GB2448561A

GB2448561A - Control circuit for discharge tube

Info

Publication number: GB2448561A
Application number: GB0717323A
Authority: GB
Inventors: Jan Simonsen
Original assignee: Cyden Ltd
Current assignee: Cyden Ltd
Priority date: 2007-04-20
Filing date: 2007-09-06
Publication date: 2008-10-22
Also published as: WO2008129314A1; GB0717323D0

Abstract

A control circuit for an electric discharge tube comprises a first circuit 110 and a second circuit 210. The first circuit comprises a first capacitor 111 and a discharge tube 112 electrically connectable via a first series of windings 113 arranged upon a core 114 of a transformer 115. The second circuit comprises a second capacitor 211 and a second series of windings 212 arranged upon the core, such that electric discharge of the first capacitor and the second capacitor creates opposed magnetic fields within the core, causing the discharge of the discharge tube to terminate.

Description

Control Circuit The present invention relates to a control circuit and

more particularly, but not exclusively, to a control circuit for controlling the operation of a discharge tube (also known as a discharge lamp).

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 11 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 11; this portion 12 no longer contributes to the laser pulse 11 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 11, in order to reduce degradation of the dye and also to reduce the amount of heat to be dissipated.

Discharge tubes are typically powered using a circuit illustrated in Figure 2 of the accompanying drawings. In such a circuit, 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 for the discharge 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 for a circuit which permits selective termination of a discharge arc from a discharge tube.

In accordance with this invention as seen from a first aspect there is provided a control circuit for an electric discharge tube, the circuit comprising a first circuit and at least one second circuit, the first circuit comprising 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 comprising 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 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 circuit 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, 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 trigger circuit, which can help to ensures bringing the transformer core 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 circuit 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 a control circuit 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 an exemplary control circuit according to the present invention.

Referring to Figure 3, there is shown a control circuit 100 for selectively controlling the operation of a discharge tube 112, in pumping a laser system 400 for example.

The control circuit 100 comprises a first circuit 110, a second circuit 210 and a trigger circuit 310. The first circuit 110 comprises a first capacitor 111 electrically connected to a discharge tube 112 via a first series of windings 113, arranged on a magnetic core 114 of a transformer 115. The capacitor 111 is to be charged via a power supply unit (not shown). The power supply unit creates a potential difference across the electrodes 116 of the discharge tube 112, which potential difference maintains a plasma discharge within the tube 112, 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 116; this high voltage is produced by the trigger circuit 310. The trigger circuit 310 includes a capacitor 311 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 114 as the first series of windings 113.

The second circuit 210 includes a capacitor 211 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 114.

In the static state, the tube 112 glows under the influence of the power supply unit (not shown) provided for the first circuit 110, due to the plasma discharge. To produce the arc discharge, the switch 313 in the trigger circuit 310 is closed to cause the capacitor 311 to discharge through the third series of windings 312 on the transformer core 114. This discharge creates a current within the third series of windings 312 and therefore sets up a magnetic field within the transformer core 114.

This magnetic field induces a high voltage spike in the first series of windings 113, by virtue of there being more turns on core in the first circuit 110 than the trigger circuit 310, causing the gas 117 within the discharge tube 112 to ionise. This lonisation creates a conduction path between the electrodes 116 within the tube 112, thereby enabling the capacitor 111 provided in the first circuit 110 to discharge across the tube 112 and produce an intense arc.

However, as the capacitor 111 discharges, the current flowing through the first series of windings 113 increases. This is because the core 114 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 113, 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 211 to discharge across the second series of windings 212. However, the polarity of the capacitor 211 is reversed with respect to the capacitor 111 such that the magnetic field created by the capacitor 211 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 114 in the opposite sense to the first series of windings 113 to create opposed magnetic fields.

This opposed flux reduces the resultant magnetic flux within the core 114 and so brings the core out of magnetic saturation. As a result, the impedance of the first series of windings 113 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 with reference to Figure 1), enables the capacitor 111 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 117 within the discharge tube 112 is fully de-ionised. If the gas 117 within the discharge tube 112 fails to de-ionise before the charging voltage is re-applied, then current will continue to flow through the tube 112, 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 circuit 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 circuit according to the present invention further enables power wastage as heat to be minimised. In addition, the use of the control circuit 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. A control circuit for an electric discharge tube, the circuit 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 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 said at least one second capacitor creates opposed magnetic fields within the core, causing the discharge of the discharge tube to terminate.

2. A control circuit 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.

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

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

5. A control circuit 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

6. A control circuit 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.

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

8. A control circuit according to any preceding claim, in which the first series of windings are arranged to be energised by the at least one first capacitor.

9. A control circuit according to any preceding claim, in which the second series of windings are arranged to be energised by the at least one second capacitor.

10. A control circuit according to any preceding claim, in which a power supply is provided to charge the at least one first capacitor which charged capacitor is sufficient to maintain a discharge plasma within the discharge tube, but is insufficient to start a discharge arc.

11. A control circuit according to any preceding claim, in which the discharge tube is such that it will generate an output pulse having a range of wavelengths at least in the visible spectrum.

12. A control circuit according to any preceding claim, in which the second series of windings are wound upon the transformer core in an opposite direction to the first series of windings to create the opposed magnetic field.

13. A control circuit according to any preceding claim, 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 opposite directions, to create opposed magnetic fields.

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

15. A control system according to claim 14, wherein the sensor causes 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,

16. A control circuit 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 controlled by a control circuit according to any preceding claim.