EP1103165B1 - Electronic ballast for at least one low-pressure discharge lamp - Google Patents

Abstract

An electronic ballast for a low-pressure discharge lamp (LA) contains an inverter that is fed with direct voltage (UBUS) and the output of which is connected to a load circuit containing terminal contacts for the lamp (LA), a heating transformer that has a primary winding (Tp), which is connected to the output of the inverter, and a respective secondary winding (Ts1, Ts2), which is located in a heating circuit with a coil (W1, W2), for heating each of the two electrodes of the lamp (LA), and a series circuit arrangement that is connected in parallel with the load circuit and which contains the primary winding (Tp) of the heating transformer and an electronic switch arrangement (S3, S4). For the purpose of identifying the type of lamp and the state of the lamp, the currents in one of the two heating circuits and in the primary winding (Tp) of the heating transformer are measured and evaluated.

Description

The present invention relates to an electronic ballast for operation at least one low-pressure discharge lamp according to the preamble of claim 1.

Ballasts (ECGs) are usually used nowadays high-frequency alternating voltage to the gas discharge lamps or fluorescent tubes submit. Apart from the power supply, such electronic ones are used Ballasts also to preheat the electrodes of the gas discharge lamps and gently ignite and operate the lamps. With their help the Efficiency of the lamps increased, a longer lifespan achieved and a Operation under reduced lamp power (dimming) enables.

Here, before the ignition voltage is applied to the discharge lamp Electrodes or the filaments of the lamp usually for a certain time preheated, which means a gentler lamp start and thus a longer service life the lamp is achieved. The preheating takes place with the help of a spiral heating, which causes a current to flow through the two coils. In one from EP 0 707 438 A3 known ballast, a heating transformer is used, the Primary winding is connected to the output of an inverter and the two Has secondary windings, each with one of the two lamp filaments are coupled. Before the discharge lamp is ignited, one is opposite the Resonance frequency of the series resonance circuit such a changed frequency for the the AC voltage output set that the at the Discharge lamp applied voltage initially does not cause the lamp to ignite. Meanwhile flows through the two secondary heating circuits with the lamp filaments an essentially constant current, which preheats them. After one for the preheating period, the frequency of the Series resonance circuit supplied AC voltage in the direction of the long Resonance frequency shifted until the thereby increasing on the discharge lamp applied voltage causes the lamp to ignite. According to the EP 0 748 146 A1 or DE 295 14 817 U1 can then be opened in series by opening one with the primary winding lying the coil heating after the ignition of the Lamp are switched off in order to avoid otherwise occurring power losses to reduce.

The requirements for electronic ballasts are always extensive. So it is common, for example, also a dimmer for the Provide gas discharge lamp. A strong dimming would, however, cool down the Lamp electrodes below their emission temperature and thus premature aging the lamp. In order to counteract this effect, the electrodes of the Gas discharge lamp to a certain extent even when it is already on be heated. In particular, it is advantageous to heat the electrodes in Depending on the degree of dimming so that they are heated the more, depending the lamp is dimmed more, the darker it is. According to the EP 0 707 438 describes the heating of the electrodes during dimming regulated that the switch in the series with the primary winding temporarily is closed.

The ballast should also monitor the condition of the lamp Take up a function in order to be able to record any malfunctions and take appropriate measures. A malfunction can, for example then exist if one of the two coils or both are defective or if the lamp has been completely removed. In that described in EP 0 707 438 electronic ballast, the voltage drop is in series with the Primary winding of the transformer resistance and thus the heating current measured to detect whether there is a filament break or whether the lamp is out of the Arrangement was removed.

The procedure just mentioned provides information about the condition of the lamp, but not about what type of lamp it is. Lamps are often different Not externally, but have different electrical parameters and one different power consumption. Then accidentally one in their Features not compatible with the electronic ballast incorrect control can occur. This affects in lighting in simpler cases, but may also be in more serious cases damage the lamp. Such problems could be avoided by determining the type of lamp in a short control measurement before ignition and appropriate measures are initiated. This can mean that the Lamp is not preheated and ignited if it is of the wrong type or even better, that one corresponding to the performance characteristics of the lamp Activation takes place.

EP / A / 889675 describes an electronic ballast with an additional one Circuit for recognizing the lamp type using a filament resistance determination known.

Finally, from EP / A / 722263 is a circuit arrangement for coil preheating known from fluorescent lamps.

Starting from the above-mentioned prior art, it is therefore the task of present invention, an electronic ballast for operating a Low pressure gas discharge lamp specify that with the lowest possible Material and circuit expenditure the functions just described, that is Lamp detection, lamp condition detection and controllable in their power Filament heating, fulfilled.

This object is achieved by a ballast which has the features of claim 1 has, solved. An essential feature of the ballast is an evaluation circuit, those for recognizing the type and condition of the lamp by the primary winding of the heating transformer current and in addition also through at least one of the two heating circuits flowing current detected and evaluated. The identification the lamp type is done by measuring the lamp filament flowing current, which is a suitable measure of the spiral resistance. The In turn, filament resistance is a characteristic feature of using lamps distinguish the same appearance but different performance features. The Current through the primary winding, however, provides information about the condition of the lamp. The transformer transforms the heating voltage on the primary winding to the Lamp down strongly, so that the filament resistances in turn to Primary winding are transformed upwards. The behavior of the transformer therefore strongly depends on whether the coils are intact or whether, for example, one The coil is defective and the associated secondary heating circuit is interrupted.

A first development of the invention is an optimal control of the heating current and thus to enable the filament heating. This is according to claim 5 thereby achieved that the connection of the primary winding of the heating transformer to the Output of the inverter through one consisting of two switches bidirectional switch is regulated, the between the two switches Primary winding of the heating transformer and a coupling capacitor are arranged. The bidirectional switch can be switched by two series and opposite oriented field effect transistors are formed, which are preferably by a common pulse width modulated signal can be controlled, the Duty cycle of this signal determines the degree of heating. With the help of this arrangement an intermediate discharge of a coupling capacitor contained in the heating circuit avoided and thus achieved a symmetrical heating voltage.

Developments of the invention are the subject of the dependent claims. For determination of the lamp type - as already mentioned - the resistance value is one of the two coils. This is above the peak value of the so-called pin current certainly. To detect a filament break or to remove the lamp the current at the primary winding and at the same time the pin current measured and both currents in relation to each other. This procedure enables one of possible voltage fluctuations independent statement about the state of the Lamp. It is preferably first checked whether an intact lamp is present and only then determine the lamp type. To the reliability of the To increase the lamp determination, the measurement can be carried out twice, once before and once after preheating the lamp. The measured Resistance values can be compared with internally stored reference values and then be assigned to known lamp types. Furthermore, before the start of the Filament preheating and lamp detection a short test can be carried out whether the filaments are actually cold. This way misinterpretations can be made during lamp detection, which can occur after a short-term power failure, be avoided. The current measurements are preferably carried out in each case Measurements of the voltage drops across two in the heating circuit of the primary winding or Measuring resistors arranged in the secondary heating circuit of a lamp filament.

In a development of the invention, the electronic ballast is designed such that that the control of the filament heating and adjusting the frequency of the AC voltage, which is applied to the load circuit with the lamp, depending from the previously determined lamp type. Depending on the filament heating the degree of dimming of the lamp can be determined that the Dimming the lamp causes the lamp current to drop due to the Heating current should be substantially balanced. The height of the setpoint for the Pen current, i.e. for the sum of lamp current and heating current, is based on this according to the electronic parameters of the lamp. Preferably also after Ignition of the lamp performed a control measurement at regular intervals detect a possible breakage of the filament or removal of the lamp.

Fig. 1 das

Ausführungsbeispiel einer erfindungsgemäßen Schaltung;

Fig. 2

ein Taktschema der Steuersignale und die dazugehörigen Zustände der Schalter während des Normal-/Dimmbetriebs der Lampe;

Fig. 3

ein Taktschema der Steuersignale vor dem Zünden der Lampe; und

Fig. 4

ein mögliches Flußdiagramm der unterschiedlichen Betriebsphasen der Lampe.

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

Fig. 1 that

Embodiment of a circuit according to the invention;

Fig. 2

a timing diagram of the control signals and the associated states of the switches during normal / dimming operation of the lamp;

Fig. 3

a timing diagram of the control signals prior to lamp ignition; and

Fig. 4

a possible flow chart of the different operating phases of the lamp.

According to FIG. 1, the inverter of the ballast is formed by a half bridge from two electronic switches S1 and S2 connected in series, one switch each consisting of a MOS field effect transistor. The two switches S1 and S2 are activated via two connections A1 and A2 connected to the gates of the transistors, which lead to a control / evaluation circuit, not shown. The lower output of the half-bridge is grounded, while the DC voltage U _BUS is present at its input, which can be generated, for example, by shaping the usual mains voltage using a combination of radio interference suppressor and rectifier. As an alternative, however, any other DC voltage source can also be present at the input of the half bridge.

At the exit of the half bridge, i.e. to the common junction of the two Switches S1 and S2 is the load circuit containing the discharge lamp LA connected. This consists of a series resonance circuit consisting of a Choke coil L1 and a resonance capacitor C2 is composed. Between the Choke coil L1 and the resonance capacitor C2 is also a coupling capacitor C1 arranged. At the connection node between the two capacitors C1 and C2 is the upper of the two cathodes of the low-pressure gas discharge lamp LA connected. The two cathodes of the lamp LA each have two connections, between each of which a heating coil W1 or W2 for heating the cathodes is provided. The lower cathode of the LA lamp is in turn over two in series switched resistors R1 and R3 connected to ground. The same is the second Connection of the resonance capacitor C2 connected to ground, so that the lamp LA and the resonance capacitor C2 are parallel to each other. The function of the second Resistor R3 will be described later.

A heating transformer is provided for heating the two coils W1 and W2. that of a primary winding Tp and two secondary windings Ts1 and Ts2 consists. The secondary windings Tsl and Ts2 are each connected in series a filament W1 or W2 of the lamp LA connected, so that two separate secondary Heating circuits are formed. The resistor R3 is inside the secondary Heating circuit of the lower filament W1 arranged so that both one through the lamp LA flowing lamp current as well as the flowing through the lower filament W1 The heating current flows in the same direction through the measuring resistor R3. The Primary winding Tp is part of a series circuit that also has a Coupling capacitor C3 and two controllable switches S3 and S4, between which the Primary winding Tp and the coupling capacitor C3 are arranged. This Series connection is at its lower end via a further resistor R2 to ground connected and at its upper end to the common node of the two switches S1 and S2 of the half-bridge connected, so that they are parallel to the Load circuit and the lower branch of the half-bridge lies. Also the two switches S3 and S4 each consist of a field effect transistor, but are - like FIG. 1 can be removed - oriented opposite to each other, so that a bidirectional switch is formed. Furthermore, the two are in the circuit diagram Free-wheeling diodes D3 and D4 of the two transistors S3 and S4 are shown.

The gates of the two switches S3 and S4 are controlled by the control / evaluation circuit with a pulse width modulated signal via connection A3. There is also a diode D1 between the two gates. The common one Node between the output of diode D1 and the gate of the switch S3 is parallel via a capacitor C4 and one to this capacitor C4 switched resistor R4 with the common node of the two switches S1 and S2 of the half-bridge connected. In conclusion, the circuit shows three with the Control / evaluation circuit connected outputs A4, A5 and A6 to that for measurement of voltage drops across resistors R2 and R3 are used.

The measuring signals at the outputs A4, A5 and A6 are used to recognize the Lamp type and to record the status of the lamp, i.e. to check whether it is intact or whether one of the two coils is broken. On the On the other hand, the control / evaluation circuit regulates the clock signals to the Connections A1 and A2 the AC voltage supplied to the load circuit and through the Pulse width modulated signal at connection A3 heating the coils W1 and W2.

In the following, the function of the from the two field effect transistors S3 is to be described first and S4 formed bidirectional switch for heating the filaments W1 and W2 and the control of the lamp LA are explained in more detail.

2 shows a typical timing diagram of the control signals present at the three inputs A1, A2 and A3 and the resultant state of the four switches S1 to S4 for an already ignited and slightly dimmed operating state of the lamp LA. In this case, alternating signals are regularly applied to the connections A1 and A2 of the two half-bridge switches S1 and S2 between a high level H and a low level L, such that one of the two switches S1 or S2 is opened (I) and the other is closed (0) is. In this way, a high-frequency AC voltage with the period length τ ₀ or the frequency 1 / τ _{0 is} generated at the center of the half-bridge and fed to the load circuit. The degree of dimming of the gas discharge lamp is essentially determined by the deviation of the frequency 1 / τ _{0 of} the AC voltage from the resonance frequency of the load circuit. A high deviation means high dimming.

In the example shown in FIG. 2, it is now assumed that the selected period length τ ₀ actually causes a certain dimming of the lamp. In order to counteract premature aging of the lamp, the two electrodes must be heated by an additional heating current so that they are kept at their emission temperature. The heating takes place by a low-frequency connection of the primary heating circuit to the center of the half-bridge at regular intervals τ _H and for a predetermined period τ _HH . In these heating phases τ _HH , capacitor C3 then decouples the DC voltage component, so that a symmetrical square-wave voltage with a peak value of U _BUS / 2 results in the primary winding Tp of the heating transformer. Even during a longer phase τ _{HL of} the heating transformer, the coupling capacitor C3 should not be discharged so that a symmetrical voltage signal can be generated at the primary winding Tp at any time. This is particularly important in those cases in which a multi-lamp device is formed in which the peak value of the primary voltage has to be placed close to the transverse discharge voltage of the low-resistance filaments. If the heating circuit were connected to the center of the half-bridge with the help of a single switch (for example only through the lower transistor S4), the coupling capacitor C3 would discharge via the internal freewheeling diode D4 of this transistor in the periods in which the lower switch S2 of the Half bridge is closed.

Therefore, in the present exemplary embodiment, a bidirectional switch is formed from the two field effect transistors S3 and S4, the gates of the two transistors S3 and S4 being controlled by the common pulse-width-modulated signal A3. The mode of operation of this bidirectional switch can also be seen from the curves in FIG. 2. If the signal A3 has a low level L, both switches S3 and S4 are open and the filament heating is switched off. If the control signal A3 changes to a high level H at the beginning of a heating pulse τ _HH , the lower transistor switches through and switch S4 is thus closed (I). However, as long as the upper switch S1 of the half-bridge is closed (I), the transistor S3 remains blocked and the second switch S3 is open (0). In this phase, current then flows through the internal free-wheeling diode D3 of this transistor S3, as a result of which the coupling capacitor C3 is charged. If the cycle of the half bridge now changes, ie switch S1 closes (0) and switch S2 opens (I), the source potential of transistor S3 is connected to ground and switch S3 closes (I) as well. The coupling capacitor C3 can then discharge and release its energy again.

To switch off the heating phase τ _HH , the PWM signal A3 is switched to a low level and the transistor S4 is thus blocked. The gate of transistor S3 is then no longer controlled via diode D1, and transistor S3 is now kept passively blocked via resistor R4. The additional capacitor C4 ensures that there is no unwanted switching on of the transistor S3 during the off phase τ _HL due to the Miller capacitance. During this period τ _HL , both switches S3 and S4 are thus open and discharging of the coupling capacitor C3 via one of the two freewheeling diodes D3 or D4 is also excluded. In this way, at regular intervals τ _H or with the frequency 1 / τ _H for a predetermined period τ _HH in the primary winding Tp of the heating transformer and in the secondary heating circuits of the two lamp filaments W 1 and W2, an alternating voltage with that of the inverter output frequency 1 / τ ₀ generated.

The period length τ _{H of} the signal A3 is significantly longer than the period length τ _{0 of} the high-frequency clock signals A1 and A2. The choice of low frequency 1 / τ _H depends on several considerations. On the one hand, a frequency 1 / τ _H that is too high or a period τ _H that is too short should not be selected, since otherwise the heating power is roughly graded. Since switching on the heating circuit influences the light output of the lamp, flickering can occur. On the other hand, the frequency 1 / τ _{H must} not be chosen too low, since the two filaments W1 and W2 otherwise cool down too much during the off phase τ _HL , which can have a negative effect on the life of the lamp LA. The frequency 1 / τ _{H of} the pulse width modulated signal A3 should therefore be chosen in any case so that an essentially constant electrode temperature is established.

The effective value of the heating voltage and thus the degree of heating power is determined by the pulse duty factor of the pulse-width-modulated signal A3 or by the temporal relationship between high-phase τ _HH and low-phase τ _HL . It is preferably set in accordance with the degree of dimming and the type of lamp LA. The corresponding procedure for setting the heating output will be explained later. If the already lit lamp LA is operated in the vicinity of the resonance frequency of the load circuit and thus with almost maximum power, the filament heating can be switched off completely in order to reduce power losses. This does not significantly affect the life of the lamp LA, since in this case the operating temperature of the electrodes is sufficient. In contrast, a relatively high heating power is selected during the preheating of the filaments W1 or W2 in order to enable a short preheating time and a quick ignition of the lamp LA. During the preheating, the half-bridge is also operated at a very high frequency 1 / τ ₀ of almost 120 kHz. Since this frequency is far above the resonance frequency of the load circuit, premature and unwanted ignition is avoided.

The lamp LA is ignited in a known manner. If closer to that for explanatory recording of the lamp status and the lamp detection none Malfunctions are detected, the after a predetermined heating time Frequency of the alternating voltage emitted by the half-bridge is reduced and the Approximate resonance frequency of the load circuit. This increases the lamp LA applied voltage until finally ignition occurs.

A simple procedure to adjust the heating output depending on the degree of dimming Regulating lamp LA will now be briefly explained. This procedure is to control the current flowing from the lower coil W1. This so-called pen stream is made up of two parts, one of which is from the ignited lamp LA flowing lamp current and on the other hand from that of the heating transformer generated average heating current. The goal now is to have this pen flow roughly on one to keep the specified setpoint or within a specified range. Because namely the lamp LA is dimmed by changing the AC voltage frequency, this reduces the lamp current and the electrode temperature. A measure of the additional heating of the electrodes can now be selected, for example, that the current reduction caused by the dimming by the heating current should be balanced again. The control / evaluation circuit is therefore preferred formed so that it measures the pin current and the pulse width of the control signal on Port A3 modulated accordingly. The current is measured by a Brief measurement of the voltage drop across the measuring resistor R3 by one to the Outputs A5 and A6 connected (not shown) voltmeter, the one Part of the control / evaluation circuit or the measurement result to it forwards.

The value specified for the pin current depends, among other things, on the type and the power consumption of the LA lamp. The electronic ballast is like this trained that the lamp type with its special electrical parameters (e.g.

Preheating current, lamp current, lamp power) automatically detects and the control then the lamp LA and the filament heating via the signals A1, A2 and A3 accordingly. Because lamps with different parameters are external can often distinguish very little or not at all, can by an automatic Lamp detection also incorrect control at the same time, resulting in a unsatisfactory light output or even damage can be avoided.

In the ballast according to the invention, the lamp is recognized by a Measure the resistance of one of the two coils. This spiral resistance is a sufficient feature around lamps that fit in a common socket have different performance parameters. With knowledge of the Supply voltage supplied to the inverter is the easiest option To determine coil resistance, a measurement of the peak value of the pin current, the in the circuit shown in Fig. 1 also by the voltage drop on Measuring resistor R3 is detected via outputs A5 and A6. This is preferably done Measurement of the coil resistance at the beginning and at the end of the preheating phase. There during preheating - before lighting the LA lamp - none yet Lamp current flows, in this case the voltage drop between the Connection A6 and mass are measured. During lamp detection, a relatively low heating output (approx. 5% duty cycle) set to a too strong Avoid heating the coils W1, W2. The half-bridge opens at this time a high frequency of about 120kHz.

The pin current is preferably measured at the end of the switch-on phase of the upper switch S 1 of the half bridge and possibly averaged. The measured peak values are then compared in each case with a stored reference value and the lamp type is determined on the basis of the comparison result. Two resistance reference values are therefore required for each lamp type, one for the cold filaments W1, W2 and one for the preheated filaments W1, W2. It should be noted that the pin current depends not only on the coil resistance, but also on the coil voltage and thus on the bus voltage U _BUS supplied to the inverter. In order to avoid possible fluctuations and incorrect measurements, the coil detection is therefore only carried out after the system has settled and the bus voltage U _{BUS has} stabilized. Alternatively, however, the bus voltage U _BUS could also be determined in a separate measurement and the voltage drop across the measuring resistor R3 could be set in relation thereto, for example by forming the differential voltage. In this way it would even be possible to carry out the lamp detection independently of such fluctuations.

A further misinterpretation when determining the lamp can occur if the mains voltage supplying the electronic ballast briefly fails or is briefly switched off and on again. This is done by a ballast in each Case interpreted as restarting the lamp LA and thus one more time Preheating and lamp detection performed. However, the coils are W1, W2 in this case not yet cooled and therefore have a different resistance. The Lamp detection then leads to an incorrect result. To this possibility before the resistance is determined, it is checked whether the coil W1, W2 is cold or hot. If the filament W1, W2 is actually still hot, the lamp turns on LA deliberately preheated with a slightly lower heating output and one Lamp detection only based on the resistance measurement at the end of the preheating phase carried out. The slightly different preheating can be accepted that this case rarely occurs. The distinction between a hot one and a cold coil W1, W2 is carried out by measuring the change in Helix resistance within a predetermined short period of time, for example 10ms. If the change is negative, a hot or warm coil W1, W2 assumed and the reduced preheating performed. However, is not a change Ascertainable, this is interpreted as the presence of a cold coil W1, W2 and therefore the usual preheating and lamp determination performed. This too Control measurement is carried out by two short samples of the pen current and the Voltage drop across the measuring resistor R3, the height of the Changes in resistance, for example, can be assessed using a Schmitt trigger can. Since the coil resistance is also measured in these, the two represent Control measurements also the first resistance measurement for the Lamp detection.

Before the lamp detection and the associated preheating of the filaments W1 and W2 is performed, it is still checked whether there is in the system there is a lamp LA at all and whether it is also intact. For this, the Peak value of the pen current measured and with the peak value of the primary current of the Heating transformer compared. The pen current is used just like when controlling the Filament heating and as with lamp detection via the voltage drop on Measuring resistor R3 determined. The through the primary winding Tp of the heating transformer current, however, is due to the voltage drop across resistor R2 certainly. For this reason, between the switch S4 and the measuring resistor R2 the output A4 connected to the control / evaluation circuit is provided. As well as when the lamp is detected, the half-bridge with a as high a frequency as possible of about 120 kHz, around that of the lamp LA to keep the supplied voltage as low as possible and to ignite it prematurely avoid. Likewise, a low duty cycle of the pulse width modulated Control signal set at terminal A3 so that the two coils W1 and W2 are not to be heated too much. Since a current flowing through the primary winding Tp is to be measured, a measurement time is selected at which at the connection A3 a high level H is present and the coupling capacitor C3 is charged. As with the Lamp detection is therefore also carried out shortly before the end of the switch-on phase of the upper switch S1 of the half bridge.

If both filaments W1 and W2 of lamp LA are intact, the relationship applies to the peak values of two measured currents: I R2 = I R3 * N * 1 / u ü denotes the gear ratio and n the number of intact filaments W1, W2. The transformation ratio ü of the heating transformer results from the maximum filament voltage. Care should be taken to ensure that this ratio ü does not become too large, since otherwise the capacitive currents when the preheating is switched off cause excessive coil losses during operation. To evaluate the lamp status, the primary current I _R2 is then related to the pin current divided by the transformation ratio I _R3 · 1 / ü and the result that theoretically results in the number of filaments n is evaluated. The simplest way to do this is to compare the result with a reference value. If, for example, a value of less than 1.3 results, there is a high probability of a spiral break. Since current still flows through the lower secondary heating circuit of coil W1, the upper coil W2 must therefore be defective. On the other hand, if no current flows through the measuring resistor R3, either the lower filament W1 is defective or there is no lamp LA at all. In this way, the possible lamp states can thus be detected in a simple and quick manner.

Another advantage of this method is that it provides a statement about the lamp state that is independent of possible fluctuations in the supply voltage U _BUS . A fluctuation in the U _{BUS influences} the measurement result of the pin current, but the primary heating current is also changed. It is not necessary to wait until the system has settled and the supply voltage U _{BUS has} stabilized. Furthermore, the influence of possible spiral resistance tolerances is reduced. In the same way, the lamp state can then be checked at regular intervals during normal operation of the lamp LA in order to detect a filament break occurring during this time. For this purpose, however, the lamp current should not influence the heating current too strongly, for example it should not be more than 10% of the pen current. If a filament break occurs during operation of the lamp or if the lamp is removed, this control measurement can be carried out repeatedly until an intact lamp is recognized in the system again. A restart can then be initiated automatically.

A possible chronological sequence of these measurements for lamp detection and for detecting the lamp state just described is shown in the timing diagram in FIG. 3. The lamp starts at time T ₀ . It is assumed here that at this point in time T ₀ the system has already settled in and the supply voltage U _{BUS has} stabilized. Immediately after the lamp has started, the control measurement is first carried out to determine whether an intact lamp is inserted or whether there is a broken filament. Since the pin current at resistor R3 is compared here with the primary current of the filament heater at resistor R2, this measurement must be carried out at a time T _W at which the control signals at terminals A1 and A3 are at a high level H. As has already been said, all measurements are preferably carried out shortly before the signals A1 and A2 change. Furthermore, a frequency of almost 120 kHz is chosen for these signals.

If an intact lamp was detected, two shortly consecutive measurements of the filament resistance at times T _L1 and T _{L1 'are then} carried out in order to determine whether the filaments W1, W2 are warm or cold. Since temperature changes or changes in resistance are to be observed, a low duty cycle is selected for the control signal at connection A3 during this time. The distance between T _L1 and T _{L1 '} is approx. 10 ms.

Subsequently, the coils W1, W2 are preheated in the period τ _VH , the heating output taking place in accordance with the state of the coils W1, W2, that is to say a higher heating output is set, for example, if the resistance measured at the later time T _{L1 'is} not lower than that resistance value measured at time T _L1 . After the preheating time, a measurement of the filament resistance is carried out again at time T _L2 and the lamp type is then determined on the basis of the measurement results at times T _L1 , T _{L1 '} and T _L2 . If the filaments W1, W2 were warm, only the result of the third measurement is taken into account. If the filaments were cold, all three measurements can be used for the lamp determination. The ignition of the lamp LA, which is not shown, is then initiated.

A summary of the described functions of the invention Electronic ballast is shown in Fig. 4. This shows a simplified one Flow diagram of the individual phases during lamp operation. After this Switching on 100 of the mains voltage or a short mains failure is first in the Query 101 just carried out described whether there is a spiral break. If this is the case or if there is no lamp at all in the system, the query is asked 101 continuously repeated until an intact lamp is finally recognized.

If an intact lamp was recognized, the next step 102 will make it short successive pin current measurements checked whether the filaments are cold. are the filaments are actually cold, the lamp is preheated normally and the Lamp detection based on the measurement results before and after the preheating phase 103 carried out. If a warm coil was recognized instead, only a reduced one Preheating 104 performed and the lamp type determined at the end. After this Preheating 103 and 104, finally, ignition 105 of the lamp is carried out, the control of the four switches depending on the detected Lamp type is done.

After ignition 105, the system is in normal or dimming operation 106 by one corresponding to the lamp type and the desired degree of dimming AC voltage frequency and heating power from the control / evaluation circuit is set. During this phase, in addition, at regular intervals once a query 107 is carried out as to whether a spiral break has possibly occurred or whether the lamp was removed. If this is the case, the normal / dimming operation ended and the system in the state of the original spiral break query 101 set back. However, it would also be conceivable for the inverter to recognize one Turn off the filament break or another defect in the lamp. With the help of a suitable circuit could then be monitored whether the defective lamp by a new one has been replaced. Finally, an intact lamp in the system detected, a restart can be initiated automatically. Will in the Control measurement 107 no change in the lamp state is found, the The lamp is controlled in normal / dimming mode until it is finally switched off becomes. The flowchart in FIG. 4 shows only one possibility of the process of the various control measurements and phases of the lamp. Of course, would be conceivable also very many other control procedures in which the different measurements are made too take place at other times.

It is thus used to control the lamp as a whole in the manner just described Current measurements at two different points in the circuit (in one of the two secondary spiral heating circuits and in the primary heating circuit) as well as a controllable one Switch device required to switch on the primary heating circuit. The The cost of materials for such an expansion is relatively low. From the Descriptions of the various acquisition measurements show that the Voltage measurements on the two measuring resistors R2 and R3 also others current measuring methods can be used, as for lamp detection and Detection of a spiral break only the respective current strengths can be determined have to. In addition, the arrangements shown for the measuring resistors R2 and R3 not mandatory. For example, the measuring resistor R2 are also located between the two switches S3 and S4. Likewise, the pen stream also in the heating circuit of the upper coil W2 and thus the coil resistance of the upper coil W2 can be measured.

Claims (22)

1. Electronic ballast for at least one low-pressure discharge lamp (LA), having an inverter that is fed with direct voltage (U_BUS) and the output of which is connected to a load circuit containing terminal contacts for the lamp (LA), having a heating transformer that has a primary winding (Tp), which is connected to the output of the inverter, and a respective secondary winding (Ts1, Ts2), which is located in a heating circuit with a coil (W1, W2), for heating each of the two electrodes of the lamp (LA), having a series circuit arrangement that is connected in parallel with the load circuit and which contains the primary winding (Tp) of the heating transformer and an electronic switch arrangement (S3, S4), and having an evaluating circuit arrangement that measures the current flowing through the series circuit arrangement with the primary winding (Tp) and the electronic switch arrangement (S3, S4), characterised in that the evaluating circuit arrangement, additionally to the current through the said series circuit arrangement, at the same time also measures the current flowing through at least one of the two heating circuits and evaluates the amplitudes or the time characteristic of the two measured currents in order to identify the type of lamp and the state of the lamp.
2. Electronic ballast according to claim 1, characterised in that for the purpose of measuring the current through the series circuit arrangement with the primary winding (Tp) and the electronic switch arrangement (S3, S4), connected in series with the latter there is a first measuring resistor (R2), and in that the evaluating circuit arrangement evaluates the voltage which is generated at the first measuring resistor (R2) by the current (I_R2) which flows through the latter.
3. Electronic ballast according to claim 1 or 2, characterised in that for the purpose of measuring the current through one of the two heating circuits this heating circuit contains a second measuring resistor (R3), and in that the voltage that drops across this second measuring resistor (R3) and which is generated by the current (I_R3) flowing through the latter is fed to the evaluating circuit arrangement.
4. Electronic ballast according to claim 3, characterised in that the second measuring resistor (R3) is arranged in one of the two heating circuits in such a way that a lamp current flowing through the lamp (LA) after the lamp (LA) has been ignited flows in the same direction in which a heating current that is generated by the heating transformer flows through the second measuring resistor (R3).
5. Electronic ballast according to any of claims 1 to 4, characterised in that the electronic switch arrangement (S3, S4) is formed by two field-effect transistors (S3, S4) which are orientated in opposition to each other, and in that the primary winding (Tp) of the heating transformer and also a coupling capacitor (C3), which is connected in series with the latter, are arranged between the two field-effect transistors (S3, S4).
6. Electronic ballast according to claim 5, characterised in that the gates of the two field-effect transistors (S3, S4) are activated by way of a common pulse-width modulated signal (A3).
7. Electronic ballast according to claim 4 and claim 6, characterised in that after the lamp (LA) has been ignited a pulse duty factor is set for the pulse-width modulated signal (A3) in such a way that the current (I_R3) flowing through the second measuring resistor (R3) is substantially equal to a rated value.
8. Electronic ballast according to claim 7, characterised in that the rated value is established by the type of lamp detected by the evaluating circuit arrangement.
9. Electronic ballast according to any of claims 1 to 8, characterised in that the inverter contains a half-bridge consisting of two electronic switches (S1, S2) that are connected in series and which are alternately opened and closed, and in that the load circuit, which contains the lamp (LA), and the series circuit arrangement with the primary winding (Tp) and the electronic switch arrangement (S3, S4) are connected in parallel with one of the two electronic switches (S1, S2).
10. Electronic ballast according to claim 9 and any of claims 5 to 8, characterised in that a diode (D1) is arranged between the two gates of the field-effect transistors (S3, S4), and in that the gate of one of the two field-effect transistors (S3, S4) is connected to the output of the inverter by way of a resistor (R4).
11. Electronic ballast according to claim 10, characterised in that a further capacitor (C4) is connected in parallel with the resistor (R4).
12. Electronic ballast according to any of the preceding claims, characterised in that it contains a rectifier that is connected to the mains and which generates the direct voltage (U_BUS) that is to be fed to the inverter.
13. Electronic ballast according to any of the preceding claims, characterised in that the load circuit contains a choke coil (L1) which is connected in series with the lamp (LA), and a resonant capacitor (C2) which is connected in parallel with the lamp (LA).
14. Electronic ballast according to any of the preceding claims, characterised in that for the purpose of identifying the type of the lamp (LA) the current (I_R3) that flows through one of the two heating circuits and which is dependent upon the respective coil resistance is measured and evaluated by the evaluating circuit arrangement.
15. Electronic ballast according to claim 14, characterised in that for the purpose of identifying the type of the lamp (LA) the evaluating circuit arrangement compares the peak value of the current (I_R3), measured in one of the two heating circuits, with reference values.
16. Electronic ballast according to claim 14 or 15, characterised in that measurements for the purpose of identifying the lamp type are carried out in each case at the beginning and at the end of a preheating phase of the lamp (LA).
17. Electronic ballast according to claim 16, characterised in that in a check measurement for distinguishing between a warm and a cold coil (W1, W2) before the preheating phase of the lamp (LA) the evaluating circuit arrangement compares the amplitudes or the peak values of two currents (I_R3), measured shortly one after the other, through one of the two heating circuits.
18. Electronic ballast according to claim 16 and 17, characterised in that only the result of the measurement at the end of the preheating phase of the lamp (LA) is used in order to identify the type of lamp if a warm coil (W1, W2) has been identified in the check measurement.
19. Electronic ballast according to any of the preceding claims, characterised in that for the purpose of identifying a change of lamp or lamp defect the evaluating circuit arrangement evaluates simultaneously measured peak values of the currents (I_R2, I_R3) through the series circuit arrangement with the primary winding (Tp) and the electronic switch arrangement (S3, S4) and also through one of the two heating circuits.
20. Electronic ballast according to claim 19, characterised in that the evaluating circuit arrangement sets the simultaneously measured peak values in relation to each other and evaluates the result.
21. Electronic ballast according to claim 19 or 20, characterised in that a measurement is effected in order to identify a change of lamp or lamp defect immediately after the ballast has been switched on.
22. Electronic ballast according to any of claims 19 to 21, characterised in that after the lamp (LA) has been ignited a measurement is carried out at regular intervals to identify a change of lamp or lamp defect.

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