Control unit for rapidly starting the illumination of a high-pressure gas-discharge lamp

A d.c. voltage source delivers the voltage at its output terminals. This voltage is converted to a higher voltage level by a d.c./d.c. converter. This higher voltage level forms the input voltage to a controlled power-supply circuit. From this, the power-supply circuit forms the supply voltage and supply current for a high-pressure gas-discharge lamp to supply it with burning energy. A control circuit is provided to control the power-supply circuit, and an ignition circuit (9) is provided for the ignition. The power-supply circuit is controlled by the control circuit so that the high-pressure gas-discharge lamp is operated with a defined overload when illumination is started. In this context, the overload is defined so as to prevent flickering or light-saddles. The overload is controlled by the control circuit as a function of the state of the high-pressure gas discharge lamp. In so doing, the control circuit takes into account the temperature of the high-pressure gas-discharge lamp, subdivided according to the temperature of the electrodes and the temperature of the gas of the high-pressure gas-discharge lamp.

FIELD OF THE INVENTION 
The present invention relates to a control unit for rapidly starting an 
illumination of a high-pressure gas-discharge lamp, in particular for use 
in motor vehicle headlights. 
BACKGROUND INFORMATION 
German Patent Application No. 41 36 486 describes a ballast for starting 
and operating a.c. high-pressure gas-discharge lamps, the ballast is 
supplied from a d.c. voltage source, for example, from the battery of a 
motor vehicle. In this context, initially a stabilized and/or 
power-regulated d.c. voltage, having a higher voltage level, is produced 
by means of a d.c./d.c. converter from the d.c. voltage of a d.c. voltage 
source. Then the alternating current required to operate the lamp is 
produced from this voltage with the aid of a controlled bridge circuit. 
The bridge circuit can be operated as a direct-current and as an 
alternating-current circuit, and it forms the power-supply circuit for the 
high-pressure gas-discharge lamp. The bridge circuit is controlled by a 
control circuit, and an ignition device is provided to ignite the 
high-pressure gas-discharge lamp. 
In this conventional ballast, in order to achieve a rapid and reliable 
ignition, as well as to rapidly and reliably achieve a stable burning 
state without the lamp flickering, during initial operation, the bridge 
circuit is operated as a d.c. circuit. A signal is generated by a 
status-defining device when the high-pressure gas-discharge lamp is 
successfully ignited. This signal is fed via a time-delay circuit to the 
control circuit for the bridge circuit, ensuring that, only after the 
requisite period of delay, the bridge circuit is switched over from d.c. 
operation to a.c. operation. The approach described above differs from the 
approach provided by the present invention. 
For the open-loop control when starting a high-pressure gas-discharge lamp, 
the Vedilis curve is usually preset as the current-/voltage curve, in the 
sense of a reference curve. The Vedilis curve is described in the "System 
Specifications for Field Test" of the VEDILIS Eureka Project No. 273 on 
Page B 1/3, and it is preset as the current-/voltage characteristic for 
gas-discharge lamps used in motor vehicles. "Vedilis" in this connection 
is an acronym standing for "Vehicle Discharge Light System." Thereupon, 
for the closed-loop control of the lamp power of a gas-discharge lamp, the 
lamp voltage U.sub.L is measured in the start-up or burn phase and the 
corresponding lamp current I.sub.L, belonging to the respective lamp 
voltage U.sub.L, is determined from the Vedilis curve. This lamp current 
I.sub.L then serves as a reference value for the subsequent current 
closed-loop control circuit by means of which the lamp power is controlled 
in closed loop to a constant value such as 35 W. 
To use a gas-discharge lamp in motor vehicle headlights it is usually 
necessary to provide as much illumination as possible within a short 
period of time after switching the control unit on. This is described as a 
rapid starting of the illumination. As shown by the above-described 
Vedilis curve, for this purpose, the gas-discharge lamp can be operated 
with a specific overload immediately after being turned on. In this 
context, this overload is driven back as a function of the lamp voltage. 
The lamp should not be destroyed or have its service life adversely 
affected as a result of such an overload. This requirement must be 
observed for all operating states. Furthermore, the control must be 
selected such that the light output does not swing back again from an 
already attained value, because this is generally perceived as an 
unpleasant flickering. This swinging back of the light output is also 
characterized as a light-saddle effect. 
It has been shown in practice that a control in accordance with the 
above-mentioned Vedilis curve cannot avoid generating light-saddles. 
Consequently, alternative methods are sought. It is generally possible to 
conduct the overload operation of the gas-discharge lamp by using a simple 
timing. In this connection, however, it should be seen as particularly 
disadvantageous that the respective state of the gas-discharge lamp is 
fully ignored. This can result in the destruction of the gas-discharge 
lamp or, at least, in a shortening of its service life. 
SUMMARY OF THE INVENTION 
In contrast, the control unit of the present invention, which is used to 
rapidly start an illumination of a high-pressure gas discharge lamp, has 
the advantage that it precisely adjusts the overload operation of the 
high-pressure gas-discharge lamp to its respective state, while avoiding 
light-saddles and any harmful effects on the service life of the 
high-pressure gas-discharge lamp. 
According to the present invention, the control circuit controls the 
power-supply circuit such that the high-pressure gas-discharge lamp, when 
the illumination is started, is operated with a defined overload so that 
flickering and light-saddles are avoided, and that this overload is 
controlled by the control circuit as a function of the state of the 
high-pressure gas-discharge lamp, especially as a function of the 
temperature of the high-pressure gas-discharge lamp. 
For that purpose, in an advantageous embodiment of the present invention, 
control circuit takes into account the temperature of the high-pressure 
gas-discharge lamp, sub-divided according to the temperature of the 
electrodes and the temperature of the gas of the high-pressure 
gas-discharge lamp. In accordance with a further embodiment of the present 
invention, in order to ascertain the state of the high-pressure 
gas-discharge lamp, the control circuit monitors the current and/or 
voltage characteristic of the high-pressure gas-discharge lamp and of the 
power-supply circuit, respectively, immediately after the ignition 
process, and the current-/voltage profiles, characteristic for the various 
states of the high-pressure gas-discharge lamp, arising in connection with 
the internal resistance of the power supply circuit are determined, the 
current-/voltage characteristics differ significantly with the varying 
states of the high-pressure gas-discharge lamp. 
In another embodiment of the present invention, in ascertaining the state 
of the high-pressure gas-discharge lamp by the control circuit with the 
assistance of the current- and/or voltage characteristic of the 
high-pressure gas-discharge lamp and of the power-supply circuit, 
respectively, not only current- and/or voltage limiting values are taken 
into consideration, but also, in particular, the dynamic curve shapes in 
the initial time period after the ignition process. 
In yet another embodiment of the present invention, the control circuit 
contains a microcontroller which carries out in the control circuit the 
observations required for ascertaining the state of the high-pressure 
gas-discharge lamp and determines the appertaining states. 
In a further embodiment of the present invention, the microcontroller, as a 
function of the detected state of the high-pressure gas-discharge lamp, 
carries out the following essential control measures: 
When the high-pressure gas-discharge lamp is cold, maintaining the output 
current of the power-supply circuit at a high level using a 
time-restricted overload function, 
When the high-pressure gas-discharge lamp is hot, lowering the lamp power 
to the nominal value immediately after recognition, and 
When the high-pressure gas-discharge lamp is warm, lowering the lamp power 
to a weak overload. 
In an advantageous embodiment of the invention, the overload function for 
controlling the overload applied is a combination of a time-program 
control and of a control dependent upon the operating voltage. It is 
beneficial if the overload function is time-restricted.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a schematic block diagram of a control unit according to the 
present invention. The d.c. voltage source is designated as 1, and is 
potentially the battery of a motor vehicle. D.C. voltage source 1, 
delivers voltage U.sub.B at its output terminals. This voltage U.sub.B is 
converted or stepped up to a higher voltage level by a d.c. voltage/d.c. 
voltage converter 2 (d.c./d.c. converter). This higher voltage level 
U.sub.D is available at the output terminals of d.c./d.c. converter 2. 
D.C/D.C. converter 2 forms the input voltage to a controlled power-supply 
circuit 3, which can be a bridge circuit or contain a bridge circuit. 
Power supply circuit 3 generates a controlled alternating current and an 
alternating voltage, respectively, or a controlled direct voltage. This 
alternating voltage or direct voltage is available at output terminals 4 
and 5 and forms the power-supply voltage U.sub.L of a high-pressure 
gas-discharge lamp 6 to supply high-pressure gas-discharge with lamp 
burning energy. To controlled power-supply circuit 3, a control circuit 7 
supplies its control pulses via lines 8, to controlled power-supply 
circuit 3. Additionally, an ignition circuit 9, via line 10 indicated by 
an arrow, supplies igniting voltage via terminal 4 to high-pressure 
gas-discharge lamp. 
Accordance to the present invention, power-supply circuit 3 is controlled 
by control circuit 7 such that high-pressure gas-discharge lamp 6 is 
operated with a defined overload when illumination is started. The 
overload in this context is set so as to avoid flickering and 
light-saddles. Furthermore, this overload is controlled by control circuit 
7 as a function of the state of high-pressure gas-discharge lamp 6, in 
particular of the temperature of high-pressure gas-discharge lamp 6. In an 
advantageous manner, control circuit 7 takes account of the temperature of 
high-pressure gas-discharge lamp 6, sub-divided according to the 
temperature of electrodes 11, 12 and the temperature of gas 13 of 
high-pressure gas-discharge lamp 6. 
There are three essential states of the high-pressure gas-discharge lamp 6, 
specifically cold, hot, and warm, which are explained below with their 
characteristics and peculiarities. 
A cold high-pressure gas-discharge lamp 6 is present when high-pressure 
gas-discharge lamp 6 has not been switched on for a longer period and has 
completely cooled off. Electrodes 11 and 12 and gas 13 are then cold. The 
cold high-pressure gas-discharge lamp 6 can be operated with marked 
overload so as to produce a large amount of light more quickly. In this 
context, control circuit 7 ensures that there is no thermal erosion of 
electrodes 11 and 12 as a result of the defined electrical overload which 
would shorten the service life of high-pressure gas-discharge lamp 6. A 
cold high-pressure gas-discharge lamp 6 exhibits a very slight operating 
voltage immediately after being switched on, the operating voltage amounts 
to somewhat less than half the standard operating voltage. 
A hot high-pressure gas-discharge lamp 6 is present when the lamp has 
initially achieved full temperature due to a longer burning operation, and 
then is subsequently switched off for a short time. The typical duration 
of the switch-off time in this example amounts to about one second. 
Electrodes 11 and 12 and gas 13 are then hot. If a hot high-pressure 
gas-discharge lamp 6 is then switched on or switched on again, it does not 
have to be operated with an overload in order to achieve a rapid start of 
illumination. Such overload operation also is not acceptable, since it 
would reduce the life expectancy of high-pressure gas-discharge lamp 6 
because of the thermal overload. 
A warm high-pressure gas-discharge lamp 6 is present when the lamp has 
initially reached full temperature due to a longer burning operation, and 
is then shut off for several seconds. The typical duration of the 
switch-off time in this example amounts to about 10 seconds. Gas 13 still 
has a high temperature because of the relatively good insulation. As a 
consequence, there is also still high pressure in high-pressure 
gas-discharge lamp 6. Electrodes 11 and 12 are already highly cooled off, 
however, because their metal terminals dissipate heat very efficiently. 
When a warm lamp is switched on again, a precisely defined weak overload 
is used according to the present invention in order to achieve rapid start 
of illumination. 
For the reliable takeover of high-pressure gas-discharge lamp 6 from the 
ignition state into the static operating state on the one hand, and, on 
the other hand, to preserve the service life of high-pressure 
gas-discharge lamp 6, it is important to differentiate reliably among the 
various states of high-pressure gas-discharge lamp 6. 
Advantageously, control circuit 7, according to the present invention, 
contains a microcontroller. This microcontroller is not shown in FIG. 1. 
Using this microcontroller, which acts as an observer, control circuit 7 
performs the necessary observations for determining the state of 
high-pressure gas-discharge lamp 6, and determines the appertaining 
states. For this purpose, control circuit 7 first sets output current 
I.sub.L of power-supply circuit 3 to a high level. This is necessary so 
that even a cold high-pressure gas-discharge lamp 6 can reliably assume 
the burning operation. Since the ignition process commences with a time 
delay after is switched on, the no-load voltage arises at power-supply 
circuit 3. 
Immediately after the ignition process, e.g., in the first 5 milliseconds, 
control circuit 7 determines the current- and/or voltage profiles of 
high-pressure gas-discharge lamp 6 and of power-supply circuit 3, 
respectively, and the current-/voltage profiles resulting at the same time 
in connection with the internal resistance of power supply circuit 3. 
These profiles are characteristic for the various states of high-pressure 
gas-discharge lamp 6, and they differ significantly according to the 
different states of high-pressure gas-discharge lamp 6. In an advantageous 
manner, in determining the state of high-pressure gas-discharge lamp 6 by 
control circuit 7 with the assistance of the current- and/or voltage 
characteristic of high-pressure gas-discharge lamp 6 and of power-supply 
circuit 3, respectively, not only the current- and/or voltage limiting 
values are taken into consideration, but also, in particular, the dynamic 
curve shapes in the initial time period after the ignition process. 
Observing the dynamic profiles makes for a more accurate differentiation 
than can be achieved through merely sampling one point in time. 
Control circuit 7, in particular using the microcontroller, according to 
the present invention and as a function of the detected state, carries out 
the following essential control measures: 
When high-pressure gas-discharge lamp 6 is cold, maintaining the output 
current of the power-supply circuit at a high level by a time-restricted 
overload function. The overload function applied is precisely defined. It 
can be a combination of a time-program control and a control dependent 
upon the operating voltage. It is limited in time. Since, when 
high-pressure gas-discharge lamp 6 is cold, the interior gas pressure must 
first be built up, this state is especially sensitive as regards 
light-saddles. A time-controlled compensation can prevent a drop in the 
light power. When high-pressure gas-discharge lamp 6 is hot, lowering the 
lamp power to the nominal value immediately after recognition. Thus, it is 
switched immediately into the standard, static, burning operation at 
nominal capacity. Thermal overload is thereby avoided. 
When high-pressure gas-discharge lamp 6 is warm, lowering the lamp power to 
a weak overload. This overload also is precisely defined. It also can be a 
combination of a time-program control and a control dependent upon the 
operating voltage. However, in this case, it is not necessary to 
compensate for the possibility of light-saddles, since there is still gas 
pressure in high-pressure gas-discharge lamp 6. 
The control unit configured according to the present invention provides for 
high-pressure gas-discharge lamp 6 optimally with respect to both 
functionality and service life, under all states which occur.