GB2113031A

GB2113031A - Power supplies

Info

Publication number: GB2113031A
Application number: GB08231389A
Authority: GB
Inventors: Thomas Keith Hemingway; Thomas Patrick Veasey
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1981-11-04
Filing date: 1982-11-03
Publication date: 1983-07-27
Also published as: DE3240759A1; FR2515894A1

Abstract

A pulse power supply comprises an inductor for storing energy received from a primary source, and a transfer device for passing the stored energy to a capacitor from whence the energy is discharged as an output pulse. To protect the supply from the result of no or incomplete discharge of the capacitor which can cause overvoltage on the capacitor at subsequent transfer steps, a switch is arranged to become conductive when such overvoltage occurs and thereby transfer energy from the inductor back into the primary source. A preferred use of such a supply is to pump a CO2 laser. <IMAGE>

Description

SPECIFICATION Power supplies This invention relates to power supplies wherein energy derived from a primary power supply is built up in an inductive device and then transferred to a capacitor from whence the energy may be discharged into a load. By way of example, the load could be a CO2 gas laser into which pulses of energy from the capacitor are discharged by a switch to pump or drive the laser.

In one basic form, such a power supply may comprise say a cored transformer the primary winding of which is connected via a primary switch to a current source and the secondary winding of which is connected to the capacitor.

When the primary switch is closed, current builds up in the primary winding and hence also flux energy in the transformer core so that, when the switch is opened, the energy of the previously built-up flux is transferred to the capacitor via the transformer secondary winding.

Various proposals have been made in connection with power supplies of the general nature described above in our U.K. patent applications Nos. 8112210,8222623 and 8221177.

In particular our patent application No.

8112210 discloses one method of dealing with the problem of incomplete transfer of energy from the power supply to the load, for example due to faulty closure of a capacitor discharge switch or to an effect within the load itself.

Such incomplete or even failed energy transfer may result in the voltage at the capacitor and hence also at the primary side of the supply becoming excessive. If the fault is transient, the next successful discharge may then damage the load or at best waste energy. A possibly excessive voltage increase at the primary side of the power supply may also arise if the capacitor becomes open-circuit or reduced in value. The solution proposed in application No. 8112210 comprises the provision of means for monitoring the primary side voltage and a heavy-current switch such as a thyristor arranged to be switched on by the monitoring means if the primary side voltage becomes excessive and thereby to shunt the primary energy supply, this supply being itself protected against the effect of the shunting by a fuze or current limiting arrangement.Although this solution is effective as a protection means, it is sometimes unsatisfactory particularly where the nature of the use to which the power supply is being put and/or the kind of work which is being done, particularly development work, is such that energy transfer failures or "misfires" are not uncommon, the reason being that each such misfire shuts down the power supply altogether until the protection circuit is manually reset.

Accordingly it is an object of the invention to provide a means which protects a power supply of the kind described hereinbefore, including the described basic form of such a supply but also developments or modifications thereof such as are disclosed in our above-mentioned patent applications, from the effects of misfiring without shutting-down the supply. A further preferred object is to make such a supply possibly somewhat more efficient by using rather than wasting the energy associated with such a misfire.

According to the invention there is provided a power supply including an inductive component, a capacitor, primary energy supply means, transfer means for causing energy derived from the primary energy supply means to be built-up in the inductive component and then to be transferred to said capacitor, and circuit means coupled to said inductive component and said primary energy supply means, and operable to derive energy from said inductive component and feed it back to said primary energy supply means.

By way of example, the circuit means can comprise an auxilliary winding associated with said inductive component and a diode connected between the auxilliary winding and the primary energy supply means, the auxilliary winding being such as to bias the diode just short of conduction during normal operation of the power supply, and when the voltage induced in the auxilliary winding exceeds its normal value the diode conducts current from the auxilliary winding to the primary energy supply means in opposition to the normal direction of current therefrom.

For a better understanding of the invention reference will now be made, by way of example, to the accompanying drawing the single figure of which is a simplified circuit diagram of a power supply connected to a load.

The illustrated power supply comprises a cored transformer 1 of which a main primary winding 2 is connected via a primary switch 3, a transistor switch for example, to a primary source 4 which makes available a direct voltage Vs.. The transformer secondary winding 5 is connected via diode 6 to a series combination of a capacitor 7, and a load 8, a CO2 gas laser for example. A switch 9, a semiconductor or spark gap switch for example is connected across the capacitor/load combination. An auxilliary transformer winding 10 is connected in series with a diode 11 across the primary source 4. In normal operation, switch 3 is periodically closed at high speed and, while it is closed, current from source 4 builds-up in winding 2 while flux energy is stored in the transformer core and windings.Each time switch 3 is opened, the stored flux energy collapses and a voltage pulse appears across secondary winding 5 and is transferred via diode 6 to capacitor 7, the load 8 here being such as to provide a charging path for the capacitor. Where the load is not like this it could be shunted by a small inductor or a diode say. The switch 9 is also periodically closed, in one for one synchronism with switch 3 or otherwise as desired, to discharge the capacitor 7 through load 8.

During the normal operation, voltage is also developed across the auxilliary winding 10 and this winding and diode 11 are so poled and connected to supply 4, and the winding has such number of turns that, normally, the operation of closing and then opening switch 3 biasses diode 11 to just short of full conduction.

In the illustrated exemplary case, this condition is obtained by having the cathode of diode 11 connected to the positive side of supply 4 and its anode connected to that side of winding 10 at which positive voltage peaks appear when switch 3 opens.

It can be shown that the voltage Vc applied to capacitor 7 when switch 3 is opened is given by I L where I is the current flowing in winding 2 at the instant of opening the switch, Lf is the inductance of winding 2 and C is the capacitance of capacitor 7. Meanwhile, the voltage induced in winding 2 will be Ve N N1/N2 (so the voltage across switch 3 will rise to Vs+Vc N1/N2) and the voltage induced in winding 10 will be Vc-N3/N2, where N 1, N2 and N3 are the numbers of turns of the windings 2, 5 and 10 respectively. The voltage induced in winding 10 is applied to the anode of diode 11 of which the cathode is at the source potential Vs.Thus, the number of turns N3 of winding 10 is chosen, relative to the source voltage and the number of turns of winding 5, so that Vc N3/N2kVs. The relative numbers of turns of windings 2 and 5 and the source voltage themselves are of course chosen primarily in view of the voltage Vc to be generated.

If now misfiring occurs, e.g. if capacitor 7 ceases to properly discharge into load 8, then each time switch 3 opens to transfer more energy to the capacitor, the voltage applied to the anode of diode 11 will increase. Dependent on the chosen amount of different between Ve N3/N2 and Vs, sooner or later diode 11 will start to conduct and the energy stored in the transformer will be returned to source 4. In the limiting case where Ve N3/N2 is equal to Vs, diode 11 will in effect clamp the capacitor voltage to a fixed value, i.e., the capacitor voltage Vc on opening switch 3 after a misfire will be virtually unchanged and, assuming a theoretical resistance-free circuit, the energy stored in transformer 1 will be transferred without loss back to the source.

A similar result is obtained if the capacitor 7 drops in value or an open-circuit occurs between the capacitor and the transformer, i.e. the voltage applied to the anode of diode 11 increases and the diode then conducts the energy stored in the transformer back to the source. The voltage across switch 3 will be limited to (Vs+VD) (N1/N3)+Vs while the voltage across winding 5 is limited to (Vs+VD) (N2/N3), VD being the voltage across diode 11 when it is conducting.

For best operation of the illustrated circuit windings 2 and 10 are preferably close-coupled.

The coupling of these windings to winding 5 is less important however.

As will be realised, the illustrated circuit can be designed so that it is effectively protected from the effect of misfiring but is not actually closed down by such a misfire. Instead with the exception of the misfired pulse, the circuit continues to operate normally. Also, the energy which should have been conveyed to the load, and which was not so conveyed, is retained for the next triggering attempt, and the excess energy which would be transferred to the capacitor on the next transfer cycle is returned instead to the primary source thus substantially avoiding energy loss from the primary source due to a misfire.

As already mentioned, the invention is not just applicable to the basic form of inductive/capacitor power supply as shown herein but also to various modifications such as some of those forming the subject of our aforementioned patent applications. The source 4 need not be a cell or battery as shown but could comprise say a rectifier/capacitor type of supply.

Furthermore, it will be appreciated that since the effect of the diode 11 and auxilliary winding 10 is to clamp the voltage Vc to a predetermined level, these items can be used as a means of controlling Vc rather than simply as a means of protecting the circuit from misfiring. Thus, the circuit components can be chosen so that, in the absence of winding 10 and diode 11, the capacitor voltage Vc would be somewhat higher than is required (even in the absence of misfiring) and winding 10 and diode 11 are made operable so that even during normal operation diode 11 conducts and clamps Vc to the predetermined required level. Vc is thereby given a measure of stability and the diode is still effective to protect the circuit in the event of misfiring.

As will be appreciated, it is not essential that the transformer 1 should have a core-a transformer without a core could equally well be used. Also, the circuit, with appropriate rearrangement of the components, could comprise an inductor instead of a transformer as shown, or a transformer with one winding acting as an inductor and replacing the windings 2 and 5 of the illustrated transformer, and only one other winding to carry out the function of winding 10.

Claims

1. A power supply including an inductive component, a capacitor, primary energy supply means, transfer means for causing energy derived from the primary energy supply means to be builtup in the inductive component and then to be transferred to said capacitor, and circuit means coupled to said inductive component and said primary energy supply means, and operable to derive energy from said inductive component and feed it back to said primary energy supply means.

2. A power supply according to claim 1, wherein said circuit means comprises an auxilliary winding associated with said inductive component and a diode connected between the auxilliary winding and the primary energy supply means, the auxilliary winding being such as to bias the diode just short of conduction during normal operation of the power supply, and when the voltage induced in the auxilliary winding exceeds its normal value the diode conducts current from the auxilliary winding to the primary energy supply means in opposition to the normal direction of current therefrom.

3. A power supply according to claim 1, wherein said circuit means comprises an auxilliary winding associated with said inductive component and a diode connected between the auxilliary winding and the primary energy supply means, the auxilliary winding being operable to bias the diode to its conductive state whereby the diode becomes operable to clamp the voltage induced in the inductive component.

4. A power supply according to claim 1, wherein said inductive component is a transformer.

5. A power supply substantially as hereinbefore described with reference to the accompanying drawing.