EP1705964B1 - Ballast with dimmer - Google Patents

Description

Technical area

The present invention relates to an electronic ballast with a dimming device for controlling the lamp brightness of a low-pressure discharge lamp, and to a method for controlling the lamp brightness of a low-pressure discharge lamp.

State of the art

Electronic ballasts for operating low-pressure discharge lamps are known in many designs. Usually, they include a rectifier circuit for rectifying an AC supply and charging a capacitor, often referred to as a smoothing capacitor. The voltage applied to this capacitor DC voltage is used to power an inverter or inverter (hereinafter inverter), which operates the low-pressure discharge lamp. Basically, an inverter from a rectified AC power supply or a DC power supply generates a power supply for the lamp, which has a much higher frequency than the mains frequency. Similar devices are also known for other lamp types, for example in the form of electronic transformers for halogen lamps.

Dimming devices for operating electronic ballasts for controlling the brightness of low-pressure discharge lamps are known per se.

A known possibility of brightness control is to adjust the regulation of the amplitude of the lamp current, the lamp power and thus the lamp brightness. This can be done by approximating or removing the operating frequency of the inverter from resonant frequencies of the lamp-inverter system.

The document US-A1-2002 / 0060537 discloses dimming from a gas discharge lamp using different dimming strategies in three different dimming ranges. The three dimming strategies are analog operation, pulsed operation and superimposition of these two modes. In all dimming ranges pulse width modulation (PWM) is used.

Presentation of the invention

The invention is based on the technical problem of providing an improved electronic ballast with regard to lamp brightness control.

• mit abnehmender Helligkeit werden sowohl die Maxima als auch die Minima der Einhüllenden des Lampenstromes kleiner;
• bei weiter abnehmender Helligkeit wird die periodische Modulation der Einhüllenden des Lampenstromes durch eine, optional Null entsprechende, untere Begrenzung (MIN) der Lampenstromamplitude überlagert, so dass eine periodische Modulation der Einhüllenden des Lampenstromes in Lampenstrompulspakete mit über der unteren Begrenzung (MIN) liegenden Lampenstromamplituden entsteht,
• wobei mit weiter abnehmender Helligkeit die Pulspaketbreiten abnehmen und die Abstände zwischen den Pulspaketen mit den begrenzten Lampenstromamplituden (MIN) zunehmen.

This object is achieved by an electronic ballast with a dimming device for controlling the brightness of a low-pressure discharge lamp. The electronic ballast is designed to operate the low-pressure discharge lamp with periodically modulated lamp current, characterized in that the dimming device is designed to set the lamp current for controlling the lamp brightness as follows:

• with decreasing brightness both the maxima and the minima of the envelope of the lamp current become smaller;
• with further decreasing brightness, the periodic modulation of the envelope of the lamp current is superimposed by a, optionally zero corresponding, lower limit (MIN) of the lamp current amplitude, so that a periodic modulation of the envelope of the lamp current is formed in Lampenstrompulspakete with lying above the lower limit (MIN) lamp current amplitudes .
• with further decreasing brightness, the pulse packet widths decrease and the distances between the pulse packets increase with the limited lamp current amplitudes (MIN).

Preferred embodiments of the invention are specified in the dependent claims and are explained in more detail below. The disclosure always refers to both the process category and the device category of the invention.

To control the lamp brightness, the invention provides that the high-frequency lamp current is amplitude-modulated with a modulation signal and thus the average lamp current is varied.

Starting from the maximum brightness of the low-pressure discharge lamp, at least the minima and, at least above a certain brightness value, the maximums of the envelopes are reduced to reduce the brightness. The amplitude difference between the minima and the maxima of the envelopes of the lamp current can be constant over a certain brightness range, but need not be. Preferably, the shape of the modulation signal is maintained in this brightness range. It only changes the DC component of the modulation signal.

As the brightness further decreases, the minimums of the envelope of the lamp current reach a lower limit. This lower limit is not undershot by the envelope. The lower limit may, depending on the embodiment of the invention, assume a positive finite value or else be set to zero. Between the phases with greater amplitude of the envelope, there are thus phases in which the amplitude of the envelope of the lamp current corresponds to the value of the lower limit.

The phases in which the amplitude of the envelope is greater than the value of the lower bound define "pulse packets". These are from each other separated by phases with minimum amplitude of the envelope. A pulse packet corresponds to a coherent period in which the amplitude of the envelope is greater than the minimum value and consists of several high-frequency lamp current oscillations. Between the pulse packets, the amplitude of the envelope corresponds to the lower limit, so it flows with positive finite lower limit, a high-frequency lamp current. If the lower limit is set to zero, no lamp current flows.

If the brightness is further reduced, the time expansion of the pulse packets decreases and the distances between the pulse packets become longer.

To increase the brightness, starting from lower brightnesses, the above scheme is used in reverse order.

When operating a low-pressure discharge lamp with periodically modulated lamp current, the low-pressure discharge lamp is operated at several operating points of the lamp voltage / lamp current characteristic, that is to say the lamp characteristic. During the lower lamp current phases, the operating point is in a steeper region of the higher voltage characteristic line, while the larger lamp current phase is the lower operating point in a flatter lower voltage characteristic line FIG. 1 rather left or rather right.

The advantages of such an operation become clear when compared to a brightness control by means of a modulation-free amplitude adjustment of the lamp current.

At low levels of brightness, small lamp currents flow without amplitude modulation, so the operating point is in FIG. 1 rather left on the lamp characteristic. In this characteristic range, the lamp voltage depends strongly on the lamp current. If the brightness is further reduced, so does the lamp current continues to decrease and the lamp voltage increases very strongly. Above all, in the field of small continuous lamp currents, the dependence of the lamp voltage on the lamp current is also very strongly temperature-dependent. For larger lamp currents, the lamp voltage is only weakly dependent on the lamp current.

From a certain lamp voltage, the inverter can no longer provide the lamp voltage continuously.

During operation of a low-pressure discharge lamp with periodically modulated lamp current, the operating point lies in a range of lower characteristic slope and lower burning voltages during the phases with a larger lamp current and in a range of greater characteristic slope and higher lamp voltages during the phases with smaller lamp current. At low levels of brightness, the low-pressure discharge lamp is periodically operated only briefly in the critical range of small lamp currents, the burning voltage increases only slightly.

Thus, lower brightnesses can be achieved than with unmodulated current, because in the phases with larger lamp currents enough charge carriers are provided for the discharge, and thus in phases with very small lamp currents, a complete recombination of the charge carriers is avoided.

There may be embodiments of the invention in which the lamp current disappears periodically entirely. Nevertheless, charge carriers are retained by the periodically recurring maxima of the envelope. So very low brightnesses can be achieved.

A preferred embodiment of the invention provides that a finite minimum lamp current amplitude is not undershot, but preferably should be so small that it only facilitates a rapid subsequent increase in amplitude.

In a further preferred embodiment, the envelope is limited to large values. The envelope does not exceed a maximum value. Starting from low brightnesses, this means that with increasing maxima and minima of the envelope of the lamp current, the maxima first reach this upper limit and then also do not exceed the corresponding value. A further increase in brightness can be achieved by increasing the minima. Phases with the maximum value of the envelope are interrupted by minima in the envelope and their surroundings. The maximum brightness is reached when the amplitude of the envelope always assumes its maximum value corresponding to the upper limit. Basically, this is a mirror image approach in relation to the lower limit explained above.

During operation of the low-pressure discharge lamp with modulated high-frequency lamp current, the rising edge of a phase with larger lamp current amplitudes is preferably carried out relatively steeply in comparison with the corresponding falling edge. A rapid reduction of the envelope of the lamp current does not correspond to the inherent dynamics of the system of inverter and low-pressure discharge lamp. Because there are still many charge carriers in the discharge space of the low-pressure discharge lamp, the burning voltage of the low-pressure discharge lamp increases only slowly and the inverter can continue to couple power into the low-pressure discharge lamp.

However, swelling of small lamp currents to large lamp currents is quickly possible within a few lamp-current oscillations, and the rising edges of the modulation can be made very steep. At low lamp currents, the low-pressure discharge lamp damps the Inverter only weak, it can be generated suddenly high voltages, which have a large lamp current result. As a result, a large lamp current can be built up very quickly after phases with a relatively small lamp current.

Preferably, an embodiment of the invention is adapted to modulate the envelope of the lamp current sawtooth or rounded sawtooth, the rising edge is much steeper than the falling.

The ratio between the peak and average value of the lamp current, the crest factor, can be kept small if the amplitude amplitude of the envelope of the lamp current is adequately selected. This suggests a longer life of the low pressure discharge lamp and inverter system.

Preferably, an embodiment of the invention comprises a signal generator for generating a periodic signal and a circuit arrangement for limiting the periodic signal. The limited periodic signal is used as a reference for controlling the modulation of the lamp current. The signal is limited by a lower bound and possibly an upper bound. The frequency response and the amplitude of the signal generated by the signal generator should preferably be able to be adapted to the particular embodiment of the invention. For example, it may be useful to make the frequency of the signal generated by the signal generator of the work area, ie the average lamp current dependent. For small lamp currents, it may be useful, for example, to increase the frequency of the sequence of maxima and minima of the envelope of the lamp current in order to give the charge carriers present in the discharge less time for recombination, even at a lower charge carrier density.

In a preferred embodiment of the invention, the output signal of the signal generator is synchronized with the phase position of the low-frequency, for example, as a result of rectification of a mains voltage supply voltage of the inverter. In this way, any beats noticeable as flickering of the lamp brightness can be avoided.

A preferred embodiment of the invention provides to control the inverter via a control loop. For this purpose, the invention has a measuring device which measures the lamp current and converts it into a controlled variable. Alternatively, this measuring device may also measure the operating frequency of the inverter or another variable related to the lamp current to convert it to a controlled variable. Next, a controller is provided. The controller receives the controlled variable of the measuring device and a signal, which is related to the desired brightness, as an input signal (reference variable). From control variable and reference variable, the output signal of the controller determines the control of the inverter.

A further preferred embodiment of the invention provides a circuit arrangement for measuring the lamp resistance, for example in EP 0 422 255 B1 described. The measured variable is converted into a controlled variable, for example a voltage signal, and serves as an additional input to the regulator. With a greatly increasing resistance of the discharge lamp, the controller can control the inverter so that a tearing of the gas discharge by increasing the lamp current is prevented.

Since the invention can make do without additional power components in the load circuit, compact can be built if required. Therefore, the invention is preferably suitable for integration of the electronic ballast in low-pressure discharge lamps, in particular compact fluorescent lamps (CFL).

Brief description of the drawings

Figur 1

zeigt die Abhängigkeit der Lampenspannung einer erfindungsgemäßen Niederdruckentladungslampe vom Lampenstrom.

Figuren 2 a-f

zeigen den modulierten Lampenstrom bei verschiedenen Lampenhelligkeiten einer erfindungsgemäßen Niederdruckentladungslampe.

Figur 3

zeigt eine erfindungsgemäße Schaltungsanordnung zur Steuerung der Lampenhelligkeit.

Figuren 4 a-f

zeigen wie ein moduliertes Signal für den Betrieb einer Niederdruckentladungslampe erfindungsgemäß generiert wird.

In the following, the invention will be explained in more detail using an exemplary embodiment. The individual features disclosed may also be essential to the invention in other combinations. The above and following description refers to the device category and the method category of the invention, although not explicitly mentioned in detail.

FIG. 1

shows the dependence of the lamp voltage of a low-pressure discharge lamp according to the invention on the lamp current.

Figures 2 af

show the modulated lamp current at different lamp brightnesses of a low-pressure discharge lamp according to the invention.

FIG. 3

shows a circuit arrangement according to the invention for controlling the lamp brightness.

Figures 4 af

show how a modulated signal for the operation of a low-pressure discharge lamp according to the invention is generated.

Preferred embodiment of the invention

In FIG. 1 the lamp voltage of a low-pressure discharge lamp according to the invention is shown as a function of the lamp current, ie the lamp characteristic. The lamp voltage initially increases only moderately from a minimum at maximum lamp current when the lamp current is reduced; the dependence of the lamp voltage on the lamp current is low: brightness range 1 in FIG. 1 , With a further reduction of the lamp current, the lamp voltage increases more and more; the dependence of the lamp voltage on the lamp current is increasing more pronounced: brightness ranges 2 and 3 in FIG. 1 , When a minimum lamp current is undershot, the gas discharge breaks down when the required voltage from the inverter can not be continuously supplied. The limited output voltage of the inverter thus defines the minimum lamp current at which the lamp can still be operated continuously, and thus the minimum brightness of the lamp at unmodulated lamp current.

Based on FIG. 1 and to illustrate the idea according to the invention, the entire brightness range is subdivided into three brightness ranges.

In a first brightness range between the maximum possible brightness and a medium brightness, the lamp current is modulated in a sawtooth manner as the brightness decreases, wherein the amplitude of the sawtooth-shaped modulation increases with decreasing brightness and the maxima of the sawtooth are "cut off" by an upper limit.

Figure 2 a shows the lamp current just below the maximum brightness, FIG. 2 b shows the lamp current at a lower brightness than FIG. 2 a. It can be seen that the amplitude deviation of the sawtooth-shaped modulation changes.

Subsequent to the end of the first brightness range, the brightness is further reduced in a second range. The amplitude deviation of the saw-tooth modulation is not changed, but the DC component of the modulation and the effective value of the lamp current continue to decrease by reducing the maximum lamp current amplitude, such as FIGS. 2 c and 2 d demonstrate.

Figure 2 c shows the lamp current just at the border to the first brightness range, FIG. 2 d shows the lamp current at a lower brightness than FIG. 2 c.

The second brightness range is followed by a third brightness range. This extends to the minimum brightness. The maximum amplitude of the lamp current is further reduced, decreasing the stroke of the sawtooth modulation and cutting off the sawtooth at values below the lower limit. Between the maximums of the envelopes of the lamp current, the lamp current amplitudes assume a presettable minimum value (MIN). As a result, the envelope of the lamp current assumes a pulsed shape. Each phase in which the amplitude of the envelope takes a value greater than the minimum value (MIN) defines a pulse packet. The further the maximum current amplitude and thus the stroke is reduced, the longer are the times with the minimum lamp current amplitudes. The minimum lamp current amplitudes can be very small and even identical to zero, so that no or almost no lamp current flows. As the brightness decreases, the amplitudes in the pulse packets become smaller, the pulse packet durations less, and the distances between the pulse packets larger. The frequency of the modulation signal can increase.

FIG. 2 e shows the lamp current at a brightness close to the boundary to the second brightness range; Figure 2 f shows the lamp current at a lower brightness.

FIG. 3 shows a circuit arrangement according to the invention for controlling the lamp brightness. To control the lamp brightness, a first signal DL is used, which behaves strictly monotone to the desired brightness. This signal is fed to a sawtooth generator STG. The sawtooth generator STG can be designed as a self-oscillating circuit.

The signal DL determines the DC component of the sawtooth signal, for example, DL can be proportional to the DC component of the sawtooth signal. The sawtooth generator generates a signal ST, which is supplied to a clamping circuit. The clamping circuit CL provides an output signal RV which is limited up and down. If ST assumes values which are greater than the value MAX, the output signal RV is limited to the value MAX (clamped). If ST assumes values smaller than MIN, RV is limited to MIN (clamped). The original signal ST can also be completely above the value MAX or below the value MIN. In these cases, the output RV of the clamping circuit CL corresponds to a constant signal with the value MAX or MIN.

The clamped sawtooth signal RV is fed as a reference variable to a regulator REG. The regulator REG can be implemented as a PI controller.

The regulator REG controls via its output signal MV, the operating frequency of the inverter INV, which operates the low-pressure discharge lamp. Further, the inverter INV provides a size CV which depends on the lamp current. The size CV may in particular be the lamp current itself or the operating frequency of the inverter.

The measuring device ME generated from the size CV, a signal AV, which is supplied to the regulator REG as a controlled variable.

The minimum value of the reference variable RV for the regulator REG corresponds to the value MIN. The value MIN should preferably not be too small from a control engineering point of view. The REG regulator should always be held in an active operating state instead of allowing its output signal to drop (rise) to a final value due to the supply voltage of the REG regulator. As a result, larger transient events can be avoided with increasing (falling) edge of the sawtooth-shaped modulation signal.

The Figures 4 af show the change of the sawtooth signal ST and the reference variable RV for driving the regulator REG with a change in the desired brightness from just below the maximum brightness to a low brightness. Near the maximum brightness, a large part of the sawtooth signal is above the maximum value MAX. Those Parts of the sawtooth signal which are above the value MAX are clamped to the value MAX. The clamping device CL generates the reference variable RV, which largely corresponds to the maximum value MAX. At the times when the signal ST is smaller than the value MAX, the reference variable RV corresponds to the signal ST, such as FIG. 4 a shows. If the desired brightness is reduced, so does the DC component of the sawtooth voltage ST. The stroke of the amplitude modulation of the controlled variable RV increases, but only up to a maximum value which corresponds to the stroke of the sawtooth signal ST, such as FIG. 4 b shows.

If the DC component of the sawtooth voltage is further reduced, then initially not the stroke of the modulation of the reference variable RV changes, but their DC component, like the FIGS. 4 c and d demonstrate.

As the brightness decreases further, phases occur in which the sawtooth signal ST drops below the minimum value for the reference variable RV defined by the value MIN. During these phases, the reference variable RV is clamped by the clamping circuit CL to the minimum value MIN. The phases with decreasing DC component of the sawtooth signal ST become longer, like the FIGS. 4 e and f demonstrate. If the sawtooth signal ST is always smaller than the value MIN, the reference variable RV corresponds to the value MIN.

Usually, when the inverter is supplied with an intermediate circuit voltage, it will not be constant in time, but will have corresponding fluctuations in the periodicity of the supply network. The frequency of the modulation signal is much larger. This can cause beats, which can be perceived as flickering of the low-pressure discharge lamp. To prevent this, the phase angle of the sawtooth signal can be synchronized with the phase position of the line frequency. For example, it can be achieved by a suitable circuit that a rising edge of the sawtooth signal is always generated at the time of the network maximum.

The size of the MIN signal should be kept as small as possible in order to achieve the lowest possible brightness. With a small signal DL or MIN increases the risk of extinguishing the discharge. To prevent that from happening EP 0 422 255 B1 known circuit can be used to measure the discharge resistance. If this increases strongly, a demolition of the discharge is imminent. Based on the knowledge of the discharge resistance, the regulator REG can be supplied with an additional controlled variable, so that the lamp current is increased in the event of an imminent extinction of the lamp.

Claims (12)

1. Electronic ballast having a dimming device for the purpose of controlling the brightness of a low-pressure discharge lamp, designed for operation with periodically modulated lamp current, characterized in that, in order to control the lamp brightness, the dimming device is designed to adjust the lamp current as follows:

   - with decreasing brightness, both the maxima and the minima of the envelope of the lamp current become smaller;

   - with further decreasing brightness, the periodic modulation of the envelope of the lamp current is superimposed by a lower limit (MIN), optionally corresponding to zero, of the lamp current amplitude, resulting in periodic modulation of the envelope of the lamp current in lamp current pulse packets with lamp current amplitudes above the lower limit (MIN),

   - with further decreasing brightness, the pulse packet widths decreasing and the intervals between the pulse packets increasing, the amplitude of the envelope corresponding to the lower limit (MIN) in the intervals.
2. Electronic ballast according to Claim 1, in which the dimming device is designed such that the lower limit (MIN) corresponds to a positive value for the envelope of the lamp current.
3. Electronic ballast according to Claim 1 or 2, in which the dimming device is designed such that the envelope of the lamp current is restricted by a maximum value (MAX) at a high and maximum brightness.
4. Electronic ballast according to Claim 1, 2 or 3, in which the dimming device is designed such that the respectively rising edges of the periodic modulation are steep compared with the respectively falling edges.
5. Electronic ballast according to Claim 4, in which the dimming device is designed to modulate the envelope of the lamp current in saw-tooth form or rounded saw-tooth form for the purpose of controlling the brightness.
6. Electronic ballast according to one of the preceding claims, in which the dimming device has a signal generator (STG) for the purpose of generating a periodic signal (ST) for the modulation of the lamp current and a circuit arrangement (CL) for the purpose of restricting the periodic signal (ST) corresponding to, if appropriate, a maximum boundary and the minimum boundary (MAX, MIN).
7. Electronic ballast according to one of the preceding claims, in which the dimming device has a signal generator (STG) for the purpose of generating a periodic signal (ST) for the modulation of the lamp current and a device for synchronizing the periodic signal with the supply voltage of an inverter (INV) for the purpose of producing the lamp current, the output signal from the signal generator (STG) being synchronized with the phase angle of the supply voltage of the inverter (INV) which is fluctuating at a low frequency.
8. Electronic ballast according to one of the preceding claims having

   - an inverter (INV) for the purpose of producing the lamp current,

   - a measuring device (ME) for the purpose of measuring the lamp current or a variable dependent on the lamp current and for the purpose of producing a controlled variable (AV),

   - a regulator (REG) controlled by the dimming device for the purpose of controlling the inverter (INV).
9. Electronic ballast according to Claim 8 having a device for preventing interruption of the gas discharge, which is designed to measure the lamp resistance and to convert the lamp resistance into an additional controlled variable.
10. Low-pressure discharge lamp having an integrated electronic ballast according to one of the preceding claims.
11. Method for controlling the brightness of a low-pressure discharge lamp by means of an electronic ballast having a dimming device, designed for operation with periodically modulated lamp current, characterized in that, in order to control the lamp brightness, the dimming device adjusts the lamp current as follows:

    - with decreasing brightness, both the maxima and the minima of the envelope of the lamp current become smaller;

    - with further decreasing brightness, the periodic modulation of the envelope of the lamp current is superimposed by a lower limit (MIN), optionally corresponding to zero, of the lamp current amplitude, resulting in periodic modulation of the envelope of the lamp current in lamp current pulse packets with lamp current amplitudes above the lower limit (MIN),

    - with further decreasing brightness, the pulse packet widths decreasing and the intervals between the pulse packets increasing, the amplitude of the envelope corresponding to the lower limit (MIN) in the intervals.
12. Method according to Claim 11 using a ballast according to one of Claims 1 to 10.

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