FIELD OF THE INVENTION

The invention relates to a lamp driver for supplying an HID lamp equipped with

input terminals for connection to a supply source,

a circuit part I coupled to the input terminals for igniting the lamp and for generating a lamp current out of a supply voltage supplied by the supply source and equipped with lamp connection terminals.

BACKGROUND OF THE INVENTION

In the operation of HID lamps different stages exist. First of all there is the ignition stage during which a high voltage is applied to the lamp to ignite it.

After ignition the next stage is "take over", during which the discharge in the lamp stabilizes. After the "take over" the lamp is in the "run-up" stage during which the lamp warms up and the composition of the plasma changes. When the lamp has reached stable operating conditions it enters the stage known as "stationary operation". Apart from these stages that the lamp is subsequently in, in case of normal operation, the lamp may also be in an unusual stage such as an "end of life" stage. In such an "end of life" stage the lamp has deteriorated so much that normal operation is no longer possible or save. In all the operational stages mentioned (apart from stationary operation and run-up), the average voltage that is present across the lamp is higher than during stationary operation. In case such a

comparatively high voltage is maintained for a long time, the lamp and/or the lamp driver might be damaged or become too hot leading to a dangerous situation.

SUMMARY OF THE INVENTION

The object of the invention is to prevent dangerous situations in every operational phase of an HID lamp.

According to the invention a lamp driver for an HID lamp is equipped with a circuit part II coupled to the circuit part I for generating a signal S 1 representing the amplitude of the lamp voltage,

a circuit part III coupled to the circuit part II for generating a stop signal S2 after a time interval Atl determined by the magnitude of signal SI,

a stop circuit coupled to circuit part I and circuit part III for stopping the operation of circuit part I under the influence of the stop signal S2,

a reactivation circuit coupled to circuit part I for reactivating circuit part I after a predetermined time interval At2.

During lamp operation the circuit part II generates signal S 1 representing the amplitude of the lamp voltage. The circuit part III generates a stop signal after a time interval Atl determined by the magnitude of signal S 1. In general, the time interval Atl can be longer when the magnitude of the signal SI is smaller and vice versa. The time interval Atl is chosen such that the stop signal is generated before the circuitry of the lamp driver or the lamp is damaged or a dangerous situation arises. The stop circuit stops the operation of circuit part I under the influence of the stop signal S2. It is generally desirable to activate the circuit part I again and try to obtain stationary operation. This reactivation of the circuit part I, however, should not be done immediately after operation of the circuit part I has been stopped, since both the driver and the lamp will still be hot, but only after a predetermined time interval. Therefore in a lamp driver according to the invention, the reactivation circuit only reactivates circuit part I after a predetermined time interval At2. Of course the duration of the predetermined time interval At2 is chosen such that a sufficient cooling down of lamp and circuitry can take place. The duration of the predetermined time interval At2 can for instance be chosen dependent on the magnitude of signal SI before the stopping of circuit part I but can also be chosen at a constant value independent of the magnitude of signal SI.

The stop circuit can be realized in a cheap and effective way in case it comprises a circuit part IV, being operable in a first and a second operational state, the stop circuit maintaining the circuit part I inoperative in case the circuit part IV is in the second operational state and maintaining the circuit part I in an active state in case the circuit part IV is in the first operational state.

Preferably, the operational state of the circuit part IV is changed from the first to the second operational state by the stop signal S2 and reset can be reset from the second operational state to the first operational state by temporarily lowering the supply voltage of circuit part IV.

Preferably, the reactivation circuit comprises circuitry for lowering the supply voltage of circuit part IV when circuit part IV is in the second operational state and for reinstating the supply voltage at the end of a time lapse equal to the predetermined time interval At2 and starting when the operational state changed from the first to the second operational state.

In this case, the reactivation circuit preferably comprises circuitry for lowering the supply voltage of circuit part IV during a time lapse equal to the predetermined time interval At2 and for increasing the supply voltage back to its original value at the end of this time lapse.

The circuit part IV preferably comprises an integrated circuit or is part of an integrated circuit. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of a lamp driver according to the invention will be further discussed making reference to a drawing.

In the drawing, Fig. 1 shows an embodiment of a lamp driver according to the invention, and

Fig. 2 shows the relation between the magnitude of signal SI and the duration of the time interval Δίΐ .

DETAILED DESCRIPTION OF THE DRAWINGS

In Fig. 1, Kl and K2 are input terminals for connection to a supply source. Diodes D3-D6, capacitors CI and C4-C6, switching elements Ml and M2, choke LI and lamp connection terminals K3 and K4 as well as a driving circuit CC for generating control signals for the switching elements M 1 and M2 together form a circuit part I for igniting the lamp and for generating a lamp current out of a supply voltage supplied by the supply source.

Resistors R3 and R4, capacitor C2 and diodes Dl and D2 together form a circuit part II coupled to the circuit part I for generating a signal SI representing the amplitude of the lamp voltage. Resistor R5, capacitor C3, comparator COMP and reference voltage source Vref generating a reference voltage VI, together form a circuit part III coupled to the circuit part II for generating a stop signal S2 after a time interval Atl determined by the magnitude of signal S 1. The output terminal of the comparator COMP is connected to an input terminal of driving circuit CC. Part of the driving circuit CC forms a stop circuit for stopping the operation of circuit part I under the influence of the stop signal S2. The stop circuit in turn comprises a circuit part IV, being operable in a first and a second operational state, the stop circuit maintaining the circuit part I inoperative in case the circuit part IV is in the second operational state and maintaining the circuit part I in an active state in case the circuit part IV is in the first operational state.

Vsup is a circuit part for generating a supply voltage. A first terminal of circuit part Vsup is connected to ground. A second terminal of circuit part Vsup is connected to the positive terminal of capacitor C6.

An input terminal of circuit part RC is coupled to an output terminal of driving circuit CC that is also connected to a control electrode of switching element M2. An output terminal of circuit part RC is coupled to an input terminal circuit part Vsup. A supply voltage input terminal of driving circuit CC is connected to an output terminal of circuit part Vsup.

Circuit part RC forms a reactivation circuit for reactivating circuit part I after a predetermined time interval At2.

The operation of the lamp driver shown in Fig. 1 is as follows. When input terminals l and K2 are connected to a supply voltage, driving circuit CC renders the switching elements Ml and M2 alternately conductive and non-conductive, causing a high frequency voltage to be present across the lamp connected between the lamp connection terminals. This voltage across the lamp causes a current to flow through resistor R3 and capacitor C2. The positive part of this current flows through diode D2 and resistor R4, the negative part flows through diode Dl. As a consequence, positive voltage pulses with an amplitude depending on the amplitude of the lamp voltage are present across resistor R4.

These positive voltage pulses cause capacitor C3 to be charged. In case the lamp voltage amplitude is comparatively low, as is the case during stationary operation, the voltage across C3 never reaches the value VI of the voltage generated by the reference voltage source Vref. As a consequence the output signal of the comparator COMP never changes from low to high so that the stop circuit is not activated and the operation of circuit part I is not stopped.

However, in case the lamp voltage is big enough, the voltage across resistor R4 causes the capacitor C3 to be charged to a value bigger than VI and the stop circuit is activated so that lamp operation is stopped. Activation of the stop circuit is realized by the circuit part IV changing from its first to its second operational state under the influence of the signal S2.

It is important to note that the voltage across resistor R4 will increase when the voltage across the lamp increases, so that the time lapse during which capacitor C3 is charged to VI will be shorter. In other words a bigger lamp voltage is present during a shorter time lapse before the stop circuit is activated. This effect is illustrated in Fig. 2. Along the horizontal axis in Fig. 2 lamp voltage is plotted in arbitrary units. Along the vertical axis time is plotted in arbitrary units. The curve shows the relation between the duration of the time lapse, during which this lamp voltage can be present before the stop circuit is activated. It can be seen that this time lapse shortens when the lamp voltage increases. For low values of the lamp voltage (stationary operation), the time duration is infinitely long, in other words the stop circuit is not activated. The larger the lamp voltage, the sooner the lamp and/or part of the lamp driver will become too hot or damaged. The chosen relation between lamp voltage and time duration before the stop circuit is activated ensures that for each value of the amplitude of the lamp voltage the lamp driver is stopped before damage is done to driver or lamp. Comparatively high lamp voltages occur during ignition, take-over and end of life.

When the stop circuit is activated and lamp operation has stopped, this is sensed by the circuit part RC because no control signals for the lower switch M2 are present at its input. As a result the circuit part RC raises the signal at its output terminal from high to low causing the supply voltage generated by circuit part Vsup to be lowered. As a consequence the voltage present at the supply voltage input of driving circuit CC and thus also the supply voltage of circuit part IV is lowered. After a predetermined time interval At2, the signal at the output terminal of circuit part RC is raised from low to high, causing the supply voltage generated by the circuit part Vsup to be raised to its normal value. As a result, the operational state of circuit part IV is reset to the first operational state and the driving circuit CC is supplied with the proper supply voltage so that it will start a new lamp operating sequence starting with ignition and after that supplying of the lamp with a lamp current.

The subsequent action of the stop circuit and the reactivation circuit causes a comparatively high lamp voltage to be present across the lamp for a time interval Atl, then the stop circuit ends the lamp operation and after a predetermined time interval At2 lamp operation is resumed. This sequence is called a burst. The predetermined time interval At2 can be timed for instance by a digital timer comprised in circuit part RC and started when the circuit part RC senses that lamp operation has stopped. Alternatively, this timing can of course be done with a resistor capacitor combination or in any other known way.

In case for instance the lamp fails to ignite, the ignition voltage will be present across the lamp during the lamp voltage determined time interval Δΐΐ, after which the operation of the lamp driver is interrupted for the predetermined time interval At2 (lamp voltage independent). When the time interval At2 has timed out the ignition voltage is once more applied to the lamp and so on. Of course, in case the lamp ignites and the lamp voltage drops before the time interval At\ has timed out, the stop circuit is not activated and lamp operation is continued.

A similar burst operation is taking place in case of unsuccessful take-over of the lamp or at end of life.

At end of life, the inner discharge vessel is leaky. This means that the gases of the inner vessel are now present in the outer-bulb. This realizes an easy ignition in this outer- bulb. Issues will appear when the outer-bulb ignitions will result in a sustained outer-bulb discharge. This can be realized when the temperature of the internal supporting wires exceeds a high value. This will happen when the driver dehvers too much energy to the outer-bulb. To avoid this high energy the stop circuit and the reactivation circuit will introduce a burst mode whenever EOL is reached. (On-time determined by EOL-lamp voltage, off-time is fixed) The following events will be observed:

EOL will result in an easy breakdown in the outer-bulb. Since there will however be no take-over in the outer-bulb the typical rms voltage will be very high. Fig. 2 shows that the stop-restart circuitry will be triggered since the voltage will be higher than 130V. Graph 1 shows that the driver will shut-down after typically 5 seconds.

A burst-mode will appear since the driver will be automatically be restarted after 7 seconds. (5 seconds on, 7 seconds off) The burst- mode is not sufficiently long to allow take-over or thermionic emission of the supporting wires. This means that it is not possible to have a strong discharge in the outer-bulb.

There will be a sputter- or transport mechanism of metal parts to the wall of the outer-bulb of the lamp. This mechanism is triggered by the burst-mode. The result will be a metalized outer-bulb. This large metal surface will start to act as a getter and collect the gas (argon) in the outer-bulb. Reducing the amount of Argon in the outer bulb by gettering will increase the ignition voltage and will finally result in the absence of any breakdown events.

The absence of outer-bulb breakdown events will guarantee a safe EOL of the system. The burner will never suffer from outer-bulb discharges or an incandescent mode.