Drive circuit for a flash tube and a method for controlling the drive circuit

Abstract

The invention discloses a drive circuit and a method for controlling a drive circuit for a flash tube. The drive circuit comprises a capacitor and a first inductor switch group comprising a first inductor and at least one first switch connected in series with each other. The first inductor switch group is configured to be connected in series with the flash tube and the capacitor. The drive unit further comprises at least a second inductor switch group comprising a second inductor and at least one second switch connected in series with each other and configured to be connected in series with the flash tube and the capacitor, and in parallel with the first inductor switch group. The method further comprises the steps of receiving input parameters related to a desired flash characteristics, and controlling the first and second switches separately based on the received input parameters to obtain the desired flash characteristics.

Claims

1. A drive circuit for a flash tube comprising: a capacitor; a first inductor switch group comprising a first inductor and at least one first switch connected in series with each other, the first inductor switch group being configured to be connected in series with the flash tube and the capacitor; a control unit comprising receiving means for receiving at least one input parameter related to a desired flash characteristics; and a second inductor switch group comprising a second inductor and a second switch connected in series with each other, the second inductor switch group being configured to be connected in series with the flash tube and the capacitor, and being connected in parallel with the first inductor switch group, wherein the control unit is configured to control the first and second inductor switch groups separately based on the at least one input parameter to obtain the desired flash characteristics, and wherein each inductor switch group is associated with a preset maximum current, the control unit being configured to, based on the input parameter: determine a flash current required to be fed to the flash tube in order to achieve the desired flash characteristics, determine, based on the determined flash current, a switch algorithm used to ensure that no individual inductor switch group has to carry an electric current exceeding the preset maximum current of that inductor switch group, and use the determined switch algorithm to feed the flash current to the flash tube.

2. The drive circuit according to claim 1 wherein the drive circuit comprises at least a third inductor switch group each inductor switch group comprising an inductor and at least one switch connected in series with each other, wherein each inductor switch group is configured to be connected in series with the flash tube and the capacitor, and connected in parallel with the first inductor switch group as well as the second inductor switch group wherein the control unit is configured to control each inductor switch group separately.

3. The drive circuit according to claim 1 wherein the input parameter relates to any of, or any combination of: a desired color temperature of the flash, a desired light intensity of the flash, and a total amount of light emitted during the flash.

4. The drive circuit according to claim 1 wherein the input parameter relates to a predefined switch algorithm wherein the control unit is configured to use the predefined switch algorithm during the flash.

5. The drive circuit according to claim 1 wherein the input parameter is a measured parameter relating to the properties of the light emitted from the flash tube and/or a measured state of the drive circuit wherein the control unit is configured to use the measured parameter for feedback regulation during the flash.

6. The drive circuit according to claim 1 wherein the control unit is configured to, based on the input parameters: determine another switch algorithm wherein the at least one switch of each inductor switch group is activated sequentially at different points in time.

7. A method for controlling a drive circuit for a flash tube, the drive circuit comprising a capacitor, a first inductor switch group comprising a first inductor and at least one first switch connected in series with each other, the first inductor switch group being configured to be connected in series with the flash tube and the capacitor, and at least a second inductor switch group comprising a second inductor and at least one second switch connected in series with each other and configured to be connected in series with the flash tube and the capacitor, and in parallel with the first inductor switch group, the method comprising: receiving input parameters related to a desired flash characteristics, and controlling the first and second inductor switch groups separately based on the received input parameters to obtain the desired flash characteristics, and wherein each inductor switch group is associated with a preset maximum current, wherein the method comprises the steps of, based on the input parameter; determining a flash current required to be fed to the flash tube in order to achieve the desired flash characteristics, determining, based on the determined flash current, a switch algorithm used to ensure that no individual inductor switch group has to carry an electric current exceeding the preset maximum current of that inductor switch group, and using the determined switch algorithm to feed the flash current to the flash tube.

8. The method according to claim 7 wherein the drive circuit comprises at least a third inductor switch group each inductor switch group comprising an inductor and at least one switch connected in series with each other, wherein each inductor switch group is configured to be connected in series with the flash tube and the capacitor, and connected in parallel with the first inductor switch group as well as the second inductor switch group, the method comprising the step of: controlling each inductor switch group separately.

9. The method according to claim 7, wherein the input parameter relates to any of, or any combination of: a desired color temperature of the flash, a desired light intensity of the flash, and a total amount of light emitted during the flash.

10. The method according to claim 7, wherein the input parameter relates to a predefined switch algorithm, wherein the predefined switch algorithm is used during the flash.

11. The method according to claim 7, wherein the input parameter is a measured parameter relating to the properties of the light emitted from the flash and/or a measured state of the drive circuit wherein the measured parameter fir feedback regulation used during the flash.

12. The method according to claim 7 wherein the method comprises the step of, based on the input parameters: determining another switch algorithm wherein at east one switch of each inductor switch group is activated sequentially at different points in time.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will become more fully understood from the detailed description provided hereinafter and the accompanying drawings which are given by way of illustration only. In the different drawings, same reference numerals correspond to the same element.

(2) FIG. 1 illustrates a flash apparatus, according to an exemplary embodiment of the present disclosure;

(3) FIG. 2 illustrates a drive circuit according to an exemplary embodiment of the present disclosure;

(4) FIG. 3a illustrates a schedule over the state of the at least one switch of the inductor switch groups illustrated in FIG. 2;

(5) FIG. 3b illustrates a chart showing the current led through the at least one switch of the inductor switch groups;

(6) FIG. 3c illustrates a chart showing the current led through the inductors of the inductor switch groups;

(7) FIG. 3d illustrates a chart showing the current led through the flash tube when the schedule according to FIG. 3a is used; and

(8) FIG. 4 illustrates a flow chart of the method according to the invention.

DETAILED DESCRIPTION

(9) FIG. 1 illustrates a flash apparatus 200 for use in the field of photography. The apparatus comprises a drive circuit 10 for a flash tube 1 and a control unit 100. The flash apparatus 200 is arranged to receive a flash tube 1. The flash apparatus 200 may be a stand-alone unit or may be arranged in a camera with a built in flash tube.

(10) FIG. 2 illustrates a drive circuit 10 for a flash tube 1. The drive circuit comprises a capacitor 4, a control unit 100, receiving means 5a, 5b for a flash tube 1 and at least two inductor switch groups 20a, 20b, 20c, 20d, . . . , 20x. The capacitor 4 is the power source of the circuit 10. The capacitor may be of different types such as a foil type or an electrolytic type. According to the embodiment illustrated in FIG. 2, only one capacitor 4 is illustrated. However, according to one exemplary embodiment, several capacitors are connected to the capacitor circuit, in parallel, enabling more energy to be stored and discharged through the flash tube.

(11) The drive circuit 10 comprises connection points 5a, 5b, to which a flash tube 1 can be connected. Further, the drive circuit 10 comprises a first inductor switch group 20a comprising a first inductor 14a and a first switch 15a. According to the illustrated embodiment, only one switch 15a, 15b, 15c, 15d, . . . , 15x is included in each inductor switch group. However, according to a non-illustrated embodiment of the present disclosure, each inductor switch group may include one or more switches. The first inductor 14a and the first switch 15a are connected in series with each other and flash tube 1 and the capacitor 4. The inductor switch group 20a is connected to the control unit 100 via a link L17a and a bus L17. A component, such as a diode 16 which only allows current in one direction is connected in series with the flash tube 1 and the inductor 14a with one connection point at the conductor between the first switch 15a and the first inductor 14a and the other connection point at the conductor between the capacitor 4 and the flash tube 1. The component 16 has a polarity opposite to a direction of energy supply from the capacitor 4 to the flash tube 1. Further, the drive circuit 10 comprises up to x inductor switch groups (20 b, 20c, 20d, . . . , 20x). Each inductor switch group is connected in series with the flash tube 1 and the capacitor 4 and in parallel with the other inductor switch groups. Each inductor switch group 20a, 20b, 20c, 20d, . . . , 20x has a component, such as a diode 16 which only allows current in one direction. The component is connected in series with the flash tube 1 and the inductor with one connection point at the conductor between the switch 15b, 15c, 15d, . . . , 15x and the first inductor 14b, 14c, 14d, . . . , 14x and the other connection point at the conductor between the capacitor 4 and the flash tube 1. The inductor switch groups 20b, 20c, 20d, . . . , 20x are individually connected to the control unit 100 via links L17b, L17c, L17d, . . . , L17x and a bus L17.

(12) The control unit 100 comprises receiving means 18 arranged to receive input parameters.

(13) Further, the apparatus 200 normally comprises a trigger circuit (not illustrated) arranged to trigger the flash tube 1.

(14) When a flash is desired, the control unit 100 sends a command to the at least one switch of each inductor switch groups 15a, 15b, 15c, 15d, . . . , 15x to close. After a certain period of time, typically around 10 microseconds, the flash tube 1 is triggered via a triggering circuit or the like. The triggering current causes a number of molecules in the fluid in the flash tube 1 to be ionized. As the molecules in the fluid in the flash tube 1 starts to ionize, the fluid starts to conduct current. At the moment when the fluid becomes a conductor, the capacitor 4 will start to discharge causing a pulse of current through the flash tube 1. The pulse of current will cause the fluid in the flash tube 1 to emit light, and a flash is created.

(15) According to the exemplary embodiment illustrated in FIG. 2-4, each inductor switch group comprises one switch. However, each inductor switch group may comprise more than one switch. If each inductor switch group comprises a number of switches, these switches are controlled as a unit, where all the switches in each inductor switch group are turned on or off at the same point in time.

(16) The characteristics of the light emitted during the flash, such as the colour temperature of the light, the intensity of the light and the total amount of light emitted during the flash, may be controlled by varying the current flow to the flash tube 1. According to one exemplary embodiment, this is done by using a switch algorithm. The control unit 100 controls the at least one switch 15a, 15b, 15c, 15d, . . . , 15x of each inductor switch group in the drive circuit according to the switch algorithm. The switch algorithm may be derived from input parameters. According to one exemplary embodiment, the input parameters are parameters derived from the characteristics of the emitted light or the state of the drive circuit during the flash, such as an electric current through or a voltage across a component connected in series with the flash tube. Hence, the switch algorithm may be adjusted during a flash in order to achieve the desired characteristics of the emitted light. According to one exemplary embodiment, the input parameter is a parameter which may be chosen by a user of the flash apparatus before the flash. Typically, the flash apparatus may comprise controls for one or several desired flash characteristics, such as the colour temperature of the emitted light, the intensity of the emitted light, the total amount of emitted light during the flash etc. By varying the controls, the input parameters are varied, and hence, different predefined switch algorithms are chosen.

(17) According to one exemplary embodiment both a predefined switch algorithm and a switch algorithm based on input parameters from the emitted light is used during a flash. Typically, a predefined switch algorithm is used initially. Further, the switch algorithm may be adjusted due to the characteristics of the emitted light and/or the state of the drive circuit of the light.

(18) The switch algorithm may be used during the whole flash or during selected parts of the flash.

(19) Each inductor switch group 20a, 20b, 20c, 20d, . . . , 20x is associated with a maximum current. If the maximum current is exceeded, the components of the inductor switch group may break or stop functioning. In addition, the life time of the components may decrease. The switch algorithms are created so that no individual inductor switch group 20a, 20b, 20c, 20d, . . . , 20x has to carry an electric current exceeding the maximum current associated with each inductor switch group 20a, 20b, 20c, 20d, . . . , 20x. However, due to the construction of the drive circuit 10 and the switch algorithms, each individual inductor switch group 20a, 20b, 20c, 20d, . . . , 20x may have a maximum current tolerance at a level lower than the total amount of current flowing from the capacitor to the flash tube. According to one exemplary embodiment, the switch algorithm controls the at least one switch of each inductor switch group 20a, 20b, 20c, 20d, . . . , 20x so that each at least one switch or each inductor switch group is activated at different points in time, sequentially. By turning the at least one switch of each inductor switch group on sequentially, a very accurate control of the current to the flash tube is achieved and the components may be protected from too high levels of current flow.

(20) According to one exemplary embodiment, the switch algorithm controls the switches of at least two inductor switch groups 20a, 20b, 20c, 20d, . . . , 20x to be activated at the same point in time. Hence, the switch algorithm may be constructed to control an arbitrary number of inductor switch groups as a unit for which the switches are turned on or off at the same point in time during at least part of the flash.

(21) Typically, the switches have a maximum switching rate. Thus, switch algorithms are created so that the desired current is fed to the flash tube at each point in time without exceeding the switching rate for each individual switch. FIG. 3a illustrates a section of a switch algorithm according to one exemplary embodiment. For the at least one switch 15a, 15b, 15c, 15c of each inductor switch group, the state of the switch is illustrated along a time t.sub.0-t.sub.5. The state of the each switch is either on or off. According to one exemplary embodiment, the drive circuit 10 comprises four inductor switch groups 20a, 20b, 20c, 20d and hence at least four switches 15a, 15b, 15c, 15d. At a point in time t.sub.0, which is at a random point in time during the execution of the switch algorithm, switch 15a is off, switch 15b is turned off, switch 15c is on, and switch 15d is turned on. The period during which the switches are turned on may vary according to the switch algorithm used. According to one exemplary embodiment, the maximum switching rate of each switch is between 10 and 40 kHz. As time passes, the switches 15a, 15b, 15c, 15d will be turned off and on depending on the switch algorithm used in that point in time. As the switch algorithm may vary during the flash, the pattern may change during a flash. In addition, each switch 15a, 15b, 15c, 15d may have an individual maximum switching rate. The switch algorithm is constructed taking the characteristics of the components of each inductor switch group into consideration. As mentioned before, according to this exemplary embodiment, each inductor switch group comprises one switch. However, each inductor switch group may comprise more than one switch. If each inductor switch group comprises a number of switches, these switches are controlled as a unit, such that all the switches in each inductor switch group are turned on or off at the same point in time.

(22) The switch algorithm illustrated in FIG. 3a is merely an example. The switch algorithm may be varied freely as long as the restrictions of the components are taken into consideration in order to achieve the desired characteristics of the emitted light.

(23) FIG. 3b schematically illustrates the current through each switch 15a, 15b, 15c, 15d when the switch algorithm according to FIG. 3a is executed in the drive circuit. At the time t.sub.0, switch 15a is turned on. Due to the laws of electronic, from the point in time when the switch in the inductor switch group is closed or turned on, the current through the switch increases with a certain rate depending on for example the properties of the inductor in the inductor switch group and also on the voltage across the inductor. The switch algorithm is constructed so that before the current through the switch 15a reaches the maximum level for that specific component, the switch is turned off. Switch 15a is turned off at t.sub.1. The switch 15a is turned off until t.sub.2 when the switch 15a is turned on again. According to the switch algorithm, the switches are turned off before the current through the switches reaches a maximum level for each individual switch thereby increasing the lifetime of the drive circuit and the components therein.

(24) FIG. 3c illustrates the current flow through the inductors of each inductor switch group using the switch algorithm as illustrated in FIG. 3a. As can be seen, when the switches of each individual inductor switch group is turned off, due to the laws of electronics, the current through the inductors will continue for a period of time after the switch has been turned off. After the switches have been turned off, the current will continue to circulate through the inductor, the diode connected in series with the inductor and the flashtube for some time before the current stops to flow.

(25) FIG. 3d schematically illustrates with a solid curve the current flow to the flash tube in the drive circuit between t.sub.0 and t.sub.5 using the switch algorithm as illustrated in FIG. 3a. According to the algorithm, the current sent to the flash tube is never turned off completely, but is kept between a level c.sub.3 and a level c.sub.4. As can be seen, the current flow through the flash tube can be kept at a rather even level with only small variations. According to one embodiment, an almost constant current flow through the flash tube may be achieved by adjusting the switch algorithm.

(26) The dashed curve in FIG. 3d illustrates the current flow to the flash tube using prior art where the switches of a single inductor switch group are controlled to switch on or off at the same time. As can be seen, the result of this method is much larger variation of the current flow through the flash tube, and, the changing rate is notably slower compared to the current flow when using the switch algorithm in FIG. 3a.

(27) FIG. 4 is a flow chart illustrating a method for controlling a drive circuit 10 for a flash tube 1, according to one embodiment of the present disclosure.

(28) In a first step, S1, input parameters related to a desired flash characteristic are received. A switch algorithm is either chosen from a number of predefined switch algorithms, or, a switch algorithm is created based in the input parameters. The input parameters may be the characteristics of the emitted light during the flash, the state of the drive circuit of the flash tube or alternatively or a desired characteristics of the light emitted during the flash. A desired characteristics of the light emitted may be a certain total amount of light emitted during the flash and this desired characteristics may be chosen by varying a control on a flash device comprising the drive circuit 10, such as a button for example. By setting the button in a specific position, a certain desired characteristics of the future flash has been entered into the control unit. The control unit may create a switch algorithm based on the input parameter or, a predefined switch algorithm stored in the control unit may be used.

(29) In a second step, S2, at least a first and a second switch of at least a first and a second inductor switch group forming part of the drive circuit 10 are controlled via the switch algorithm.