GB2424752A

GB2424752A - Power supply circuit for lasers

Info

Publication number: GB2424752A
Application number: GB0506233A
Authority: GB
Inventors: Christopher Frederick Parsons
Original assignee: GSI Lumonics Ltd; GSI Group Ltd
Current assignee: Novanta Technologies UK Ltd
Priority date: 2005-03-29
Filing date: 2005-03-29
Publication date: 2006-10-04
Also published as: GB0717972D0; GB2438559A; WO2006103409A2; WO2006103409A3; GB0506233D0; CN101167223A

Abstract

A power supply circuit for a lamp 5 which pumps a laser is disclosed, comprising an input for a mains supply, an output, respective positive and negative paths from the input to the output and switches 25, 41 provided in both negative and positive paths. A differential current sensor 43 may be provided to detect current imbalance due to faults such as faults to earth (ground) at the output. A control unit 44, with inputs from the differential current sensor 43 and from a further current sensor 34 are used to control the switches 25, 41. As shown, the circuit has a 3-phase supply 21, a rectifier 22 and a capacitor 24 acting as an energy store, although single-phase and DC arrangements are possible.

Description

Power Supply Circuit for Lasers This invention relates to a power supply

circuit for lasers.

In particular, it relates to a power supply circuit for supplying power to the pumping lamps of a solid state laser.

Figure 1 shows schematically a solid state laser apparatus. A solid state laser medium 1, such as a YAG rod is arranged in a pumping chamber between two optical resonator mirrors 3 and 4, one or both of which is partially reflective so that a generated laser beam can be emitted from this. The rod 1 is pumped by one or more pumping means, usually pumping lamps of which one lamp 5 is shown in the drawing for clarity. The lamps are powered by a power supply 6.

A typical power supply is shown in Figure 2. The figure shows a power supply using a three phase mains supply, although similar power supplies are available with single phase supplies. DC power supplies are also sometimes used.

A three phase AC mains supply 21 is applied to a diode rectifier 22 which, for a three phase supply, may comprise 6 diodes 22A to 22F. The rectified current is supplied to a capacitor or bank of capacitors 24. Capacitors 24 acts as an energy reservoir and this energy is supplied through means of a switching element 25 to the lamp 5. It may be pulsed or supplied continuously. The energy supplied to the lamp then depends on the duty and frequency of the switch frequency and can be varied by varying this. Typically, the switching frequency may be of the order of 5 - 5OKz but may be less or greater than this.

An inductor 27 and capacitor 28 smooth the output and a diode 26 provides a path for the current in inductor 27 when switch 25 is off.

There are, however, two main problems associated with such previouslyproposed power supplies. Firstly, when the supply is first switched on, or if there is a momentary interruption of the mains supply 21, then there is a large inrush current into the capacitor bank through the rectifier. This can cause damage to or failure of components. Secondly, the power supply may be damaged by a fault, such as a short circuit to ground or earth, at point A, ie at the lamp, caused for example by breakage of the lamp. The circuit provides no or little protection against this and such a fault could even damage the laser pumping chamber itself. A failure such as a fault to earth at point B may be protected against by switching off switching element 25 provided it is done quickly enough.

A further example of a conventional lamp supply is shown in Figure 3. This is similar to the circuit of Figure 2 except for also including a boost stage comprising an inductor 31 diode 32 and a further switching element 33 in addition to a current sensor 34 positioned between the rectifier 22 and inductor 31, for example.

Such a boost stage serves to further stabilise the voltage at the lamp. The boost stage increases the voltage across the capacitor bank 24 to a value somewhat higher than mains voltage. Such a circuit is generally considered to be an improvement upon the circuit of Figure 2 as it may help to protect against momentary interruptions in mains supply, if the interruption is such that the voltage across the capacitor bank 24 does not fall below the mains input voltage. However, it can still be vulnerable to damage by inrush currents at switch on, and can still be damaged by a fault to earth at point A. US 6,154,473 discloses a power supply for a laser pumping apparatus. The supply is restricted to use with a single-phase AC input and includes means to protect against failure of output switching. It does not provide any soft-start protection.

The present invention arose in an attempt to provide an improved power supply circuit with better protection.

According to the present invention, there is provided a power supply circuit for laser apparatus, comprising an input for a mains supply, an energy storage means, an output, respective positive and negative paths from the input to the output and switch means provided in both negative and positive paths.

The user of a further switch enables full protection even in the event of a short- circuit.

Preferably, a differential current sensor is arranged to detect current imbalance, and both switches are operated in the event of such an imbalance being detected.

The switch on one side is preferably mounted between the input and the energy storage means, and is on the opposite side to the output switching device.

The energy storage means is preferably a capacitor or bank of capacitors.

Embodiments of the invention will now be described, by example only, with reference to the accompanying drawings, in which; Figure 1 shows schematically a laser pumping chamber; Figure 2 shows a previously proposed power supply arrangement; Figure 3 shows an alternative previously proposed power supply arrangement; Figure 4 shows a power supply arrangement, according to the present invention; Figure 5 shows a simplified power supply circuit; Figure 6 shows an alternative power supply arrangement; and Figures 7 and 8 show schematically alternative differential current sensors.

Referring to Figure 4 a power supply, is shown which includes many similar components to the circuits shown in Figures 2 and 3. In particular, a power input (in this case a three phase main supply 21 applied through a rectifier 22) is applied to an energy store in the form of a capacitor or capacitor bank 24 and this is used to power a lamp or other pumping means 5 either in Continuous (CW) or pulsed laser mode. Instead of the three phase supply, the supply may be a single phase or may even be a DC supply or other power input. The lamp 5 is used to pump a laser pumping chamber of the type shown schematically in Figure 1. Diode 26, inductor 27 and capacitor 28 smooth the power output from the capacitor bank 24. Figure 4 shows a circuit in which a boost stage similar to that of Figure 3 is used to improve performance and this boost stage includes inductor 31, diode 32, a further switching element 33 and current sensor 34. The circuit includes a switch 25 similar to that of Figures 2 and 3 which, on its own, can serve to protect against faults such as a fault to earth (ground) at point B by being turned off.

The circuit shown in Figure 4 differs, however, by also comprising a further switching element 41, a capacitor 40 and diode 42. A differential current sensor 43 is also supplied and the figure also shows a control unit 44 (which would normally also be present in the previously available power supply) but which receives input from the two current sensors.

The differential current sensor is a device which senses net current. In normal conditions, the net current is zero. In the event of a fault to earth or other conditions, there may be a current imbalance. The differential sensor can sense this and output an appropriate signal to the control unit to activate the switch. Many different types of differential current sensor may be used, typical examples of which are Hall Effect or magneto-resistive sensors.

The second current sensor 34 (which is similar to that of the current sensor in Figure 3, for example) would typically be a Hall Effect device, although again other types of sensors may be used if appropriate. This sensor simply senses the current level passing through it, not a differential current.

The outputs from one or both of current sensors 34 and 43 are used to control the switching of switch 41 and switch 25, under control of control unit 44.

Figure 5 is an explanatory diagram showing conceptually the effect of the second switch 41. As is shown in the figure, the power input (which may be either DC or rectified AC) is used to charge a capacitor or capacitor bank 24 and ultimately this feeds the positive and negative sides of the output. In previously proposed power supplies, switch 25 has been used to stop current flowing from the lamp and has been provided generally on the negative path to the output. This can protect against failures to earth in the negative side of the output but not to failures occurring in the positive side. The provision of a switch anywhere in the positive side between the input and output also serves to protect against a fault at the positive side of the output and therefore in the event of a fault being detected, if both switches are switched then full protection is achieved.

In principle, therefore, switch 41 may be located anywhere from the mains input to the output on the positive side.

However, in Figure 4, and indeed in most embodiments, the switch will be provided before capacitor bank 24 and in Figure 4 it is provided between the rectifier 22 (or other power input) and inductor 31. Location of the switch 41 before capacitor bank 24 protects against inrush currents as well as protecting against a fault to earth at point A. Referring more specifically to Figure 4, in normal operation, switching element 41 is on continuously and therefore power is supplied to the boost stage and the capacitor bank 24 as before. However, if the current flowing through the current sensor 34 exceeds a threshold value, then the switch is operated (typically via the control unit) and turns off.

This then protects the capacitor bank and other components.

Such a circumstance may arise for many reasons, non-limiting examples of which are: because of a fault at the power supply; as a current spike or current surge following a brown out for example, or during start up. During start up, it is often important that a soft start is achieved so that the current does not rise above a certain level and switch 41, in conjunction with current sensor 34, may be used to achieve this. Again, the current sensor 34 may be placed in any convenient place where it can sense the current.

A further function of the additional switching stage is to interrupt the current to point A in the figure (ie the positive side of the lamp) in the event of a fall to earth. In order to achieve this, the differential current sensor 43 is used. As described above, the differential current sensor detects an imbalance in the supply current. This corresponds to a fault to earth for example at point A or other parts in the circuit and, upon the differential current sensor detecting such an imbalance, control unit 44 is used to rapidly switch off both switching elements 41 and 25 to prevent damage to the power supply or to the laser pumping means. Since this has to occur quickly, both switches 41 and 25 need to be fast acting switches. Typically, they should be able to operate within a few tens of a microsecond.

If there are no faults of this type, then the differential current sensor 43 will not sense any imbalance. Since the positive and negative currents will balance.

Many different types of configuration of differential current sensors may be used.

In some examples, these could be configured in the form of a component through which each of the input wires from a mains power supply 21 pass. In the circuit of Figure 4, where a three phase source is supplied, then there are three wires from the power source 21 to the rectifier 22 but for single phase supplier then there will be two wires and both of these will pass through the sensor. Figure 7 shows schematically one type of differential current sensor in which all three input wires from mains supply 21 pass through a bore in a sensor block 43a and circuitry in this compares the signals. Figure 8 shows schematically an alternative known arrangement in which a sensor is provided for each conductor. Thus, for a three phase AC source having three wires 101, 102, 103, three sensors 431, 432 and 433 are provided, one for each respective wire. The output from these are summed at unit 450 to measure the net, or differential, current. These types of sensors could be a magnetic type, eg a Hall Effect type device, current sensing resistors (shunts) or other types of devices.

Figure 6 shows an alternative method embodiment in which an alternative method of differential current sensing, using separate sensors, is used. In this case, instead of the differential current sensor 43 which is connected to the adjacent wires from the power input 21 to the rectifier 22, a further current sensor 60 is mounted on the negative line from the negative side of the diodes 22 and 42, switch 25 (and switch 33). A control unit is not shown in this figure for clarity but in practice the control unit (or a separate control circuit) receives signals from both the sensors 34 and 60 which, as can been seen on the figure, are on the positive and negative paths between input and output and therefore a comparison of the signals on them can indicate whether there is any current imbalance. As described above, a current imbalance would generally represent a fault to earth (ground) and therefore the combination of the two sensors 34 and 60 can be used to provide the differential current sensor.

in embodiments of the invention, it will usually be important that the switch 41 is of the type which can be switched off while it is conducting current. This will usually be a device which has a gate through which a first signal can be sent to turn the switch ON and a second signal can be sent to turn the switch OFF and therefore stop current flowing between two terminals. Any type of transistor switch may be applicable, as can other types of electronic switches, but the switch 41 in a currently preferred embodiment is an insulated gate bipolar transistor (IGBT).

For soft start protection, or to protect against other momentary problems, the current sensor 34 is measured and if the current exceeds a threshold value then switch 41 is turned OFF. The switch 41 may be turned ON again when the current has fallen in an acceptable value. The value of fall is principally governed by the value of Inductor 31.

Note that although the embodiment of Figure 4 for example shows switch 41 on the positive side (between the input and capacitor tank) and switch 25 on the negative side, this could of course be reversed. The switch between the input and capacitor could equally be on the negative side and the other switch on the positive side. The advantages of the invention apply for either polarity'.

Claims

Claims 1. A power supply circuit for laser apparatus, comprising an input

for a mains supply, an energy storage means, an output, respective positive and negative paths from the input to the output and switches provided in both negative and positive paths.
2. A power supply circuit as claimed in Claim 1, comprising current sensing means arranged to operate one or both switches upon a specified fault condition being detected.
3. A power supply circuit as claimed in Claim 2, comprising a differential current sensor arranged to detect current imbalances on the positive and negative paths and to open both switches upon a current imbalance being detected.
4. A power supply circuit as claimed in Claim 3, wherein one of the switches is provided between the power input and energy storage means.
5. A power supply circuit as claimed in Claim 4, wherein the switch on the positive path is provided between the input and energy storage means.
6. A power supply circuit as claimed in claim 4, wherein the switch on the negative path is provided between the input and energy storage means.
7. Apparatus as claimed in any of Claims 3 to 6, wherein the differential current sensor is provided at the input.
8. Apparatus as claimed in Claim 7, wherein a mains input is applied to a rectifier and the differential current sensor is mounted between the mains input and rectifier.
9. Apparatus as claimed in Claim 8, wherein the means input is AC.
10. Apparatus as claimed in Claim 9, wherein the means input is threephase AC.
II. A power supply circuit as claimed in any of Claims 2 to 6, wherein the differential current sensor comprises separate current sensors in each path between each input and output and including means for detecting current unbalance.
12. Apparatus as claimed in any of Claims 2 to 9, wherein a current sensor and a switch are arranged before the energy storage means and including means for turning the switch OFF when the current through the sensor exceeds a threshold.
13. A power supply circuit as claimed in Claim 10, wherein an inductor is provided between the current sensor and the energy storage device.
14. A power supply circuit as claimed in any of Claims 2 to 11, wherein at least one current sensor is a Hall Effect device.
15. Apparatus as claimed in any preceding claim, including a control means for receiving sensed current and operating one or both switch means in accordance with signals from the or each sensor.
16. Apparatus as claimed in any preceding claim, wherein at least one switch is a transistor device, preferably an IGBT.
17. A method of protecting a power supply circuit as claimed in any preceding claim from faults to earth at the output, comprising detecting a current imbalance between the positive and negative paths from the input to the output and operating switch means to open one or both paths.
18. A method of protecting a power supply circuit as claimed in any of Claims 1 to 14 from overload, comprising detecting when current through a sensor exceeds a threshold value and operating one or both switches in accordance with this detection.
19. A method of protecting a pumping means for a laser, comprising powering the pumping means by a power supply circuit as claimed in any preceding claim.
20. A power supply circuit substantially as hereinbefore described with reference to, and as illustrated by, any of Figures 4 to 8 of the accompanying drawings.
21. A method of protecting a power supply circuit or protecting laser pumping apparatus, which method is substantially as hereinbefore described with reference to the accompanying drawings.