EP0707438A2 - Ballast for at least one discharge lamp - Google Patents

Abstract

The circuit has inverter switches (S1,S2) coupled in series, and operated in an alternating mode using signals from a control circuit (1). The switches connect with the load circuit that has series resonance stage having a coil (L1) and capacitor (C3). The circuit connects to the lighting tube via a coupling capacitor (C1). The other electrode of the lamp is connected to a heating transformer (T) that has a pair of primaries (T1,T3) and a pair of secondaries (T2,T4). A switch (S3) in the supply line controls the generation of the heating current.

Description

The invention relates to a ballast for at least one gas discharge lamp according to the preamble of claim 1, and a method for operating a gas discharge lamp with such a ballast.

In today's high-quality ballasts for gas discharge lamps, the lamp electrodes are usually preheated before the ignition voltage is applied between them. It has been shown that the lamp life is considerably extended by this measure.

Such as. described in EP 0 594 880 A1, the gas discharge lamp is generally operated on a series resonant circuit, the resonant circuit capacitor generally lying parallel to the discharge path of the gas discharge lamp. The electrodes of the lamp are designed as heating coils through which the current of the resonant circuit flows when the lamp is not ignited. In the preheating mode, the frequency is changed in relation to the resonance frequency of the resonance circuit in such a way that the voltage across the resonance capacitor and thus above the gas discharge lamp does not cause the gas discharge lamp to ignite. In this way, a substantially more constant current flows through the lamp electrodes designed as filaments, so that they are preheated. After the preheating phase, the frequency is set in the vicinity of the resonance frequency of the resonance circuit, whereby the voltage across the resonance capacitor increases so that the gas discharge lamp ignites.

With high-quality ballasts, it is common today to provide dimming for the gas discharge lamp. It has now been shown that premature aging of the gas discharge lamp begins with strong dimming. For this reason, it is necessary to provide an arrangement in which the electrodes of the gas discharge lamp can also be heated in ignited operation. In particular, it is advantageous to set the heating of the electrodes as a function of the degree of dimming, i.e. the more the lamp is dimmed - the darker it is - the more it needs to be heated.

A suitable circuit arrangement is described for this from EP 0 589 081 A1, from which the preamble of claim 1 is based. This points in Resonance circuit on the primary winding of a heating transformer, the secondary windings are connected in parallel to the connections of the heating coils. In this way, it is possible to supply the heating coils with energy even in ignited operation. Furthermore, a controllable switch is provided parallel to the primary winding of the heating transformer, which bridges the primary winding if necessary and thus simultaneously prevents the heating of the heating coils.

However, this arrangement has the disadvantage that the provision of the primary winding of the heating transformer in the series resonant circuit means that the latter is at least required for as long as heating the filaments, i.e. the switch connected in parallel to the primary winding is open, dampens the resonance circuit. This leads to a detuning of the resonance circuit, so that reliable ignition and thus reliable operation can no longer be guaranteed without restriction.

The invention is therefore based on the object of providing a ballast for at least one gas discharge lamp, so that reliable operation which is gentle on the lamp is always ensured.

This object is achieved according to claim 1 in that an inverter which has two series switches connected to a DC voltage source and connected in push-pull mode, with a load circuit consisting of a series resonance circuit and the lamp and a heating circuit for the power supply to the inverter the lamp filament is connected. The heating circuit also has a further controllable switch for controlling the heating current. Furthermore, the heating circuit is also connected in parallel to one of the two switches of the inverter.

Furthermore, the object is achieved according to claim 16 in that when the gas discharge lamp is preheated, the inverter is operated at the maximum clock frequency, while the clock for the additional controllable switch is selected such that the gas discharge lamp is operated with the maximum permissible heating power.

With the circuit arrangement according to the invention it is ensured that the series resonant circuit can operate unaffected and thus undamped. Furthermore, it is possible by means of the method according to the invention, depending on the Inverters determine the dimming level of the gas discharge lamp to set the required heating voltage. In addition, when using different lamps on one and the same ballast, it is possible to provide the heating power required for the lamp selected in each case without influencing the operating conditions of the lamp.

Further advantageous embodiments of the invention are specified in the subclaims.

So it is particularly advantageous for the energy balance of the ballast according to the invention that the further switch is switched so that the lamp is only supplied with heating current when it has to be heated due to its operating state. This means that only as much energy as is absolutely necessary for heating the lamp is used.

Claims 2-4 relate advantageously to configurations of the ballast according to the invention.

According to subclaims 5-8, configurations are provided which ensure that the control of the additional switch is kept as simple as possible while taking into account the control of the inverter and at the same time remains variable in order to be able to set the necessary heating power as precisely as possible.

According to subclaims 9-14, configurations of the heating circuit are specified which use the advantageous use of a heating transformer and also ensure the simplest and most reliable circuit design possible.

According to subclaim 17, operation of the gas discharge lamp is guaranteed within the permissible operating range for the heating power.

The invention is explained in more detail below on the basis of exemplary embodiments with reference to the drawings.

• Fig. 1 ein Ausführungsbeispiel eines elektronischen Vorschaltgerätes,
• Fig. 2 Steuersignale der Schalter S1 und S3 gemäß Fig. 1,
• Fig. 3 eine Variante des in Fig. 1 vorgesehenen Heiztransformators,
• Fig. 4 ein Beispiel eines einstellbaren Heizstromes in Abhängigkeit vom Dimmgrad,
• Fig. 5 eine Darstellung des effektiven Heizstroms in Abhängigkeit von der Pulsanzahl und
• Fig. 6 eine Darstellung der effektiven Heizspannung bzw. Heizleistung in Abhängigkeit von der Pulsanzahl.

Show it:

• 1 shows an embodiment of an electronic ballast,
• 2 control signals of the switches S1 and S3 of FIG. 1,
• 3 shows a variant of the heating transformer provided in FIG. 1,
• 4 shows an example of an adjustable heating current as a function of the degree of dimming,
• Fig. 5 shows the effective heating current as a function of the number of pulses and
• 6 shows the effective heating voltage or heating power as a function of the number of pulses.

FIG. 1 shows the essential parts of an exemplary embodiment of a ballast for a gas discharge lamp. This initially has an inverter, which consists of the controllable switches S1 and S2, which are controlled in a push-pull manner by means of an inverter control circuit 1. This means that one switch is switched on and the other is switched off. The two inverter switches S1 and S2 are connected in series between a positive supply voltage and ground. The load circuit is connected to the common node of the two inverter switches S1, S2. This consists of a series resonance circuit, which is composed of a resonance circuit coil L1 and a resonance circuit capacitor C3. The resonant circuit capacitor C3 has one terminal connected to ground. One connection of a coupling capacitor C2 is connected to the connection node between the resonant circuit capacitor C3 and the resonant circuit coil L1. The other connection of the coupling capacitor C2 is connected to one of two cathodes, a gas discharge lamp LA. The two cathodes of the gas discharge lamp LA each have two connections, between each of which a heating coil is provided for heating the respective cathode. The electrode of the gas discharge lamp LA which is not connected to the coupling capacitor C2 is connected to ground with a connection K3.

Furthermore, a heating transformer T is provided which has two windings T1 and T3 on the primary side and two windings T2 and T4 on the secondary side. The one winding T1 on the primary side is connected with its one connection to the connecting node of the two inverter switches S1 and S2 and with its second connection it is connected to the second winding T3 on the primary side. This is again connected to a connection of a further controllable switch S3. The second connection of the further controllable switch S3 is in turn connected to a resistor R1, which on the other hand is connected to ground. This results in a series connection of the two primary-side windings of the heating transformer, the further controllable switch S3 and the resistor R1, which are connected in parallel to the inverter switch S2.

The further controllable switch S3 is in turn operated by a switching controller 2 assigned to it.

The two windings T2 and T4 on the secondary side of the heating transformer T are connected in series with respective diodes D3 and D4 to one of the two electrodes of the gas discharge lamp LA. Thus, the winding T2 is connected via the diode D3 to the heating coil connections K3 and K4 of the one electrode and the winding T4 is connected via the diode D4 to the heating coil connections K1 and K2 of the second electrode.

Finally, a diode D1 with its anode is connected to the connection node between the further switch S3 and the secondary winding T3, the cathode of which is connected to the positive supply voltage. Finally, a diode D2 is connected in parallel to the inverter switch S2, the anode connection of which is connected to ground. This diode can be omitted when using FET transistors, provided that the transistors used have already integrated a diode circuit.

An advantageous operation of the circuit arrangement described above is described below with reference to FIGS. 1 and 2.

In FIG. 2, control signals for the inverter switch S1 are shown in curve I, which periodically change between levels L and H. The period length is PO, the signal being at level H over the duration tO. Now it is assumed that the inverter switch S1 is closed as long as it is driven with the H level. At the same time, the second inverter switch S2 switches, as already explained, alternating with the switch S1.

For the sake of completeness, it should be mentioned that an oscillation of the period of the control signals at the inverter switches forms in the connected series resonance circuit, when the period PO is close is the resonance frequency of the series resonance circuit, the gas discharge lamp LA ignites.

By increasing the switching frequency of the inverter above the resonance frequency of the resonance circuit, the lamp is dimmed, i.e. the more the period PO deviates from the resonance frequency of the series resonance circuit, the more the lamp is dimmed, i.e. the darker it is. As already mentioned, such an operation alone would have the consequence that the lamp would age to an increased extent.

It is therefore provided that the control circuit 2 of the further controlled switch S3 is coupled to the inverter control circuit 1 via a coupling 3. According to FIG. 3, this is done in such a way that the further controllable switch S3 is only switched on when the inverter switch S1 is also switched on. is. This corresponds to the synchronization of the control signals according to curve I and the control signals for switch S3 according to curves II to VIII.

Curves I and II are identical, so that switches S1 and S3 switch in common mode and are simultaneously switched on and off. It follows from this that when the switch S1 is switched on, a primary-side current simultaneously flows through the windings T1 and T3 of the heating transformer T via the closed further controllable switch S3 and the resistor R1 from the positive supply voltage connection to ground. Since, according to curve II in FIG. 3, there is an interrupted current, ie no direct current, a voltage is induced with the same clock rate on the secondary side of the heating transformer, ie in the windings T2 and T4, according to the laws of magnetic induction. which results in a current which, as described, flows through the heating windings connected to it and thus leads to heating of the electrodes of the gas discharge lamp LA. As soon as the further controllable switch S3 is opened, the current flow through the windings T1 and T3 is interrupted, so that, due to the known physical laws, there is an abrupt voltage increase at the connection node between the switch S3 and the winding T3. This voltage rise is limited by the diode D1 to the value of the positive supply voltage. In the switching phases in which the switch S3 is open, the discharge of the energy stored in the heating transformer thus results in demagnetization. Diodes D3 and D4 are also necessary for demagnetizing the heating transformer T.

As already mentioned above, the gas discharge lamp LA can ultimately be dimmed by changing the clock frequency of the inverter. However, in order not to unnecessarily reduce the service life of a gas discharge lamp, it is necessary to adapt the heating power of the electrode heater in accordance with the degree of dimming. This can be done according to curves II to VII in FIG. 2 in that the further controlled switch S3 is not also switched on each time the inverter switch S1 is switched on. Rather, longer clock periods P1 to P5 can be provided, for example, the period durations P1 to P5 being an integer multiple of the clock period of the inverter switch S1.

In other words, with undimmed or weakly dimmed operation of the lamp, no or only a low heating power is necessary, so that, for example, a switching period P5 according to curve VII could be provided for the further controlled switch S3. In addition, there is the possibility that in undimmed operation the heat development of the electrodes is so great that no additional heating of the electrodes is necessary. In this case, the further controllable switch S3 remains open, so that no energy is consumed for additional heating power, as a result of which the arrangement operates with the greatest possible efficiency. It is therefore only necessary to apply heating power when dimming does not generate enough heat.

Along with a stronger dimming of the gas discharge lamp LA, in order to provide a higher heating power, i.e. of a more frequently recurring heating current, the cycle period can be reduced at the further switch S3 until it reaches a maximum heating power at the highest dimming level. This corresponds to curve II, in which, as already mentioned above, switches S3 and S1 operate in common mode.

Another alternative and also additionally applicable option for setting the heating power is to change the switching period in which the switch S3 is switched on. While in curves II to VII the switch S3 is switched on for as long as the switch S1, and only the time in which the switch S3 is switched off is varied, according to curve VIII the switch S3 is switched on significantly shorter. This also reduces the heating current that can be achieved by the heating coil.

In the illustration according to FIG. 4, curves 1 and 3 show the maximum and minimum heating current in recommended by the manufacturer of a gas discharge lamp Dependence on the degree of dimming is shown. Curve 2 shows the heating current that can be achieved with the circuit and control method described above. It is thus clear from FIG. 4 that a heating current for the gas discharge lamp is easily adjustable, which is just above the minimally required heating current.

FIG. 5 shows a representation which shows the adjustable heating current as a function of the number of switch-on pulses omitted with respect to the switch S1, i.e. which represents a correspondingly extended period. If the number of missed switch-on pulses is 0 - as in II of Figure 2 - a maximum heating current can be achieved. This value can be reduced almost continuously.

Furthermore, according to FIG. 6, there is a maximum heating voltage, a maximum heating output being achievable with this voltage, as is also shown in FIG. 6. Corresponding to the continuously changeable heating current and heating voltage, a continuous change in the heating power can thus be set, as shown.

This shows that the heating power can be adjusted using simple means, essentially independently of the operation of the gas discharge lamp. This leads to the fact that the circuit explained above can be used not only to adjust the degree of dimming but also to adapt the heating power when using different lamps to their requirements both during preheating and in dimming operation.

According to Figure 3, a circuit variant of the heating transformer T is shown. In this case, there are not two windings connected in series on the primary side, but rather a single winding T1 'wound over a core, which windings operate the secondary-side windings T2 and T4.

The circuit arrangements described above are initially purely a controller, in which the dependency between the switching frequency PO of the inverter and the switching frequency of the further controllable switch S3 is predetermined. Nevertheless, regulation is of course also conceivable. In such a case, the lamp current is measured in a manner known per se and supplied to the control circuits 1 and 2 (not shown). In addition, a voltage proportional to the heating current can be measured at the resistor R1 and the value of the heating current via a heating current detector 4 as a signal Inverter control circuits are supplied. In this way, the heating current can be set directly as a function of the lamp current by means of regulation.

This arrangement also has the advantage that it is also possible to detect when there is a filament break, that is to say the lamp is defective and when the lamp has been removed from the arrangement. Through this detection via the heating current detector 4 and the forwarding of the information to the inverter control circuit 1, the lamp voltage can be reduced immediately by changing the inverter frequency. Likewise, a reinsertion of the lamp LA is detected, so that the inverter can be operated in such a way that the lamp LA automatically ignites after the insertion.

Claims (17)

Ballast for at least one gas discharge lamp an inverter which has two switches (S1, S2) connected in series to a DC voltage source and connected in push-pull, - A load circuit connected to the inverter, which contains a series resonance circuit (L1, C1) and the lamp (LA), and a heating circuit (T, S3, R1) also connected to the inverter for powering the lamp filaments, which contains a further controllable switch (S3) for controlling the heating current,
characterized,
that the heating circuit is connected in parallel to one of the two switches of the inverter. Ballast according to claim 1,
characterized,
that the further controllable switch (S3) is switched so that the lamp (LA) is only supplied with heating current when the inverter switch (S2), to which the load circuit is connected in parallel, is open. Ballast according to claim 1 or 2, wherein the load circuit is parallel to a switch of the inverter,
characterized,
that the heating circuit (T, S3, R1) is connected in parallel to the switch of the inverter, to which the load circuit (L1, C1, LA) is also connected in parallel. Ballast according to claim 1 or 2,
characterized,
that the heating circuit (T, S3, R1) is connected in parallel to the switch (S1), which is connected in series with the switch (S2) of the inverter to which the load circuit (L1, C1, LA) lies in parallel. Ballast according to one of claims 1 to 4,
characterized,
that the switching period of the further controllable switch (S3) can be changed by an integral multiple of the clock period of the inverter. Ballast according to one of claims 1 to 5,
characterized,
that the period in which the further controllable switch (S3) supplies the heating device with heating current is shorter than the period in which the inverter switch (S2), to which the load circuit is parallel, is open. Ballast according to claim 6,
characterized,
that the time period in which the further controllable switch (S3) supplies the heating circuit with current is adjustable. Ballast according to claim 7,
characterized,
that the period and / or the switching period is set as a function of the current in the load circuit (L1, C1, LA). Ballast according to one of claims 1 to 8,
characterized,
that the heating circuit has a heating transformer (T) which is connected in parallel on the primary side to one of the two inverter switches (S1, S2) and is connected on the secondary side to the lamp filaments. Ballast according to claim 9,
characterized,
that the heating transformer (T) has its own secondary-side winding (T2, T4) for each lamp filament and has a common winding (T1 ') on the primary side or the windings (T1, T3) corresponding to the secondary-side windings (T2, T4), which are connected in series with one another are connected. Ballast according to one of claims 9 and 10,
characterized,
that the further controllable switch (S3) is connected to the primary side of the heating transformer (T) in series connection. Ballast according to one of claims 1 to 11,
characterized,
that in series with the further controllable switch (S3) there is an impedance, preferably an ohmic resistance, the voltage drop across this impedance being a detection signal for a flowing heating current and thus for a heating coil break or the absence or a gas defect of the lamp (LA) is used. Ballast according to claim 12,
characterized,
that the voltage drop across this impedance is used as a detection signal for reinstalling a lamp. Ballast according to one of claims 9 to 12,
characterized,
that a circuit arrangement for demagnetizing the heating transformer (T) is provided which demagnetizes the heating transformer (T) when it does not supply any heating current to the lamp filaments. Method for operating a ballast according to one of the preceding claims,
characterized,
that with the further controllable switch after ignition of the lamp, the heating is switched off until further notice. Method for operating a gas discharge lamp with a ballast according to one of claims 1 to 15,
in which, when the lamp electrodes are preheated, the switches (S1, S2) of the inverter are operated at the maximum clock frequency and
in which, to ignite the gas discharge lamp (LA), the clock frequency is reduced to near the resonance frequency of the series resonance circuit (L1, C3),
characterized,
that when preheating the further controllable switch (S3) is operated at a clock frequency which enables heating with the maximum permissible heating power. A method according to claim 16,
characterized,
that after the ignition of the gas discharge lamp (LA), the clock frequency of the further controllable switch (S3) is set as a function of a dimming state of the gas discharge lamp (LA), so that the heating power between that for the Gas discharge lamp (LA) is the maximum permitted and the minimum required heating power.

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