WO2011051846A1

WO2011051846A1 - Driving an electrodeless discharge lamp

Info

Publication number: WO2011051846A1
Application number: PCT/IB2010/054576
Authority: WO
Inventors: Renatus Willem Clemens Van Der Veeken
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-10-28
Filing date: 2010-10-11
Publication date: 2011-05-05

Abstract

An electrodeless discharge, ED, lamp is driven by a driver circuit at a predetermined high frequency, f. The driver circuit comprises DC power input terminals and lamp output terminals to supply power to a coupling coil of the ED lamp. A switching circuit comprises a series arrangement of a switching device and an inductor, and the lamp output terminals are coupled across the switching device. A shunt capacitor is coupled across the switching device, and has a shunt capacitor capacitance, C1a. The driver circuit is coupled to the ED lamp by a transmission line having a transmission line inductance, L2a, and a transmission line capacitance, C1b, and through a compensating series capacitor having a compensating capacitor capacitance, C3a. The shunt capacitor capacitance, C1a, is matched to the transmission line capacitance, C1b, to obtain a designed shunt capacitance, C1= C1a + C1b, across the switching device. The compensating capacitor capacitance, C3a, is matched to the transmission line inductance, L2a to obtain a designed compensating capacitance, C3, having an impedance of 1/ωC3 = -ωL2a + 1/ωC3a, at the predetermined high frequency.

Description

DRIVING AN ELECTRODELESS DISCHARGE LAMP

FIELD OF THE INVENTION

The present invention relates to the field of lamp driver circuits, and more specifically to driver circuits for an electrodeless discharge lamp. The invention further relates to a lighting system comprising a driver circuit and an electrodeless discharge lamp.

BACKGROUND OF THE INVENTION

Inductively coupled electrodeless discharge, ED, lamps (also referred to as electrodeless fluorescent lamps, EFLs, or electrodeless high intensity discharge, HID, lamps) usually comprise a discharge vessel containing a specific mixture of gases and salts, and an antenna, also referred to as coupling coil, consisting of one or more turns of an electrical conductor wound around the discharge vessel.

Efficient power generation for driving an electrodeless discharge lamp is offered by a power driver circuit having a switching-mode operation of RF power converters.

The driver circuit is connected to the coupling coil to couple electromagnetic energy into the discharge vessel. The electromagnetic energy excites the mixture in the discharge vessel, and causes the lamp to emit light. The coupling coil is fed by the driver circuit with a high frequency (radio frequency, RF) current. The supply current frequency may be selected in the

Industrial-Scientific-Medical (ISM) bands (e.g., 13.56 MHz) because of the relatively high emission limits allowed by the governing bodies.

An ignition of the electrodeless discharge lamp may be facilitated by adding an ignition appendix to the lamp construction. The ignition appendix is filled with a relatively low-pressure gas and is therefore easily ignitable.

At the high frequencies used for operating the ED lamp, wave-effects appear, and special measures need to be taken to transport energy from the driver circuit to the ED lamp. Since ED lamps have a very high efficiency, a loss in the driver circuit and a loss of energy in the transport of energy from the driver circuit to the ED lamp needs to be prevented as much as possible to take an optimum advantage of the high efficiency of the ED lamp.

ED lamps represent highly inductive loads which lead to a high quality factor.

Therefore, in order for the driver circuit to be able to deliver sufficient active power to the load, an impedance matching network is necessary to match the highly inductive ED lamp load to an optimum impedance expected by the driver circuit. RF driver circuits are usually designed for 50 Ohm standard load matching, which is convenient for measurements and cabling. However, for driving an ED lamp, it is not necessary an advantage.

Since the driver circuit and the ED lamp usually need to be spaced apart to avoid undesired thermal and electromagnetic al interaction between the two, a wiring between the driver circuit and the ED lamp is necessary. Using simple wiring between the driver circuit and the ED lamp will lead to a considerable loss of energy in the transport of energy from the driver circuit to the ED lamp. Here, a proper matching of the ED lamp to the driver circuit is critical.

At high frequencies, the electromagnetic coupling between wires, which connect the ED lamp to the driver circuit, and a luminaire containing the lamp, becomes significant. As an example, at a lamp voltage of 1 kV, and a typical capacitance between the wires and the luminaire which can be represented by a parasitic capacitor of 20 pF, an electric current of 1.75 A will flow through the parasitic capacitor at 13.56 MHz. Such a reactive current reduces the efficiency of the lighting system as a whole.

Furthermore, the position of the wiring usually is not fixed, and also for this reason the performance of the lighting system which comprises the driver circuit and the ED lamp, is sub-optimum. The positioning of the wiring is difficult to control, which means that the matching of the driver circuit and the ED lamp is cumbersome and consequently costly. It is further noted that the wiring needs to be isolated to withstand the high voltage in the lighting system, which further adds to the cost of the system.

SUMMARY OF THE INVENTION

It would be desirable to provide a driver circuit for operating an electrodeless discharge lamp with optimum matching of the driver circuit and the lamp, having a predetermined length of wiring between the driver circuit and the lamp. It would also be desirable to provide a driver circuit for operating an electrodeless discharge lamp through a predetermined length of wiring creating at low losses.

To better address one or more of these concerns, in a first aspect of the invention a driver circuit for driving an electrodeless discharge, ED, lamp at a predetermined high frequency, f, is provided, the driver circuit comprising:

power input terminals configured for receiving a DC power supply; lamp output terminals configured to supply power to a coupling coil of the ED lamp;

a switching circuit coupled between the input terminals and the output terminals, the switching circuit comprising a series arrangement of a switching device and an inductor, the lamp output terminals being coupled across the switching device;

a shunt capacitor coupled across the switching device, the shunt capacitor having a shunt capacitor capacitance, CI a,

wherein the driver circuit is adapted to be coupled to the ED lamp by a transmission line having a transmission line inductance, L2a, and a transmission line capacitance, Clb, and through a compensating capacitor having a compensating capacitor capacitance, C3a, coupled in series with the ED lamp,

wherein the shunt capacitor capacitance, CI a, is matched to the transmission line capacitance, Clb, to obtain a designed shunt capacitance, CI = Cla + Clb, across the switching device, and

wherein the compensating capacitor capacitance, C3a, is matched to the transmission line inductance, L2a to obtain a designed compensating capacitance, C3, having an impedance of l/coC3 = -coL2a + l/coC3a, at the predetermined high frequency, f, with ω = 2πΐ.

According to the present invention, electrical components in the driver circuit are split and rearranged. Groups of these electrical components are identified as an equivalent model of a transmission line. These groups of components in the electrical network of the lighting system can be replaced by defined transmission lines. This allows to connect an electrodeless lamp at some physical distance to a driver circuit without distorting a matching and the performance of the lighting system.

In a second aspect of the invention, wherein the driver circuit is a balanced driver circuit, the driver circuit comprises:

a first switching leg and a second switching leg arranged in parallel between the power input terminals, wherein the first switching leg comprises a series arrangement of a first switching device and a first inductor having a common first node, wherein the second switching leg comprises a series arrangement of a second switching device and a second inductor having a common second node, and wherein the lamp output terminals are coupled between the first node and the second node; a first shunt capacitor coupled across the first switching device, the first shunt capacitor having a first shunt capacitor capacitance, CI a;

a second shunt capacitor coupled across the second switching device, the shunt capacitor having a second shunt capacitor capacitance, C2a;

wherein the first switching leg of the driver circuit is adapted to be coupled to the ED lamp by a first transmission line having a first transmission line inductance, L2a, and a first transmission line capacitance, Clb,

wherein the second switching leg of the driver circuit is adapted to be coupled to the ED lamp by a second transmission line having a second transmission line inductance, L2b, and a second transmission line capacitance, C2b,

wherein the first and the second switching legs of the driver circuit further are adapted to be coupled to the ED lamp through a compensating capacitor having a

compensating capacitor capacitance, C3a, coupled in series with the ED lamp,

wherein the first shunt capacitor capacitance, CI a, is matched to the first transmission line capacitance, Clb, to obtain a designed first shunt capacitance, CI = Cla +

Clb, across the first switching device,

wherein the second shunt capacitor capacitance, C2a, is matched to the second transmission line capacitance, C2b, to obtain a designed second shunt capacitance, C2 = C2a + C2b, across the first switching device, and

wherein the compensating capacitor capacitance, C3a, is matched to the first transmission line inductance, L2a, and the second transmission line inductance, L2b, to obtain a designed compensating capacitance, C3, having an impedance of l/coC3 = -coL2a -coL2b + l/coC3a, at the predetermined high frequency, f, with ω = 2πΐ.

In a third aspect of the present invention, a lighting system is provided comprising the driver circuit according to the present invention, coupled to an ED lamp through at least one transmission line having a transmission line inductance and a

transmission line capacitance. The ED lamp comprises a coupling coil connected between two lamp terminals, and a series capacitor is arranged in series between the coupling coil and at least one of the lamp terminals

According to this aspect of the invention, high voltages across parasitic capacitors are avoided by placing an extra capacitor very close to the lamp, and making a compact lamp unit from the combination of the capacitor and the lamp. The physical dimensions of such a lamp unit can be made very small compared to the wavelength of the signals in the lighting system. Furthermore, when the lighting system is balanced, the influence of the parasitic elements in the lighting system can be reduced even further.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a circuit diagram of an embodiment of a lighting system comprising a driver circuit and an ED lamp.

Figure 2 depicts an embodiment of a lighting system according to the present invention comprising a driver circuit connected to an ED lamp by wiring.

Figure 3 depicts an embodiment of a lighting system according to the present invention comprising a driver circuit connected to an ED lamp by transmission lines.

Figure 4 depicts a part of a lighting system comprising a part of a driver circuit connected to an ED lamp provided in a luminaire.

Figure 5 depicts a part of a lighting system according to the present invention comprising a part of a first type of driver circuit connected to an ED lamp provided in a luminaire.

Figure 6 depicts a part of a lighting system according to the present invention comprising a part of a second type of driver circuit connected to an ED lamp provided in a luminaire. DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts a basic topology of a lighting system comprising a driver circuit and an electrodeless discharge, ED, lamp 100 modeled as a series arrangement of a lamp inductor 110 and a lamp resistor 112. The ED lamp 100 has lamp terminals 113, 114 which are terminals of a coupling coil of the ED lamp 100. The driver circuit comprises power input terminals 121, 122 for supplying DC power to the driver circuit. The driver circuit comprises a first switching leg having a first inductor 131 arranged in series with a parallel arrangement of a first switching device 132, a first diode 133 (which may represent an intrinsic switching device diode and/or an extrinsic diode) and a first shunt capacitor 134. The driver circuit further comprises a second switching leg having a second inductor 141 arranged in series with a parallel arrangement of a second switching device 142, a second diode 143 (which may represent an intrinsic switching device diode and/or an extrinsic diode) and a second shunt capacitor 144. The first switching leg and the second switching leg of the driver circuit, representing a push-pull class E converter, are arranged in parallel between the power input terminals 121, 122. A common node 124 of the first inductor 131, the first switching device 132, the first diode 133 and the first shunt capacitor 134 is coupled to the lamp terminal 113, while a common node 125 of the second inductor 141, the second switching device 142, the second diode 143 and the second shunt capacitor 144 is coupled to the lamp terminal 114. The first and second switching devices 132, 142 may be metal oxide

semiconductor field effect transistors, MOSFETs, having a drain coupled to the first and second inductor 131, 141, respectively, and having a source coupled to the power input terminal 122. An anode of the first and second diodes 133, 143 is coupled to the power input terminal 122, while a cathode of the first and second diodes 133, 143 is coupled to the common nodes 124, 125, respectively. The first and second switching devices 132, 142 are driven to be conducting alternatingly (in anti -phase).

The lighting system comprises a compensating capacitor 150 having a capacitance C3 to largely compensate the high inductance LI of the coupling coil

(represented by the lamp inductor 110) of the ED lamp 100. In effect, the impedance of the lamp inductor 110 at a high frequency, e.g. 13.56 MHz, is reduced by the compensating capacitor 150. The compensating capacitor 150 may be integrated with the ED lamp 100, or may be integrated with the driver circuit.

It is noted that the compensating capacitor 150 may be implemented by two partial capacitors each having twice the capacitance of the compensating capacitor 150, and each arranged in series with the ED lamp 100, e.g. one at the lamp terminal 113, and the other one at the lamp terminal 114. It is also noted that such partial compensating capacitors may be integrated with the ED lamp 100, arranged in series with the coupling coil 110, between the lamp terminals 113, 114. It is also noted that one such partial compensating capacitor need not have the same value as the other one, as long as the combined capacitance of the partial compensating capacitors is the same as the capacitance of the compensating capacitor 150. It is further noted that the partial capacitors may be arranged in series between one of the legs of the driver circuit and the corresponding lamp terminal 113, 114, where one of the partial capacitors is integrated with the driver circuit, and the other one of the partial capacitors is integrated with the ED lamp 100. Also, a combination of both alternatives of partial capacitors may be implemented resulting in four partial capacitors each having four times the capacitance of the compensating capacitor 150, where one pair of such partial capacitors is arranged at the lamp terminal 113, and the other pair of such partial capacitors is arranged at the lamp terminal 114, while one partial capacitor of each pair of partial capacitors is integrated with (a leg of) the driver circuit, and the other partial capacitor of each pair of partial capacitors is integrated with the ED lamp 100.

In the topology shown in Figure 1, the driver circuit and the ED lamp 100 must be very close to each other to avoid unwanted effects (wave or antenna effects resulting in energy losses) of wires and cables between the driver circuit and the ED lamp 100 at high frequencies. Such wires and cables are located between compensating capacitor 150 and lamp terminal 113 (assuming that compensating capacitor 150 is integrated in the driver circuit), and between lamp terminal 114 and second shunt capacitor 144.

Figure 2 depicts an embodiment of the invention in which the driver circuit and the ED lamp 100 may be spaced apart at a distance which is convenient in practice, while reducing wave or antenna effects resulting in energy losses to a minimum. Also, the electrical performance of the lighting system is kept the same as in Figure 1.

According to Figure 2, the first and the second shunt capacitors 134, 144 each are split up. The first shunt capacitor 134 having a capacitance CI is split up into a first partial first shunt capacitor 134a having a capacitance CI a, and a second partial first shunt capacitor 134b having a capacitance Clb, where the first partial first shunt capacitor 134a and the second partial first shunt capacitor 134b are arranged in parallel. Likewise, the second shunt capacitor 144 having a capacitance C2 is split up into a first partial second shunt capacitor 144a having a capacitance C2a, and a second partial second shunt capacitor 144b having a capacitance C2b, where the first partial second shunt capacitor 144a and the second partial second shunt capacitor 144b are arranged in parallel. For the capacitances CI, Cla, Clb, C2, C2a, C2b the following relationships according to equations (1) - (2) are valid:

CI = Cla + Clb equation (1) C2 = C2a + C2b equation (2) Furthermore, a first partial inductor 160a having an inductance L2a, and a second partial inductor 160b having an inductance L2b are added. The first partial inductor 160a is arranged in series with the lamp terminal 113, and the second partial inductor 160b is arranged in series with the lamp terminal 114. A modified compensating capacitor 150a has a capacitance C3a. At the operating frequency, f, of the driver circuit, the following relationship according to equation (3) is valid, determining a capacitance C3 of the compensating capacitor 150: l/coC3 = -coL2a -coL2b + l/coC3a

with ω = 2-7 f ... equation

(3)

In Figure 2, combinations of lumped elements can be recognized, as indicated with dashed-dotted lines. A first combination 181 of second partial first shunt capacitor 134b and partial inductor 160a, and a second combination 182 of second partial second shunt capacitor 144b and partial inductor 160b both may be regarded as a representation of a part of a transmission line with the same electrical properties. As a result, the circuit diagram of Figure 2 may be redrawn as shown in Figure 3.

In Figure 3, the first combination 181 at the lamp terminal 113 has been replaced with a first transmission line 191 with a predetermined length. Likewise, the second combination 182 at the lamp terminal 114 has been replaced with a second transmission line 192 with a predetermined length which is not necessarily the same as the length of the transmission line 191. Since transmission lines (which may e.g. consist of parallel wires, or coaxial cables) are characterized by a capacitance per unit length (e.g. 100 pF/m) and an inductance per unit length (e.g. 250 nH/m), a predetermined length of the first transmission line 191 at the lamp terminal 113 corresponds with a capacitance Clb of the second partial first shunt capacitor 134b and an inductance L2a of the partial inductor 160a, while a predetermined length of the second transmission line 192 at the lamp terminal 114 corresponds with a capacitance C2b of the second partial second shunt capacitor 144b and an inductance L2b of the partial inductor 160b. From a known or selected or designed capacitance Clb, determined by a length of the first transmission line 191, the capacitance Cla of the first partial first shunt capacitor 134a can be determined by equation (1) above. Likewise, from a known or selected or designed capacitance C2b, determined by a length of the second transmission line 192, the capacitance C2a of the first partial second shunt capacitor 144a can be determined by equation (2) above. Next, the capacitance C3a of the modified compensating capacitor 150a can be determined or designed by equation (3) above, on the basis of a designed capacitance C3.

As long as the length L of the transmission lines 191, 192 fulfils the relationship L/λ < about 0.1, where λ is the wavelength of the high frequency signal generated by the driver circuit, the modeling of the combinations 181, 182 by the transmission lines 191, 192, respectively, remains valid. As an illustration, at a frequency of 13.56 MHz, λ ¾ 15 m in a coaxial cable. This implies that cables (transmission lines) having a length up to about L/10

= 15/10 = 1.5 m can be applied in the lighting system.

It is noted that the first transmission line 191 need not be identical to the second transmission line 192. The first and second transmission lines 191 and 192 may differ both in electrical characteristics and in physical dimensions. They may e.g. have different lengths. Consequently, the capacitance Cla of the first partial first shunt capacitor 134a, and the capacitance C2a of the first partial second shunt capacitor 144a need not be the same.

With reference to Figures 1-3, it is noted that the driver circuit need not comprise a switching circuit comprising a first switching leg and a second switching leg. The switching circuit may also comprise one switching leg. Referring to Figures 2 and 3, in the case of the switching circuit comprising one switching leg, the lamp terminal 114 may be directly connected to ground, and the components 141, 142, 143, 144a, 144b and 160b are omitted.

In accordance with the above, the present invention allows a proper matching of a driver circuit and an ED lamp which are coupled using transmission lines, to a large extent (in practice) irrespective of the electrical characteristics or the physical dimensions of the transmission lines, as long as they are predetermined.

In practice, an ED lamp 100 operated at high frequencies is to be placed in a metallic housing, also indicated as a luminaire, to achieve electromagnetic compatibility, EMC, and to avoid electromagnetic interference, EMI, problems. The luminaire surrounds the ED lamp at least partially, with the exception of at least a window through which the light generated by the ED lamp is transmitted from the luminaire. As a consequence of the metallic structure of the luminaire, the lighting system will experience a capacitive (and some inductive) coupling with the luminaire. Figure 4 illustrates this phenomenon.

Figure 4 shows a basic topology of a lighting system coupled to a power source 400 having output terminals 402, 404. The power source 400 is configured to supply an alternating voltage U and an alternating current I of a high frequency (e.g. 13.56 MHz) at the output terminals 402, 404. An ED lamp 100, modeled as a series arrangement of lamp inductor 110 and lamp resistor 112, has lamp terminals 113, 114. Wires 410, 412 extend between an output terminal 402 and a lamp terminal 113, and between an output terminal 404 and a lamp terminal 114, respectively. A capacitive coupling of the ED lamp 100 to the luminaire is modeled as luminaire coupling capacitors 420, 422 having a capacitance C4a, C4b, respectively. The wire 412 is connected to ground, and the wire 410 has a capacitance C5 to ground, modeled as a ground coupling capacitor 424. The capacitances C4a, C4b usually are much smaller than capacitance C5, e.g. a factor of five less.

In a practical lighting system, the voltage U generated by the power source 400 can be as high as 1 kV at 300 W. Such a high voltage is required due to the relatively high inductance, typically 2μΗ, of the lamp inductor 110. With a typical capacitance of 20 pF of the ground coupling capacitor 424, a current of about 1.7 A has been measured to flow in the 'parasitic' ground coupling capacitor 424 at 13.56 MHz. This current magnitude is significant in the design of the lighting system, since it lowers the efficiency of the driver circuit and the lighting system, and causes problems of isolation. The wires 410, 412 must be kept away from the (metallic parts of the) luminaire, and expensive wires must be applied.

Referring to Figure 5, depicting an embodiment of the present invention, a series capacitor 500 is arranged in series between a lamp terminal 113 and the coupling coil 110 of the ED lamp 100. In this embodiment, the series capacitor 500 is placed very close to the ED lamp 100. Accordingly, a lamp unit 510 may be constructed, e.g. by placing the series capacitor 500 in a base of the ED lamp, or by placing the series capacitor 500 in an outer vessel of the ED lamp, where the outer vessel surrounds the discharge vessel and the coupling coil 110 provided on the discharge vessel.

Referring to Figures 2 and 3, the series capacitor 500 may represent the modified compensating capacitor 150a. As a result of the inclusion of the series capacitor 500 in the lamp unit, the voltage U across the connecting wires 410, 412 is reduced considerably, e.g. with a factor of at least three. The effect of series capacitor 500 is that it provides a voltage compensation. Consequently, less insulation is needed for the wires 410, 412, since a high voltage is only present at a node 520 interconnecting the series capacitor 500, the coupling coil 110, and the luminaire coupling capacitor 420. A high voltage at node 520 is relatively easy to master since it is confined to a small, defined place. A further result of the inclusion of the series capacitor 500 is that the reactive current in the ground coupling capacitor 424 is reduced substantially, e.g. with a factor of at least three. The lower voltage U leads to this lower reactive current, thereby lowering losses in the wires 410, 412 squared. This means that the influence of parasitic elements, such as the ground coupling capacitor 424, on the lighting system is reduced. It is noted that this result is achieved without changing the wiring between the driver circuit and the lamp unit 510.

In order to keep the coupling between the luminaire and the wiring, as expressed by the luminaire coupling capacitors 420, 422, as small as possible, the outside dimensions of the lamp unit 510 should be smaller than λ/100, where λ is the wavelength of the high frequency signal provided to the lamp unit 510. Further, a resonance frequency of the luminaire coupling capacitor 420, 422 and the coupling coil 110 should be much higher than the operating frequency of the lighting system (as an example, with a representative capacitance of a luminaire coupling capacitor 420, 422 of 5 pF, and a representative inductance of the coupling coil 110 of 2 μΗ, a resonance frequency of 50 MHz results. A resonance frequency of the series capacitor 500 and the coupling coil 110 should be below this value. If, for example, a capacitance of the series capacitor 500 is chosen to be 70 pF, the resonance frequency of the series capacitor 500 and the coupling coil 110 will be about 13.5 MHz.

It is noted that the series capacitor 500 may be implemented as one series capacitor, as in the above embodiment, or as several partial series capacitors which together have a capacitance equal to a predetermined capacitance of the series capacitor 500 in another embodiment. Thus, for example, one partial series capacitor may be integrated with the lamp unit 510, and another partial series capacitor may be integrated with the driver unit, between the power source 400 and the output terminal 402. The first partial series capacitor does not need to have a capacitance equal to the capacitance of the second partial series capacitor. By applying partial series capacitors, costs may be saved. However, a reduction of the voltage U across the wires 410, 412 will be less.

In a further embodiment illustrated in Figure 6, a further reduction of the voltage U between the wires 410, 412, and the current I through ground coupling capacitor 424 can be achieved. For this purpose, the lighting system is balanced, and the ED lamp 100 electrically floats in the luminaire (which is grounded).

Figure 6 illustrates that the wire 410, through which a current II flows, has a capacitive coupling to ground, modeled by a ground coupling capacitor 610. A voltage Ul between the wire 410 and ground is applied. The wire 412, through which a current 12 flows, has a capacitive coupling to ground, modeled by a ground coupling capacitor 620. A voltage

U2 between the wire 412 and ground is applied. As in the embodiment shown in Figure 5, the ED lamp 100 is comprised in a lamp unit 510 (which may include at least one (possibly partial) series capacitor 500), where the lamp unit 510 is comprised in the luminaire 600, which is grounded.

In the embodiment of Figure 6, a power source is required which will deliver a voltage Ul and a voltage U2. Usually, the voltage U2 will have an opposite phase when compared to the voltage Ul. It is noted that both the voltage Ul and the voltage U2 attain only half of the amplitude when compared to the voltage U in Figure 5. This reduces a parasitic current through the ground coupling capacitors 610, 620. Furthermore, the insulation requirements of the wires in the luminaire 600 are reduced by a factor two. Balanced power sources are well-known, e.g. implemented as a combination of a single power source and a transformer having a balanced secondary winding, or by applying two E-class converters operating in opposite phases (cf. Figures 2, 3). It is noted that the phase difference between Ul and U2 may differ from 180 degrees to compensate for phase differences in the different paths (one path through wire 410, and the other path through wire 412) to the lamp unit 510.

Additionally or alternatively, Ul may differ from U2 in its amplitude.

In relation to Figure 5 and Figure 6, it is noted that the series capacitor 500 may be implemented as two partial series capacitors in series with each lamp terminal 113, 114. In this way, the lamp unit may be made completely symmetrical from an electrical point of view, where one of the partial series capacitors need not be equal to the other one of the partial series capacitors. The voltage stress across each partial series capacitor may be reduced by a factor of about two, which saves costs. As has been explained in detail above, an electrodeless discharge, ED, lamp is driven by a driver circuit at a predetermined high frequency, f. The driver circuit comprises DC power input terminals and lamp output terminals to supply power to a coupling coil of the ED lamp. A switching circuit comprises a series arrangement of a switching device and an inductor, and the lamp output terminals are coupled across the switching device. A shunt capacitor is coupled across the switching device, and has a shunt capacitor capacitance, CI a. The driver circuit is coupled to the ED lamp by a transmission line having a transmission line inductance, L2a, and a transmission line capacitance, Clb, and through a compensating series capacitor having a compensating capacitor capacitance, C3a. The shunt capacitor capacitance, CI a, is matched to the transmission line capacitance, Clb, to obtain a designed shunt capacitance, CI

= Cla + Clb, across the switching device. The compensating capacitor capacitance, C3a, is matched to the transmission line inductance, L2a to obtain a designed compensating capacitance, C3, having an impedance of l/coC3 = -coL2a + l/coC3a, at the predetermined high frequency.

The present invention applies to inductively coupled electrodeless discharge lamps, which may be applied when very high lumen output is required, such as in horticulture or other high-bay applications.

As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

Claims

CLAIMS:

1. A driver circuit for driving an electrodeless discharge, ED, lamp at a predetermined high frequency, f, the driver circuit comprising:

power input terminals configured for receiving a DC power supply;

lamp output terminals configured to supply power to a coupling coil of the ED lamp;

2. The driver circuit of claim 1, wherein the switching circuit comprises:

a first switching leg and a second switching leg arranged in parallel between the power input terminals, wherein the first switching leg comprises a series arrangement of a first switching device and a first inductor having a common first node, wherein the second switching leg comprises a series arrangement of a second switching device and a second inductor having a common second node, and wherein the lamp output terminals are coupled between the first node and the second node;

a first shunt capacitor coupled across the first switching device, the first shunt capacitor having a first shunt capacitor capacitance, CI a;

wherein the second switching leg of the driver circuit is adapted to be coupled to the ED lamp by a second transmission line having a second transmission line inductance,

L2b, and a second transmission line capacitance, C2b,

compensating capacitor capacitance, C3a, coupled in series with the ED lamp,

wherein the first shunt capacitor capacitance, CI a, is matched to the first transmission line capacitance, Clb, to obtain a designed first shunt capacitance, CI = Cla + Clb, across the first switching device,

3. The driver circuit of claim 1 or 2, wherein the compensating capacitor comprises at least a first partial compensating capacitor and a second partial compensating capacitor arranged in series, and wherein the first partial compensating capacitor is part of the driver circuit.

4. The driver circuit of any of the preceding claims, wherein the length, L, of a transmission line does not exceed 0.1 ·λ, wherein λ is a wavelength of a power supply signal.

5. A lighting system comprising the driver circuit of any of claims 1-4, coupled to an ED lamp through at least one transmission line having a transmission line inductance and a transmission line capacitance.

6. The lighting system of claim 5, wherein the ED lamp comprises a coupling coil connected between two lamp terminals, and wherein the compensating capacitor is arranged in series with the coupling coil between the lamp terminals.

7. The lighting system of claim 6, wherein the ED lamp comprises a coupling coil connected between two lamp terminals, wherein a first partial compensating capacitor is arranged in series between the coupling coil and one of the lamp terminals, wherein a second partial compensating capacitor is arranged in series between the coupling coil and the other one of the lamp terminals, and wherein the combined capacitance of the first partial compensating capacitor and the second partial compensating capacitor equals the capacitance of the compensating capacitor.