GB2343065A

GB2343065A - Improved power supply for series connected lamps

Info

Publication number: GB2343065A
Application number: GB9821391A
Authority: GB
Inventors: Philip John Rimmer
Original assignee: Tunewell Technology Ltd
Current assignee: Tunewell Technology Ltd
Priority date: 1998-10-01
Filing date: 1998-10-01
Publication date: 2000-04-26
Also published as: GB9821391D0

Abstract

A power supply for a plurality of series connected resistive loads 9, 10, 11, 12 such as incandescent lamps supplied from the mains through a converter 2 to convert the supply to a high frequency square wave. This converted supply is then fed through a leakage transformer 3 to the lamps 9, 10, 11, 12 which are connected in series with the transformers secondary coil 5 and there being a capacitor 8 in parallel. The selection of resistive load 9, 10, 11, 12, leakage flux of the secondary coil 5 and capacitance 8 form a resonant circuit substantially with the same frequency as the high frequency supply. Each of the lamps is provided with an automatic bypass circuit 13, 14, 15, 16 so that in the event of a single lamp failure/removal the remaining lamps will still function there is also provision for bypass a set of lamps using a switch 18.

Description

DESCRIPTION OF INVENTION "IMPROVEMENTS IN OR RELATING TO A POWER SUPPLY" THE PRESENT INVENTION relates to a power supply, and more particularly relates to a power supply adapted to supply power to a plurality of power consuming units connected in series.

There is a demand for a power supply adapted to supply power to a plurality of power consuming units connected in series. The power consuming units may, for example, comprise low voltage incandescent lamps.

It has been proposed previously to provide a power supply for power consuming units connected in series which comprises a constant current feed circuit. The constant current feed circuit has, typically, required a servocontrol arrangement. This is relatively complex, and consequently relative costly.

The present invention seeks to provide an improved power supply.

According to this invention there is provided power supply for connection to a series connection of a plurality of resistive loads, the power supply comprising a transformer having a primary coil adapted to be connected to a source of alternating current of a predetermined frequency, and having a secondary coil adapted to be connected to the said series connection, there being a capacitor connected across said secondary coil, the transformer having a predetermined flux leakage to provide the effect of an inductance in series with the output coil of the transformer, the effective inductance being of such a magnitude that the resonant circuit constituted by the secondary coil of the transformer and the associated effective inductance and the said capacitor is of a magnitude which is substantially equal to the said predetermined frequency of the said source of alternating current.

The power supply may incorporate a converter connected to the input coil of the transformer and adapted to be connected to an external electricity supply, the converter being adapted to provide, to the input coil of the transformer, said alternating current of said predetermined frequency and of a predetermined waveform.

The predetermined waveform may be a square waveform of a substantially sinusoidal waveform.

Conveniently the transformer comprises a first pair of limbs receiving respectively the primary coil and the secondary coil of the transformer, said limbs being inter-connected by inter-connecting limbs, the inter-connecting limbs each having a large surface area and being located adjacent each other.

Preferably the inter-connecting limbs are each of slab-like form, having a width greater than the diameter of the limbs of said first pair of limbs, the first pair of limbs being located at opposite ends of the inter-connecting limbs.

Advantageously the transformer is contained within a conductive housing.

Conveniently connected to a series connection of a plurality of resistive loads.

Advantageously each resistive load is an incandescent lamp.

Preferably each resistive load is connected in series with a voltage operated shorting device, adapted to provide a short circuit in the event that the resistive load should go open circuit.

Advantageously the voltage operated shorting device comprises two leads connected to the series connection of resistive loads on opposed side of the said relevant resistive load, the leads being inter-connected by a connection incorporating at least one device adapted to break down when subjected to a predetermined potential, and also being inter-connected by an initially nonconductive connection between the two leads, means being provided, responsive to a current flow between the leads through said element which is adapted to break down, to render conductive said initially non-conductive connection between the two leads.

Conveniently the element adapted to break down may comprise a zener diode.

Advantageously the normally non-conductive connection comprises the controlled current path of a triac between main terminal one and main terminal two of the triac, and wherein the zener diode is connected to one said lead by a diode, and to the other said lead through a resistor, the node between the zener diode and the resistor being connected to the gate of the triac.

In an alternative embodiment the normally non-conductive connection comprises a controlled switch, one of said leads being coupled, by an appropriate core, to a coil such that current flowing through the lead provides a current through the coil, said switch being adapted to be closed in response to current flowing through the coil.

Conveniently the connection incorporating said device adapted to break down comprises two oppositely orientated zener diodes.

Advantageously the coil is in a series circuit with a diode and a resistor, the node between one terminal of the resistor and the diode being connected to the gates of two field effect transistors that constitutes said switch, the other terminal of the resistor being connected to the two sources of the field effect transistors, the drains of the field effect transistors being connected respectively to the two leads.

Alternatively the coil is connected to a series circuit containing a diode and a capacitor, there being a relay that constitutes said switch, the relay having an operating coil connected across the capacitor, the relay incorporating a mechanically operable switch which constitutes the normally non-conductive connection between the two leads.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which: FIGURE 1 is a diagram illustrating one embodiment of the invention, FIGURE 2 is a diagram illustrating the constant current feed principle, FIGURE 3 is a diagram illustrating a voltage operated shorting device suitable for use in the embodiment of Figure 1, FIGURE 4 illustrates an alternative form of voltage operated shorting device for use in the embodiment of Figure 1, FIGURE 5 illustrates a further voltage operated shorting device for use in the embodiment of Figure 1, and FIGURE 6 illustrates a transformer suitable for use in the embodiment of Figure 1.

Referring now to Figure 1, a source of mains power 1 is connected to an optional converter 2 adapted to convert the mains power into a high frequency square wave or sinusoidal wave having a predetermined frequency between 20 and 200 khz. A frequency of between 50 and 100 kHz is preferred. The output of the converter 2, which may have a voltage typically of 160 VRMS, is supplied to the primary coil 3 of a"leaky"transformer 4 which will be described in greater detail hereinafter. The transformer has a secondary coil 5, the centre turn of which is tapped and is connected by a line 6 to earth. The output of the secondary coil 5 of the transformer 4 is typically at a lower level than the input voltage and may be, for example, 50 VRMS or less.

The transformer 4 not only serves to transform the input voltage to a lower voltage as described above, but also serves, effectively, to insert an inductance in the circuit in series with its secondary coils. This effective inductance does not physically exist as a separate component, but the inductance is provided as a result of the construction of the transformer, which causes a defined amount of flux leakage. The construction of a typical transformer will be described below with reference to Figure 6.

A capacitor 8 is connected across the secondary coil 5 of the transformer, and consequently the secondary coil of the transformer, together with the effective inductance provided thereby, and the capacitor 8, define a resonant circuit. The resonance frequency of the resonance circuit is chosen to be equal to or just below the output frequency of the converter 2.

It is to be understood that any resistive load connected between the output terminals of the transformer 4 will conduct a substantially constant current for values of resistance from zero up to some predetermined upper limit. The upper limit is determined by various factors including a degree of similarity between the output frequency of the converter 2 and the resonance frequency of the resonance circuit constituted by the secondary coil of the transformer, and the inductance defined thereby together with the capacitor 8.

As shown in Figure 1, the secondary coil of the transformer 5 is connected to a series connection comprising a plurality of low voltage incandescent lamps 9,10,11,12. Whilst four lamps are shown, any number of lamps may be present in the series connection.

Connected in parallel with each individual lamp is a respective voltage operated shorting device 13,14,15,16. As will be described in more detail hereinafter, each voltage operated shorting device is so devised that in the event that the incandescent lamp, with which it is connected in series, should fail and go'; open circuit", the shorting device will go"short circuit"and consequently the series connection of the lamps will not be broken although, of course, the lamp that has failed will no longer be illuminated.

It is also possible to incorporate in the series connection of the lamps individual switches, such as the switch 17 which can be closed to provide a short circuit across one lamp, which will enable that one lamp to be extinguished simply by closing the relevant switch. Of course, it is also possible to incorporate a switch such as the switch 18 which provides a short circuit across a plurality of lamps, thus permitting the plurality of lamps to be extinguished simply by closing the switch 18 whilst leaving the remaining lamps within the circuit illuminated. Thus the lamps within the series circuit may be controlled individually or in groups.

Referring now to Figure 2, the constant current feed principle will be described. In Figure 2, a voltage generator 20 is illustrated adapted to produce an alternating output of a predetermined frequency is shown together with a separate inductor 21. A capacitor 22 is connected across the combination of the voltage generator 20 and the inductor 21. A load shown as a resistor 23 is connected across the capacitor 22. The voltage generator 20, inductor 21 and capacitor 22 defined a resonant circuit having a specific resonance frequency.

If the voltage generator operates with a frequency which is near, typically above, the resonance frequency of the resonant circuit, the load current, that is to say the current flowing through the resistor 23, will be constant for a range of values of the resistor from zero upwards. The constant current is nominally the current that will flow when the value of the resistor 23 is zero. In such a situation, the capacitor 22 will pass no current.

Referring now to Figure 3, a voltage operated shorting device which may be used in the arrangement of Figure 1 is illustrated. The device incorporates two leads 30,31, which are to be connected to the series circuit on opposite sides of an incandescent lamp. Connected between the leads 30,31 is a triac 32. The controlled current path of the triac 32, between main terminal one and main terminal two, which is normally non-conductive, is connected to the leads 30,31. The gate of the triac 32 is connected to a node 33. The node 33 is connected by the parallel connection of a resistor 34 and a capacitor 35 to the lead 30. The node 33 is also connected by the series connection of a reverse biased zener diode 36, and a diode 37 to the lead 31.

In ordinary use of the apparatus described incorporating a voltage operated shorting device as illustrated in Figure 3, the controlled current path between main terminal 1 and main terminal 2 of the triac 32 will be nonconductive. A voltage equivalent to the operating voltage of the incandescent lamp will be present across the leads 30,31. In the event of failure of the incandescent lamp the voltage across the leads 30,31 will rise. The increased voltage will be applied to the series connection of the resistor 34, the reverse biased zener diode 36 and the diode 37. The voltage will be of such a magnitude that the reverse biased zener diode will break down, permitting current to flow. Current thus flows through the series connection of the resistor 34, the reverse biased zener diode 36 and the diode 37, and a potential is generated across the resistor 34. The potential may be slow to develop as capacitor 35 must be charged. When the potential develops, the potential present on the node 33 is not the same as the potential applied to the main terminal 2 of the triac. As a result the triac becomes conductive, thus providing a"short circuit"between the leads 30 and 31. The need to charge the capacitor 35 to develop the potential reduces the risk of the triac being rendered conductive in response to spurious signals.

The triac is thus rendered conductive in response to a current flow between the leads 30,31 through the zener diode 36 that has broken down.

Triacs are relatively slow to return to the non-conductive state, and thus the triac remains conductive even thought the current flow repeatedly falls to zero.

Also the potential that is stored in the capacitor 35 connected in parallel with the resistor 34 will help to maintain the potential that is present on the node 33, thus also serving to maintain the triac in a conductive state. Thus the triac only needs to be triggered once at lamp failure.

Figure 4 illustrates a second type of voltage operated shorting device. In the embodiment of Figure 4, two leads 40,41 are provided adapted to be connected to the opposed sides of an incandescent lamp in the series connection of incandescent lamps of Figure 1. The leads 40,41 are connected to a series connection of two zener diodes 42,43. The diodes are connected in series in the opposite sense so that the forward direction of one diode is opposed to the forward direction of the other diode.

Connected in parallel with the two diodes 42,43 is a switch constituted by the controlled current paths of two MOSFET transistors 44,45. The two sources of the MOSFET transformers are connected to a central node 46. The node 46 is connected by a resistor 47 to second node 48. The node 48 is connected to the two gates of the MOSFET transistors 44 and 45 and is also connected by means of a diode 49 to one end of a coil 50 which is wound around a part of an annular ferrite core 51, the other end of the coil 50 being connected to the node 46. The coil 50 is thus in a series connection with the resistor 7 and the diode 49. The lead 41 passes through the ferrite core 51.

In ordinary use of the voltage operated shorting device of Figure 4, the incandescent lamp connected between the leads 40 and 41 is operational, and the voltage drop between the leads 40 and 41 is not sufficient to break down either of the zener diodes 42 or 43. In the event of failure of the incandescent lamp, however, the potential applied across the leads 40 and 41 rises, which, on each path cycle of operation, causes one of the zener diodes 42 or 43 to break down. Thus an alternating current is initiated which flows through the leads 40 and 41. Since the current flows through the part of the lead 41 that passes through the annular ferrite core 51, flux is generated within that core which creates a current flow within series connection that incorporates the coil 50.

The current flow within the series connection is uni-directional because the outputs of the coil 50 are connected in series with the diode 49. The unidirectional current flow creates a potential across the resistor 47, leading to a potential at the node 48, which is higher than the potential of the sources of the MOSFETS 44,45. The potential at the node 48 is applied to the gates of the MOSFETS 44 and 45, thus rendering the controlled current path of each MOSFET, i. e. the source-drain path, conductive. The gates of the MOSFETS have a certain inherent capacitance, and the MOSFETS are retained in a conductive state even though the uni-directional current flow through the coil 50 and the associated diode 49 is effectively a half-wave current.

It is to be appreciated that the arrangement shown in Figure 4 is"selflocking"since once the MOSFETS 44 and 45 are conductive, a current will flow through the part of the lead 41 passing through the annular ferrite core 51, thus maintaining a current flow through the coil 50 and maintaining the potential at the node 48 that is applied to the gates of the MOSFETS to keep the controlled current paths of the MOSFETS conductive.

Referring now to Figure 5, it is possible to use a low-cost mechanical relay in place of the switch constituted by the MOSFETS IN the embodiment described above. Thus, in the embodiment of Figure 5, leads 40 and 41 are present, as described above, together with the associated zener diodes 42,43.

Again the lead 41 passes through an annular ferrite core 51 which is associated with a coil 50. The coil 50 in the described embodiment is connected in a series connection with a diode 49 and a capacitor 52. A mechanical relay 53 is provided, the operating coil of which is connected across the capacitor 52. The controlled switch 54 of the relay is connected between the lead 40 and the lead 41. The switch is normally open, forming a normally non-conductive connection between the leads 40,41. The switch 54 is adapted, when closed, to form a short circuit between those leads.

It is to be appreciated that in operation of the embodiment illustrated in Figure 5, in the event of failure of a lamp connected between the leads 40 and 41, the diode 42 or the diode 43 will break down, in the manner described above, causing an initial current to flow through the part of the lead 41 passing though the ferrite core 51. A current is thus generated in the coil 50. The current is a uni-directional current because of the presence of the diode 49 connected in series with the coil 50, but the current will serve to charge up the capacitor 52. The potential generated across the capacitor 52 is applied to the coil of the relay 53. The relay is thus actuated to close the switch 54, thus providing a short circuit between the lead 40 and the lead 41.

Figure 6 of the accompanying drawings is an exploded view of the transformer 4 of the arrangement shown in Figure 1. The transformer 4 is provided with a ferrite core of a particular configuration. In the embodiment illustrated, the ferrite core is constituted by two core components 60,61 which are of the same form. The component 61 will be described. The component 61 comprises a rectangular plate or slab of ferrite material 62 provided with two up-standing posts 63,64 of cylindrical form. The posts are illustrated as being of circular cross-section, but may be of other alternative cross-section, such as a square cross-section. The posts are located at opposed ends of the slab 62.

The diameter of each post 63,64 is substantially less than the width of the slab 62. Consequently the slab 62 extends laterally beyond the posts 63,64. Preferably the width of the slab 62 is substantially three times the diameter of the posts 63,64, but the ratio of widths to diameter may be in the range 21/2 to 31/2, or even in the range H4 to 5.

The element 61 is adapted to be located in contact with the element 60, with the posts 63,64 of the element 61 abutting the corresponding posts of the element 60, the slab 62 of the element 61 thus being spaced away from the corresponding slab of the element 60. The primary coil 3 of the transformer is wound around one abutted set of posts, incorporating the post 63, and the secondary coil 5 of the transformer 6 is wound around the other pair of posts, including the post 64.

It is preferred that the width of the slab 62 should be at least equal to, or preferably slightly larger than, the external diameter of the primary coil 3 and the secondary coil 5. This provides an ideal package with good coupling properties.

It is to be appreciated that in a transformer having a core of this configuration, there is a substantial flux leakage field, but the leakage field is almost totally contained with the outline of the transformer, the leakage field thus being present in the volume between the slabs of the element 61,62 in the region between the posts of the elements 61,62. Thus, in contrast with other types of transformer which have substantial leakage of flux, there is not a large external field.

The transformer of Figure 6 consequently has a first pair of parallel limbs, constituted by the aligned posts of the two elements on which are wound the coils of the transformer, with those limbs being inter-connected by interconnecting limbs constituted by the slabs located at the top and bottom of the transformer. The slabs are relatively large in surface area, and are relatively close together. These design feature help ensure that the leaked flux occurs within the body of the transformer. The leaked flux is thus not effected by the presence of any external conductors, thus ensuring that the transformer has uniform properties, regardless of its positioning, and making the transformer substantially unresponsive to the presence of conductive items outside the transformer.

It is to be appreciated, that in the arrangement of the invention it is essential for the inductance provided by the transformer, which is dependent upon the leakage of the transformer, to be precisely known, since otherwise it would be impracticable to achieve the desired resonance frequency between the secondary coil 5 of the transformer and the capacitance 8.

Utilising a transformer of the type illustrated in Figure 6, it has been found possible to produce a high quality inductance at low cost. The transformer may be contained within a conductive housing or casing 65, indicated in phantom. The casing will eliminate directly radiated EMI and will help in maintaining the stability of the leakage inductance value wherever the power supply is located. Conventional transformers with high leakage have a large external magnetic field and cannot be packaged in this way.

Claims

CLAIMS: 1. A power supply for connection to a series connection of a plurality of resistive loads, the power supply comprising a transformer having a primary coil adapted to be connected to a source of alternating current of a predetermined frequency, and having a secondary coil adapted to be connected to the said series connection, there being a capacitor connected across said secondary coil, the transformer having a predetermined flux leakage to provide the effect of an inductance in series with the output coil of the transformer, the effective inductance being of such a magnitude that the resonant circuit constituted by the secondary coil of the transformer and the associated effective inductance and the said capacitor is of a magnitude which is substantially equal to the said predetermined frequency of the said source of alternating current.
2. A power supply according to Claim 1 incorporating a converter connected to the input coil of the transformer, adapted to be connected to an external electricity supply, the converter being adapted to provide, to the input coil of the transformer, said alternating current of said predetermined frequency and of a predetermined waveform.
3. A power supply according to Claim 2 wherein said predetermined waveform is a square waveform.
4. A power supply according to Claim 2 wherein the predetermined waveform is a substantially sinusoidal waveform.
5. A power supply according to any one of the preceding Claims wherein the transformer comprises a first pair of limbs receiving respectively the primary coil and the secondary coil of the transformer, said limbs being interconnected by inter-connecting limbs, the inter-connecting limbs each having a large surface area and being located adjacent each other.
6. A power supply according to Claim 5 wherein the inter-connecting limbs are each of slab-like form, having a width greater than the diameter of each limb of said first pair of limbs, the first pair of limbs being located at opposite ends of the inter-connecting limbs.
7. A power supply according to Claim 6 wherein the width of the interconnecting limbs is from 1/2 to 5 times as wide as the diameter of each limb of the first pair of limbs.
8. A power supply according to Claim 6 wherein the width of the interconnecting limbs is from 21/2 to 31/2 times as wide as the diameter of each limb of the first pair of limbs.
9. A power supply according to Claim 6 wherein the width of the interconnecting limbs is substantially three times as wide as the diameter of each limb of the first pair of limbs.
10. A power supply according to any one of Claims 6 to 9 wherein the width of the inter-connecting limbs is at least equal to the external diameter of said primary and secondary coils.
11. A power supply according to any one of the preceding Claims wherein the predetermined frequency is between 20 and 200 kHz.
12. A power supply according to any one of the preceding Claims wherein the predetermined frequency is between 20 and 200 kHz.
13. A power supply according to any one of the preceding Claims wherein the transformer is contained within a conductive housing.
14. A power supply according to any one of the preceding Claims connected to a series connection of a plurality of resistive loads.
15. A power supply according to Claim 14 wherein each resistive load is an incandescent lamp.
16. A power supply according to Claim 14 or 15 wherein each resistive load is connected in series with a voltage operated shorting device, adapted to provide a short circuit in the event that the resistive load should go open circuit.
17. A power supply according to Claim 16 wherein the voltage operated shorting device comprises two leads connected to the series connection of resistive loads on opposed side of the said relevant resistive load, the leads being inter-connected by a connection incorporating at least one device adapted to break down when subjected to a predetermined potential, and also being inter-connected by an initially non-conductive connection between the two leads, means being provided, responsive to a current flow between the leads through said element which is adapted to break down, to render conductive said initially non-conductive connection between the two leads.
18. A power supply according to Claim 17 wherein the element adapted to break down comprises a zener diode.
19. A power supply according to Claim 18 wherein the normally nonconductive connection comprises the controlled current path of a triac between main terminal one and main terminal two of the triac, and wherein the zener diode is connected to one said lead by a diode, and to the other said lead through a resistor, the node between the zener diode and the resistor being connected to the gate of the triac.
20. A power supply according to Claim 17 wherein the normally nonconductive connection comprises a controlled switch, one of said leads being coupled, by an appropriate core, to a coil such that current flowing through the lead provides a current through the coil, said switch being adapted to be closed in response to current flowing through the coil.
21. A power supply according to Claim 20 wherein the connection incorporating said device adapted to break down comprises two oppositely orientated zener diodes.
22. A power supply according to Claim 20 or 21 wherein the coil is in a series circuit with a diode and a resistor, the node between one terminal of the resistor and the diode being connected to the gates of two field effect transistors that constitutes said switch, the other terminal of the resistor being connected to the two sources of the field effect transistors, the drains of the field effect transistors being connected respectively to the two leads.
23. A power supply according to Claim 20 or 21 wherein the coil is connected to a series circuit containing a diode and a capacitor, there being a relay that constitutes said switch, the relay having an operating coil connected across the capacitor, the relay incorporating a mechanically operable switch which constitutes the normally non-conductive connection between the two leads.
24 A power supply substantially as herein described with reference to and a shown in Figures 1 and 6 of the accompanying drawings.
25 A power supply substantially as herein described with reference to and a shown in Figures 1 and 6 of the accompanying drawing, as modified by Figure 3.
26. A power supply substantially as herein described with reference to and a shown in Figures 1 and 6 of the accompanying drawing, as modified by Figure 4.

27 A power supply substantially as herein described with reference to and a shown in Figures 1 and 6 of the accompanying drawing, as modified by Figure 5.

28 Any novel feature or combination of features disclosed herein.