FIELD OF THE INVENTION

The invention relates to a lamp circuit as described in the pre-characterizing part of claim 1.

BACKGROUND OF THE INVENTION

A gas discharge lamp circuit of this type is disclosed in US 4,959,593. In this lamp circuit, the alternating voltage (AC) supply source is the mains, so that the lamp- operating frequency has a low value of, for example, 50Hz or 60Hz. The inductor and the capacitor, which are connected in series with the lamp, are both part of a ballast circuit. Consequently, the capacitor has a value which represents a significant impedance for the lamp-operating frequency. An ignition circuit is connected parallel to the lamp. The ignition circuit comprises a series arrangement of two networks of a resistor and a capacitor, a voltage-dependent device and possibly an inductor. The voltage-dependent device may be a bilateral semiconductor switch, for example, a sidac. When the lamp circuit is turned on, the ignition circuit generates ringing (on/off) voltages. The ringing voltages distort the supply by the AC supply source, and a high- voltage, high-frequency ignition pulse is generated across the lamp so as to ignite it.

Since ringing must occur, tuning must be provided, so that proper operation of an ignition device of the above-mentioned type is dependent on specifications of the ballast and the lamp connected therewith. This puts a burden on design and logistics. In addition, said specifications, in particular of the lamp, may change with time and by changes in temperature so that the operation of the ignition device may be affected.

Other conventional high- intensity discharge (HID) lamps are usually supplied with a square-wave current at a low lamp-operating frequency of 50Hz to 500Hz. The lamp is ignited by superimposing a high- voltage ignition pulse on the low-frequency voltage supplied to the lamp. A pulse transformer is therefore connected in series with the lamp so as to induce such an ignition pulse to the low-frequency lamp supply voltage. When such a circuit is used for high lamp-operating frequencies, such as in the megahertz range, a transformer of this

type represents a very high impedance which significantly affects normal lamp operation. Moreover, the resistance of such a transformer causes energy losses.

OBJECT OF THE INVENTION It is an object of the invention to solve the drawbacks of the prior art as described above.

SUMMARY OF THE INVENTION

The object of the invention is achieved by providing a lamp circuit as defined in claim 1.

The capacitor acts as a DC blocking device. It breaks the DC current path between the lamp and the AC source. It thereby prevents a DC current flow through the AC source. In this way, a high DC voltage can be built up across the lamp. This high DC voltage results in ignition of the lamp. The capacitor may have a comparatively low impedance for the (HF) AC frequency, in which case it would not be a significant component in the ballasting function. In another implementation, the capacitor may have a comparatively high impedance, representing a significant part of the ballasting impedance.

When the lamp circuit is turned on, the lamp is not conducting and represents a very high impedance. The DC voltage source will then charge said capacitor in a relatively short time of typically 10ms, thereby generating a high voltage across the lamp, so that the lamp may ignite. After its ignition, the lamp will represent a much lower impedance. As a result, a steady HF supply current may flow from the AC source through the lamp. The resistor connected in series with the high- voltage DC source has a value which is sufficient to limit an AC current through the DC source. This AC current-limiting function can also be fulfilled by an inductor constituting a similar impedance for the AC frequency.

Tests have shown that the DC voltage may have a much lower amplitude than an ignition pulse used in the prior art to ignite the lamp. The lower amplitude may lead to the condition that the lamp circuit must meet more easily attainable requirements of a lower safety class. In addition, the lower amplitude mitigates isolation requirements and allows a smaller size of the lamp circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be apparent from the following description, given by way of example, with reference to the accompanying drawing. In the drawings:

Fig. 1 shows a diagram of a first embodiment of a gas discharge lamp circuit according to the invention; and

Fig. 2 shows a diagram of a second embodiment of a gas discharge lamp circuit according to the invention.

DESCRIPTION OF EMBODIMENTS

The gas discharge lamp circuit according to the invention shown in Fig. 1 comprises a high-frequency (HF) power supply source 2. An inductor 4, a capacitor 6 and a gas discharge lamp 8 are connected in series with the HF supply source 2. The HF supply source 2 may be, for example, a half bridge, a lull bridge, and a class-E DC- AC converter, comprising switches which are alternately switched on and off to provide a substantial AC voltage.

A series circuit of a resistor 10 and a direct-voltage (DC) source 12 is connected parallel to the lamp 8. The above-mentioned entities and components have, for example, the following values.

HF voltage source 2: 400 V, 2 MHz. inductor 4: 40μH capacitor 6: 22OpF lamp 8: to infinite if not ignited; 60Ω if ignited resistor 10: 50MΩ

DC voltage source 12: 15kV

It will be clear from the above examples that the capacitor has a suitable rating of maximum voltage and capacitance. If the ballast were to operate at a low frequency of e.g. 100Hz, the capacitor should have a value of 4.4μF to reach a similar impedance. At the 15kV voltage rating, such a capacitor would become excessively large. A quick calculation shows that the required power rating of the DC voltage source in the case of such a large capacitor is excessive and impractical: with I = C*dV/dt and with the above values, I = 4.4*10 ^"6 * 15*10 ³ / 10 ^"2 = 6.6A.

This would in turn lead to a peak power rating for the DC voltage source of: P = 6.6 * 15*10 ³ = 99kVA.

Furthermore, if the capacitor had such a comparatively large capacitance, the stored energy inside it would destroy the electrodes of the lamp upon ignition. The stored energy would be: Q = C*V ² , so that Q = 4.4*10 ^~6 * (15*10 ³ ) ² = 99J.

It will be apparent from the above that the invention specifically relates to circuits having a comparatively high operating frequency. Such circuits, with high lamp- current frequencies, have significant advantages of cost and size when compared to low- frequency ballasts. In certain cases, the properties of the lamp, such as efficacy, may be positively influenced by the high frequency of operation. This is, for example, the case with fluorescent lamps.

It will be clear from the examples described above that, in case the lamp 8 has not ignited yet, i.e. is not conducting, upon switching on the lamp circuit, the DC source 12 will charge capacitor 6, thereby developing a high voltage across the lamp 8. At some point during this charging of capacitor 6, the voltage across the lamp 8 will be such that the lamp 8 will break down, i.e. it ignites and will conduct. The lamp 8 will then have an impedance which is very small with respect to the value of resistor 10. Consequently, the DC voltage across lamp 8 will collapse. Furthermore, with the lamp 8 being ignited, a HF current will flow through the lamp 8. It will be appreciated from the description above that said series circuit of resistor 10 and DC source 12, together with capacitor 6, operates as an ignition circuit, or igniter, which is simple, energy-efficient and small-sized.

The second embodiment of the gas discharge lamp circuit according to the invention comprises an inductor 14 instead of resistor 10. Inductor 14 provides an impedance which is comparable to that of resistor 10 for the AC frequency.

To further reduce energy consumption, the ignition circuit of the second embodiment shown in Fig. 2 also comprises a control circuit 16. The control circuit enables the ignitor when the circuit is switched on. After the lamp 8 has ignited, the igniter can be disabled and no DC current will flow through the DC voltage source 12 and resistor 10. Consequently, during normal operation of the lamp 8, no DC power from DC voltage source 12 will be consumed.

Control circuit 16 may comprise a timer to disable the DC voltage source after, for example, several milliseconds, from the time a DC current starts flowing. This represents a simple and inexpensive embodiment, in particular in cases where lamp

specifications and changes with time are known in advance. An example of such a case is the field of automotive lighting with Xenon lamps.

Control circuit 16 may also comprise a sensor or detector or a detecting circuit generally used to detect run-up or a steady state of operation of the lamp 8, often referred to as normal operation. The detection circuit may comprise a light sensor for detecting light emitted by the lamp 8. It may also comprise an AC current detector for detecting an AC current flowing through the lamp. Moreover, it may comprise a DC current detector for detecting a DC current flowing through the lamp 8 and/or through resistor 10. Upon detecting such an AC and/or DC current, the control circuit 16 may control the DC voltage source to be rendered inoperative.

Tests conducted with a lamp circuit according to the invention showed that the DC voltage provided by DC source 12 may have a significantly lower amplitude than an ignition pulse as used in prior-art lamp circuits. This has the advantage that isolation requirements are easier to meet, and/or smaller distances between conductors can be applied. It may also result in that the lamp circuit must meet requirements of a different safety class which might require less effort and cost.

Tests also showed that the ignition circuit according to the invention may also be used in a lamp circuit having an inductor 4 with a low value while maintaining the advantages as described above.