WO2008129325A1

WO2008129325A1 - Drive circuit for a discharge tube and a method of driving a discharge tube

Info

Publication number: WO2008129325A1
Application number: PCT/GB2008/050279
Authority: WO
Inventors: Jan Simonsen
Original assignee: Cyden Ltd
Current assignee: Cyden Ltd
Priority date: 2007-04-20
Filing date: 2008-04-21
Publication date: 2008-10-30
Anticipated expiration: 2009-10-20
Also published as: DK2181568T3; HK1143486A1; EP2181568A1; EP2181568B1; ES2374420T3; ATE532388T1

Abstract

The drive circuit comprises a primary circuit and a secondary circuit, both arranged to deliver power to the discharge tube. The primary circuit comprises a first power supply and a first controller for controlling the delivery of power from the first power supply to the discharge tube.The secondary circuit comprises a power storage arrangement, and at least one second controller for controlling the discharge of the power storage arrangement. The second controller is arranged to selectively discharge the power storage arrangement at intervals determined by the first controller, such that the selective discharge maintains the voltage across the discharge tube above a threshold voltage necessary to maintain a discharge arc.

Description

Drive Circuit for a Discharge Tube and a Method of Driving a Discharge Tube

The present invention relates to a drive circuit and particularly, but not exclusively, to a drive circuit for a discharge tube and a method of driving 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.

Discharge tubes are typically powered using a circuit such as that illustrated in Figure 1 of the accompanying drawings. In such a circuit, a capacitor 11 is charged by a direct current (dc) source 10. The dc supply 10 and capacitor 11 are electrically connected to a discharge tube 12 by a first series of windings arranged upon the core of a transformer (not shown). However, the discharge tube 12 does not produce any output in this initial state, since there is no conductive path through the gas 13 between electrodes 14.

The conductive path is created by ionising the gas within the tube 12, and this is performed using a trigger circuit 15. The trigger circuit 15 induces a high voltage supply on the first series of windings (not shown) causing the gas 13 within the discharge tube 12 to break down. The trigger circuit is typically controlled by a controller 16 and comprises a second series of fewer windings on the same transformer core as the first series of windings, to create a step up in voltage. This high voltage produced across the tube creates a conduction path between the tube electrodes, thereby allowing the capacitor 1 1 to discharge and thus produce an intense arc.

It is also known to power discharge tubes using alternating current (ac) sources. However, in such systems it is first necessary to rectify the ac signal, for example with a diode bridge. The well-known waveform 17 of a full wave rectified sinusoidal voltage is shown in Figure 2 of the accompanying drawings. The figure further provides an indication of the time interval (illustrated as the interval t_rt₂) over which the voltage is sufficient to maintain a discharge arc. In this manner, when the voltage of the rectified sinusoidal waveform reaches a threshold (Vt_h) voltage of, for example, 70V at time t-i , an optical output (that is, an arc) will be produced that will continue until the rectified waveform voltage drops to, for example 50 volts at time t₂.

US Patent 6,965,203 discloses a method and circuitry for repetitively firing a flash lamp that is powered with a periodic voltage signal. A problem with powering discharge tubes using a rectified mains supply, however, is that the optical output is limited to a flash rate of twice the mains frequency (that is 100 Hz for a 50 Hz mains supply and 120 Hz for a 60 Hz mains supply) and the optical output from the discharge tube will only continue while the mains voltage exceeds the threshold voltage. Once the mains voltage drops below the threshold voltage, the optical output from the discharge tube will terminate.

A known solution to this problem is to incorporate a capacitor bank after the diode bridge in order to hold the voltage above the threshold voltage. Such a capacitor bank is found to smooth the voltage supply to the discharge tube. However, such an arrangement is prone to fluctuations in the voltage supply 18 as shown in Figure 3 of the accompanying drawings and this manifests as a fluctuation in the optical output of the discharge arc.

Moreover, certain applications require the application of a steady pulse of radiation from a discharge tube for a time period less that the duty cycle of a rectified mains supply.

We have now devised a drive circuit for a discharge tube and a method of driving a discharge tube which alleviates these problems.

In accordance with the present invention as seen from a first aspect, there is provided a drive circuit for a discharge tube, the drive circuit comprising a primary circuit and a secondary circuit both arranged to deliver power to the discharge tube, the primary circuit comprising a power supply and a first controller for controlling the delivery of power from the first power supply to the discharge tube, the secondary circuit comprising power storage means and at least one second controller for controlling the discharge of the power storage means, the second controller being arranged to selectively discharge the power storage means at intervals determined by the first controller, such that the selective discharge maintains the voltage across the discharge tube above a threshold voltage necessary to maintain a discharge arc.

By this means, the drive circuit according to the invention is enabled to supply a substantially constant power supply to the discharge tube.

The primary circuit preferably includes a diode bridge for rectifying ac mains supply. Such a diode bridge preferably provides full wave rectification of the ac mains.

Preferably, the primary circuit includes a trigger mechanism for inducing a high voltage spike across the discharge tube to ionise the gas within the discharge tube.

The first controller preferably comprises a timing circuit to time the triggering of the trigger mechanism with the rectified ac mains supply. Such a timing circuit preferably enables the trigger mechanism to be triggered only in those intervals when the rectified ac mains voltage is above a threshold voltage.

Preferably the threshold voltage is the minimum voltage necessary to sustain a discharge arc.

Preferably, the rectified mains supply is continuously monitored by the first controller.

In accordance with the first embodiment of the present invention the secondary circuit preferably comprises a power supply such as a direct current (dc) supply for charging the power storage means.

Preferably, the power storage means comprises at least one capacitor, such as a capacitor bank.

The at least one second controller preferably causes such a capacitor bank to discharge in order to compensate for the drop in voltage from the rectified ac mains supply and to maintain the voltage across the discharge tube above the threshold voltage, between duty cycles of the rectified mains voltage. Preferably, the discharge arc is terminated by a switch, such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

Preferably, the first controller and the at least one second controller are arranged in electronic communication to enable synchronous discharge of the capacitor (such as the capacitor bank) with the rectified voltage.

Preferably, the first embodiment of the present invention provides for the discharge of a substantially uniform pulse of radiation from the discharge tube for durations greater than the given duty cycle of the rectified mains voltage.

In accordance with the second embodiment of the present invention, the power storage means preferably further comprises an inductor.

The at least one second controller preferably comprises a semiconductor switch, such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), which in the "OFF" state prevents the current from the power supply in the primary circuit and the charge from the capacitor bank from passing through the discharge tube. In this state, the inductor preferably discharges across the discharge tube to maintain a discharge while the MOSFET is "OFF". While the MOSFET is "OFF", this further enables the capacitor bank to preferably recharge from the power supply in the primary circuit, such that it can subsequently discharge across the tube when the MOSFET is switched back "ON".

Preferably, the switched state of the MOSFET is controlled by the first controller. Preferably, the source terminal of the MOSFET is connected to substantially zero voltage, or ground.

Preferably, the second embodiment of the present invention provides for the discharge of a substantially uniform pulse of radiation from the discharge tube for durations less than a given cycle period of the power supply signal in the primary circuit. In accordance with the present invention as seen from a second aspect, there is provided a method of driving a discharge tube, the method comprising providing a drive circuit according to the invention: powering the discharge tube using the primary circuit; - selectively powering the discharge tube using the secondary circuit at times when the first power supply is below a threshold value.

Preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a circuit diagram of a conventional drive circuit for a discharge tube, as described above;

Figure 2 is a graphical representation of the voltage waveform of the rectified ac mains supply using such a conventional circuit, as described above;

Figure 3 is a graphical representation of the voltage waveform of a rectified ac mains supply, following smoothing by a capacitor bank as described above;

Figure 4 is a circuit diagram of an exemplary drive circuit according to a first embodiment of the present invention; and,

Figure 5 is a circuit diagram of an exemplary drive circuit according to a second embodiment of the present invention.

According to the first embodiment of the present invention as shown in Figure 4, there is provided a drive circuit 100 comprising a primary circuit 101 and a secondary circuit 102a, for powering a discharge tube 103. The primary circuit 101 powers the discharge tube 103 and the secondary circuit 102a compensates for the variation in the rectified ac power supply in the primary circuit 101 to provide a substantially constant voltage between duty cycles of the rectified power supply.

The primary circuit 101 comprises a diode bridge 106, which rectifies the incoming ac mains power supply 104 from an isolation transformer 105, to provide a full wave rectified voltage signal. The output of the diode bridge 106 is electrically connected to the discharge tube 103 via a single diode 107 which ensures unidirectional flow of current toward the discharge tube 103 from the power supply 104. However, the rectified voltage is insufficient to initiate a discharge within the tube 103, since there is no conduction path between the electrodes 108 of the discharge tube 103.

The primary circuit 101 includes a controller 109 for controlling a trigger circuit 1 10. The controller 109 receives an input signal from the output of the diode bridge 106 and monitors the variation in waveform via digital signal processing means (not shown). The controller 109 includes an output for outputting a voltage as input to the trigger circuit 110. The trigger circuit 110 typically includes a step-up transformer (not shown) which produces a high voltage spike across the tube electrodes 108.

The high potential difference created across the electrodes 108 by the trigger circuit 1 10, causes the gas 11 1 within the tube 103 to ionise, thereby reducing the impedance of the tube 103. However, in order to ensure that the trigger circuit 1 10 applies the high voltage spike at the correct time, that is, when the rectified voltage is sufficient to create a discharge arc, the controller 109 monitors the rectified voltage via connection 112 and outputs a signal to the trigger circuit 1 10 at a time determined by digital signal processing means (not shown) to cause the trigger circuit 1 10 to produce the voltage spike. However, with such a system, there is no control over the duration or frequency of the discharge arc.

Accordingly, the circuit 100 according to the present invention further comprises a secondary drive circuit 102a which includes a second (dc) power supply 113 arranged in series with the rectified voltage supply, and which used to charge a capacitor bank 1 14.

The secondary circuit further includes a second controller 1 15 arranged in electronic communication with the first controller 109 to control selective discharge of the capacitor bank 1 14.

In use, the first controller 109 causes the trigger circuit 110 to produce a high voltage across the electrodes 108, causing the gas 1 11 within the tube 103 to ionise, when the rectified voltage across the tube electrodes 108 from a given duty cycle reaches the minimum value necessary to maintain a discharge arc. As the rectified voltage subsequently falls below the threshold voltage within the same duty cycle, the second controller 115 causes the capacitor bank 1 14 to progressively discharge, so as to compensate for the fall in voltage on the rectified voltage input. This maintains a constant voltage across the tube electrodes 108 and therefore a uniform discharge arc.

Initially, the discharge of the capacitor bank 114 is quite small. However, as the rectified voltage is reduced further from the threshold voltage, the rate of discharge of the capacitor bank 1 14 increases, so as to offset the fall in rectified voltage. As the rectified voltage begins to rise again on the next duty cycle, the capacitor bank 1 14 begins to progressively recharge from the second power supply 1 13, so that it is ready to discharge again in accordance with the fall in rectified voltage.

The second controller 115 monitors the first controller 109 in order to time the selective charge and discharge of the capacitor bank 1 14. In this manner, the cyclic charge and discharge of the capacitor bank 114 can be used to maintain the discharge arc for a desired time period.

When it is desired to terminate the arc, a semiconductor switch 116, such as a metal oxide semiconductor field effect transistor (MOSFET), arranged in electrical connection with the cathode of the discharge tube 103, is switched to the "OFF" state by the first controller 109 to inhibit the flow of current between the electrodes 108 of the discharge tube 103.

In accordance with a second embodiment of the present invention as shown in Figure 5, the primary circuit 101 can be seen to comprise those components of the primary circuit according to the first embodiment. The secondary circuit 102b however, comprises a first secondary controller 117 and a second secondary controller 118. The first secondary controller 1 17 is arranged in electrical communication with the first controller 109 arranged in the primary circuit 101 and is arranged to discharge a capacitor bank 119 arranged in the secondary circuit 102b at a time which corresponds to a period following the application of a pulse to the trigger circuit 110. The first controller 109 arranged in the primary circuit 101 monitors the variation of the rectified mains voltage and outputs a pulse to the trigger circuit 110 when the voltage level of the rectified mains voltage passes above a threshold value for producing a discharge arc, which may be 60-70V. The trigger pulse causes a high voltage spike to be applied across the electrodes 108 of the discharge tube 103 which ionises the gas 11 1 within the tube 103 and thus creates a conduction path between the electrodes 108 such that the charge stored on the capacitor bank 119 can discharge across the tube 103 to produce a pulse of radiation.

The current which exits the tube 103 passes through an inductor coil 120 arranged in the secondary circuit 102b and subsequently though the second secondary controller 118, which may comprise a semiconductor switch, such as a MOSFET, also arranged in the secondary circuit 102b, before passing to ground. The gate terminal of the MOSFET 118 is connected to the first controller 109 arranged in the primary circuit 101 , such that the first controller 109 arranged in the primary circuit 101 can control the "ON/OFF" state of the switch 118 and thus control the current passing from the capacitor bank 119 and the primary circuit 101 through the tube 103.

Following the discharge of the capacitor bank 119, which may take place over a time period significantly less that the duty cycle of the rectified mains voltage, the

MOSFET 1 18 is switched to the "OFF" state. This causes the magnetic field in the inductor 120 to collapse creating a high voltage spike which is applied across the electrodes 108 of the tube 103 so as to maintain a voltage above the threshold. This collapse of magnetic field causes a current to flow within the secondary circuit 102b through diode 121 and between the electrodes 108 of the tube 103 to maintain a discharge arc. During this period, the capacitor 119 is recharged by the voltage from the rectified mains voltage within the same duty cycle and once charged, the

MOSFET 120 is then switched back "ON", so as to discharge the capacitor 119 across the tube 103. In this manner, by applying a modulated pulsed signal to the gate terminal of the MOSFET 120 once the voltage of the rectified mains signal initially passes above a threshold value, it is possible to maintain a discharge arc for a desired period of time. Moreover, and in common with the first embodiment, it is possible to produce and maintain a discharge arc over selected periods within a given duty cycle of the rectified mains supply as with the second embodiment or over periods comprising several duty cycles of the rectified mains supply, as with the first embodiment, with the application of only a single trigger pulse.

From the foregoing therefore, it is evident that the drive circuit of the present invention enables a discharge tube, powered from a mains supply, to produce a discharge arc of a desired period of time.

Claims

1. A drive circuit for a discharge tube, the drive circuit comprising a primary circuit and a secondary circuit both arranged to deliver power to the discharge tube, the primary circuit comprising a power supply and a first controller for controlling delivery of power from the power supply to the discharge tube, the secondary circuit comprising power storage means and at least one second controller for controlling discharge of the power storage means, the second controller being arranged to selectively discharge the power storage means at intervals determined by the first controller, such that the selective discharge maintains the voltage across the discharge tube above a threshold voltage necessary to maintain a discharge arc.

2. A drive circuit according to claim 1 , wherein the primary circuit comprises a diode bridge for rectifying ac mains supply.

3. A drive circuit according to claim 2, wherein the diode bridge provides full wave rectification of the ac mains.

4. A drive circuit according to any preceding claim wherein the first controller is arranged to continuously monitor the variation in the power supply.

5. A drive circuit according to any preceding claim, wherein the primary circuit includes a trigger mechanism for inducing a high voltage spike across the discharge tube so as to ionise gas within the discharge tube.

6. A drive circuit according to any preceding claim, wherein the first controller comprises a timing circuit to time the triggering of the trigger mechanism with the rectified ac mains supply.

7. A drive circuit according to claim 6, wherein the timing circuit is arranged to enable the trigger mechanism to be triggered only in those intervals when the rectified ac mains voltage is above the threshold voltage.

8. A drive circuit according to any preceding claim, wherein the threshold voltage is the minimum voltage necessary to sustain a discharge arc.

9. A drive circuit according to any preceding claim, wherein the secondary circuit comprises a power supply, such as a direct current supply, for charging the power storage means.

10. A drive circuit according to any preceding claim, wherein the power storage means comprises at least one capacitor.

1 1 A drive circuit according to claim 10, wherein the second controller is arranged to cause the or each said capacitor to discharge in order to compensate for the drop in voltage from the rectified ac mains supply and to maintain the voltage across the discharge tube above the threshold voltage.

12. A drive circuit according to claim 10 or 11 , wherein the first controller and the second controller are arranged in electronic communication to enable synchronous discharge of the capacitor with the rectified voltage.

13. A drive circuit according to any preceding claim, wherein the discharge arc is arranged to be terminated by a switch, such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

14. A drive circuit according to any preceding claim, which provides for the discharge of a substantially uniform pulse of radiation from the discharge tube for durations greater than a given duty cycle of the rectified voltage.

15. A drive circuit according to any of claims 1 to 8, wherein the power storage means comprises an inductor.

16. A drive circuit according to claim 15, wherein the power storage means further comprises a capacitor.

17. A drive circuit according to claim 15 or 16, wherein the second controller comprises at least one semiconductor switch, such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

18. A drive circuit according to claim 17, wherein the switched state of the semiconductor switch is arranged to be controlled by the first controller.

19. A drive circuit according to claim 17 or 18, wherein semiconductor switch has a source terminal connected to substantially zero voltage, or ground.

20. A drive circuit according to any of claims 1 to 8 or 15 to 19, which provides for the discharge of a substantially uniform pulse of radiation from the discharge tube for durations less than a given duty cycle of the rectified voltage

21. A method of driving a discharge tube, the method comprising; providing a drive circuit according to any of claims 1 to 20; powering the discharge tube using the primary circuit; selectively powering the discharge tube using the secondary circuit only at times when the first power supply is below a threshold value.