Gas discharge lamp unit

FIELD OF THE INVENTION

The invention relates to a gas discharge lamp unit, e.g. a backlighting unit, such as for an LCD display.

BACKGROUND OF THE INVENTION

WO-A-02/104084 discloses an inverter circuit for a gas discharge lamp having a primary circuit having a DC voltage supply, a transformer, a switching circuit including a first switch and a second switch for controlling a conduction state of the inverter circuit; a tank circuit having a resonant inductor and a resonant capacitor, the lamp load being coupled with the resonant capacitor; and a capacitor coupled to the first and second switches for maintaining a voltage across a primary winding of said transformer.

SUMMARY OF THE INVENTION

It is, inter alia, an object of the invention to provide a gas discharge lamp circuit that is relatively cheap in comparison with the prior art. The invention is defined by the independent claims. Advantageous embodiments are defined in the dependent claims.

One aspect of the invention provides a gas discharge lamp unit, comprising a transformer for generating a secondary voltage in the order of magnitude of some hundreds of volts, and an LC circuit for generating from the secondary voltage an output voltage in the order of magnitude of a few kilovolts. A plurality of EEFL gas discharge lamps may be coupled in parallel to an output of the LC circuit. The invention is preferably used as a backlighting unit for a flat panel display, such as an LCD display.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an embodiment of a power supply architecture with integrated inverter in accordance with the present invention;

Fig. 2 shows an embodiment of a basic mains isolated inverter in accordance with the present invention;

Figs. 3a and 3b shows embodiments of a mains isolated inverter with symmetrical output with integrated output coils in accordance with the present invention; and Fig. 4 shows a preferred embodiment of a lamp driver in accordance with the present invention.

DESCRIPTION OF EMBODIMENTS

As shown in Fig. 1, one aspect of the invention is to use an inverter or lamp driver LD that is directly operated from 400Vdc as obtained by a power factor correction circuit PFC, and that includes mains isolation MI as indicated by the dotted line. An auxiliary power supply unit Aux PSU may be coupled to the power factor correction circuit PFC in order to drive other circuitry (indicated by "-> Aux") in a flat panel display unit, such as an LCD unit. Advantageously, there is only a single transformer T between the mains input voltage and the CCFL or EEFL lamps. As indicated by "-> L" EEFL lamps can be connected directly in parallel to the outputs, while CCFL lamps need to be connected thru series capacitors.

As shown in Fig. 2, the mains isolated inverter LD is build around a so-called half bridge converter (also a full bridge can be used). The switches Sl and S2 are operated at 50% duty cycle. The transformer T provides mains isolation and converts the input voltage essentially into a square wave voltage of appropriate amplitude. The resonance network, comprising a coil Ls and a capacitor Cs, filters out the first harmonic of this square wave (resulting in sine wave shaped voltages and currents through the lamps) and swings up the voltage to the level required by the lamps L. The inductor Ls can be integrated into the transformer T when desired. Several EEFL lamps can be connected in parallel to the output of the circuit.

It is an important feature of the present invention that the secondary voltage of the transformer T is in the order of magnitude of several hundreds of volts, e.g. 300 V, rather than in the order of magnitude of some kilo volts. As a result, it is not necessary to use a transformer that is dimensioned to deal with kilovolts, so that a much cheaper and much smaller transformer can be used. The LC circuit Ls,Cs at the secondary side of the transformer T is used to get the kilovolts (e.g. 2 kV) needed for driving the lamps from the few hundreds of volts that are available at the secondary side of the transformer.

For reasons of symmetry (EMC!) the inverter can be build as shown in Figs. 3a or 3b. The inductor Ls is split up into two coils LsI, Ls2 divided over the output branches. In preferred embodiment shown in Fig. 3b, LsI and Ls2 are integrated onto one core.

The advantage of the embodiment of Fig. 4 is that the voltages across the coils

LsI and Ls2 are forced to be equal. Thereby the lamp electrode voltages are symmetrical with respect to ground, irrespective of component tolerances and lamp wiring differences. The extra windings on the coil can be very easily added on the coils during the winding process. By measuring the voltage Vovp on the extra winding a very effective and cheap over voltage protection can be realized.

By measuring the current in the winding (e.g. via a simple resistor), an asymmetrical current Iasym can be detected and by that touching of one of the outputs by a human. This can be explained as follows. The cross-coupled windings in Fig. 4 force the voltages across the secondary coils to be equal. Due to component tolerances (that are always present) in the inductance values of the secondary coils LsI, Ls2 and the capacitance values of the output capacitors CsI, Cs2, a certain current flows in the cross-coupled windings to force the coil voltages to be equal. As a result, the output voltages become symmetrical with regard to ground. The peak current flowing in the cross-coupled windings therefore depends on the amount of mismatch present in the component values of the secondary inductances and capacitances. If the secondary coil inductances were equal, and also the output capacitor values were equal, the current in the cross-coupled windings were zero. In practice, this is never the case, and a current Iasym flows in the cross-coupled windings. When a human body touches one of the outputs, this can be modeled as an impedance being connected between ground and the output being touched. Then, the aforementioned asymmetry becomes worse! As a result, the current Iasym flowing in the cross-coupled windings increases significantly. This increase of the Iasym current can easily be measured, and can be used to switch off the lamp inverter. As such, a very simple touch protection can be realized.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The use of CCFL or EEFL lamps is an example; the invention can also be used with HCFL lamps. The use of a mains voltage as input voltage, with only one transformer as shown in Fig. 1, is very advantageous, but the basic idea of using a transformer that is not dimensioned for a

secondary voltage of several kilo volts but merely for a secondary voltage of some hundreds of volts, followed by an LC circuit to get the kilovolts that are needed for driving the lamps, can also be used in a configuration that starts from 24 V. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. 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.