EP1324643B1 - 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 fluorescent tubes, are often power-controlled. In this type of control, the intermediate circuit voltage supplied to the inverter is kept substantially constant, while the current flowing through the inverter is regulated by changing the operating frequency. This is done, for example, by a shunt resistor provided in the half-bridge of the inverter, the voltage dropping across this shunt resistor being fed as the actual value for the half-bridge current to a control or regulating circuit. The control or regulating circuit sets the operating frequency of the inverter so that the average current through the shunt resistor or the proportional mean voltage above it remain constant. With the DC link voltage kept constant and the current regulated in this way to a constant value, the lamp is always supplied with the same power.

Fluorescent tubes have a negative characteristic when their voltage is a function of the current. This means that at a certain temperature T1, the lamp voltage U _LA drops as the lamp current I _LA increases, as is the case, for example, in the characteristic curve U _{LA, T1} illustrated in FIG. If the dependency between current and voltage at a certain power is entered into the same diagram, since the power is the product of current and voltage, the hyperbola P also shown in FIG. 2 results. If the lamp now has a certain power regulated, so sets on the characteristic of the lamp an operating point, which corresponds to the intersection between the characteristic and the power hyperbola. In the example shown in FIG. 2, in which the power P is to be set, the operating temperature which corresponds to the lamp current I _T1 results, for example, at the temperature T1. Usually, the lamps are designed so that the operating point is optimal at a certain temperature. This means that optimum light output is guaranteed when the lamp is operated at a certain temperature, typically between 30 ° C and 40 ° C.

Compared with the previously described ideal case at normal operating temperature, however, the situation may occur that the temperature at which the lamp is actually operated, is significantly higher. This could be used, for example, in factory buildings a high heat development will be the case. Increasing the temperature, however, results in a new characteristic curve for the lamp and thus also a new operating point. In the example shown in FIG. 2, for example, the new characteristic curve U _{LA, T2} results at the higher temperature T2 _, and a new operating point is established which corresponds to the increased lamp current I _T2 . This new operating point is achieved in the context of the power control described above by reducing the operating frequency for the inverter.

By changing to the new operating point with an increase in the operating temperature is achieved while ensuring that the power supplied to the lamp remains constant, but at the same time increases due to the higher power dissipation and the luminous efficacy of the lamp is reduced. If the operating frequency is significantly reduced compared to the frequency at normal temperature, there is even a risk that the current will rise to areas in which the device itself is endangered.

Therefore, to prevent the current from assuming dangerous levels due to a rise in temperature, usually the frequency range within which the lamp current is allowed to be controlled is limited downwards. Practically, a stop frequency is set, which must not be fallen below. When the stop frequency is reached, the ballast acts as a constant current source and no longer as a constant power source. As a result, the maximum current corresponding to the stop frequency can not be exceeded, thus preventing destruction or damage of some or all of the components of the ballast.

However, setting the stop frequency is also problematic, since several factors must be taken into account. On the one hand, the highest possible stop frequency is desirable since the safety function described above must not be used too late and thus at too high currents. For example, if the lamp is operated at normal temperature with a frequency of about 45 kHz, then a stop frequency of about 41 kHz should be selected.

On the other hand, however, it should be noted that the operating frequency at normal temperature may fluctuate around the ideal value of 45 kHz, for example, 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 power, then each ballast will have a slightly different operating frequency, as shown for example in Figure 3. The curve I shown in FIG. 3 shows that the actual operating frequency of all Ballast is distributed around the optimum frequency f _nm 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 the curve II. The reason for this again lies in certain tolerances of the components responsible for determining the stop frequency.

It may now be the case that the operating frequency at normal temperature is already below the stop frequency. This is the case in the region F shown hatched in FIG. 3, in which the curves for the operating frequency (I) at normal temperature and the stop frequency (II) overlap. All ballasts that fall within this hatched area do not allow for proper lamp operation and are therefore considered scrap. In order to keep this proportion of such faulty ballasts low, the lowest possible stop frequency is therefore desirable, which, however, is in contradiction to the above-mentioned preference for the highest possible stop frequency.

The problem described above could be avoided by carrying out an adjustment for each device immediately after production, in which a stop frequency suitable for the respective operating frequency is determined individually. In electronic ballasts that allow dimming the lamp, such an adjustment is carried out anyway, so that there is no additional work required. For non-dimmable devices, however, such an adjustment is not absolutely necessary, so that an individual determination of the stop frequency means additional work, by which the manufacturing cost is increased.

The present invention is therefore an object of the invention to provide an electronic ballast, in which on the one hand early and reliable onset of current limiting is made possible and on the other hand ensures that the stop frequency in the initial state in each case below the operating frequency lies.

The object is achieved by an electronic ballast having the features of claim 1. This is characterized by the fact that the stop frequency is temperature-dependent and - starting from a base stop frequency - increases with increasing temperature. In this way, it is possible to choose a correspondingly low base stop frequency, which is far enough below the operating frequency, so that in each case a proper lamp start is guaranteed. Starting from the base stop frequency then increases the stop frequency with increasing temperature, so that at higher temperatures in time uses a current limit, which avoids damage to the ballast. An adjustment of the ballast after its production is thus no longer necessary, which is why the invention contributes to a reduction in manufacturing costs, especially in non-dimmable ballasts.

Further developments of the invention are the subject of the dependent claims. Thus, the power control preferably takes place in that a control or regulating circuit detects the voltage drop across a resistance arranged at the base of the half-bridge, compares this actual value of the half-bridge current with a reference value and determines an operating frequency for the inverter on the basis of the comparison result. The increase of the stop frequency with increasing temperature can be achieved in a simple and elegant way in that the control or regulating circuit derives the control signals for the inverter from a fundamental frequency, this fundamental frequency rising with increasing temperature. Here, the stop frequency, which takes into account the control or regulating circuit internally when determining the frequency for the inverter, remains unchanged, while the effectively set at the inverter minimum frequency still increases. Alternatively, however, there is also the possibility that the control or regulating circuit determines the currently present temperature - 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 with increasing temperature.

A particularly advantageous embodiment of the ballast according to the invention is to form the control or regulating circuit for the inverter digital. This is achieved in that within the control or regulating circuit, an analog / digital converter is provided, which converts the operating parameters detected by the control or regulating circuit into a digital value consisting of at least 2 bits. On the basis of this digital value, an operating frequency for operating the inverter is then calculated in a digital computing block and converted by means of a driver circuit into corresponding control signals for the switching elements of the inverter. This solution enables extensive integration of the ballast controls. At the same time, the implementation of the analog measured operating parameters into digital values with high accuracy enables a high stability in the control of the lamp power. This digital embodiment can be extended, for example, to the control circuit for the smoothing circuit.

The base stop frequency for the ballast can be specified by a connectable to the control or regulating circuit reference resistance whose size is determined by a provided in the control or regulating circuit analog / digital converter, which after connecting an internal power source, converts the voltage dropping across this reference resistor into a digital value also consisting of at least 2 bits. If the control circuits for the inverter and the intermediate circuit voltage are also digital in the manner described above, a further reduction of the components can be achieved by merely converting in control or regulation circuit the detected operating parameters and the voltage drop across the reference resistor a single analog / digital converter is provided which operates in time division multiplex. Preferably, the digital values converted by the analog / digital converter have an accuracy of 12 bits.

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.

In the following the invention will be explained in more detail 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 graph illustrating 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 Figure 1 is the input side connected via a high-frequency filter 1 to the mains supply voltage U ₀ . At the output of the high-frequency filter 1 is a rectifier circuit 2 in the form of a full-bridge rectifier, which converts the mains supply voltage U ₀ in 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 boost converter having an inductance L1, a controllable switch in the form of a MOS field-effect transistor S1 and a diode D1. Instead of the boost converter illustrated here, other known smoothing circuits can also be used.

By means of a corresponding switching of the MOS field-effect transistor S1, a storage capacitor C2 adjoining the smoothing circuit 3 is applied DC _link voltage U _z generated, which is supplied to the inverter 4. This inverter 4 consists of two arranged in a half-bridge arrangement MOS field effect transistors S2 and S3. By an alternating high-frequency driving of the two field-effect transistors S2, S3, an alternating voltage is generated at the midpoint of the half-bridge, which is supplied to the load circuit 5 with the gas discharge lamp LA connected thereto. 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 this to a driver circuit 7 which converts this control information into corresponding control signals for the gates of the three MOS transistors. Field effect transistors S1-S3 converts. The control information is determined based on operating parameters that 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 falling across the storage capacitor C2 is determined, on the other hand the shunt resistor R1 which drops across this resistor R1 or the average half-bridge current and thus ultimately that of the lamp is connected via a shunt resistor R1 arranged at the base of the half-bridge of the inverter 4 LA supplied power determined.

Within the control or regulating circuit 6, two control loops are formed, one for the intermediate circuit voltage U _z and one for the lamp power. The intermediate circuit voltage U _z is thereby converted by a first analog / digital converter ADC1 into a digital value which is fed to a first digital arithmetic block 8. This calculation block 8 calculates by means of the from the analog / digital converter ADC1 obtained actual value of the intermediate circuit voltage U a suitable switching frequency for the MOS field effect transistor S1 _for the boost 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 drop across the shunt resistor R1 at the base of the half-bridge voltage is converted by a second analog-to-digital converter ADC2 and fed to a comparison block 9. This - also digitally operating - comparison block 9 compares the current actual value of the lamp power with a predetermined reference value P _ref and determines from the comparison result, whether the frequency of the inverter 4 must be increased or reduced to operate the lamp LA with the desired power. This information will be available later described in more detail logic block 10 to an output block 11, which outputs corresponding control information to the driver circuit 7, which in turn drives the two MOS field-effect transistors S2 and S3 of the inverter 4. In this way, the operating frequency of the inverter 4 is adjusted so that the lamp LA is operated at the desired power.

As already explained, 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 height of the resistor R2 is a measure of the _stop frequency f _stop . It is determined by the fact that the terminal of the resistor R2 is connected to a provided in the control or regulation circuit 6 internal current source I _s via a switch S4. The voltage which then drops 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, firstly the result supplied by the comparison block 9 and secondly the value determined by the third analog / digital converter ADC3, which is a measure of the stop frequency is to be supplied. 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 , then it is transmitted unchanged to the output block 11, which drives the inverter 4 with the aid of the driver circuit 7. In this case, therefore, the _stop frequency f _stop does not affect the control operation for the lamp power.

However, if the operating frequency f _run determined by the comparison block 9 for setting the desired lamp power lies 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 indicates 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, whereby the occurrence of excessive currents and damage to the ballast is avoided.

As previously mentioned, the reference resistance R2 is designed such that the _stop frequency f _stop predetermined by it is sufficiently below the operating frequency necessary for the desired lamp power at normal operating temperatures f _run lies. This ensures that in each case a regular lamp start can be performed and the current limit is not already used at start of operation.

Only with an increase in the temperature of the lamp LA or the ballast, the _stop frequency f _{stop to be} raised to allow timely insertion of the current limit. This is achieved by providing the central clock 12, which is provided in the control or regulation circuit 6, in a temperature-dependent manner. This clock 12 transmits to all components of the control or regulating circuit 6 a clock signal to enable a synchronous operation of the various units. In particular, this clock signal is also transmitted to the output block 11 of the lamp power control circuit; 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 12 is temperature-dependent applies to the frequency f _{base of} the transmitted from him to all components of the control or regulating circuit 6 clock signal: $${\mathrm{f}}_{\mathrm{Base}}={\mathrm{f}}_{\mathrm{Base}.0}-\mathrm{TK}\times \left(\mathrm{T}-{\mathrm{T}}_0\right)$$

The frequency f _{base, 0} represents the frequency of the clock signal generated by the clock 12 at normal temperature T _0. Since the frequency f _{base of} the clock signal at the same time represents the fundamental frequency from which the output block 11 derives the control signals for the driver circuit 7, which applies same temperature dependence for the transmitted from the control or regulating circuit 6 to the driver circuit 7 signals. In particular, this means that in the event that the stop frequency obtained 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: $${\mathrm{f}}_{\mathrm{Stop}}={\mathrm{f}}_{\mathrm{Stop}.0}-\mathrm{TK}\times \left(\mathrm{T}-{\mathrm{T}}_0\right)$$

The temperature dependence of the clock 12 thus has the consequence that the actually used stop frequency increases with increasing temperature.

The solution presented here is characterized in that the increase of the stop frequency is achieved in a particularly simple and elegant way. The temperature-dependent behavior of the clock 12 can be achieved without much effort. Of course, there is also the possibility of others To take measures to ensure an increase in the stop frequency with increasing temperature.

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

The illustrated in the present embodiments, digital embodiment of the control or regulating circuit 6 advantageously allows extensive integration of the entire circuit. The integration can be further enhanced by using only a single analog-to-digital converter which operates to time-multiplex the various input signals. The analog / digital converter (s) 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, it is also possible to design the circuit as a so-called application-specific integrated circuit (ASIC).

The solution according to the invention thus ensures reliable operation of the electronic ballast, in which it is ensured that the desired current limit occurs in time to avoid damage. In addition, the production costs for the ballast are reduced because no additional adjustment in the production is necessary for the realization of a reliably functioning current limit.

Claims (13)

1. Electronic ballast for at least one gas discharge lamp (LA), preferably for a fluorescent tube, having a rectifier circuit (2) connectable 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 an inverter (4), fed with the intermediate circuit voltage (U_z), to the output of which a terminal for the load circuit (5) containing the lamp (LA) is connected,
   and having a control or regulation circuit (6) which detects an operating parameter of the inverter (4) or of the load circuit (5) which corresponds to the power of the lamp (LA) and in dependence upon detected operating parameter varies the operating frequency (f_run) of the inverter (4), wherein the variation of the operating frequency (f_run) is bounded downwardly by means of a stop frequency (f_stop),
   characterized in that,
   the stop frequency (f_stop) is temperature dependent and, starting from a basis stop frequency (f_stop,0), increases with increasing temperature.
2. Electronic ballast according to claim 1,
   characterized in that,
   the inverter (4) is formed by means of two controllable switch elements (S2, S3) arranged in a half-bridge arrangement, and the control or regulation circuit (6) detects the current flowing via the half-bridge.
3. Electronic ballast according to claim 2,
   characterized in that,
   the control or regulation circuit (6) detects the voltage dropping via a resistance (R1) arranged at the foot point of the half-bridge.
4. Electronic ballast according to any preceding claim,
   characterized in that,
   the control or regulation 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.
5. Electronic ballast according to any preceding claim,
   characterized in that,
   the control or regulation circuit (6) initially determines, in dependence upon the detected operating parameter, an operating frequency (f_run) for the inverter (4), then compares the determined operating frequency (f_run) with the stop frequency (f_stop) and controls the inverter (4) in dependence upon the comparison result either with the determined operating frequency (f_run) or the stop frequency (f_stop).
6. Electronic ballast according to any preceding claim,
   characterized in that,
   the control or regulation circuit (6) derives the control signals for the inverter (4) from a base frequency (f_basis), wherein the base frequency (f_basis) increases with increasing temperature.
7. Electronic ballast according to any of claims 1 to 5,
   characterized in that,
   the control or regulation circuit (6) determines the temperature currently prevailing and on the basis of the obtained temperature information determines the stop frequency (f_stop).
8. Electronic ballast according to claim 7,
   characterized in that,
   for determining the temperature, the control and regulation circuit (6) detects a temperature dependent reference voltage (V_ref).
9. Electronic ballast according to any preceding claim,
   characterized in that,
   the control or regulation circuit has an analog/digital converter (ADC2) for converting the detected operating parameter into a digital value consisting of at least two bits, calculates on the basis of this digital value, in a digital computation block (8), an operating frequency (f_run) for the operation of the inverter (4) and passes this on to a driver circuit (7), which transforms the switching information into corresponding control signals for control of the inverter (4).
10. Electronic ballast according to claim 9,
    characterized in that,
    the smoothing circuit (3) is formed by means of a switched regulator and the control or regulation circuit (6) detects at least one operating parameter (U_z) of the smoothing circuit (3) and controls a controllable switch (S1) in the switched regulator in dependence upon the value of the detected operating parameter (U_z),
    wherein the control or regulation 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 two bits,
    and wherein the control or regulation circuit calculates on the basis of this digital value, in a digital computation block (7), switching information for the operation of the controllable switch (S1), and passes this on to the driver circuit (7), which transforms this switching information into a corresponding control signal for the control of the switch (S1).
11. Electronic ballast according to claim 9 or 10,
    characterized in that,
    the basis stop frequency (f_stop,0) can be predetermined by means of a reference resistance (R2), which can be connected to the control or regulation circuit (6), the size of which resistance is determined via an analog/digital converter (ADC3) provided in the control or regulation circuit (6), which converter after the connection of an internal current source (I_s) transforms the voltage dropping via the reference resistance (R2) into a digital value consisting of at least two bits.
12. Electronic ballast according to any of claims 9 to 11,
    characterized in that,
    the control or regulation circuit (6) has, for the transformation of the detected operating parameter and the voltage dropping via the reference resistance (R2), a single analog/digital converter operating in a time multiplex manner.
13. Electronic ballast according to any preceding claim,
    characterized in that,
    the control or regulation circuit (6) is constituted as an application specific integrated circuit.

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