EP1324643A1 - Electronic ballast with over temperature protection - Google Patents

Abstract

The stop frequency (fstop), starting from a basic-stop frequency (fstop,0), rises with increasing temperature.

Description

The present invention relates to a power-controlled electronic ballast for operating at least one gas discharge lamp.

Modern ballasts for operating gas discharge lamps, in particular from Fluorescent tubes are often performance-controlled. With this type of regulation the DC link voltage supplied to the inverter is kept essentially constant, while the current flowing through the inverter by changing the Operating frequency is regulated. This is done for example by a in the Half-bridge of the inverter provided shunt resistor, the over this Shunt resistance dropping voltage as actual value for the half-bridge current one Control or regulating circuit is supplied. The control circuit regulates the Operating frequency of the inverter so that the average current through the shunt resistor or the proportional mean voltage above this constant stay. With the DC link voltage kept constant and in this way to a constant regulated current, the lamp always has the same power fed.

Fluorescent tubes have a negative characteristic if one shows their voltage as a function of the current. This means that at a certain temperature T1, with increasing lamp current I _LA, the lamp voltage U _LA drops, as is the case, for example, with the characteristic curve U _{LA, T1} shown in FIG. 2. If the dependency between current and voltage at a certain power is entered in the same diagram, the hyperbola P also shown in FIG. 2 results, since the power is the product of current and voltage. If the lamp is now at a certain power regulated, an operating point is set on the characteristic curve of the lamp, which corresponds to the intersection between the characteristic curve and the power hyperbola. In the example shown in FIG. 2, in which the power P is to be set, the operating point which corresponds to the lamp current I _T1 results, for example, at the temperature T1. The lamps are usually designed in such a way that the operating point is optimal at a certain temperature. This means that an optimal light yield is guaranteed if the lamp is operated at a certain temperature, which is typically in the range between 30 ° and 40 ° C.

Compared to the ideal case described above at normal operating temperature, however, the situation can arise that the temperature at which the lamp is actually operated is significantly higher. This could be the case, for example, in factory buildings that generate a lot of heat. However, increasing the temperature results in a new characteristic curve for the lamp and thus a new operating point. In the example shown in FIG. 2, the new characteristic curve U _{LA, T2} results, for example, at the higher temperature T2 and a new operating point is set which corresponds to the increased lamp current I _T2 . This new operating point is achieved within the scope of the power control described at the beginning by reducing the operating frequency for the inverter.

By changing to the new working point when increasing the Operating temperature is reached that the power supplied to the lamp is constant remains, but at the same time the power loss increases due to the higher current and the luminous efficacy of the lamp is reduced. Is compared to the operating frequency frequency is significantly reduced at normal temperature, there is even a risk that the current rises to areas where the device itself is at risk.

Therefore, to prevent the electricity due to a rise in temperature assumes dangerous orders of magnitude, usually the frequency range, within which the lamp current can be regulated is limited. Practically a stop frequency is set which must not be undercut. Will the If the stop frequency is reached, the ballast acts as one from this point in time Constant current source and no longer like a constant power source. This has to As a result, the maximum current corresponding to the stop frequency is not exceeded can be destroyed and consequently damage or damage to some or all Components of the ballast is prevented.

However, setting the stop frequency is also problematic because of this several factors need to be considered. On the one hand, one is possible high stop frequency desirable because of the previously described safety function not too late and thus not allowed to start with too high currents. Will the lamp operated at normal temperature with a frequency of about 45 kHz, for example for example, a stop frequency of approx. 41 kHz should be selected.

On the other hand, however, it must be taken into account that the operating frequency at normal temperature can fluctuate around the ideal value of, for example, 45 kHz due to the fact that the various elements of the electronic ballast are subject to tolerances. If all devices are to be operated at normal temperature at a certain output, each ballast will have a slightly different operating frequency, as is shown, for example, in FIG. 3. Curve I shown in FIG. 3 shows that the actual operating frequency of all ballasts is distributed around the optimal frequency f _run of 45 kHz and is approximately between 43 and 47 kHz. In the same way, the _stop frequency f _stop is distributed around an ideal value, as shown by curve II. The reason for this again lies in certain tolerances of the components responsible for determining the stop frequency.

It can now happen that the existing at normal temperature Operating frequency is already below the stop frequency. This is the case in FIG. 3 hatched area F the case in which the curves for the operating frequency (I) at normal temperature and the stop frequency (II) overlap. All ballasts, that fall within this shaded area do not allow proper Lamp operation and are accordingly to be regarded as a committee. To this Keeping the proportion of such faulty ballasts low is therefore one possible low stop frequency is desirable, which contradicts the above mentioned preference for the highest possible stop frequency.

The problem described above could be avoided by being immediate after manufacture, a comparison is carried out for each device, in which one for the respective operating frequency suitable stop frequency is individually determined. at electronic ballasts that allow the lamp to be dimmed such adjustment carried out anyway, so that no additional Workload arises. Such is the case with non-dimmable devices Comparison is not absolutely necessary, so that an individual determination of the Stop frequency means additional work, through which the manufacturing costs increase.

The present invention is therefore based on the object of an electronic Ballast to specify, on the one hand an early and reliable use of the current limit is made possible and on the other side it is ensured that the stop frequency in the initial state in each Case is below the operating frequency.

The task is performed by an electronic ballast, which has the characteristics of the Has claim 1 solved. This is characterized in that the Stop frequency is temperature-dependent and - starting from a basic stop frequency increases with increasing temperature. In this way, there is a possibility to choose correspondingly low base stop frequency, which is far enough below the Operating frequency is, so that a proper lamp start in any case is guaranteed. Starting from the base stop frequency, the stop frequency then increases with increasing temperature, so that at higher temperatures a Current limitation that prevents damage to the ballast. On Adjustment of the ballast after its manufacture is therefore no longer necessary which is why the invention in particular in the case of non-dimmable ballasts Reduction of manufacturing costs contributes.

Developments of the invention are the subject of the dependent claims. This is how the Power control preferably in that a control or regulating circuit voltage drop across a resistor located at the base of the half-bridge detected, compares this actual value of the half-bridge current with a reference value and an operating frequency for the inverter based on the comparison result certainly. The stop frequency can increase with increasing temperature simple and elegant way that the tax or Control circuit the control signals for the inverter from a basic frequency derives, this fundamental frequency increases with increasing temperature. This remains the stop frequency, which the control or regulating circuit internally when determining the Frequency for the inverter is taken into account unchanged, while the effective one the minimum frequency set in the inverter still rises. Alternatively However, there is also the possibility that the control or regulating circuit is the current one current temperature determined - for example by measuring a temperature-dependent reference voltage - and then based on this Temperature information determines a stop frequency, which increases according to the invention Temperature rises.

A particularly advantageous development of the ballast according to the invention consists of digitally controlling or regulating the inverter train. This is achieved in that within the control or regulation circuit an analog / digital converter is provided, which is the one of the control or Control circuit recorded operating parameters in a 2 bit or more Implements digital value. Based on this digital value, a digital Computing block calculates an operating frequency for operating the inverter and with the help of a driver circuit in corresponding control signals for the switching elements of the inverter implemented. This solution enables extensive integration the control elements of the ballast. At the same time, by implementing the analog measured operating parameters in digital values with a high accuracy great stability in the regulation of lamp power. This digital Embodiment can, for example, also on the control loop for the smoothing circuit be expanded.

The basic stop frequency for the ballast can be set to the control or Control circuit connectable reference resistance are specified, its size via an analog / digital converter provided in the control or regulating circuit is determined after connecting an internal power source that is connected to this Reference resistance falling voltage in a likewise from at least 2 bit existing digital value. Are also the control loops for the inverter and the intermediate circuit voltage digitally formed in the manner described above, so a further reduction in the components can be achieved in that Control or regulating circuit for implementing the recorded operating parameters and the only a single voltage drop across the reference resistor Analog / digital converter is provided, which works in time-division multiplex. Preferably have the digital values converted by the analogue / digital converter or converters Accuracy from 12 bit.

Fig. 1

ein erstes Ausführungsbeispiel eines erfindungsgemäßen Vorschaltgeräts;

Fig.2

die Kennlinien einer Leuchtstofflampe bei zwei verschiedenen Temperaturen;

Fig. 3

eine Grafik zur Verdeutlichung der Schwierigkeiten bei der Festlegung der Stoppfrequenz; und

Fig. 4

ein zweites Ausführungsbeispiel eines erfindungsgemäßen Vorschaltgeräts.

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

Fig. 1

a first embodiment of a ballast according to the invention;

Fig.2

the characteristics of a fluorescent lamp at two different temperatures;

Fig. 3

a graphic to illustrate the difficulties in determining the stop frequency; and

Fig. 4

a second embodiment of a ballast according to the invention.

The electronic ballast shown in FIG. 1 is connected on the input side to the mains supply voltage U ₀ via a high-frequency filter 1. At the output of the high-frequency filter 1 there is a rectifier circuit 2 in the form of a full-bridge rectifier, which converts the mains supply voltage U ₀ into a rectified input voltage for the smoothing circuit 3. The smoothing circuit 3 is used for harmonic filtering and smoothing the rectified input voltage and comprises a smoothing capacitor C1 and a step-up converter having an inductor L1, a controllable switch in the form of a MOS field-effect transistor S1 and a diode D1. Instead of the step-up converter shown here, other known smoothing circuits can also be used.

A corresponding switching of the MOS field-effect transistor S1 generates an intermediate circuit voltage U _z which is present across the storage capacitor C2 connected to the smoothing circuit 3 and is supplied to the inverter 4. This inverter 4 consists of two MOS field effect transistors S2 and S3 arranged in a half-bridge arrangement. An alternating high-frequency activation of the two field effect transistors S2, S3 generates an AC voltage at the center of the half-bridge, which is fed to the load circuit 5 with the gas discharge lamp LA connected to it. The gas discharge lamp LA is in particular a fluorescent tube.

The three MOS field-effect transistors S1-S3 of the smoothing circuit 3 and the inverter 4 are driven by a control or regulating circuit 6, which generates corresponding control information and transmits it to a driver circuit 7, which converts this control information into corresponding control signals for the gates of the three MOS Field effect transistors S1-S3 implemented. The control information is determined on the basis of operating parameters which are taken from the smoothing circuit and the inverter 4 or the load circuit 5. On the one hand, the intermediate circuit voltage U _z dropping across the storage capacitor C2 is determined, on the other hand, the voltage dropping across this resistor R1 or the mean half-bridge current and thus ultimately that of the lamp is determined via a shunt resistor R1 arranged at the base of the half-bridge of the inverter 4 LA power determined.

Two control loops are formed within the control or regulating circuit 6, one for the intermediate circuit voltage U _z and one for the lamp power. The intermediate circuit voltage U _z is converted by a first analog / digital converter ADC1 into a digital value, which is fed to a first digital arithmetic block 8. This arithmetic block 8 uses the actual value of the intermediate circuit voltage U _z obtained from the analog / digital converter ADC1 to calculate a suitable switching frequency for the MOS field-effect transistor S1 of the step-up converter. This frequency is transmitted to the driver circuit 7, which drives the gate of the transistor S1 accordingly. In this way, the intermediate circuit _voltage U _{z is} kept constant at a certain value.

The voltage drop across the shunt resistor R1 at the base of the half-bridge is converted by a second analog / digital converter ADC2 and fed to a comparison block 9. This - also digitally working - comparison block 9 compares the current actual value of the lamp power with a predetermined reference value P _ref and uses the comparison result to determine whether the frequency of the inverter 4 has to be increased or reduced in order to operate the lamp LA with the desired power. This information is transmitted via a logic block 10, which will be described in more detail later, to an output block 11, which outputs corresponding control information to the driver circuit 7, which in turn controls the two MOS field effect transistors S2 and S3 of the inverter 4. In this way, the operating frequency of the inverter 4 is set such that the lamp LA is operated at the desired power.

As already explained at the beginning, the frequency of the inverter 4 should not fall below a certain minimum value in order to avoid excessive currents in the ballast and the lamp LA. This stop frequency is determined by an external reference resistor R2, which is connected to the control or regulating circuit 6. The level of the resistance R2 is a measure of the _stop frequency f _stop . It is determined in that the connection of the resistor R2 is connected to an internal current source I _s provided in the control or regulating circuit 6 via a switch S4. The voltage then falling across the resistor R2 is converted by a third analog / digital converter ADC3.

In order to achieve the desired current limitation, the logic block 10 was inserted into the digital control circuit for the lamp power, on the one hand the result supplied by the comparison block 9 and on the other hand the value determined by the third analog / digital converter ADC3, which is a measure of is the stop frequency. The logic block 9 now determines whether the operating frequency f _run determined by the comparison block 9 is above or below the _stop frequency f _stop . If the operating frequency f _{run is} above the _stop frequency f _stop , it is transmitted unchanged to the output block 11, which controls the inverter 4 with the aid of the driver circuit 7. In this case, the _stop frequency f _stop does not influence the control process for the lamp power.

However, if the operating frequency f _run determined by the comparison block 9 for setting the desired lamp power is below the _stop frequency f _stop , the _stop frequency f _{stop is} transmitted to the output block 11 instead of this operating frequency f _run . In this case, the _stop frequency f _stop specifies the maximum current at which the lamp LA is operated. The control loop now changes to a state in which the ballast is a constant current source for the lamp LA, thereby avoiding the occurrence of excessive currents and damage to the ballast.

As previously mentioned, the reference resistor R2 is designed in such a way that the _stop frequency f _stop that it specifies is sufficiently below the operating frequency f _run necessary for the desired lamp power at normal operating temperatures. This ensures that a regular lamp start can be carried out in any case and that the current limitation does not start at the start of operation.

The _stop frequency f _stop should only be raised when the temperature of the lamp LA or the ballast rises in order to enable the current limitation to be set in good time. This is achieved in that the central clock generator 12 provided in the control or regulating circuit 6 is designed to be temperature-dependent. This clock generator 12 transmits a clock signal to all components of the control or regulating circuit 6 in order to enable the various units to operate synchronously. In particular, this clock signal is also transmitted to the output block 11 of the control circuit for the lamp power, which also uses this clock signal to convert the frequency value obtained from the logic block 10, which is in digital form, into corresponding high-frequency control signals for the driver circuit 7. Since the clock generator 12 operates in a temperature-dependent manner, the following applies to the frequency f _{basis of} the clock signal transmitted by it to all components of the control or regulating circuit 6: f Base = f basis, 0 - TK x (TT 0 )

The frequency f _{basis, 0} represents the frequency of the clock signal generated by the clock generator 12 at normal temperature T _0. Since the frequency f _{basis of} the clock signal also represents the fundamental frequency from which the output block 11 derives the control signals for the driver circuit 7, this applies same temperature dependency also for the signals transmitted from the control or regulating circuit 6 to the driver circuit 7. In particular, this means that in the event that the stop frequency received by the analog / digital converter ADC3 is transmitted from the logic block 10 to the output block 11, the following applies to the stop frequency: f Stop = f stop, 0 -TK x (TT 0 )

The temperature dependency of the clock generator 12 therefore has the consequence that the actual stop frequency used increases with increasing temperature.

The solution shown here is characterized in that the increase in Stop frequency is achieved in a particularly simple and elegant way. The Temperature-dependent behavior of the clock generator 12 can be done without great effort can be achieved. Of course, there is also the possibility of others Take measures to increase the stop frequency with increasing temperature guarantee.

In the exemplary embodiment shown in FIG. 4, the clock generator 12 is designed, for example, to be temperature stable, so that it supplies a base frequency that is independent of the temperature. In addition, however, a further analog / digital converter ADC4 is now provided, which measures a deliberately temperature-dependent internal reference voltage V _ref and supplies the temperature information obtained in this way to logic block 10. This determines a suitable stop frequency on the basis of this temperature information and takes this into account in the manner described above when transmitting the operating frequency determined by the comparison block 9 to the output block 11. According to the invention, the stop frequency determined by the logic block 10 on the basis of the temperature information increases with increasing temperature.

The digital embodiment of the illustrated in the present exemplary embodiments Control or regulating circuit 6 advantageously enables extensive Integration of the entire circuit. This can further increase integration be that only a single analog / digital converter is used to Converting the various input signals works in time-division multiplex. The or the Analog / digital converters preferably form digital values with an accuracy of 12 bits, so that a very precise regulation of the intermediate circuit voltage and the Lamp power is obtained. In particular, there is also the possibility of Circuit as a so-called application-specific integrated circuit (Application Specific Integrated Circuit - ASIC).

The solution according to the invention thus ensures reliable operation of the electronic ballast, which ensures that the desired Current limitation to avoid damage occurs in good time. Furthermore the manufacturing costs for the ballast are reduced because of the realization a reliably functioning current limitation, no additional adjustment at the Manufacturing is necessary.

Claims (13)

Electronic ballast for at least one gas discharge lamp (LA), preferably for a fluorescent tube, with a rectifier circuit (2) that can be connected to a supply voltage source, a smoothing circuit (3) connected to the output of the rectifier circuit (2) for generating an intermediate circuit voltage (U _z ) and one inverter (4) supplied with the intermediate circuit voltage (U _z ), to the output of which a load circuit (5) containing connections for the lamp (LA) is connected,
and with a control or regulating circuit (6) which detects an operating parameter of the inverter (4) or the load circuit (5) corresponding to the power of the lamp (LA) and, depending on the detected operating parameter, the operating frequency (f _run ) of the inverter ( 4) changed, the lower variation of the operating frequency (f _run ) being limited by a stop frequency (f _stop ),
characterized in that the stop frequency (f _stop ) - starting from a base _stop frequency (f _{stop, 0} ) - increases with increasing temperature. Electronic ballast according to claim 1,
characterized in that the inverter (4) is formed by two controllable switching elements (S2, S3) arranged in a half-bridge arrangement and the control or regulating circuit (6) detects the current flowing over the half-bridge. Electronic ballast according to claim 2,
characterized in that the control or regulating circuit (6) detects the voltage drop across a resistor (R1) arranged at the base of the half-bridge. Electronic ballast according to one of the preceding claims,
characterized in that the control or regulating circuit (6) determines the operating frequency (f _run ) for the inverter (4) on the basis of a comparison of the detected operating parameter with a reference value. Electronic ballast according to one of the preceding claims,
characterized in that the control or regulating circuit (6) first determines an operating frequency (f _run ) for the inverter (4) as a function of the detected operating parameter, then compares the determined operating frequency (f _run ) with the stop frequency (f _stop ) and controls the inverter (4) depending on the comparison result either with the determined operating frequency (f _run ) or the stop frequency (f _stop ). Electronic ballast according to one of the preceding claims,
characterized in that the control or regulating circuit (6) derives the control signals for the inverter (4) from a basic frequency (f _basis ), the basic frequency (f _basis ) increasing with increasing temperature. Electronic ballast according to one of claims 1 to 5,
characterized in that the control or regulating circuit (6) determines the current temperature and determines the _stop frequency (f _stop ) on the basis of the temperature information received. Electronic ballast according to claim 7,
characterized in that the control or regulating circuit (6) detects a temperature-dependent reference voltage (V _ref ) for determining the temperature. Electronic ballast according to one of the preceding claims,
characterized in that the control or regulating circuit has an analog / digital converter (ADC2) for converting the detected operating parameter into a digital value consisting of at least 2 bits, on the basis of this digital value in an digital arithmetic block (8) an operating frequency (f _run ) calculated to operate the inverter (4) and transmitted to a driver circuit (7), which converts this switching information into corresponding control signals for controlling the inverter (4). Electronic ballast according to claim 9,
characterized in that the smoothing circuit (3) is formed by a switching regulator and the control or regulating circuit (6) detects at least one operating parameter (U _z ) of the smoothing circuit (3) and a controllable switch (S1) of the switching regulator depending on the value of the detected Controls operating parameters (U _z ),
The control or regulating circuit (6) has at least one further analog / digital converter (ADC1) for converting the detected operating parameter (U _z ) into a digital value consisting of at least 2 bits,
and wherein the control or regulating circuit (6) calculates switching information for operating the controllable switch (S1) of the switching regulator on the basis of this digital value in a digital computing block (7) and transmits it to the driver circuit (7), which converts this switching information into a corresponding control signal implemented to control the switch (S1). Electronic ballast according to claim 9 or 10,
characterized in that the base _stop frequency (f _{stop, 0} ) can be predetermined by a reference resistor (R2) which can be connected to the control or regulating circuit (6), the size of which is via an analog provided in the control or regulating circuit (6) / Digital converter (ADC3) is determined, which, after connecting an internal current source (I _s ), converts the voltage drop across the reference resistor (R2) into a digital value consisting of at least 2 bits. Electronic ballast according to one of claims 9 to 11,
characterized in that the control or regulating circuit (6) for converting the detected operating parameters and the voltage drop across the reference resistor (R2) has a single time-multiplexing analog / digital converter. Electronic ballast according to one of the preceding claims,
characterized in that the control or regulating circuit (6) is designed as an application-specific integrated circuit.

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