NL9000680A - HIGH-FREQUENT BALLAST. - Google Patents

Description

NEDAP N.V.

Groenlo.

High frequency ballast

The invention relates to a high-frequency ballast for medium and high-pressure metal vapor gas discharge lamps. High-frequency ballasts can be very compact and are easily adjustable.

High-frequency ballasts for the said type of lamps are known, inter alia, from publications DE 3445817, DE 3623306, US 4170747 and EUR 0240049.

The ballasts described in publication DE 3445817 and EUR 0240049 contain an adjustable DC-DC converter, followed by an inverter.

This solution avoids acoustic resonances in the gas discharge arc, but the ballast becomes relatively expensive and bulky, because two series-connected power converters must be used.

Although the ballasts described in publication DE 3623306 and US 4170747 consist of a single bridge converter, separate control circuits are used to drive one side of the bridge at a high frequency and the other side at a low frequency. This entails additional costs compared to a bridge converter in which one drive frequency is used. Furthermore, a separate circuit is used in all said publications to ignite the lamps, which makes the circuit relatively expensive and bulky.

It is known, inter alia from publication DE 3511661, to ignite gas discharge lamps by using the oscillation of the voltage in a series resonant circuit. However, the known circuits cannot simply be used for large powers.

None of the above-mentioned ballasts have taken special measures to stabilize lamp power, light intensity or lamp temperature, as is desirable in many industrial applications of medium and high pressure medium discharge gas discharge lamps of the order of 500 W to 3000 W.

Furthermore, none of the aforementioned publications pay attention to the problem of the power factor. This problem arises when the input DC voltage of the bridge converter is obtained by rectifying and smoothing a mains AC voltage.

This results in higher harmonic currents in the supply network, which can lead to additional losses in the AC voltage distribution network, and which can trigger fuses earlier. This can also cause deformation of the voltage form in the supplying AC voltage network.

Circuits are known to deal with this problem. However, use is made of an additional inductance and an additional semiconductor switching element with associated control circuits, which leads to additional costs and makes the device more extensive.

Such a method is described, for example, in article "Simplified Control Algorithm for Active Power Factor Correction" by Neil J. Barabas, Proceedings of the tenth international PCI-85 Conference, pp. 1-9. A simpler method for power factor correction, in particular also for high-frequency ballasts, is further described in "A power Factor Corrected, MOSFET Multiple Output,

Flyback Switching supply "by J.J. Spangler, Proceedings of the tenth international PCI-85 Conference, pp. 19-32.

However, this method also requires additional power components and therefore entails additional costs.

From NL-A-8600812 and DE-A-3505182 it is known to keep and control the lamp power of a high-pressure gas discharge lamp. However, a low lamp frequency is used for this.

It is generally known to use so-called full bridge circuits for converting a DC voltage into a high-frequency AC voltage. Control transformers are often used for controlling the switching elements of the bridge circuit, if necessary followed by a buffer circuit in order to be able to drive the control connection of the switching element with low resistance.

However, these buffer circuits then require supply voltage, for which separate windings from a separate transformer are used, which entails quite a lot of extra costs and extra space.

The object of the invention is to provide an improved high-frequency ballast, which operates reliably, is compact and can be built relatively inexpensively. More particularly, the invention contemplates a high-frequency ballast comprising a single bridge converter, the frequency of which can be varied over a wide range in order to control the lamp power. Another object of the invention is a high-frequency ballast which can ignite the lamps without special additional starting circuits, and which prevents acoustic resonances.

Another object is to enable simple adjustment and stabilization of the lamp power or the light intensity. Yet another object is to allow controlled forced cooling of the lamp, which allows to design still more compact lamps with a high power density, while at the same time maintaining an optimum lamp temperature over a wide power range.

Yet another object is to provide such a control circuit for the bridge converter power switching elements that no separate power transformer is required for the buffer stages connected behind the control transformer. Such a control circuit therefore entails little additional costs and can be well miniaturized.

Yet another object is to obtain a good power factor in the order of 0.8 to 0.9 without using additional components when supplying power from an alternating voltage network.

The following measures can contribute to achieving the above goals.

In the first place, use can be made of a bridge converter consisting of power MOSFETS or other power switching elements in which the lamp is connected in series on the output side of the bridge circuit with one or more current-limiting self-inductors, while one or more capacitors are arranged parallel to the lamp. Prior to ignition of the lamp, the drive frequency of the bridge above the resonant frequency of the series circuit, formed by said current-limiting self-inductors and capacitors, is selected and this frequency is then gradually lowered, until at a drive frequency that is close to the resonant frequency , a strong winding of the voltage across the lamp occurs, which can ignite it. In normal operation, frequency modulation of the output voltage can be applied, so that the acoustic vibration circuits in the lamp cannot be strongly impacted.

In particular, the frequency modulation can be synchronized with the double mains frequency. A high frequency gives a low power to the lamp due to an increase in the impedance of the current-limiting coils. By now choosing the frequency modulation so that the most discourse is delivered to the lamps around the positive and negative peaks of the mains voltage, the smoothing capacitor which follows the mains rectifier can be kept small. This then essentially only serves to supply a residual DC voltage around the zero crossings of the mains voltage in order to keep the lamps in a conducting state via the bridge converter, but with a low instantaneous power. The said frequency modulation can be provided by a microprocessor included in the control of the inverter and therefore does not cost any additional components.

Furthermore, the operating condition of the lamps is checked by measuring the lamp voltage, input current and input voltage of the converter and, if necessary, the light intensity of the lamps. Data stored in the microprocessor program then allows the drive frequency for the bridge converter and fan speed to be set so that the lamps provide the desired power or the desired light intensity and also maintain the desired lamp temperature.

Furthermore, the buffer circuit for the bridge converter power switching elements, which buffer circuit permits a wide frequency range for driving the bridge converter, is fed by a capacitor, which is between the buffer circuit of the power switching element which follows the output AC voltage and one of the input supply poles or the associated buffer circuit is connected.

According to the invention, a ballast for gas discharge lamps is particularly suitable for medium and high pressure gas discharge lamps, comprising a bridge converter fed by a DC voltage source via two input DC voltage terminals with at least two power switching elements, each of which is provided with a control electrode. a series circuit consisting of at least one self-inductance and at least one capacitor connected in parallel with at least one gas discharge lamp is connected on the output side of the bridge converter, and a buffer circuit is connected to each control electrode, which is connected in operation via a coupling element by a a control unit-controlled control circuit is provided with control signals, characterized in that for supplying supply voltage to at least those buffer circuits which are connected to the control electrode of a power switching element for which it applies that the The current flow of the field element is substantially in phase with the output AC voltage of the bridge converter, each time a capacitor is provided, one terminal of which is connected to the anode of a first diode, the cathode of which is connected to the positive supply terminal of the relevant buffer circuit and to the cathode of a second diode, the anode of which is connected to the negative power supply terminal of the respective buffer circuit, the second terminal of the capacitor being connected to a point having approximately the potential of one of the input DC voltage terminals.

In the following, the invention will be further elucidated with reference to the accompanying drawing of some exemplary embodiments.

Fig. 1 shows a block diagram of a possible embodiment of a ballast according to the invention; Fig. 2 and Fig. 3 schematically show alternative embodiments of a ballast according to the invention; Fig. 4 shows an example of a supply circuit for buffer stages that can be used in a ballast according to the invention; Fig. 5 schematically shows an exemplary embodiment of a buffer stage; Fig. 6 illustrates in a block diagram a possible way of controlling and controlling a ballast according to the invention; Fig. 7 shows some important waveforms of currents and voltages in a ballast according to the invention; and FIG. 8 shows some important parameters as a function of time, during the ignition and heating phase of the lamp.

In the circuit according to Fig. 1, an input DC voltage IK, which prevails between input terminals 1 and 2, and e.g.

can be obtained by rectifying 220 V AC voltage in an input stage 70 with a rectifier 7 and smoothing it with a capacitor 72, converted into a high-frequency block voltage. Controllable switching elements 2 and 5, respectively. 3 and 4, which form a bridge converter (bridge inverter), are alternately brought into the conductive state. The obtained block voltage is applied to a series circuit which in this example is formed by inductors 6 and 7 and capacitor 8. The capacitor 8 is connected in parallel with the lamp 9.

Before igniting the lamp, the drive frequency of the bridge inverter is above the resonant frequency of the series circuit formed by inductors 6 and 7, and capacitor 8. The series circuit is now inductive and the waveforms of voltage (at point A) and the currents through the switching elements are as shown in FIG.

7. Each switching element is provided with a freewheeling diode 12,13,14,15. A negative value of the current i ^ / ii ^, i ^ resp. It is confirmed that the freewheeling diodes 12, 13, 14 or 15 are conductive.

In high-frequency operation of the bridge, it is important that the diodes first conduct, and then the controllable power switching elements, because the diodes can then recover freely and are not brutally resected by the opposite switching element, which leads to very large peak currents and even failure of the inverter may result. The drive frequency is then gradually lowered and as it approaches the resonant frequency of the above series circuit, the voltage across capacitor 8, and thus across lamp 9, becomes very high, causing the lamp to ignite. The lamp now loads the circuit and dampens it overcritically, so that elements 6,7 and 8 now form a low-pass filter. The drive frequency can now be reduced even further, while the load on the bridge remains inductive. The resistance of the ignited medium or high pressure gas discharge lamp 9 is initially very low. Therefore, a relatively high frequency of the voltage across circuit 6, 7, 8, 9 is still required, in order to keep the impedance of inductance 6 and 7 so high that the current at one for the switching elements 2, 3, 4, 5 and lamp 9 an acceptable value remains limited. As the lamp heats up, the resistance of the lamp increases, so that the driving frequency can be lowered even if the lamp current remains the same.

The final operating state of the lamp is determined by the control unit shown in Fig. 6, which will be described later. The control unit determines which control frequency should be selected on the basis of lamp voltage, input current of the bridge, supply voltage of the bridge, and external information, e.g. desired lamp power, or difference between desired and actual light intensity. A lower frequency hereby leads to a lower impedance of the self-inductors 6 and 7 and therefore to higher currents with the same lamp resistance. The peak value of lamp voltage is always smaller than the value of the supply voltage that is applied to terminals 1 and 2.

Fig. 2 shows an alternative embodiment. In this embodiment, additional capacitors 16,17,18 and 19 are connected between the terminals of the lamp and one of the supply lines. The capacitors 16,18 are connected as capacitive voltage divider with resistors 20 and 21 connected in parallel, which is frequency independent. , and which reduces the peak voltage as it may arise on one side of the lamp, e.g.

2000 V to 10 V, for example. Capacitors 17.19 and resistors 22.23 are similarly connected. A peak voltage detector IQ is connected to the node between capacitors 16 and 18, to said supply line, and to the node between capacitors 17 and 19.

The peak voltage detector can give a control signal 24 to the control unit when a certain value of the lamp voltage is exceeded, after which the control of the bridge is blocked and all power switching elements are blocked. The voltage threshold at which the driving is stopped is lower than the value that can occur at resonance of the series circuit, which is usually determined by nuclear saturation phenomena of self-inductances 6 and 7. The threshold value is chosen such that on the one hand the currents through the switching elements are at acceptable values remain limited, while on the other hand the lamp can still ignite reliably.

By proceeding in the above manner, damage to the ballast if the lamp is not connected or is still very hot and then cannot ignite due to the high gas pressure.

Yet another embodiment is shown in Figure 3. Here, compared to an example of Fig. 2, only half a bridge comprising the switching elements 2 and 3, and the freewheeling diodes 12 and 13 has been used. An additional coupling capacitor 11 is also provided to block a DC voltage component in the drive voltage.

In this case, the lamp voltage has half the value of that in Figs. 1 and 2 in proportion to the source voltage across terminals 1 and 2, for otherwise equal operating conditions.

Fig. 4 shows how the buffer circuits 39 and 40 via which the power switching elements 2,3 are controlled can be supplied. A capacitor 30 is connected between the nodes of diode pair 28, 29 on the one hand and diode pair 31, 32 on the other. Before the inverter starts to operate, the buffer circuits 39, 40 are supplied with voltage via resistors 35 and 36. The supply voltage is stabilized by zener diodes 34 and 38 connected in parallel with the buffer circuits and decoupled for high frequencies by means of capacitors 33 and 37. It is important that the buffer circuits 39 and 40 in the switched-off state have a low current consumption in order to avoid unnecessary energy dissipation in the resistors 35 and 36. As soon as the converter is put into operation and control signals 01 and 02 activate a control circuit 25, the buffer circuits are driven via torque transformers 26 and 27 energized by the control circuit 25. At point A the in .fig. 7 block voltage UA shown. This voltage causes alternating charging and discharging of capacitor 30, alternating diodes 28 and 32, respectively. diodes 29 and 31 conduct.

The capacitance value of capacitor 30 is chosen to be amply sufficient to accommodate the charge required for the gate-source and gate-drain capacities of the power switching elements, e.g., as MOSFETs, IGBTs, or as Darlington combination of MOSFET and BJT , to be able to deliver. Capacitor 30 also serves as a dV / dt limiter, and reduces the switching losses in the power switching elements. In principle, the buffer circuit 40 could be supplied from the same supply voltage as, for example, the control circuit 25, and one connection of capacitor 30 could be connected directly to the negative supply line instead of to the junction of diode 31 and 32. However, as a result of the very fast switching (about 30 nsec), large voltage peaks occur, so that it is preferable to isolate the power supply for buffer circuit 40, and directly at the switching element 3, respectively. 13 and disconnect it by capacitor 37.

An exemplary embodiment of the buffer circuit 39 is shown in Fig. 5. The same circuit can be used for the other buffer circuits. The output voltage of transformer 26 is dipped through a network consisting of diodes 42,43 and zener diodes 41,44 and then has the form shown in FIG. 7 at U26, respectively. U27 for buffer circuit 40. Switch element 2 is turned on by a positive voltage from transformer 26, whereby transistor 47 is turned on. The emitter of the transistor is connected to the control electrode of the switching element and to the main circuit of a P-channel field effect transistor 51.

It is important here that the control voltage on the base of transistor 47 is lower than the voltage on the collector of this transistor, so that it does not saturate and can be dissipated very quickly. To switch off the switching element, the output voltage of transformer 26 becomes negative and P-channel FET 51 is turned on. A diode 46 connected between the secondary winding of the transformer and the base of the transistor now blocks and the action of a diode 48 connected between the base and the emitter and a resistor 52 bridging the diode 46 causes a negative bias of about 0.7 V at the base-emitter junction of transistor 47 is presented.

As can be seen from Fig. 7, switching on of the switching element takes place in normal operation when the parallel-connected freewheeling diode is already conducting. As a result, the buffer circuit does not have to load the Miller-drain-gate capacity and a relatively high-resistance value can be selected for the resistor 49 connected to the control electrode. This choice then prevents an excessive peak current from flowing when the switching element is switched on for the first time as a result of the discharge of capacitor 30. However, the Miller drain-source capacity is effective when the switching element is switched off. In order to minimize the switching losses at high operating frequencies, a low switching resistance is required, which in this case is formed by the channel resistance of MOSFET transistor 51.

A sizing example follows below.

MOSFET switches 2,3,4,5: 500 V types with an on-resistance of 0.3 ohm and egg drain current of 13 A continuous and 50 A peak.

Supply voltage: 250-380 VDC Resistance value of resistor 49: 47 Ohm.

Channel resistance of FET 51: 10 Ohm.

Output voltage of transformer 26: + 12V and -12V,

Supply voltage of buffer circuit 39 and 40: 15V.

Dead time td: 500 nsec.

Lamp power: 1200 W, Fire voltage 150 V effective.

A possible embodiment of the control device of the ballast is shown in Fig. 6.

A control unit 54, which can be equipped with a microcomputer, receives information from the outside world 56 via an optical coupling member 55. This information can be: lamp on / off, desired lamp power, desired light intensity, measured light intensity. Furthermore, this control unit measures the following internal variables: the input voltage U. between terminals 1 and 2; the average input current of the bridge inverter by measuring the voltage drop across a resistor 63 with terminals 61 and 62 if necessary via a low-pass filter (not shown); and the lamp voltage measured by circuit 10 and transmitted via signal 64. Furthermore, the microcomputer, which can be provided with its own battery supply if necessary, can retain data from the past. These data may include the burning time, the time of extinction, etc.

Furthermore, depending on signal 64, the control unit 54 can control a fan 66 for lamp cooling via a control circuit 65. The main output quantity of the control unit consists of a signal 58 for a voltage-controlled oscillator 57, which in turn generates the input signals 01 and 02 (59 and 60) for the control circuit 25. The control unit ensures that prior to lamp ignition, the driving frequency is above the series resonant frequency of the aforementioned series circuit, and that this frequency is gradually decreased until the lamps ignite, or the or the maximum peak voltage value for the series circuit, measured by circuit 10 is exceeded. Furthermore, the control unit then determines the desired output frequency on the basis of the measured lamp voltage, the input current of the inverter and the input voltage of the inverter. This will generally be chosen as low as possible in order to bring the lamp to temperature as quickly as possible, without thereby exceeding the aforementioned operating parameters.

If forced cooling is used, the control unit will keep the lamp voltage constant from the moment the lamp voltage has reached the value corresponding to the measured power corresponding to the optimum working temperature of the lamp by controlling the fan appropriately. Should this not happen, the gas pressure in the lamp would increase further due to an increase in temperature, which would result in a higher lamp voltage.

The lamp voltages and associated temperatures are previously measured on one or a few copies of the lamps to be fed, after which this data is processed in the control program of the microcomputer. The control signal for the voltage controlled oscillator can be amplitude modulated with a frequency of, for example, about 100 Hz to about 1 kHz. The control signal for the bridge inverter is hereby modulated in frequency, so that acoustic resonances in the lamps can be prevented. It is possible to modulate the duty cycle of the controller or both the frequency and duty cycle with comparable frequencies. In that case, a low-frequency "off-set" current will also pass through the lamp, which may possibly prevent resonances in the gas.

In particular, the frequency modulation can be synchronous with the double mains frequency, in which case a large power is supplied to the lamps around the negative and positive peaks of the mains voltage by selecting a low driving frequency, and a small power around the zero crossings. The smoothing capacitor connected to the mains rectifier then need only have a small value. The reference signal required to synchronize this frequency modulation may be the DC ripple of the smoothed mains voltage, which is also the supply voltage for the bridge. In this way a power factor of 0.8 I 0.9 can be realized.

A typical temporal progression of the parameters which occur during ignition, starting, burning and cooling of the lamps is shown in Fig. 8. The progression of the peak value of the lamp voltage ü-lamp, the peak value of the lamp current i-lamp, the drive frequency of the inverter "freq.", the air displacement of the fan "a (ir) f (low)", whereby very low speeds can be achieved by on / off control of the fan, and the light intensity, weighted according to a certain spectral distribution, "l (ight) o (utput)".

The time scale in interval A is 10 msec / division, in interval B 1 min / division. At time tO the inverter is started up under the influence of an external control command "on". The desired light intensity has level 1. The inverter is started at the highest frequency, after which the frequency is continuously decreased. At time t1 the frequency is close to the resonant frequency frÊS and the voltage across the series resonant circuit is so high that the lamp ignites. The frequency reduction continues until the maximum permissible value of the lamp peak strip is reached at time t2.

This value is not measured directly, but determined on the basis of the input power of the converter (product input current and input voltage) and the lamp voltage. Then the lamp heats up, causing the lamp voltage to increase. The i-lamp value is now kept approximately constant, based on the indirect measurement described above.

The basis for the calculation can be that the lamp power P-lamp is given by: P-lamp = Cl. i-lamp.ü-lamp

Cl is a constant which depends on the waveform of lamp voltage and lamp current. It is also assumed that the lamp can be regarded as a purely resistive, linear element. If, during the warm-up, the waveform remains constant, as does the efficiency η of the inverter, and it is further assumed that the inverter is supplied with a pure DC voltage Ui and that it draws an average current Ii from this source, then: P-lamp = η. Pi or

Cl i lamp ü lamp = η.ϋ-i.I-i or

With known η and Cl, therefore, i-lamp can indeed be calculated by the processor present in the control unit from U-i, I-i and ü-lamp which are measured.

When the previously described frequency modulation is applied synchronously with the double mains frequency, the above-mentioned relationships remain valid for the time being, although the parameters themselves vary periodically within half a mains period. The microprocessor can now control the frequency, by means of the calculated instantaneous powers, in such a way that the average lamp power is kept constant, that the maximum permissible currents through the switching elements are not exceeded and that the largest input current is around the maximum and minimum of the mains voltage. . The microprocessor program may therefore contain a number of pre-programmed modulation curves or an algorithm to generate these modulation curves, in particular the ripple of the DC voltage U-i can be used as a reference signal for the frequency modulation, so that no additional components are required. At time t3 the desired value of the light intensity has been reached. However, the lamp temperature is not yet optimal. The optimum working temperature of the lamp is reached at time t4.

The lamp voltage now increases a little further and the lamp current drops slightly, using the lamp current / lamp voltage ratio as the control variable for the air displacement of the fan. A thermally stable situation has arisen at time t5. At time t6, a new desired light intensity 2 is prescribed by an external signal. This is achieved almost immediately by lamp voltage and lamp current by means of adjust frequency control.

The lamps are already at the right temperature. A slight change in the lamp voltage / lamp current ratio again serves as a control variable for the air displacement of the fan, so that a new thermal equilibrium is finally reached at time t7.

As mentioned earlier, an optimal control program must be compiled on the basis of temperature measurements on lamps. These temperature measurements take place once in advance. If desired, correction can also be made when the lamps age. Also, a certain lamp power / light output ratio can be used to indicate that the lamps have reached the end of their life and need to be replaced. This can then be made known to the outside world 56 by a signal via coupling member 55.

Claims (23)

A ballast for gas discharge lamps, particularly suitable for medium and high pressure gas discharge lamps, comprising a bridge converter fed by a DC voltage source via two input DC voltage terminals with at least two power switching elements, each comprising a control electrode, the output side of the bridge converter being one of at least one self-inductance and at least one capacitor series circuit connected in parallel with at least one gas discharge lamp is connected, and a buffer circuit is connected to each control electrode, which in operation is controlled via a coupling element by a control circuit of control signals controlled by a control unit is provided, characterized in that for supplying supply voltage to at least those buffer circuits (39), which are connected to the control electrode of a power switching element (2; 4), for which the current flowing through the power switching element The current is substantially in phase with the output AC voltage of the bridge converter, each time a capacitor (30) is provided, the one terminal of which is connected to the anode of a first diode (28), the cathode of which is connected to the positive supply terminal of the relevant buffer circuit is connected and to the cathode of a second diode (29), the anode of which is connected to the negative power supply terminal of the respective buffer circuit (39), the second terminal of the capacitor (30) being connected to a point approximately potential of one of the DC input terminals (1,2). Ballast according to claim 1, characterized in that the second terminal of the capacitor (30) is connected to the anode of a third diode (31), the cathode of which is connected to the positive power supply terminal of a second buffer circuit ( 40), which is connected to the control electrode of a power switching element (3? 5), for which it holds that the current flowing through the power switching element is in antiphase with the output voltage of the bridge converter, the second terminal of the capacitor ( 30) is also connected to the cathode of a fourth diode (32), the anode of which is connected to the negative power terminal of the second buffer circuit and to one of the DC input terminals. Ballast according to claim 1 or 2, characterized in that the control unit is arranged to control the bridge circuit prior to igniting the gas discharge lamp such that the frequency of the output voltage exceeds the resonant frequency of the series circuit (6.7, 8), and then gradually lower the frequency. Ballast according to any one of the preceding claims, characterized by a lamp voltage detector (10) which is connected to the lamp (9) to be supplied and which provides a control signal (24) as soon as the voltage prevailing over the lamp (9) exceeds a predetermined threshold value , which signal results in blocking of the power switching elements (2 to 5). Ballast according to claim 4, characterized in that the control signal (24) provided by the lamp voltage detector (10) is supplied to the control unit (54). Ballast according to claim 4 or 5, characterized in that the lamp voltage detector (10) is connected to the terminals of the gas discharge lamp (9) to be supplied via capacitive voltage dividers (16, 18; 17, 19). Ballast according to any one of the preceding claims, characterized in that each buffer circuit comprises a transistor (47), the collector of which is connected to the positive power supply terminal, the emitter of which, via a relatively high-resistance resistor (49), to the control electrode of a power switching element ( 2; 3; 4; 5) is connected, while 1 the emitter further via a P-channel field effect; nsistor (51) is connected to the negative power supply terminal. Ballast according to any one of the preceding claims, characterized in that the control unit (54) is connected to an optical coupling member (55), which can receive and receive external information about the current burning state of the lamp and the desired burning state of the lamp. pass it on to the control unit. Ballast according to claim 8, characterized in that the control unit (54) is adapted to measure the input voltage (LL), the average input current of the bridge converter. Ballast according to claim 8 or 9, characterized in that the control unit (54) is provided with a memory in which data about the life cycle of a lamp are stored. Ballast according to claims 4 and 9, characterized in that the control unit is adapted to determine the optimum frequency of the output voltage based on the lamp voltage measured by the lamp voltage detector (10), the input current and the input voltage (ÏL ·) of the inverter. (U ^). Ballast according to claim 11, characterized in that the control unit is arranged to calculate the lamp power (P lamp) on the basis of the measured lamp voltage, the input current and the output voltage of the inverter, and then, if after ignition of the the lamp has obtained the lamp voltage the value corresponding to the power corresponding to the optimum working temperature, to keep the lamp voltage constant by controlling a fan which can cool the lamp. Ballast according to claim 12, characterized in that the fan is controlled in such a way that the lamp voltage / lamp current ratio remains substantially constant. Ballast according to claim 11, characterized in that, during the heating of the lamps, the control unit of the inverter controls the drive frequency of the power switching elements on the basis of the measured value of the input DC voltage, input DC current and output lamp voltage, so that the maximum permissible lamp current is not exceeded. Ballast according to claim 12, characterized in that the frequency of the bridge inverter is regulated such that the lamp current has a predetermined nominal value during the heating-up. Ballast according to any one of claims 8 to 15, characterized in that the control unit is adapted to compare the measured light intensity of the lamp with the desired light intensity of the lamp and, in case of deviation, to change the average control frequency of the power switching elements that the desired light intensity is achieved. Ballast according to any one of the preceding claims, characterized in that the control unit is provided with a modulator capable of modulating the control frequency of the power switching elements. Ballast according to claim 17, characterized in that the modulator comprises a voltage-controlled oscillator (57), the control signal of which can be modulated in amplitude by the control unit and the output signals of which are fed to the control circuit (25). Ballast according to any one of the preceding claims, characterized in that the control unit is provided with a modulator which can modulate the operating cycle of the charging signal for the power switching elements. Ballast according to any one of the preceding claims, characterized in that both duty cycle and frequency of the control signal are modulated. Ballast according to claim 19 or 20, wherein the input DC voltage for the bridge converter is obtained from an AC voltage source, by rectifying and smoothing the AC voltage in two phases, characterized in that the modulation due to duty cycle and / or frequency modulation of the control signal of the bridge inverter takes place synchronously with the double frequency of the AC voltage source, the phase of the modulation being selected such that around the positive and negative peaks of the input AC voltage, the inverter absorbs the largest current, and around the zero crossings of the input AC voltage is the smallest current. Ballast according to claim 21, characterized in that the AC voltage component of the DC input voltage of the bridge converter is used as the reference signal for determining the correct phase of said modulation. A ballast according to any one of claims 12 to 22, characterized in that the control unit is arranged to provide a signal indicating that the ratio between the lamp power calculated by the control unit and the light intensity exceeds a certain value, that the lamp needs to be replaced.