WO2008129314A1

WO2008129314A1 - Control apparatus

Info

Publication number: WO2008129314A1
Application number: PCT/GB2008/050248
Authority: WO
Inventors: Jan Simonsen
Original assignee: Cyden Ltd
Current assignee: Cyden Ltd
Priority date: 2007-04-20
Filing date: 2008-04-09
Publication date: 2008-10-30
Anticipated expiration: 2009-10-20
Also published as: GB0717323D0; GB2448561A

Abstract

Control apparatus for an electric discharge tube The apparatus comprises a first circuit and at least one second circuit. The first circuit comprises at least one first capacitor and a discharge tube electrically connectable via a first series of windings arranged upon a core of a transformer. The second circuit comprises at least one second capacitor and a second series of windings arranged upon the core, such that electric discharge of the at least one first capacitor and the at least one second capacitor creates opposed magnetic fields within the core, causing the discharge of the discharge tube to terminate. The apparatus is used to generate an optical output pulse for optical cosmetic and therapeutic methods of treatment.

Description

Control Apparatus

The present invention relates to control apparatus and more particularly, but not exclusively, to control apparatus comprising circuitry for controlling the operation of a discharge tube (also known as a discharge lamp). The present invention further relates to use of such control apparatus in controlling optical output for cosmetic or therapeutic methods

Discharge tubes typically comprise an arrangement of electrodes in a gas, housed within an insulating, temperature resistant glass or ceramic envelope. Discharge tubes operate by ionising the gas with an applied voltage across the electrodes to create a conduction path within the gas between the electrodes. The electrical breakdown of the gas produces a plasma discharge with the result that upon passing a current through the plasma, an intense optical pulse is generated as the free electrons within the plasma combine with the ionised gas atoms.

The optical output produced by the discharge tube is commonly used to pump a laser medium, such as an organic dye crystal, to create a population inversion within the medium. The laser pulse generated by the population inversion typically comprises a temporal linewidth which is less than that of the pulse produced by the discharge tube. Moreover, the laser pulse typically terminates before the pulse from the discharge tube.

Referring to Figure 1 of the accompanying drawings, there is shown a graphical representation of the optical output of a discharge tube over time t, in which curve 10 shows the pulse from the discharge tube, while curve 1 1 shows the corresponding laser pulse. There is a portion of pulse 10 (represented by the shaded area 12) which remains after the termination of the laser pulse 1 1 ; this portion 12 no longer contributes to the laser pulse 1 1 and so the energy associated with this part is dissipated as heat.

Dye lasers degrade on exposure to light and so this excess light energy in portion 12 within the discharge tube pulse adds to the degradation rate of the dye. Accordingly, it would be desirable to provide a means to switch off the discharge arc after termination of the laser pulse 1 1 , in order to reduce degradation of the dye and also in order to reduce the quantity of heat to be dissipated. Discharge tubes are typically powered using circuitry such as that illustrated in Figure 2 of the accompanying drawings. In such circuitry, a capacitor 13 is charged by a power supply 14 which rectifies an AC mains supply. The power supply 14 and capacitor 13 are electrically connected to a discharge tube 15 by a first series of windings 16 arranged upon the core 17 of a transformer 18. The discharge tube 15 does not produce any output in an initial state, in which there is no conductive path through the gas 19 between electrodes within the tube 15.

In order to ionise the gas within the discharge tube 15, and thus produce a conduction path, a trigger circuit 20, which comprises a second series of windings 21 on the transformer core 17, is used to induce a high voltage supply on the first series of windings 16 causing the gas 19 within the discharge tube 15 to break down. This breakdown creates a conduction path through the tube 15 allowing the capacitor 13 to discharge across the tube electrodes thereby producing an intense arc.

When current flows from the capacitor 13 through the first series of windings 16 on the transformer core 17, the transformer core saturates, creating a low impedance to the flowing current. For low currents, that is, less than about 300A, a semiconductor switch can be used to switch the current flow.

US6965203 discloses a method and circuitry for repetitively firing a discharge tube, and particularly a current interruption circuit comprising a semiconductor transistor, for switching off the discharge tube. In high current systems, however, semiconductor switches cannot tolerate the high current and are often damaged.

There is thus a requirement in the art for circuitry and apparatus which permit selective termination of a discharge arc from a discharge tube.

In accordance with a first aspect of the present invention, there is provided control apparatus for an electric discharge tube, the apparatus comprising a first circuit and at least one second circuit as will now be described. The first circuit comprises at least one first capacitor and a discharge tube electrically connectable via a first series of windings arranged upon a core of a transformer. The second circuit comprises at least one second capacitor and a second series of windings arranged upon the core; the arrangement is such that electric discharge of the at least one first capacitor and of the at least one second capacitor creates opposed magnetic fields within the core, causing the discharge arc of the discharge tube to terminate.

Preferably, the control apparatus further includes a trigger circuit including a third series of windings arranged on the core of the transformer, for creating a high voltage across the discharge tube, to ionise the gas within the tube. It is preferred that the trigger circuit includes less windings on the core than the first circuit has, so that a step up in voltage is obtained from the third to the first series of windings.

The high voltage results in very low impedance within the first series of windings, since the associated high current within the first series of windings creates a magnetic field which saturates the transformer core. The at least one second circuit preferably reduces the magnetic flux within the core, thereby removing the core from magnetic saturation. This effectively increases the impedance of the first series of windings and thus restricts current flowing there through. The transformer thus comprises a magnetic core which preferably alternates between respective states of high and low magnetic permeability.

it is preferred that the discharge tube generates an output pulse having a range of wavelengths at least in the visible spectrum.

The second circuit may include more windings on the core than the windings of the trigger circuit. This helps to ensure that the transformer core is brought out of magnetic saturation by the second circuit.

The first series of windings are preferably energised by the at least one first capacitor, and the second series of windings are preferably energised by the at least one second capacitor.

The trigger circuit preferably comprises at least one third capacitor for energising the third series of windings. Preferably the at least one first capacitor is charged by a power supply which maintains a discharge plasma within the discharge tube, but is insufficient to start a discharge arc in the tube.

Preferably, the second series of windings are wound upon the transformer core in an opposite direction to the first series of windings, in order to create the opposed magnetic field. Alternatively, the at least one first capacitor and the at least one second capacitor are arranged such that the charged poles of the capacitors cause a current to flow in the respective windings in opposite directions with respect to each other to create the opposed magnetic fields.

The control apparatus according to the invention preferably further includes a sensor for sensing the intensity of a laser output. When the laser output falls below a predefined threshold, the sensor preferably causes the at least one second capacitor in the second circuit to discharge, to thereby terminate the arc discharge from the discharge tube.

According to a second aspect of the present invention there is provided a laser system comprising a discharge tube, in which the discharge tube is controlled by control apparatus according to the invention.

The present invention further comprises a method of providing a pulsed electrical output, the method comprising selectively discharging the first and second capacitors of apparatus according to the invention so as to create opposed magnetic fields within the core, and to cause the discharge of the discharge tube to terminate.

The present invention further comprises an optical cosmetic or therapeutic method of treatment, which comprises providing an electrical energy input to a discharge tube so as to produce an optical output pulse directed towards animal tissue, wherein the electrical energy input is provided by control apparatus according to the invention.

A preferred embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is, as indicated above, a graphical representation of the optical output of a discharge tube and laser;

Figure 2 is, as indicated above, a circuit diagram of a conventional circuit used to power a discharge tube; and

Figure 3 is a circuit diagram of exemplary control circuitry used according to the present invention.

Referring to Figure 3, there is shown a control device 100 for selectively controlling the operation of a discharge tube 1 12, in pumping a laser system 400 for example.

The control device 100 comprises a first circuit 1 10, a second circuit 210 and a trigger circuit 310. The first circuit 1 10 comprises a first capacitor 1 1 1 electrically connected to a discharge tube 1 12 via a first series of windings 1 13, arranged on a magnetic core 1 14 of a transformer 1 15. The capacitor 1 1 1 is to be charged via a power supply unit (not shown). The power supply unit creates a potential difference across the electrodes 1 16 of the discharge tube 1 12, which potential difference maintains a plasma discharge within the tube 1 12, which is evident as a weak glowing of the tube, but which is insufficient to produce an intense optical output in the form of an arc discharge.

Such a discharge arc is created by applying a high voltage across the electrodes 1 16; this high voltage is produced by the trigger circuit 310. The trigger circuit 310 includes a capacitor 31 1 to be charged via a power supply unit (not shown). The capacitor is electrically connectable via a switch 313 to a third series of windings 312 arranged on the same core 1 14 as the first series of windings 1 13.

The second circuit 210 includes a capacitor 21 1 to be charged via a power supply (not shown), the capacitor being electrically connectable via a switch 213 to a second series of windings 212 also arranged upon the transformer core 1 14. In the static state, the tube 1 12 glows under the influence of the power supply unit (not shown) provided for the first circuit 1 10, due to the plasma discharge. To produce the arc discharge, the switch 313 in the trigger circuit 310 is closed to cause the capacitor 31 1 to discharge through the third series of windings 312 on the transformer core 1 14. This discharge creates a current within the third series of windings 312 and therefore sets up a magnetic field within the transformer core 1 14.

This magnetic field induces a high voltage spike in the first series of windings 1 13, by virtue of there being more turns on core in the first circuit 1 10 than the trigger circuit 310, causing the gas 1 17 within the discharge tube 1 12 to ionise. This ionisation creates a conduction path between the electrodes 1 16 within the tube 1 12, thereby enabling the capacitor 1 1 1 provided in the first circuit 1 10 to discharge across the tube 1 12 and produce an intense arc.

However, as the capacitor 1 1 1 discharges, the current flowing through the first series of windings 1 13 increases. This is because the core 1 14 of the transformer becomes magnetically saturated. The saturation of the transformer core creates a very low impedance to the flowing current within the first series of windings 1 13, thereby enabling large currents to flow. However, these currents cannot be switched using traditional semiconductor switches as such switches are unable to tolerate the associated high currents. The circuit according to the invention overcomes this problem by incorporating a second circuit 210 as hereinbefore described.

In order to reduce the magnetic flux within the core, the switch 213 in the second circuit 210 is closed, causing the capacitor 21 1 to discharge across the second series of windings 212. However, the polarity of the capacitor 21 1 is reversed with respect to the capacitor 1 1 1 such that the magnetic field created by the capacitor 21 1 within the second series of windings 212 opposes the magnetic field created by the third series of windings 312 of the trigger circuit 310. Alternatively, the second series of windings 212 may be wound around the transformer core 1 14 in the opposite sense to the first series of windings 1 13 to create opposed magnetic fields. This opposed flux reduces the resultant magnetic flux within the core 1 14 and so brings the core out of magnetic saturation. As a result, the impedance of the first series of windings 1 13 increases, which reduces the current within the windings to a level which causes the arc discharge to terminate. In this manner, it is possible to selectively terminate a discharge arc.

This is particularly useful for pulsed operation, since terminating the arc discharge early, rather than waiting for the arc to decay (as described above with reference to Figure 1 ), enables the capacitor 1 1 1 to be recharged more quickly for another cycle. In conventional systems, it is necessary to wait until the arc has finished, in order to ensure that the gas 1 17 within the discharge tube 1 12 is fully de-ionised. If the gas 1 17 within the discharge tube 1 12 fails to de-ionise before the charging voltage is reapplied, then current will continue to flow through the tube 1 12, thus producing an afterglow. Such an afterglow results in the build-up of heat, which can be detrimental to the system.

From the foregoing therefore, it is evident that the apparatus according to the present invention permits the termination of the arc discharge when desired, thereby enabling a re-charging voltage to be re-applied more quickly. The apparatus according to the present invention further enables power wastage as heat to be minimised. In addition, the use of the apparatus of the present invention can prolong the life of laser dye crystals, when a laser is pumped with a discharge tube, by switching the arc discharge off when the laser pulse terminates or falls below a preset value.

Claims

1 . Control apparatus for an electric discharge tube, the control apparatus comprising a first circuit and at least one second circuit, said first circuit comprising at least one first capacitor and a discharge tube electrically connectable via a first series of windings

5 arranged upon a core of a transformer, said second circuit comprising at least one second capacitor and a second series of windings arranged upon the core, such that electric discharge of said at least one first capacitor and of said at least one second capacitor creates opposed magnetic fields within the core, causing the discharge of the discharge tube to terminate.

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2. Control apparatus according to claim 1 , further comprising a trigger circuit, said trigger circuit comprising a third series of windings arranged on the core of the transformer for creating a high voltage across the discharge tube to ionise the gas within the tube.

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3. Control apparatus according to claim 2, in which the trigger circuit has less windings on the core than the first circuit has.

4. Control apparatus according to claim 2 or 3, in which the trigger circuit comprises a -¹O switch for selectively controlling the passage of current through said third series of windings.

5. Control apparatus according to any of claims 2 to 4, in which the trigger circuit comprises at least one third capacitor for energising the third series of windings

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6. Control apparatus according to any preceding claim, in which the at least one second circuit is arranged to reduce the magnetic flux within the core, to remove the core from magnetic saturation.

30 7. Control apparatus according to any preceding claim, in which the transformer core is arranged to alternate between relative states of high and low magnetic permeability.

8. Control apparatus according to any preceding claim, in which the first series of windings is arranged to be energised by the at least one first capacitor. 35

9. Control apparatus according to any preceding claim, in which the second series of windings is arranged to be energised by the at least one second capacitor.

10. Control apparatus according to any preceding claim, in which a power supply is 5 provided to charge the at least one first capacitor such that the first capacitor has sufficient charge to maintain a discharge plasma within the discharge tube, but is insufficient to start a discharge arc.

1 1. Control apparatus according to any preceding claim, in which the discharge tube is 10 such that it will generate an output pulse having a range of wavelengths at least in the visible spectrum.

12. Control apparatus according to any preceding claim, in which the second series of windings is wound upon the transformer core in an opposite direction to the first

15 series of windings to create the opposed magnetic field.

13. Control apparatus according to any of claims 1 to 1 1 , in which the at least one first capacitor and the at least one second capacitor are arranged such that charged poles of said capacitors respectively cause current to flow in the respective windings in

-¹O opposite directions, to create opposed magnetic fields.

14. Control apparatus according to any preceding claim, further comprising a sensor for sensing the output intensity of a laser output from the discharge tube.

15 15. Control apparatus according to claim 14, wherein the sensor is arranged to cause the at least one second capacitor in the second circuit to discharge, to thereby terminate the arc discharge from the discharge tube, when the laser output falls below a predefined threshold,

30 16. Control apparatus according to any preceding claim, in which said second circuit comprises a switch for selectively controlling the passage of current in the second series of windings.

17. A laser system comprising an electric discharge tube, in which said discharge tube is 35 controlled by control apparatus according to any preceding claim.

18. A method of providing a pulsed electrical output, the method comprising providing control apparatus according to any of claims 1 to 16, selectively discharging said first and second capacitors so as to create opposed magnetic fields within the core, and to cause the discharge of the discharge tube to 5 terminate.

19. In combination, at least one electric discharge tube capable of generating an output pulse for medical or optical dermatology applications, and control apparatus for the discharge tube, wherein the control apparatus is as defined in any of claims 1 to 16.

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20 An optical cosmetic method of treatment, which comprises providing an electrical energy input to a discharge tube so as to produce an optical output pulse directed towards animal tissue, wherein said electrical energy input is provided by control apparatus according to any of claims 1 to 16.

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