Generally, the invention relates to the field of light sources. In particular, the invention relates to the field of self-ballasted electrodeless lamps, such as induction lamps.

BACKGROUND OF THE INVENTION

Electrodeless lamps, such as induction lamps, are known by virtue of their long lifetime and high efficiency, which is comparable with conventional gas discharge lamps such as TL tubes and the energy saving lamps using the fluorescence principle. The bulbs are estimated to have a lifetime of 100.000 hours. The electronics, however, will shorten this lifetime to over 60.000 hours.

As can be inferred from its name, electrodeless lamps do not rely on an electron current in the lamp generated by a voltage between two electrodes. Instead, an induction coil is used, in which a high frequency voltage is generated, causing current to flow in the gas which is enclosed in the glass enve- lope around the coil. The gas may e.g. be mercury vapor

(extracted from amalgam) and another (rare) gas. The light generation process entails the emission of electromagnetic

radiation by the gas as a result of the electric current and the conversion of this radiation into visible light by means of fluorescence.

In view of global environmental concerns and cost effectiveness, electrodeless lamps have been seen over the years to be used more often in residential places, like households as a substitute for incandescent lamps.

US 2005/0206322 and US 2005/0012458 disclose self- ballasted electrodeless lamps. Self-ballasted lamps, as used in the present application, include ballasts and sockets so that these lamps can directly replace incandescent lamps in terms of structure. The prior art self-ballasted lamps include a luminous bulb wherein an induction coil is placed, a ballast containing portion for accommodating the ballast circuit and a socket. The structure of the socket resembles that of an incandescent lamp in order to allow simple replacement by an electrodeless lamp.

A disadvantage of the prior art self-ballasted electrodeless lamp amounts to the restriction in the luminous solid angle resulting from the ballast containing portion of the lamp. SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved self-ballasted electrodeless lamp.

To that end, in one aspect of the invention, a self- ballasted electrodeless lamp comprising a bulb accommodating an induction coil, a ballast circuit for applying a high-frequency signal to the induction coil and a lamp socket is disclosed. The lamp socket is configured to be connected to a power supply, e.g. of a lamp armature, for supplying power to the ballast circuit. The ballast circuit is accommodated within the space provide by the lamp socket.

In another aspect of the invention, a method is disclosed for operating such a lamp.

The luminous solid angle of the lamp is enhanced by omitting the need for a ballast circuitry containing portion lo- cated between the bulb accommodating the induction coil and the lamp socket. Instead, the transparent or translucent bulb of the lamp can be directly mounted onto the lamp socket, since the ballast circuit is substantially entirely accommodated within the lamp socket.

In an embodiment of the invention, the self-ballasted electrodeless lamp has a ballast circuit comprising high- frequency driving electronics including a transformer. The transformer fits entirely into the lamp socket. The transformer component of the high-frequency driving electronics has been miniaturized in order to enable accommodating the transformer entirely within the lamp socket. The high-frequency driving electronics is configured, in an embodiment of the invention, for providing the high- frequency signal with a frequency of higher than 1 Mhz, e.g.

2.65 MHz or 13.5 MHz. Operating the induction coil at such fre- quencies enables the lamps to be of a size for consumer

applications .

In an embodiment of the invention, the transformer comprises a transformer core containing a ceramic material,

preferably ferrite. More particularly, the ferrite is a soft ferrite containing e.g. an iron oxide and manganese oxide. Smaller portions of zinc oxide and cupper oxide may also be

included. The transformer core comprises a material with a high Q-factor (e.g. > 40 at 2.65 MHz, e.g. Q=50) and low-loss characteristics at frequencies higher than 1 MHz. The transformer may be a toroid. The dimensions of the transformer are such that the outer diameter is smaller than 10 mm (e.g. 5, 7 or 8 mm) and the inner diameter is smaller than 5 mm (e.g. 4.5, 3.5 or 2.5 mm) . The height of the transformer is smaller than 7 mm (e.g. 3, 4, 5 or 6 mm) .

In an embodiment of the invention, the transformer of the electrodeless lamp comprises a first secondary winding and a second secondary winding. The first secondary winding is connected with a gate of a first n-channel MOSFET of the ballast circuit and the second secondary winding is connected with a gate of second n-channel MOSFET of the ballast circuit. Terminals of the first and second n-channel MOSFET are connected at a common connection point of the ballast circuit. This embodiment enables the ballast circuitry to obtain a smooth sinusoidal signal for the gates of the field effect transistors thereby efficiently steering the transistors to produce a square wave signal at the common connection point. The amount of heat generated within the lamp socket has been found to result in a temperature decrease of only 10 °C for a 22W lamp.

In an embodiment of the invention, the transformer com- prises a primary winding connected to a triggering means of the ballast circuitry for conducting current after exceeding a predetermined voltage threshold. Such triggering means, e.g. a DIAC, controls operation of the ballast circuitry. The triggering means are contained within the lamp socket.

In still another embodiment of the invention, the ballast circuitry is provided as an integrated circuit, e.g. using a field-programmable gate array (FPGA). The integrated circuit is accommodated within the space provided by the lamp socket.

Hereinafter, embodiments of the invention will be described in further detail. It should be appreciated, however, that these embodiments may not be construed as limiting the scope of protection for the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings :

FIGS. 1A and IB are schematic illustrations of a self- ballasted electrodeless lamp according to the prior art and according to an embodiment of the invention, respectively;

FIGS. 2-4 are illustrations of self-ballasted electrodeless lamps according to embodiments of the invention; and

FIG. 5 depicts a ballast circuit for a self-ballasted electrodeless lamp as shown in FIGS. 2-4.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-section illustration of prior art electrodeless lamp 100 comprising a bulb 102, a bal- last containing portion 104 and a lamp socket 106. The ballast containing portion 104 houses a ballast circuit 108. The dashed lines indicate that the luminous solid angle for light emitted from the bulb 102 is restricted by the ballast containing portion 104. The lamp socket 106 is mounted into a power providing portion A of e.g. a lighting armature from which power is received for operating the lamp 100. The lamp socket may e.g.

comprise a E27, E14, G9, GU10 or GX53 socket.

FIG. IB, on the other hand, is a schematic illustration of an embodiment of an electrodeless lamp 200 according to an embodiment of the invention. The self-ballasted electrodeless lamp 200 comprises a bulb 202 accommodating an induction coil (not shown in FIG. IB) and a lamp socket 206. The ballast con- taining portion coincides with or is integrated within the lamp socket 206, such that a ballast circuit 208 is housed within the lamp socket 206. The ballast circuit 208 is configured for applying a high-frequency signal to the induction coil within the bulb 202. The ballast circuit 208 may be provided as an integrated circuit, e.g. a field-programmable gate array (FPGA).

The lamp socket 206 is configured to be connected to a power supply for supplying power to the ballast circuit 208 and the ballast circuit 208 is accommodated within the lamp socket 206. The power extracted from the mains can be between 4 and 200 Watts. For that reason, the lamp socket 206 may e.g. be mounted into a power providing portion A of e.g. an lighting armature from which power is received for operating the lamp 100. The lamp socket may e.g. comprise a E27, E14, G9, GU10 or GX53 socket.

As can be seen from the dashed lines in FIG. IB, the luminous solid angle has considerably increased and may e.g. range from 250 - 350 degrees (in the cross-section shown in FIG. IB) , thereby also improving the amount of light obtained from the lamp.

FIGS. 2-4 show induction lamps 200, comprising a base 210 on which a coil 212 is wound. A glass bulb 202 encloses all the parts, including an amalgam containing portion 214. The lamp socket 206 is used to mount the lamp in a lamp holder A (as shown in FIG. IB) to provide electricity to the ballast circuit accommodated within the lamp socket 206. In FIG. 4, the part 216 can be used to cool the lamp 200 only for higher power outputs. It should be appreciated that the glass bulb 202 shown in FIGS. 2 and 3 can take any and the designs of FIGS. 2 and 3 are only examples.

FIG. 5 is an embodiment of a ballast circuit 208 as shown in FIG. IB. The ballast circuit 208 is provided within the lamp socket 206 for operating the lamp, particularly for driving the coil 212 in a manner known as such.

The ballast circuit 208 receives power Vcc via e.g. a lamp holder A. The ballast circuit 208 contains a transformer having a primary winding with terminals 1 and 2 and two secon- dary windings with terminals 3, 4 and 5, 6 respectively. The dimensions of the transformer are such that the outer diameter is smaller than 10 mm (e.g. 5, 7 or 8 mm) and the inner diameter is smaller than 5 mm (e.g. 4.5, 3.5 or 2.5 mm) . The height of the transformer is smaller than 7 mm (e.g. 3, 4, 5 or 6 mm) .

The transformer has a core C comprising a soft ferrite material. As an example, the core material C comprises 49.5% Fe ₂ 0 ₃ , 42,5% MnO, 3% Zn) and 5% CuO. The Q-factor of the core material is 50 at a freguency of 2.65 MHz (measured with a HP4342A Q-meter) . The core material has been sintered at a temperature of 1100 °C.

The first secondary winding is connected with a gate of a first n-channel MOSFET and the second secondary winding is connected with a gate of second n-channel MOSFET of the ballast circuit. Terminals 3 and 5 of the first and second secondary windings are connected with gates of the first and second n- channel MOSFETs. Terminals of the first and second n-channel MOSFET are connected at a common connection point P of the ballast circuit. This embodiment enables the ballast circuitry to obtain a smooth sinusoidal signal for the gates of the field effect transistors thereby efficiently steering the transistors to produce a sguare wave signal at the common connection point P.

The primary winding is connected to a DIAC that conducts current only after Vcc exceeds a predetermined voltage threshold in order to establish an induction field in the primary winding of the transformer. The predetermined voltage may e.g. be 70 Volts. The DIAC is also contained within the lamp socket 206.

In operation, power Vcc is applied to the circuit and CI is charged. When the voltage across CI reaches the threshold of the DIAC, an electric current will flow through the primary winding of the pulse transformer (LI, 2), generating an induction voltage. As a result, a positive pulse is generated in the secondary winding 3-4 with C2 and the G-S capacity from Ql is parallel to it. This combination defines a resonance frequency by the formula / = . With this frequency a pulse is gener- ated and the upper field effect transistor Ql starts to conduct and applies a voltage across LR1 and CR1. As a result of the positive pulse generated in winding 3-4 a negative shaped pulse is generated by Electromotive Force (EMF ) in winding 5-6. To transform the negative pulse into a positive pulse from for using the lower field effect transistor Q2, the polarity from winding 5-6 is reversed. Now 180 degrees later then the generated positive pulse in winding 3-4 winding 5-6 will make the positive pulse for the lower field effect transistor Q2 with the same resonance conditions as described for winding 3-4 as result of which the voltage across LR1 and CRl will become zero. With this sequence a square wave signal is buildup on point P between VCC and ground. This square wave signal is applied to LR1. The function of LR1 is to limit the current in the antenna within the lamp bulb. On the output points an antenna will be mounted, CRl and this antenna will be defined on such a way that reso-

1

nance will take place with the formula and a sine wave

voltage from about 800 Volt will be produced to start the lamp. After the lamp is started a current will flow limited by LR1 and the voltage over the antenna will drop to about 400 Volt.

When the voltage on the connection point P equals zero, diode Dl will conduct, resulting in CI not being charged, thus eliminating the starting process. The total process will take about 100 msec. These electronics have been successfully tested in prac- tice and provides for a stable operation of the lamp at 2.65

MHz .

It should be appreciated that FIG. 5 does not contain parts for meeting EMC requirements and the power supply other than indicating Vcc. Such parts are also contained within the lamp socket

206.