EP0596152A1 - AC ballast for electric discharge lamps - Google Patents

Abstract

An inductance (L) is connected by means of a first electronic switch (T1) to an input voltage (UN) and thereby charged. Discharging is carried out with the switch (T1) in the locked state, the discharge current flowing through the discharge lamp (EL). A control unit (SE) controls the charge and discharge cycles. The control unit (SE) specifies the cycle frequency of the charge and discharge cycles, or the maximum charge current (iO), as a function of the phase angle of the input voltage (UN) and terminates the respective cycle after the cycle duration has expired. By specifying the parameter characteristic stored in the control unit (SE), the power can be evened out without generating an impermissibly high harmonic component. <IMAGE>

Description

The invention relates to an AC ballast for electrical discharge lamps, and in particular for fluorescent lamps which have heated electrodes.

An AC ballast of the type mentioned in the preamble of claim 1 is known from DE 41 01 980 A1. In this known ballast, an inductance is charged via a first electronic switch and then this switch is blocked and the inductance is discharged via a second electronic switch and the fluorescent lamp. A third electronic switch is connected in parallel to the fluorescent lamp. If the amplitude of the input AC voltage is greater than a predetermined threshold value, the lamp is operated continuously with the third switch blocked, that is to say without a short circuit. If the input voltage is below the limit, the third electronic switch is switched on and off alternately. By switching the third switch at high frequency on and off it is achieved that the lamp voltage assumes a value, even with small values of the input voltage, which is sufficient for maintaining the lamp operation. The level of the lamp voltage that arises can be changed to the desired extent by changing the pulse duty factor and / or the frequency of actuation of the third switch. The ballast delivers a largely constant lamp power over a period of the input voltage. The higher the constancy of the lamp power, the greater the ripple that the ballast causes in the mains current. By suitable dimensioning of the circuit it can be achieved that the time course of the lamp power over a period of 360 of the input voltage has two adjacent maxima and in between goes to zero. This results in good efficiency on the one hand and low harmonic generation on the other. A disadvantage, however, is that it is difficult to change the shape of the power curve or to adapt the required maximum ripple so that the official regulations are met, but on the other hand the greatest possible and constant power is transmitted.

The invention has for its object to provide an AC ballast that allows easy determination or change of the ripple caused.

This object is achieved according to the invention with the features specified in claim 1 or in claim 3.

wobei L die Induktivität, i_o der obere Grenzwert des Lade- oder Entladestromes und f_z die Zyklusfrequenz ist. Da L für das betreffende Vorschaltgerät eine Konstante ist, ist die Leistung P proportional der Zyklusfrequenz f_z, wenn i_o konstant ist.In the ballast according to the invention, the control unit specifies the cycle frequency, ie the cycle duration of the charging and discharging processes, or the maximum charging current of the inductance, as a function of the phase of the input voltage. This means that the control unit contains either a formula or curve or a table in which a cycle frequency or cycle duration, or the upper limit value of the charging current, of the relevant charging and discharging process is contained for each phase position of the input voltage. The respective cycle is ended after the phase-dependent cycle duration. The invention is based on the idea that the power P transmitted to the lamp during a cycle of charging and discharging is proportional to the frequency of the cycle in question and proportional to the square of the maximum charging current of the inductor. The lamp power P is in a very short phase interval

where L is the inductance, i _{o is} the upper limit of the charge or discharge current and f _{z is} the cycle frequency. Since L is a constant for the ballast in question, the power P is proportional to the cycle frequency f _z if i _{o is} constant.

If one specifies a certain time course of the cycle frequency according to claim 1, the power P has the same time course. Since the time course of the power is decisive for the ripple of the input current generated by the ballast, it is possible to influence the ripple in that the cycle frequency has a predetermined course with respect to the phase of the input voltage.

The cycle frequency is significantly higher than the frequency of the input voltage. If the frequency of the input voltage (mains voltage) is 50 Hz, the cycle frequency is preferably of the order of 30 kHz. During a period of the input voltage, numerous pulses (cycles) with a varying cycle duration t _{z are} carried out, the instantaneous cycle frequency being f _z = 1 / t _z . The cycle frequency can be varied over a period of the input voltage so that a curve arises from a fundamental oscillation (with the frequency of the input voltage) and numerous harmonics, the fundamental oscillation components of which correspond to the maximum permissible degree of deformation of the fundamental oscillation.

According to claim 3, the upper limit of the charging current is changed as a function of the phase position of the input voltage, in such a way that the square of the maximum value of the charging current is varied according to a predetermined waveform having the frequency of the input voltage. The duration of the charging and discharging cycles, that is to say the cycle frequency f _z , is constant.

The maximum permissible degree of deformation of the power curve can be determined, for example, according to the VDE regulations, which provide a specific maximum fundamental component for each harmonic order. In this way it is possible to compile a power curve with which the harmonic components of the mains input current do not exceed the permissible limit values. This fulfills the desire to find a compromise between an even distribution of power over time and the maximum permissible harmonic content.

The ballast according to the invention converts the AC voltage coming from the network into an AC supply current for the discharge lamp without being converted into a DC voltage. This supply alternating current is high-frequency (e.g. 30 kHz), the envelope curve corresponds to the frequency of the input voltage (e.g. 50 Hz). The envelope of the AC supply current has a time course that lies between a sine function and a rectangular function.

The ballast according to the invention also offers the possibility of dispensing with the otherwise usual burst pulses in the ignition phase, which are generated separately by the control device in order to ignite the fluorescent lamp. Rather, the ignition can be carried out with the same high-frequency pulses with which the later stationary lamp operation also takes place. A separate ignition phase that precedes the operating phase is therefore not necessary.

Finally, the ballast according to the invention offers the possibility of dispensing with an additional switch connected in parallel with the fluorescent lamp and which would have to be controlled by the control unit. Instead of such a switch, which determines the duration of the preheating phase for the electrodes of the discharge lamp, a resistance element can be used in the ballast according to the invention, for example a PTC resistor, which becomes high-resistance after the preheating phase has ended.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawings.

• Fig. 1 eine schematische Darstellung einer ersten Ausführungsform des Vorschaltgerätes,
• Fig. 2 den zeitlichen Verlauf der aufeinanderfolgenden Zykluszeiten bei der Schaltung nach Fig. 1,
• Fig. 3 eine graphische Darstellung verschiedener Verläufe von Leistung und Eingangsstrom über der Phasenlage der Eingangsspannung bei der Schaltung nach Fig. 1,
• Fig. 4 Zeitdiagramme während der Vorheizphase, der Zündphase und der Betriebsphase,
• Fig. 5 eine zweite Ausführungsform, bei der die Induktivität ein Transformator ist und der Lade- und Entladestromkreis voneinander galvanisch entkoppelt sind,
• Fig. 6 eine dritte Ausführungsform, bei der die Induktivität in eine Parallelinduktivität und eine Serieninduktivität, die magnetisch miteinander gekoppelt sind, unterteilt ist,
• Fig. 7 eine vierte Ausführungsform mit zwei ersten elektronischen Schaltern für jeweils eine Halbwelle der Eingangsspannung,
• Fig. 8 ein Zeitdiagramm der Schaltung nach Fig. 7 und
• Fig. 9 ein Ausführungsbeispiel ähnlich demjenigen von Fig. 8, jedoch mit nur einem einzigen ersten elektronischen Schalter.

Show it:

• 1 is a schematic representation of a first embodiment of the ballast,
• 2 shows the time course of the successive cycle times in the circuit of FIG. 1,
• 3 shows a graphical representation of different curves of power and input current over the phase position of the input voltage in the circuit according to FIG. 1,
• 4 time diagrams during the preheating phase, the ignition phase and the operating phase,
• 5 shows a second embodiment, in which the inductance is a transformer and the charging and discharging circuit are galvanically decoupled from one another,
• 6 shows a third embodiment in which the inductance is divided into a parallel inductance and a series inductance which are magnetically coupled to one another,
• 7 shows a fourth embodiment with two first electronic switches for one half-wave of the input voltage,
• Fig. 8 is a timing diagram of the circuit of Fig. 7 and
• Fig. 9 shows an embodiment similar to that of Fig. 8, but with only a single first electronic switch.

1, the input voltage U _N , which is the line voltage of 50 Hz, is present at the input. This mains voltage is fed to the ballast via a filter and radio interference suppression circuit (not shown). One pole of the input voltage U _N is connected to the one main connection of the bidirectional electronic switch T1, the other main connection of which is connected to the inductance L. The inductance L lies in a transverse branch of the circuit and connects the second main connection of the switch T1 via a measuring resistor R to the other pole of the input voltage U _N.

The series circuit comprising the second bidirectional electronic switch T2 and the discharge lamp EL is parallel to the series circuit comprising the inductance L and the measuring resistor R. The discharge lamp EL has electrodes E1 and E2 at opposite ends. The electrodes E1 and E2 are connected to one another by a third bidirectional electronic switch T3.

The electronic switches T1, T2 and T3 are controlled by the control unit SE, which is a microprocessor. This microprocessor is connected to the poles of the input voltage U _N , so that it receives information about the time course of the input voltage and in particular about the respective phase position of the input voltage. It is also connected to the two ends of the measuring resistor R so that it provides information about the one flowing through the inductance L. Current i receives.

FIG. 2 shows the time course of the current i flowing through the inductance L in the operating phase. The switches T1 and T2 are operated inversely to each other. If the switch T1 is conductive and the switch T2 is blocked, the inductance L charges, which is denoted by 10 in FIG. 2. The current i increases linearly because there is practically no ohmic resistance in the series circuit which contains the electronic switch T1 and the inductance L. The value of the measuring resistor R is very small. When the current i has reached an upper limit value i _o , the charging phase is ended. Reaching the limit value i _o is determined in the control unit SE and the control unit then carries out the discharge phase of the inductance L, in which the switch T1 is blocked and the switch T2 is conductive. In this discharge phase, the inductance L discharges, striving to maintain the current that has flowed in the charging phase. The current now flows from the inductance L via the measuring resistor R, the discharge lamp EL and the conductive switch T2 back to the inductance L. Since the current flows through a consumer, namely the discharge lamp EL, it decreases in the manner of an e-function. The discharge phase 11 is ended when the cycle time t _{z has reached} a predetermined value, the size of which will be explained. After the first cycle time t _z 1 has elapsed, the next cycle time t _z 2 follows.

In the first cycle time t _z 1, the charging time in which the inductance L is charged is t _a 1 and the discharging time in which the inductance L is discharged via the discharge lamp EL is t _e 1. In the subsequent second cycle time t _z 2 is the charging time t _a 2 and the final charging time t _e 2. The duration of the charging times t _a 1 and t _a 2 depends on the current level of the input voltage U _N , that is, inter alia, also on the phase position of the input voltage. If the input voltage is large, the charging time, namely the time until the current i reaches the limit value i _o , is small, and if the input voltage is small, the charging time is large. In any case, the charging phase continues until the current has reached the limit value i _o , so that, regardless of the instantaneous level of the input voltage, the energy transferred into the inductance L is always the same. This energy is discharged in the subsequent discharge phase 11.

The charge / discharge cycles take place in the cycle time t _z . The cycle frequency f _z is 1 / t _z .

The power P transferred to the lamp is

Since the limit value i _{o is} constant and since the inductance L is also constant for a specific ballast, the lamp power P is proportional to the cycle frequency f _z .

In Fig. 3, the current i and the power P are plotted against the phase angle of the input voltage. Curve 20 shows the time profile of the power P in the case of an ohmic consumer. The power curve 20 corresponds to the square of the sine of the input voltage. With such a curve, no harmonics are generated. However, this course of the power curve 20 has the disadvantage that the power fluctuates greatly over time. It is desirable to operate the fluorescent lamp with a constant power over time.

The power curve 21 indicates lamp operation with constant power. If this power curve were to be implemented, in which the power only drops briefly in the range of 0 ° or 180 ° or 360 ° or 0 and otherwise has a constant value, large harmonic components would arise, as a result of which disturbances would be coupled into the supply network.

A compromise between the power curves 20 and 21 is the power curve 22, which can be implemented with the circuit according to FIG. 1. In this power curve 22, the mains input current contains harmonic components, the maximum value permitted according to DIN VDE 0838 being taken for each harmonic order. This standard provides the following maximum values for the fundamental oscillation components of the harmonics of the mains input current:

When utilizing these maximum permissible distortions, the power curve 22 results with the time profile shown. The integral (ie the area) of this power curve 22 is equal to that of the power curve 20 and also to that of the power curve 21. It can be seen that the essentially trapezoidal power curve 22 represents a compromise between the power curves 20 and 21 because it is flatter and wider than the power curve 20, but on the other hand contains fewer harmonic components than the power curve 21.

3 also shows the current curve 23, that is to say the time profile of the current i at the input of the ballast when using the power curve 22.

In the present exemplary embodiment, the power curve 22 has been determined on the basis of the maximum permissible input current harmonics shown in the table, that is to say to a kind of Fourier synthesis.

After the selection of the suitable power curve, the cycle frequency f _z is selected so that its time course corresponds to the time course of the power curve 22. Such a time profile of the cycle frequency f _z over the phase of the input voltage is stored in a memory of the control unit SE, for example in the form of a table. If a specific phase position of the input voltage is determined, the associated value of the cycle frequency is read from the memory. The duration of the respective cycle time t _z results from the value of the cycle frequency. After the cycle time t _z has elapsed, the respective cycle is ended and the next cycle begins with the associated new cycle duration. The control unit SE contains in a table the different cycle frequencies or the cycle times (the reciprocal values of the cycle frequencies) as a function of the phase angles of the input voltage.

The previous description was limited to the operating phase. In the present exemplary embodiment, the discharge lamp EL is a fluorescent lamp with heatable electrodes E1 and E2. With such a fluorescent lamp, preheating must be carried out before ignition. FIG. 4 shows a) the course of the current i in the preheating phase. In the preheating phase, the limit value i _o , at which the charging phase of the inductance is ended, is reduced to the value i _ov , which is lower than i _o . The representations b) and c) show the switching states of the electronic switches T1 and T2, which are controlled by the control unit SE as a function of the preheating limit value i _ov being reached. The frequency of the preheating periods is also varied in accordance with the power curve 22 in FIG. 3 depending on the phase position of the input voltage. Representation d) in Fig. 4 shows the switching state of the third electronic switch T3, which is conductive in the preheating phase. After the preheating phase, which has a predetermined duration, the ignition phase is carried out. The electronic switch T3 is permanently blocked in the ignition phase and the subsequent operating phase. In the ignition phase, the normal limit value i _{o of} the charging current is effective, which is also effective in the operating phase. The cycle duration t _z varies in accordance with the power curve 22 in FIG. 3. In the ignition phase, while the switch T1 is blocked and the switch T2 is conductive, voltage pulses U _{L appear} in the discharge lamp EL, which are shown in the representation e) of FIG. 4 are. These voltage pulses, the average frequency of which is approximately 30 kHz, cause the discharge lamp to ignite. After ignition, the operating phase arises, in which switches T1 and T2 are controlled in the same way as in the ignition phase. The lamp voltage U _L has the course shown in illustration e) of FIG. 4 in the operating phase. The time scale in illustration e) is larger in the operating phase than in the preheating phase and the ignition phase. The lamp voltage U _L varies with the 50 Hz frequency of the input voltage and is composed of numerous high-frequency pulses with an average pulse frequency of 30 kHz.

In the embodiment of FIG. 1, an actuator SG is connected to the control unit SE, with which the dimming operation of the discharge lamp EL can be controlled. During dimming operation, the limit value i _{o of} the current is reduced, so that the power supplied to the lamp is reduced.

The embodiment of FIG. 5 differs from that of FIG. 1 in that the inductance L consists of a transformer, the primary winding L1 is connected in series with the first electronic switch T1 and the measuring resistor R to the input voltage U _N and the secondary winding L2 forms a closed circuit with the switch T2 and the discharge lamp EL. The two circuits are thus galvanically decoupled from one another by the transformer. Furthermore, the third switch T3 of FIG. 1 is replaced by a resistance element R _v , which becomes high-resistance after the preheating phase has ended. This resistance element R _v is, for example, a PTC resistor which has a low resistance in the cold state during the preheating phase and which then becomes high resistance. In this way, an electronic switch that would have to be controlled by the control unit is unnecessary. The resistance element R _v can also be used in the circuit according to FIG. 1 in order to replace the third switch T3 there. The two switches T1 and T2 are controlled in FIG. 5 in the same way as was explained with reference to FIG. 1.

In the embodiment of FIG. 6, the inductor L consists of two partial inductors L3 and L4. The parallel inductor L3 is arranged in the same way as the inductor L in FIG. 1 and the series inductor L4 is arranged between the first electronic switch T1 and the second electronic switch T2. Both partial inductors L3 and L4 are magnetically coupled to one another by a common core 25. In the discharge phase, voltages U ₁ and U ₂ arise at the partial inductors L3 and L4, which add up to give the ignition voltage for the lamp. On the other hand, the voltage occurring at switch T1 only has the maximum value of U _N + U ₁ . The switch T1 therefore does not need to have such a high dielectric strength as in the exemplary embodiment in FIG. 1.

In the embodiment of FIG. 7, two first electronic switches T11 and T12 are provided, each of which is connected in series with a rectifier D1 or D2, the rectifiers D1 and D2 being polarized in opposite directions to one another. The two series connections from the switch T11 and the rectifier D1 on the one hand and the switch T12 and the rectifier D2 on the other hand form a parallel connection. The connection of the switch T11 to the rectifier D1 is connected via an inductor L5 and a measuring resistor R to the second pole of the input voltage U _N. The connection point between the switch T12 and the rectifier D2 is connected to the measuring resistor R via an inductor L6. In this embodiment, switch T11 controls the positive half-wave and switch T12 controls the negative half-wave of the input voltage, which is shown in FIG. 8. The switches T11 and T12 are controlled by the control unit SE in the same manner as was explained with reference to FIG. 1, with the difference that the switch T11 is blocked during the negative half-wave of the input voltage and the switch T12 is blocked during the positive half-wave .

In the embodiment of FIG. 9, there is only a single first switch T1, which is connected in series with a rectifier circuit D1, D2, D3, D4. The inductor L5 is connected to the cathodes of the rectifiers D1 and D3 and the inductor L6 is connected to the anodes of the rectifiers D2 and D4. In the positive half-wave, the charging current flows through the switch T1 and the rectifier D3 to the inductance L5 and the discharge current flows from the inductance L5 through the lamp EL and the rectifier D1. In the negative half-wave of the input voltage, the charging current flows through the switch T1, the rectifier D4 and the inductance L6 and the discharge current flows through the lamp EL and the diode D2. In this case there is only one electronic switch to be controlled.

Claims (13)

1. AC ballast for electric discharge lamps, with an inductor (L), a first electronic switch (T1) for charging the inductor by connecting the inductor (L) to the input voltage (U _N ), one to the first electronic switch (T1 ) inversely operated second electronic switch (T2) or rectifier (D1, D2) for discharging the inductor (L) via the discharge lamp (EL), and a control unit (SE) which controls the charging and discharging cycle of the inductor (L),
characterized,
that the control unit (SE) specifies the cycle frequency (f _z ) of the charge and discharge cycles as a function of the phase of the input voltage (U _N ) and ends the respective cycle after the phase-dependent cycle duration (t _z ). 2. AC ballast according to claim 1, characterized in that the control unit (SE) receives a signal corresponding to the charging current of the inductor (L) and ends the charging of the inductor (L) and begins to discharge when the charging current (i) one predetermined upper Limit (i _o ) reached. 3. AC ballast for electric discharge lamps, with an inductor (L), a first electronic switch (T1) for charging the inductor by connecting the inductor (L) to the input voltage (U _N ), one to the first electronic switch (T1 ) inversely operated second electronic switch (T2) or rectifier (D1, D2) for discharging the inductor (L) via the discharge lamp (EL), and a control unit (SE) which controls the charging and discharging cycle of the inductor (L),
characterized,
that the control unit (SE) receives a signal corresponding to the charging current of the inductor (L) and stops charging the inductor (L) and starts discharging when the charging current (i) reaches a predetermined upper limit value (i _o ), and that Control unit specifies the upper limit value (i _o ) as a function of the phase of the input voltage (U _N ) and ends the respective cycle after a constant cycle time (t _z ). 4. AC ballast according to one of claims 1 to 3, characterized in that the cycle frequency (f _z ), or the upper limit value (i _o ) of the charging current (i) over a phase angle range of the input voltage of 180 substantially trapezoidal course Has. 5. AC ballast according to one of claims 1 to 4, characterized in that the cycle frequency (f _z ), or the upper limit (i _o ) of the charging current (i) over a phase angle range of the input voltage (U _N ) of 180 corresponds to a curve consisting of a fundamental and numerous even and odd harmonics, the fundamental components of which correspond to the maximum permissible degree of deformation of the fundamental. 6. AC ballast according to one of claims 2 to 5, characterized in that the upper limit value (i _o ) of the charging current (i) is variable for dimming. 7. AC ballast according to one of claims 1 to 4, characterized in that in an ignition phase, the first electronic switch (T1) is controlled in the same way as in the operating phase, the cycle frequency (f _z ) on the lamp ( EL) generated pulses are used as ignition pulses. 8. AC ballast according to one of claims 2 to 7, characterized in that in a preheating phase with interconnected lamp electrodes (E1, E2) the upper limit value (i _o ) of the charging current is reduced compared to the operating phase. 9. AC ballast according to one of claims 1 to 8, characterized in that the inductor (L) consists of a transformer which galvanically separates the circuit containing the first electronic switch (T1) from the circuit containing the lamp (EL). 10. AC ballast according to one of claims 1 to 8, characterized in that the inductor (L) from a parallel to the lamp (EL) parallel inductor (L3) and from between the first electronic switch (T1) and the lamp (EL ) switched series inductance (L4), both of which have a common core (25). 11. AC ballast according to one of claims 1 to 10, characterized in that two first electronic switches (T11, T12) are provided which are connected in series with rectifiers (D1, D2) of different polarity, and that each first electronic switch (T11, T12) together with its rectifier (D1, D2) is connected to its own inductor (L5, L6). 12. AC ballast according to one of claims 1 to 10, characterized in that a single first electronic switch (T1) is provided, which is connected in series with two rectifier branches of rectifiers connected in opposite poles in series (D3, D1; D4, D2) and that each rectifier branch is connected to its own inductance (L5, L6). 13. AC ballast according to one of claims 1 to 12, characterized in that the lamp (EL), a resistance element (R _v ) is connected in parallel, which after completion of the pre heating phase becomes high-resistance due to heating.

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