube connected to the driver circuit  10  compared to if only one type of capacitor were used. 
     In these exemplary embodiments with capacitors of different types connected in parallel the capacitors can also be used individually. Using e.g. only a foil type of capacitor provides a shorter flash time compared to using an electrolytic type of capacitor of the same size. 
     As mentioned above other exemplary embodiments than the embodiment illustrated in  FIG. 3  can also comprise several inductors  14  and switches  15 . In these exemplary embodiments the inductors  14  are connected in parallel. Using several inductors  14  in parallel give the advantage that the driver circuit  10  can handle higher currents compared to if only one inductor  14  is used. Several inductors  13  in parallel also change the inductance. The switches  15  also are connected in parallel in the exemplary embodiments containing more than one switch  15 . 
     In one exemplary embodiment of the driver circuit  10  according to the present invention is the component  16  a diode  16 . The diode  16  is then connected with a polarity opposite to a direction of energy supply from the capacitor  13  to the first output  11 . In another exemplary embodiment of the driver circuit  10  according to the invention the component  16  is a MOSFET, Metal Oxide Semiconductor Field Effect Transistor, connected to a controller  17 , and wherein the controller  17  is configured to control the MOSFET so that the MOSFET does not conduct current when the switch  15  conducts current. The controller  17  is further configured for controlling the switch  15 , as will be described below. 
     The controller  17  can comprises receiving means  18  for receive parameters related to characteristics for a desired flash. These parameters are then used by the controller  17  when determining how to control the switch  15  in order to produce a flash with the desired characteristics according to the parameters received by the receiving means  18 . In one exemplary embodiment the receiving means  18  receives a desired color temperature, a desired flash time and a desired flash energy. In other exemplary embodiments the receiving means  18  is configured to receive other parameters that describe characteristics for a flash. These parameters can be e.g. one of or a combination of a desired color temperature, a flash energy and/or flash time. The parameters are then used by the controller  17  to control the switch  15  so that the flash tube  19  connected to the drive circuit  10  produces a flash with the desired flash characteristics. 
     In yet another exemplary embodiment the receiving means  18  also receives information about what type of flash tube  19  that is connected to the driver circuit  10 . In this exemplary embodiment the controller  17  is further configured to use this information when determining how to control a flash tube connected to the driver circuit. 
     In an exemplary embodiment of the driver circuit  10  the controller  17  is further configured to switch the switch  15  on and off in repetitive duty cycles in order to produce a flash with the characteristics according to the parameters received by the receiving means  18 . 
       FIG. 4  illustrates several diagrams  41 - 44  of different currents and voltages in the driver circuit  10  when the switch  15  is switched on and off in repetitive duty cycles during the flash time. The duty cycle in  FIG. 4  is 50%. The first diagram  41  illustrates the voltage over the switch  15  when the switch  15  is turned on and off by the controller  15 . As can be seen in diagram  41  the voltage over the switch  15  is approximately zero when the switch  15  is on when the switch  15  is off the voltage over the switch is approximately the same as over the capacitor  13 , except for a small voltage drop over the component  16 . The next diagram  42  illustrates the current through the first  11  and the second output  12  when the switch  15  is switched on and off. This is also the current that passed through the flash tube  19  connected to the driver circuit  10 . As can been seen in diagram  42  the current first raises to a certain level when the switch  15  first is turned on. The current falls and rises periodically with the duty cycle. The color temperature from the flash tube is dependent on the current through the flash tube connected to the driver circuit  10 . A higher current leads to a higher color from the flash tube and a lower current leads to a lower current from the flash tube. The color temperature will therefore vary with the rise and fall of the current through the flash tube. This variation is however small in comparison with the current level through the flash tube, the current variation will therefore have small impact on the color temperature. Diagram  43  illustrates the current through the switch  15 . As can be seen the current varies with the duty cycle for the switch  15 . When the switch  15  is an on state the current rises and when the switch  15  is an off state the current is zero. Next, in diagram  44  is the current through the component  16  which only allows current flow in one direction illustrated. The current through the component  16  which only allows current flow in one direction varies with the duty cycle for the switch  15 . When the switch  15  is closed the inductive energy that has been built up in the inductor  14  makes the current go through the component  16  which only allows current flow in one direction instead for through the switch  15 . 
       FIG. 5  illustrates several diagrams  51 - 54  of different currents and voltages in the driver circuit  10  when the switch  15  is switched on and off in repetitive duty cycles during the flash time. The duty cycle in  FIG. 5  is 80%. The first diagram  51  illustrates the voltage over the switch  15  when the switch  15  is turned on and off by the controller  15 . As can be seen in diagram  51  the voltage over the switch  15  is approximately zero when the switch  15  conducts current. When the witch  15  is closed the voltage over the switch is approximately the same as over the capacitor  13 , except for a small voltage drop over the component  16 . The next diagram  52  illustrates the current through the first  11  and the second output  12  when the switch  15  is switched on and off. This is also the current that passed through the flash tube  19  connected to the driver circuit  10 . As can been seen in diagram  52  the current first raises to a certain level when the switch  15  first is turned on. The current falls and rises periodically with the duty cycle. The color temperature from the flash tube follows the current through the flash tube connected to the driver circuit  10 . A higher current leads to a higher color from the flash tube and a lower current lead to a lower current from the flash tube. The color temperature will therefore vary with the rise and fall of the current through the flash tube. This variation is however small in comparison with the current level through the flash tube, the current variation will therefore have small impact on the color temperature. Diagram  53  illustrates the current through the switch  15 . As can be seen the current varies with the duty cycle for the switch  15 . When the switch  15  is an on state the current rises and when the switch  15  is an off state the current is zero. Next, in diagram  54  is the current through the component  16  which only allows current flow in one direction illustrated. The current through the component  16  varies with the duty cycle for the switch  15 . When the switch  15  is open the inductive energy that has been built up in the inductor  14  makes the current go through the component  16  instead for through the switch  15 . 
     In the exemplary embodiment of the driver circuit  10  illustrated in  FIG. 3  the controller  17  is further configured to increase the duty cycle to achieve a higher color temperature and to decrease the duty cycle to achieve lower color temperature. Increasing the duty cycle for the switch  15  imply that the switch  15  will be open during a longer period of the duty cycle and thereby will the current through a flash tube connected to the driver circuit  10  increase. A higher current through the flash tube results in a higher color temperature. 
     The driver circuit  10  according to the exemplary embodiment is further configured to increase the flash time if the same energy level is desired at a lower color temperature. If the duty cycle is reduced the color temperature from a flash tube connected to the driver circuit  10  is lowered. Thereby is also the power level from the flash tube connected to the driver circuit  10  lowered. In order to compensate for this lower power level the controller  17  in this exemplary embodiment is configured to increase the flash time. 
     In another exemplary embodiment the driver circuit  10  is further configured to change the duty cycle during the desired flash time, thereby obtaining different color temperatures during the flash time. In a first period of the flash time the controller may use a first duty cycle and then change to another duty cycle for the rest of the flash time. Using different duty cycles during the flash time results in that the color temperature will vary during the flash time. A longer duty cycle can e.g. be used in the beginning of the flash time than in the end of the flash time. This will result in that color temperature will fall during the flash time. 
     In yet another exemplary embodiment of the driver circuit  10  for different capacitor voltages, the color temperature and flash energy can be kept constant, controlled by the duty cycle. Therefore several flashes with the same color temperature can be fired independent of capacitor charging in between, as long as sufficient energy is stored in the capacitors. In this exemplary embodiment of the driver circuit, when the flash energy is changed, the voltage of the flash capacitors need not be changed before the flash is fired to get a desired color temperature, as long as sufficient energy is stored in the capacitors. 
     The description above is of the best mode presently contemplated for practicing the present invention. The description is not intended to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the present invention should only be ascertained with reference to the issued claims.