EP2468078B1 - Electronic ballast and method for operating at least one discharge lamp - Google Patents

Description

Technical area

The present invention relates to an electronic ballast for operating at least one discharge lamp having an input with a first and a second input terminal for coupling with a DC supply voltage, an output having a first and a second output terminal for coupling to the at least one discharge lamp, a bridge circuit having at least one first and a second electronic switch, wherein a series circuit of the first and the second electronic switch is coupled to form a first bridge center between the first and the second input terminal, a lamp inductor coupled between the first bridge center and the first output terminal, at least one trapezoidal capacitor coupled in parallel with one of the electronic switches, a resonant capacitor coupled in parallel with the first and second output terminals, a control device for driving g of at least the first and the second electronic switch with a drive signal, wherein the drive signal during a preheating phase of a first and a second coil of at least one connected between the first and the second output terminal discharge lamp has an operating frequency, wherein the control device is designed, during an ignition phase of Discharge lamp to lower the operating frequency, and a heater, which is coupled to the control device, wherein the heater is further coupled to the first bridge center and configured to heat the first and second filaments of the discharge lamp. It also relates to a corresponding method for operating at least one discharge lamp.

State of the art

The EP0707438 A2 shows an electronic ballast for a gas discharge lamp, wherein the ballast heats the lamp filaments according to the Dimmgrad during a dimming phase.

The DE102005013564 A1 shows a circuit arrangement and method for operating at least one lamp having a first and a second coil, wherein the lamp comprises a device for determining a size which is correlated with the temperature of at least one of the two coils.

The DE29514817U1 discloses an electronic ballast having a heating circuit that can be independently controlled and that the electronic ballast heats the lamp filaments when the electronic Vorschalgerät is in dimming mode.

The problem underlying the present invention results from the design of so-called multi-lamp ECGs (electronic ballasts), that is electronic ballasts, for the operation of different discharge lamps, in particular low-pressure discharge lamps. In a typical multi-lamp electronic ballast, the power range of the discharge lamps that can be operated with it ranges from 14 W to 80 W.

In this case, by default, the filaments of the at least one discharge lamp connected to the electronic ballast are heated during a preheating phase. Shortly before the start of the ignition phase, the heating of the coils is switched off. In order to carry out the corresponding switching operations, in known generic electronic ballasts, the control device is implemented as an ASIC (application specific integrated circuit), wherein such an integrated circuit operates as a so-called state machine. Thereafter, states can only be processed serially, that is, in the present case, the preheating phase, followed by the ignition phase.

Since the DC supply voltage is independent of the respectively connected discharge lamp, the only possibility for adaptation to the respective connected Discharge lamp is to vary the operating frequency of the drive signal for the first and the second electronic switch. Correspondingly, the spectrum of the operating frequencies in the mentioned example ranges from 90 kHz (for 14 W lamps) to 45 kHz (for 80 W lamps).

However, different boundary conditions have to be met: On the one hand, the resonance capacitor is chosen to be small, for example 2.2 nF, in order to generate the least possible losses. Since in generic electronic ballasts with particularly high efficiency preferably active components, in particular the control device, from an AC voltage source in the load circuit, preferably the trapezoidal capacitor, are supplied with energy, this is to be dimensioned so that it at the lower limit frequency, ie in the example 45 kHz, provides sufficient power. He should therefore be sized as large as possible; in the example about 1 nF.

The ratio of the capacitance of the resonant capacitor to the capacitance of the trapezoidal capacitor determines the slope of the edge of the voltage U _HBM at the first bridge center of the bridge circuit. Regarding Fig. 1 Curve 1) shows the progression for a large ratio called Curve 2) for a small ratio called. Like the presentation of Fig. 1 It can also be seen that curve 2) reduces the time period t _S2 in comparison to the time period t _S1 . Accordingly, the smaller this ratio is, the more difficult it is to ensure a switching-relieved oscillation of the bridge circuit, since the switching process triggering the oscillation has to take place within the periods t _S1 , t _S2 .

If the switching operation is performed when the voltage U _HBM at the first bridge center is still or not equal to zero again, then "hard" is switched. This results in undesirable losses and in excessive stress and thus a shortening of the lifetime of the components involved.

Particularly short is the period t _S2 when operating at high frequencies. In this context, it should be noted that the switches of the bridge circuit for the purpose of preheating and ignition must be operated at a significantly higher frequency than as for the above-mentioned normal operation. In particular, when a discharge lamp with low power is connected to the electronic ballast, the frequency must not be too low during the heating of the filaments of the discharge lamp, otherwise there is a risk of pre-ignition when the OCV (Open Circuit Voltage) of the discharge lamp is exceeded.

In sum, it can thus be stated that precisely when the electronic ballast is designed with regard to loss, reliable auxiliary voltage source and avoidance of the risk of spark ignition, there is a voltage curve of the voltage U _HBM at the first half-bridge center, which tends to follow the curve 2) in FIG Fig. 1 in fact, even has a shorter phase t _S2 . This results in such a designed electronic ballast taking into account usually fluctuating DC supply voltage - it is so usually derived from the known fluctuating mains voltage - tolerance of the components, ambient temperature, lamp aging, etc. only extremely difficult can be operated at different, connected discharge lamps in switch-unloaded operation. As analyzes have shown, this is particularly critical in the area of the ignition phase.

In order to ensure a switch-relieved operation, the resonance capacitor has been designed larger in the prior art, as actually desired, with the consequence of increased losses.

Presentation of the invention

The present invention is therefore the object of developing a generic electronic ballast or a generic method such that a reliable switch-unloaded operation with minimum losses especially in the ignition phase of the discharge lamp can be made sure.

This object is achieved by an electronic ballast having the features of claim 1 and by a method having the features of claim 10.

The present invention is based on the finding that a safe swing, corresponding to a reliable switch-unloaded operation, even with a small ratio of capacitance of the resonant capacitor to capacitance of the trapezoidal capacitor can be ensured if in the load circuit of the electronic ballast, that is, in particular by the lamp inductor, always one sufficiently large current flows, which provides the necessary reactive energy for the swinging of the bridge circuit. A closer analysis of the procedure in the prior art leads to the realization that there due to the shutdown of the consumer, ie fed from the load circuit Wendelheizung, before the start of the ignition phase of the discharge lamp leads to a rapid drop in the load circuit existing reactive energy. This is the root cause of the problems of the prior art. Since, according to the invention, the heating of the filaments remains activated beyond the beginning of the ignition phase, there is no dip in the power consumption from the load circuit since the heating device is still supplied from it. Accordingly, there is enough reactive energy for a reliable switching of the bridge circuit available.

Exactly considered, therefore, the bridge circuit was still operated during the pre-heating circuit relieved, since energy was consumed by the heating of the coils in the prior art. During the transition from the filament heater to the ignition phase, the heater was switched off before the start of the ignition phase. As a result, no power was consumed in the output circuit, since the discharge lamp had not yet been ignited. Due to the lack of power reduction, the current through the lamp choke was small and thus there was little reactive energy in the load circuit. This prolonged the swinging of the current through the lamp choke. This could lead to the point that the voltage U _HBM at the first bridge center no longer became zero. The switches of the bridge circuit switched with this no longer relieved switching. In the bridge circuit cross currents flowed, which even with appropriate monitoring could lead to the shutdown of the electronic ballast.

Due to the fact that, in the procedure according to the invention, the preheating of the filaments continues into the ignition phase, a sufficiently large current flows in the load circuit even in this phase. There is therefore a sufficient amount of reactive energy available, which ensures a safe swinging of the circuit arrangement after the ignition of the discharge lamp. This results in the advantage that in an electronic ballast according to the invention, the resonant capacitor can be made small, resulting in a reduction of the power loss. By heating the filaments during ignition, moreover, the switching numbers of the discharge lamp can be significantly increased, since the discharge lamp ignites at a lower voltage. This results from the fact that in the present case by the continued operation of the heater ignition at higher temperatures than in the prior art, since at higher temperatures more electrons are present. Moreover, in the procedure according to the invention, the lamp inductor is not loaded as heavily as in the prior art, since the load changes are not abrupt, but continuous. This leads to a further increase in the service life.

Another advantage of an electronic ballast according to the invention is that it also lamp replacement resistors can be operated without a takeover problem, as by the invention load jumps after preheating in conjunction with high operating frequencies of Bridge circuit can be reliably prevented by the continued operation of the heater.

Preferably, the heating of the coils is maintained until the discharge lamp has ignited. After ignition, a sufficiently large current flows in the output circuit anyway, which reduces the transient times and ensures that the switches of the bridge circuit are operated switch-relieved.

In a preferred embodiment, the predetermined period of time from the beginning of the reduction of the operating frequency is at least 20 ms, preferably at least 50 ms, more preferably at least 100 ms. In this case, the duration of the predetermined period of time is correlated with the speed with which the operating frequency is lowered. Even a brief overlap of the heating and the ignition phase brings advantages with regard to an improved swinging of the bridge circuit of the electronic ballast. This tends to be further improved by extending the period. Thus, in a particularly preferred embodiment, the predetermined period of time is such that it extends until after the time of ignition of the discharge lamp.

Before its lowering, that is during the preheating phase, the operating frequency is between 100 and 150 kHz. When the discharge lamp is ignited, the operating frequency is preferably between 50 and 100 kHz.

The control device is preferably designed, the operating frequency after ignition of the discharge lamp to a Set nominal frequency. This can be between 40 and 95 kHz.

As already mentioned, the electronic ballast may further comprise an auxiliary voltage source, which is coupled to its supply, in particular using a charge pump, with a trapezoidal capacitor. This provides a possibility, particularly low-loss to realize a voltage source whose amplitude is significantly lower than that of the DC supply voltage. As mentioned, the control device is particularly preferably coupled to its supply with the auxiliary voltage source.

In addition, the control device may be designed to deactivate the heating device as soon as an ignition of the discharge lamp can be detected. An ignition of the discharge lamp can be determined in a simple manner by monitoring the lamp burning voltage. Since after ignition of the lamp in the load circuit, a large current flows, so a consumer is present, the heater can be easily switched off without jeopardizing a safe swinging of the bridge circuit.

Further advantageous embodiments will become apparent from the dependent claims.

The preferred embodiments presented with reference to an electronic ballast according to the invention and their advantages apply correspondingly, if applicable, for the method according to the invention.

Short description of the drawing (s)

Fig. 1

in schematischer Darstellung den zeitlichen Verlauf der Spannung U_HBM am ersten Brückenmittelpunkt bei unterschiedlicher Dimensionierung des Resonanzkondensators nach dem Stand der Technik;

Fig. 2

in schematischer Darstellung den Aufbau eines gattungsgemäßen elektronischen Vorschaltgeräts;

Fig. 3

den zeitlichen Verlauf verschiedener elektrischer Größen des Aufbau von Fig. 2 bei einer Auslegung des elektronischen Vorschaltgeräts nach dem Stand der Technik;

Fig. 4

den zeitlichen Verlauf der Betriebsfrequenz im Ansteuersignal der elektronischen Schalter der Brückenschaltung der Schaltungsanordnung von Fig. 2 bei einem Ausführungsbeispiel einer erfindungsgemäßem Auslegung; und

Fig. 5

den zeitlicher Verlauf verschiedener elektrischer Größen des Aufbaus von Fig. 2 bei einer erfindungsgemäßen Auslegung des elektronischen Vorschaltgeräts.

In the following, an embodiment of the invention will now be described in more detail with reference to the accompanying drawings. Show it:

Fig. 1

a schematic representation of the time course of the voltage U _HBM at the first bridge center at different dimensions of the resonance capacitor according to the prior art;

Fig. 2

in a schematic representation of the structure of a generic electronic ballast;

Fig. 3

the time course of different electrical quantities of the structure of Fig. 2 in a design of the electronic ballast according to the prior art;

Fig. 4

the time course of the operating frequency in the drive signal of the electronic switch of the bridge circuit of the circuit of Fig. 2 in an embodiment of an inventive design; and

Fig. 5

the time course of various electrical variables of the construction of Fig. 2 in an inventive design of the electronic ballast.

Preferred embodiment of the invention

Fig. 2 shows a schematic representation of the structure of an electronic ballast. This structure is in and of itself known from the prior art, wherein the invention in one, in Fig. 2 initially unrecognizable interpretation of yet to be introduced control device 12 reflects.

This in Fig. 2 illustrated electronic ballast comprises an input with a first E1 and a second input terminal E2, between which a DC supply voltage, preferably the so-called intermediate _circuit voltage U _Zw , is applied. In order to support the intermediate _{circuit voltage} U _Zw , a storage capacitor C _{1 is connected in} parallel with the input. The storage capacitor C ₁ feeds an inverter 10, which comprises a bridge circuit, which in the present case is realized as a half-bridge arrangement. This in turn comprises the series connection of an electronic switch S1 and an electronic switch S2, between which a first half-bridge center HBM1 is formed, and two coupling capacitors C _K1 and C _K2 , between which a second half-bridge center HBM2 is formed. A lamp inductor L1 is coupled between the first half-bridge center HBM1 and a first output terminal A1 of the electronic ballast. A second output terminal A2 is coupled to the second half-bridge center HBM2. Between the output terminals A1, A2, a discharge lamp La is coupled, which comprises a first W1 and a second coil W2. Parallel to the switch S2, a trapezoidal capacitor C _{T is} coupled. A resonance capacitor C _R is coupled between the first output terminal A1 and the reference potential, which in the present case represents the second input terminal E2. The electronic ballast further comprises a control device 12, which via an output AL, the switch S2 and via an output AH drives the switch S1 with a drive signal having an operating frequency.

This in Fig. 2 illustrated electronic ballast further comprises a heater. For this purpose, the series connection of the primary winding La of a transformer TR, a capacitor C ₂ and an electronic switch S3 between the first half-bridge center HBM1 and the reference potential is coupled. A first secondary winding Lb1 is coupled to the first filament W1 of the discharge lamp La, and a second secondary winding Lb2 is coupled to the second filament W2 of the discharge lamp La. The switch S3 is also driven by the control device 12 and via an output AS3. Accordingly, if the switch S3 is turned on, a current flows through the primary winding La of the transformer Tr, whereby a current flow through the respective secondary winding Lb1, Lb2 is generated. This leads to heating of the coils W1, W2 of the discharge lamp La.

To realize an auxiliary voltage source U _H , a half-wave rectifier comprising the diodes D1 and D2 is coupled to the trapezoidal capacitor C _T. To integrate the voltage provided at the output of the rectifier D1, D2, a capacitor C _{3 is used} , to which a Zener diode Z1 is connected in parallel. The auxiliary voltage source U _H is coupled to the terminal VCC of the control device 12 and supplies them with energy.

The voltage drop across the output terminals A1, A2 is designated by U _La , the current flowing through the lamp inductor L1 to I _L1 , which activates the switch S3 Current with I _GS3 and the current flowing through the switch S2 current with I _S2 .

Fig. 3 shows the time course of the variables I _GS3 , U _La , I _L1 , I _S2 in a design of the electronic ballast according to Fig. 2 according to the prior art, wherein Fig. 3b a section of Fig. 3a shortly after the time t ₁ in a significantly enlarged resolution shows. How out Fig. 3a can be clearly seen, the switch S3 is turned off at the time t ₁ , whereby the preheating of the helices W1, W2 is terminated. Following this, the operating frequency of the switches S1, S2 is continuously reduced, whereby starting from above, ie starting from higher frequencies, the resonant frequency of the load circuit approximates. This leads to an increase in the voltage U _La , which leads to the ignition of the discharge lamp La at the time t ₂ . The increase of the voltage U _La during the period between t ₁ and t ₂ is correlated with an increase of the current I _S2 . After ignition of the discharge lamp La at time t ₂ , the current I _L1 increases significantly.

As Fig. 3b can be seen, the current I _S2 current peaks, which are an indication that the switches S1, S2 of the half-bridge arrangement is not soft, ie not switch relieved, are switched. If the current I _S2 , as usual, for example, using a shunt resistor, measured and fed over the shunt resistor voltage drop of the control device 12, the detection of such current peaks can lead to a shutdown of the electronic ballast. Fig. 4 shows the time course of the operating frequency in the drive signals, at the outputs AH, AL of the control device 12 to the switches S1, S2 in an inventive design of the electronic ballast according to Fig. 2 to be provided. Accordingly, the operating frequency f until the time t _{1 is} the value f _heat , the frequency commonly used for preheating. At time t ₁ , that is at the beginning of the ignition phase, the operating frequency f is gradually lowered until the time t _2, the ignition of the discharge lamp La takes place. Then the operating frequency f is lowered to a nominal frequency f _nom . In the present case, the ignition phase extends over the period of time which lies between t ₂ and t ₁ and is referred to herein as T _Z. While in the prior art, the switch S3 has been turned off at the time t ₁ , in this case extends the heating phase T _heat beyond the time t ₁ ; it may, as indicated by the dotted line, even extend beyond the time t ₂ out.

In an interpretation of the in Fig. 2 shown electronic ballast according to Fig. 4 , the result in Fig. 5 shown time profiles of the electronic variables U _La , I _GS3 , I _L1 , I _S2 ., where Fig. 5b turn a section of Fig. 5a shortly after the time t ₁ in a significantly enlarged resolution shows. The beginning of the ignition phase is again marked with t ₁ , the time of ignition with t ₂ . As can be seen in the course of the current I _GS3 , now the helices W1, W2 of the discharge lamp La are heated up to the time t ₃ , that is to say significantly beyond the time t _{2 of} the ignition of the discharge lamp La.

Fig. 5b shows in enlarged resolution the specially marked area of Fig. 5a , shortly before the ignition of the discharge lamp La in an enlarged resolution. As can clearly be seen in the course of the current I _S2 , there are now no more current peaks; the switches S1, S2 of the half-bridge arrangement are therefore switched soft.

Claims (10)

1. Electronic ballast for operating at least one discharge lamp (La) comprising

   - an input comprising a first input terminal (E1) and a second input terminal (E2) for coupling to a DC supply voltage (U_Zw);

   - an output comprising a first output terminal (A1) and a second output terminal (A2) for coupling to the at least one discharge lamp (La);

   - a bridge circuit (10) comprising at least one first electronic switch (S1) and one second electronic switch (S2), wherein a series connection of the first electronic switch (S1) and the second electronic switch (S2) with formation of a first bridge centre point (HBM1) is coupled between the first input terminal (E1) and the second input terminal (E2);

   - a lamp inductor (L1) coupled between the first bridge centre point (HBM1) and the first output terminal (A1);

   - a resonance capacitor (C_R) coupled in parallel with the first output terminal (A1) and the second output terminal (A2);

   - a control device (12) for driving at least the first electronic switch (S1) and the second electronic switch (S2) with a drive signal (AL, AH), wherein the drive signal (AL, AH) has an operating frequency (f) during a preheating phase (T_heat) of a first filament (W1) and a second filament (W2) of at least one discharge lamp (La) connected between the first output terminal (A1) and the second output terminal (A2), wherein the control device (12) is designed to reduce the operating frequency (f) during an ignition phase (T_Z) of the discharge lamp (La); and

   - a heating device (T_R, La, Lb1, Lb2, S3, C₂) coupled to the control device (12), wherein the heating device (T_R, La, Lb1, Lb2, S3, C₂) is furthermore coupled to the first bridge centre point (HBM1) and designed to heat the first filament (W1) and the second filament (W2) of the discharge lamp (La);

   characterized
   in that the control device (12) furthermore has at least one trapezoidal capacitor (CT) coupled in parallel with one of the electronic switches (S1, S2) and is designed to activate the heating device (T_R, La, Lb1, Lb2, S3, C₂), which has a series connection of a transformer (Tr), a capacitor (C2) and a switch (S3), at least during a predetermined time period of the ignition phase (T_Z), wherein the predetermined time period (t₃-t₂) is dimensioned in such a way that it extends until after the point in time of the ignition of the discharge lamp (La).
2. Electronic ballast according to Claim 1,
   characterized
   in that the predetermined time period (t₃-t₂) starting from the beginning of the reduction of the operating frequency (f) is at least 20 ms, preferably at least 50 ms, even more preferably at least 100 ms.
3. Electronic ballast according to either of the preceding claims,
   characterized
   in that the operating frequency (f) before its reduction is between 100 and 150 kHz.
4. Electronic ballast according to any of the preceding claims,
   characterized
   in that the operating frequency (f) upon ignition of the discharge lamp (La) is between 50 and 100 kHz.
5. Electronic ballast according to any of the preceding claims,
   characterized
   in that the control device (12) is designed to set the operating frequency (f) to a nominal frequency (f_nom) after ignition of the discharge lamp (La).
6. Electronic ballast according to Claim 5,
   characterized
   in that the nominal frequency (f_nom) is between 40 and 95 kHz.
7. Electronic ballast according to any of the preceding claims,
   characterized
   in that the electronic ballast furthermore comprises an auxiliary voltage source (U_H), which is coupled to the trapezoidal capacitor (C_T) for its supply, in particular using a charge pump.
8. Electronic ballast according to Claim 7,
   characterized
   in that the control device (12) is coupled to the auxiliary voltage source (U_H) for its supply.
9. Electronic ballast according to any of the preceding claims,
   characterized
   in that the control device (12) is designed to deactivate the heating device (T_R, La, Lb1, Lb2, S3, C₂) as soon as an ignition of the discharge lamp (La) is determinable.
10. Method for operating at least one discharge lamp (La) on an electronic ballast comprising an input comprising a first input terminal (E1) and a second input terminal (E2) for coupling to a DC supply voltage (U_Zw); an output comprising a first output terminal (A1) and a second output terminal (A2) for coupling to the at least one discharge lamp (La); a bridge circuit comprising at least one first electronic switch (S1) and one second electronic switch (S2), wherein a series connection of the first electronic switch (S1) and the second electronic switch (S2) with formation of a first bridge centre point (HBM1) is coupled between the first input terminal (E1) and the second input terminal (E2); a lamp inductor (L1) coupled between the first bridge centre point (HBM1) and the first output terminal (A1); at least one trapezoidal capacitor (C_T) coupled in parallel with one of the electronic switches (S1, S2); a resonance capacitor (C_R) coupled in parallel with the first output terminal (A1) and the second output terminal (A2); a control device (12) for driving at least the first electronic switch (S1) and the second electronic switch (S2) with a drive signal (AL, AH), wherein the drive signal (AL, AH) has an operating frequency (f) during a preheating phase (T_heat) of a first filament (W1) and a second filament (W2) of at least one discharge lamp (La) connected between the first output terminal (A1) and the second output terminal (A2), wherein the control device (12) is designed to reduce the operating frequency (f) during an ignition phase (T_Z) of the discharge lamp (La); and a heating device (T_R, La, Lb1, Lb2, S3, C₂) coupled to the control device (12), wherein the heating device (T_R, La, Lb1, Lb2, S3, C₂) is furthermore coupled to the first bridge centre point (HBM1) and designed to heat the first filament (W1) and the second filament (W2) of the discharge lamp (La);
    characterized by the following step:

    activating the heating device (T_R, La, Lb1, Lb2, S3, C₂) at least during a predetermined time period of the ignition phase (T_Z),

    wherein the predetermined time period (t₃-t₂) is dimensioned in such a way that it extends until after the point in time of the ignition of the discharge lamp (La).

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