FIELD OF THE INVENTION

The invention relates to a lamp driver for operating an HID-lamp, comprising: input terminals for connection to the poles of a low frequency supply voltage, rectifying means coupled to the input terminals for rectifying the low frequency supply voltage,

buffer capacitor means coupled to the rectifying means for filtering the rectified low frequency supply voltage,

a DC-AC-converter coupled to the buffer capacitor means for generating a high frequency AC lamp current with average frequency f out of the voltage across the buffer capacitor means, said DC-AC-converter comprising a circuit part I for generating a control signal with average frequency f .

BACKGROUND OF THE INVENTION

Such a lamp driver is generally known. In case of an HID-lamp, operating the lamp at a comparatively high frequency can result in acoustic resonances. Generally these acoustic resonances are undesirable since they cause instabilities in the lamp. An exception is the so-called arc straightening resonance that stabilizes the discharge. It is therefore generally desirable to choose a way of operating the lamp that makes sure that the arc straightening resonance is excited while excitation of the other acoustic resonances is suppressed. The invention aims to achieve such lamp operation.

SUMMARY OF THE INVENTION

In accordance with the invention a lamp driver as described in the opening paragraph is therefore characterized in that the lamp driver is further equipped with a circuit part II, coupled between the output terminals of the rectifying means and the circuit part I, for modulating the frequency of the control signal with a frequency equal to twice the frequency of the low frequency supply voltage.

DETAILED DESCRIPTION OF EMBODIMENTS It is remarked that the buffer capacitor means can be directly connected to the output terminals of the rectifying means as is the case in the embodiment shown in Fig. 2. In the embodiments of this first group the circuit part II is connected directly to the output terminals of the rectifying means. In a second group of embodiments, however, one of the output terminals of the rectifying means is coupled to the buffer capacitor means via a diode. In these embodiments the output terminals of the rectifying means can be connected by means of a (comparatively small) capacitor. An embodiment is shown on the left side of Fig. 1. Also in embodiments belonging to this second group, the circuit part II is generally coupled directly to the output terminals of the rectifying means. In a third group of embodiments a power factor correction circuit is coupled between the rectifying means and the buffer capacitor means. Such a power factor correction circuit can be for instance an up-converter. An embodiment is shown on the right side of Fig. 1. In this latter case one of the output terminals of the rectifying means is coupled to the buffer capacitor means via a series arrangement of a choke and a diode. In this third group of embodiments the circuit part II can be directly connected to the output terminals of the rectifying means but can also for instance be connected to the cathode of the diode and one of the output terminals of the rectifying means. In this latter case the circuit part II is coupled to one of the output terminals of the rectifying means via the series arrangement of the choke and the diode.

In a lamp driver according to the invention frequency modulation is effected making use of the rectified supply voltage or of a derived voltage such as the ripple that is present across the buffer capacitor means. It has been found that a lamp driver according to the invention is of a very simple construction. The modulation ensures that the frequency spectrum is broadened, while the amplitudes of the frequencies are low enough to avoid excitation of acoustic resonances. At the same time it appeared possible to excite the arc straightening resonance in a dependable way.

The HID lamp impedance will increase when the momentary voltage across the buffer capacitor means is low, because the momentary lamp current is small. This is caused by thermal characteristics of the HID lamp. The increased lamp impedance can cause the extinguishing of the lamp. This undesired effect can, however, be avoided making use of the modulation.

Preferably the modulation of the frequency of the control signal is synchronized with the rectified supply voltage or with the AC component of the voltage present across the buffer capacitor means, in other words with the voltage ripple. This synchronization should preferably be such that the frequency of the control signal (and thus the frequency of the lamp current) is low when the amplitude of the voltage across the buffer capacitor means is low and the frequency of the control signal is high when the amplitude of the voltage across the buffer capacitor means is high. This synchronization makes the lamp impedance more constant and increases the life of the lamp.

The circuit part I can be realized in a simple and dependable way in case it comprises a current source (CS), a capacitor CI, a comparator COMP and a switching element S.

Good results have been obtained for embodiments of a lamp driver according to the invention, wherein the circuit part II comprises a capacitor C2 and a resistor Rl coupled between the buffer capacitor means and the capacitor CI.

Tolerance analysis on both the lamp and the lamp driver showed that it is difficult to always guarantee arc- straightening when the resonant converter is operated at a fixed frequency. For this reason a modulation algorithm is applied, more in particular a frequency modulation. The specific (20W) HID burner used in the tested exemplary embodiments requires the following FM-parameters to enforce arc -straightening:

f average ⁼ 52kFIz

FMamplitude = 4kHz

FM _repetition frequency = lOOHz (Synchronized with the mains supply)

The average frequency is generated by an oscillation circuit . This oscillating circuit comprises a current source CS, a capacitor CI, a comparator COMP and a switching element S. This oscillating circuit together with circuit part CC forms circuit part I in the embodiment shown in Fig. 2

The FM algorithm, which needs to be synchronized with the mains frequency, is derived from the ripple that is present on the buffer capacitor that supplies the resonant half -bridge. (C3 in figure 2) An example of the implementation of the frequency determining circuit is shown in figure 2.

During operation of the circuitry in Fig. 2, the current source CS is supplying a constant current to a common terminal of switch S and capacitor C 1. In addition a second current is supplied to this terminal via the voltage divider formed by resistors Rl and R2, and capacitor C2 and resistor R3, together forming a circuit part II. When switch S is

nonconductive these currents charge capacitor CI. When the voltage across capacitor CI reaches a voltage that equals the voltage Vref, the signal at the output of comparator COMP changes from low to high so that switch S becomes conductive and capacitor CI is discharged. Switch S is rendered non-conductive again and the current source CS charges capacitor CI again. This sequence is continuously repeated so that a voltage with an average frequency f is present across capacitor CI. Since a voltage with a 100Hz ripple is present across the buffer capacitor means C3, the second current is also modulated with a frequency of 100Hz and this in turn causes the frequency of the voltage across capacitor CI and also the frequency of the signal at the output of comparator COMP to be modulated with a frequency of 100 Hz. Since the control signals used to control the switching elements Ml and M2 in the resonant converter are derived by the circuitry comprised in circuit part CC from the voltage at the output of comparator COMP , the frequency of these control signals and the frequency of the lamp current are also frequency modulated with a frequency of 100Hz and also have an average value of f.

The amplitude of the FM modulation is determined by several circuit components and the lamp power:

The capacity of buffer capacitor C3

Lamp power

The resistances of resistors Rl, R2 and R3 and the capacity of capacitor C2 The capacity of capacitor C 1.

Figure 3 displays the resulting FM-shape. The momentary frequency will be varied around the average frequency of 52 kHz. The HID lamp-impedance will increase when the momentary voltage across capacitor C3 is low and the lamp current is thus comparatively small. (This is caused by thermal characteristics of the HID lamp) The increased lamp- impedance might be followed by lamp extinguishing. This is a real disadvantage when the HID-lamp is operated on a resonant topology that is only supplied by the rectified mains. This undesired effect can be avoided by the modulation algorithm. The following is done:

The FM algorithm is synchronized with the mains ripple (100Hz);

The phase between the mains-ripple and the FM is defined as:

FM signal results in low half -bridge frequencies when the ripple voltage is low FM signal results in high half -bridge frequencies when the ripple voltage is high

As a result the lamp-impedance will behave more constant. This increases the life of the system.

Next to FM modulation AM modulation is caused by the ripple voltage across capacitor C3. This voltage ripple will directly result in an amplitude modulation of the lamp current. Figure 4 displays an example of the voltage across capacitor C3, consisting of a constant DC voltage with a voltage ripple superimposed upon it.

Both the FM - and the AM modulation result in a broadened frequency spectrum. This helps to trigger arc-straightening resonance of the HID-burner. The spectral components are determined by:

Average resonant half -bridge frequency;

FM -parameters ;

AM-parameters;

Resonant half -bridge components (LC-tank);

Lamp-power

The control algorithm of the resonant lamp driver is designed in a way that it will immediately after ignition generate the correct frequency spectrum that is required to trigger the arc- straightening frequency of the HID-burner. This means that the proposed design will immediately generate the required spectrum after lamp breakdown.