EP1330946A1

EP1330946A1 - Circuit arrangement for operating several gas discharge lamps

Info

Publication number: EP1330946A1
Application number: EP01982352A
Authority: EP
Inventors: Dietmar Klien; Markus Mayrhofer
Original assignee: Tridonicatco GmbH and Co KG
Current assignee: Tridonicatco GmbH and Co KG
Priority date: 2000-10-09
Filing date: 2001-09-25
Publication date: 2003-07-30
Anticipated expiration: 2021-09-25
Also published as: AU2002213959B2; US20030214252A1; EP1330946B1; DE50105646D1; ATE291342T1; DE10049842A1; AU1395902A; US6765354B2; WO2002032196A1

Abstract

A circuitry arrangement for operating n gas discharge lamps, n being a whole number greater than 1, includes a single inverter, fed with d.c. voltage, for generating an a.c. voltage alterable in its frequency, delivered to a load circuit arranged at the inverter's output. The load circuit includes a series resonant circuit of an inductance and capacitance, n gas discharge lamps connected to a common node point between the inductance and capacitance, which lamps are connected in parallel to one another, and (n-1) balancing transformers for balancing currents in two gas discharge lamps. The load circuit further has for each gas discharge lamp at least one d.c. current supply line, connected between an output side terminal of a corresponding balancing transformer winding and gas discharge lamp and via which there is delivered to each gas discharge lamp a d.c. current, so as to avoid an unintended extinguishing of a lamp.

Description

Circuit arrangement for operating a plurality of gas discharge lamps

The present invention relates to a circuit arrangement for operating at least two gas discharge lamps according to the preamble of claim 1.

The use of so-called two-lamp or multi-lamp ballasts can be reduced to a certain extent in terms of circuitry. The advantage over the use of ballasts, each of which only control a single gas discharge lamp, is that a large part of the components of the ballast, for example the rectifier, the harmonic filter, the control circuit and the inverter, can be used to operate a plurality of lamps at the same time.

The inverter and the load circuit of a known two-lamp ballast, which is disclosed in EP 0 490 329 AI, are shown schematically in FIG. 4 and are to be briefly explained below. The inverter is formed by two controllable switches S 1 and S2, which are arranged in a half-bridge arrangement, at the input of which a DC supply voltage V _BUS is present. The two switches S 1 and S2 are controlled by a control circuit 1 in such a way that they open and close alternately, so that a high-frequency alternating voltage U _ac results at the center of the half-bridge. This alternating voltage is fed to the load circuit, which on the input side has a series resonant circuit consisting of an inductor L _a and a capacitor C _r . At the common node between the inductance L _a and the capacitance C _r , the two gas discharge lamps LA1 and LA2 are each connected in parallel via a coupling capacitor C _kl and C ^.

In addition, the two gas discharge lamps LA 1 and LA 2 are preceded by a balancing transformer L _bal , the windings of which flow through the two lamp currents. This happens in opposite directions, so that deviations in the current amplitudes result in magnetization, which induces a voltage in the windings, which in turn acts symmetrically. Component tolerances as well as lamp tolerances and different temperature conditions, which would result in the two lamps LA 1 and LA 2 burning with different brightness, can thus be compensated to a certain extent by the balancing transformer L _bal . The symmetrizing effect of the transformer L ^, however, is limited and does not guarantee complete adjustment of the lamp currents. For example, the lamps are practically connected in parallel at low currents, which result at low dimming levels, since the voltage drop at the balancing transformer can only be a fraction of the operating voltage of the lamps. This is particularly evident at lower temperatures, where the operating voltage reaches a maximum with small lamp currents.

This case is shown in Figure 5. The two lamps should be operated at a brightness _that corresponds to a specific target current I _S0LL . Due to tolerances, however, the two lamps are not identical, but instead have characteristic curves U _arc ι and U _{arc2 which} are slightly shifted _relative to one _another , as shown in FIG. For example, for a given current, the second lamp basically requires a slightly higher _operating voltage U _arc2 than the first lamp. Therefore, in order to be able to operate both lamps with the target current I _S0LL , two different operating voltages U _S0LL1 and U _S0LL would be required. However, since the ballast with the inverter only provides a voltage _value U _S0LL1 , which in the example shown is determined by the lamp with the lower operating voltage, _i.e. by the first lamp with the characteristic U _arcl , this voltage U _{S0L 1 is} also due to the second lamp on. As a result, the second lamp does not assume the desired current value I _S0LL , but possibly forms a second operating point with a different current value I _aιc2 and thus of course also with a different brightness. However, there is also the risk that the second lamp with the higher operating voltage may not be able to form a fixed operating point at all and will consequently go out.

In order to avoid extinction of one of the two lamps LA1 or LA2 at low brightness values, in the ballast shown in FIG. 4 the inverter is always regulated according to the lamp LA1 or LA2 which currently has the lower lamp current. For this purpose, the ballast has two detection circuits 2 and 2, each of which detects the current flowing through a lamp LA 1 or LA2 by detecting the current through a measuring resistor R _SEIM . _SI or R _SN determine falling voltage. The actual values V _ISTI and V _1ST2 generated by the two detection _circuits 2, and 2 ₂ are then fed to a comparison circuit 3, which selects the correspondingly lower value and forwards it as the final actual value V _IST to the control circuit 1 for controlling the inverter.

A separate detection circuit is therefore necessary for each lamp in order to be able to reliably ensure that neither lamp goes out. The However, this increases circuit complexity. In addition, it must be taken into account that, due to the switching capacities of the lamps or the wiring, a capacitive current always flows through the lamps. Correct control is only guaranteed, however, if the actual active component of the lamp current is determined. This requires complex and expensive circuits. Finally, in the multi-flame systems in which more than two lamps are connected to a single inverter, a complex selection circuit is required to select the lowest actual value.

It is therefore an object of the present invention to provide a simplified circuit arrangement for operating at least two gas discharge lamps, in which the extinction of one of the lamps is reliably avoided.

The object is achieved by a circuit arrangement according to claim 1. According to the invention, n (n is an integer and greater than 1) gas discharge lamps are operated with a single inverter which is fed with a DC voltage and generates an AC voltage which can be varied in frequency and which is fed to a load circuit arranged at the output of the inverter. The load circuit contains a series resonance circuit consisting of an inductance and a capacitance and the n gas discharge lamps connected to the common node between the inductance and the capacitance. The load circuit (n-1) also contains balancing transformers for balancing the currents of two gas discharge lamps in each case.

In order to prevent one of the lamps from extinguishing, the load circuit according to the invention has a direct current supply line for each gas discharge lamp, which acts between the output-side connection of the winding of the balancing transformer and the gas discharge lamp and via which a direct current is supplied to each gas discharge lamp. Thus, in addition to the AC voltage supplied via the resonance circuit and the inverter, each gas discharge lamp also receives an independent current source which supplies the lamp with a DC current. This additional direct current preferably corresponds to approximately half of the nominal 1% current at 25 ° C - 35 ° C. It has the effect that even in the event that a stable operating point cannot form due to the predetermined alternating voltage, none of the lamps goes out. In addition, the additional direct current also prevents the occurrence of so-called running layers. Developments of the invention are the subject of the dependent claims. The DC supply lines preferably each have a resistor connected in series with the lamp and are connected to a common supply voltage at their input-side connection. This supply voltage can be obtained, for example, with the aid of a diode connected to the output of the inverter, a capacitor connected to ground preferably being arranged between the diode and the DC supply lines.

The extinction of the lamps can be reliably prevented by the measures according to the invention. However, there may be large differences in brightness due to asymmetrical wiring and lamp capacities, since the balancing transformer (s) try to compensate for the relatively large currents and, as a result, an additional active current is generated in a lamp with a lower wiring capacity. In order to avoid this and to achieve better balancing of the lamp currents, according to another development of the invention, the two windings of a balancing transformer can each be connected to one another by a series circuit comprising a capacitor and a resistor. This has the consequence that the balancing effect of the transformer for small lamp currents is reduced without influencing the direct current sources. The reduction in the balancing effect only affects the AC components of the lamp voltage, i.e. only the part that is significantly influenced by asymmetrical wiring capacities at low dimming levels.

The circuit according to the invention is characterized in that it can be easily expanded from a two-lamp system to a multi-lamp system. In addition, it is no longer necessary to provide a separate detection circuit for measuring the lamp current for each lamp. Rather, it is sufficient to use only a single detection circuit which detects the sum of the active powers of the gas discharge lamps arranged in the load circuit and generates a corresponding actual value. The inverter can then be controlled on the basis of a comparison between this actual value and a predetermined target value. In a half-bridge inverter, for example, the sum of the active powers can be detected in a simple manner by determining the voltage drop across a measuring resistor arranged at the base of the half-bridge. The DC supply lines proposed according to the invention with the ^" resistors connected in series to the lamps, which are connected on the input side to a common supply voltage, can also be used in multi-lamp lamp systems in which no balancing transformers are provided. A corresponding circuit arrangement is the subject of claim 9 ,

The invention will be explained in more detail below with reference to the accompanying drawings. Show it:

Fig. 1 shows an embodiment of a circuit arrangement according to the invention for a two-lamp system;

2 shows the effect of the direct current supply lines according to the invention;

3 shows an exemplary embodiment of a circuit arrangement according to the invention for a three-lamp system;

4 shows a known circuit arrangement of a two-lamp system; and

Fig. 5 shows the effects of lamps with different characteristics.

The basic arrangement of the circuit arrangement shown in FIG. 1 is similar to that of the known circuit shown in FIG. Again, only a single inverter consisting of two controllable switches S 1 and S2 is provided for operating the two gas discharge lamps LA 1 and LA 2. The switches S 1 and S2 arranged in a half-bridge arrangement are supplied with a DC voltage V _HUS and, by alternately opening and closing, generate a high-frequency AC voltage U _αC which is fed to the load circuit. The load circuit contains the series resonance circuit consisting of the inductance L 1 and the capacitance C _r , at the center of which the two lamps LA 1 and LA 2 are connected via two coupling capacitors C _kl and C _κ . Again the lamps LA1 and LA2 have a balancing transformer L _ba | upstream.

The DC supply lines according to the invention are each connected to a point between the lamp LA 1 or LA 2 and the output side of the corresponding winding of the balancing transformer L _lul . They each contain one _Resistor R _dcI or R _dc2 connected in series with the corresponding lamp LA 1 or LA2 and are connected on the input side to a common DC voltage source. The resistance _values for the two resistors R _dcl and R _dc2 are identical. The DC voltage source is formed in the example shown by a diode D 1 connected to the output of the inverter and a capacitor C _dc connected to ground as a low-pass filter, which form a smoothed DC voltage U _dc from the high-frequency AC voltage U _ac .

The direct current I _dcl supplied to the first lamp LA 1 is then calculated as follows:

U d

^■ ύc \ IPd R arc \

where R _{arcl is} the resistance of the gas discharge lamp LAl. The direct current supplied to the second lamp LA2 is analogous. The two resistors R _dcl and R _dc2 are designed so that the additional direct current corresponds to approximately half of the nominal 1% current at 25 ° C to 35 ° C.

Obtaining the direct voltage U _dc from the alternating voltage U _{ac of} the inverter has the further advantage that, after the inverter has been switched off, the direct current supplied to the lamps LA1 and LA2 is also deactivated, so that both lamps LA1, LA2 are safely switched off. However, it would also be possible to use a DC voltage source that is separate from the inverter. The direct current supplied to the lamps LA1, LA2 also prevents the occurrence of so-called running layers.

However, the symmetry effect of the transformer L ^ only works up to a certain dimming level. At brightness values below this dimming level, the lamp current is so low that capacitive currents can arise which are larger than the lamp currents themselves. These capacitive currents can arise, for example, in that the leads of the lamps are laid asymmetrically, as a result of which - as shown schematically in the second lamp LA2 - additional wiring capacitances C _par and thus capacitive currents I _pa . occur. If these capacitive currents I _{par are} greater than the lamp currents, the balancing transformer L _ba reacts in such a way that the asymmetry is increased. The lamp LA 1, which does not have the additional wiring capacity, is then supplied with an additional active current I _arcl , which can be estimated in the following way:

In order to counteract this, the balancing effect of the transformer L _bal for low lamp currents should be reduced without the DC sources being influenced thereby. This is achieved in that the two output-side connections of the windings of the balancing transformer L _ba are connected to one another by a frequency-dependent impedance, which in the present example consists of the series connection of a resistor R _ba) and a capacitor C _bal . This connection allows a certain compensation of small asymmetries. However, the reduction in the balancing effect only affects the AC component of the lamp voltage, i.e. only the part that is responsible for the capacitive currents at low dimming levels.

The effect of the circuit according to the invention is shown schematically in Figure 2. The graph shown here shows the lamp voltage U _arcl and U _arc2 applied to the lamps LA1 and LA2 and changing over time. Although the same AC voltage U _acl or U _{ac2 is still} supplied to both lamps, however, since they are now decoupled in terms of direct current, they can assume a different DC voltage _component U _dcl or U _dc2 . As a result, each lamp can adopt exactly the voltage that it would have to develop for the specified brightness value or lamp current. This enables both lamps to be controlled by a single inverter and still operate both at the desired brightness.

In addition, since there is no longer any risk of one of the two lamps LA1 or LA2 being accidentally extinguished, it is no longer necessary to provide a separate detection circuit for each lamp, as in the circuit arrangement shown in FIG. Instead, as shown in Figure 1, only one can

Detection circuit 2, for example in the form of a low-pass filter, can be used, which _{detects the} voltage drop across a measuring resistor R _{sι; NS} arranged at the base of the half-bridge circuit and, accordingly, an actual value

V _[ST generated. This actual value now corresponds to the sum of the active powers of both

Gas discharge lamps LA l and LA2. The one generated by the detection circuit 2

Actual value V _LST is supplied to the control circuit 1, which after a comparison of the

Actual value V, _st _controls the two switches S 1 and S2 of the inverter with a desired value V _S0LL corresponding to the desired brightness.

Another advantage of the circuit arrangement according to the invention is that it can easily be expanded to more than two lamps. This is shown in Figure 3, which shows the expansion of the system to three Gas discharge lamps LA 1, LA2 and LA3 represents. The only extension is that now several balancing transformers L _ba , ₁₂ and L ,,. ,, ^ are used, each of which symmetrize the currents of two lamps LA 1 and LA2 or LA2 and LA3. Again, the output-side connections of the balancing transformers L _balI2 and P ^ are connected to one another via the series circuit described above, consisting of a resistor R _ball2 or R _bal23 and a capacitance C _ba , ₁₂ or C _bal23 , in order to decouple the DC _components . An extension of the system to n gas discharge lamps then consists only in the fact that (n-1) balancing transformers are used, each of which balances the currents of two lamps.

In particular when expanding to more than two gas discharge lamps, the advantage of the circuit arrangement according to the invention is evident, since the use of a single detection circuit 2 is still sufficient, as a result of which the circuit is significantly simplified.

Claims

Expectations

1. Circuit arrangement for operating n gas discharge lamps (LA l, LA2, LA3), where n is an integer greater than 1, with a single inverter (S l, S2) fed with direct voltage (V _BUS ) to generate a frequency that can be changed AC voltage (U _ac ), which is supplied to a load circuit arranged at the output of the inverter (S l, S2), the load circuit having the following: a series resonance circuit n consisting of an inductance (L _a ) and a capacitance (C _r ) the common node between the inductance (L _a ) and the capacitance (C _r ) connected gas discharge lamps (LA l, LA2, LA3), which are connected in parallel to each other, and

(n- 1) balancing transformers (L _bal , L _baU2 , L ^,) for balancing the currents of two gas discharge lamps (LA l, LA2, LA3), characterized in that the load circuit also for each gas discharge lamp (LA l, LA2, LA3) has a direct current supply line, which in each case between the output-side connection of the corresponding winding of the balancing transformer (L _bj ,,, L _balI2 , and the gas discharge lamp (LA1, LA2, LA3) is connected and via which each gas discharge lamp (LA1, LA2, LA3) is supplied with a direct current.

2. Circuit arrangement according to claim 1, characterized in that the DC supply lines each have a series-connected resistor (R _dcι , R _dc2 , R _dc3 ) and that a common supply voltage (U _dc ) is present at their input-side connection, the resistance values these resistances (R _dcl , R _dc2 , R _dc3 ) are the same.

3. Circuit arrangement according to claim 2, characterized in that the common supply voltage (U _dc ) is formed by a diode (D l) connected to the output of the inverter.

4. Circuit arrangement according to claim 3, characterized in that a low-pass filter (C _dc ) is arranged between the diode (D l) and the direct current supply lines.

5. Circuit arrangement according to one of the preceding claims, characterized in that the output connections of the windings of a balancing transformer (L _bab P _{a l2} , 1- ₁ , ₀₁₂₃ ) each by a series connection of a capacitor (C _b . ", C _ball2 , C _baI23 ) and a resistor (R _bal , R _baI12 , R _baι23 ) are interconnected.

6. Circuit arrangement according to one of the preceding claims, characterized by a detection circuit which detects the sum of the active powers of the gas discharge lamps (LA 1, LA2, LA3) arranged in the load circuit and generates a corresponding actual value (V _IST ), and by a control circuit which _controls the inverter on the basis of a comparison between a setpoint (V _S0LL ) and the actual value (V _[ST ) generated by the detection circuit.

7. Circuit arrangement according to one of the preceding claims, characterized in that the inverter is formed by two switches arranged in a half-bridge arrangement (S l, S2).

8. Circuit arrangement according to claim 6 and claim 7, characterized in that the detection circuit _{detects the} voltage drop via a resistor (R _SENS ) arranged at the base of the half-bridge.

9. Circuit arrangement for operating n gas discharge lamps (LA1, LA2, LA3), where n is an integer greater than 1, with a single inverter (S I, S2) fed with direct voltage (V _BUS ) for generating an alternating voltage which can be varied in frequency (U _ac ), which is fed to a load circuit arranged at the output of the inverter (S1, S2), the load circuit having the following: a series resonance circuit consisting of an inductance (L _a ) and a capacitance (C _r ), and n to the common node between the inductance (L _a ) and the capacitance (C _r ) connected gas discharge lamps (LA l, LA2, LA3), which are connected in parallel to each other, characterized in that the load circuit further for each gas discharge lamp (LA l, LA2, LA3 ) has a direct current supply line, via which each gas discharge lamp (LA 1, LA2, LA3) is supplied with a direct current. wherein the DC supply lines each have a series-connected resistor (R _cl , R _dc2 , R _dc and a common supply voltage (U _dc ) is applied to their input-side connection.