The invention relates to a control method for semiconductor light sources which is suitable for applications that presuppose a rapid driving of the semiconductor light sources. This is the case in front and rear projection applications, for example.
Recently, powerful semiconductor light sources such as high-power light emitting diodes have increasingly been used in applications which had previously been reserved for high-pressure discharge lamps. Precisely in the field of projection, the semiconductor light sources are not driven continuously but rather are operated in pulsed fashion in order to meet the requirements in this field. Very short pulses are employed, which in return have a very high power density.
Since, in present-day semiconductor light sources, the light emission is dependent to a greater or lesser extent on the temperature of semiconductor light sources themselves, a control is necessary which ensures that the quantity of light emitted by a projection unit remains constant. Since pulsed driving methods have only been employed for a short while, the control methods usually used only take account of heating of the semiconductor light sources in relatively large time spans, that is to say over a relatively long period of consideration. As an example, one conventional method can be mentioned in which the temperature of the heat sink or heat sinks connected to the semiconductor light sources is measured, and the driving of the semiconductor light sources is adapted in accordance with the heat sink temperature. This takes account only of long-term effects, but not of the immediate heating of the semiconductor light sources that takes place during driving with short, powerful pulses. Since the pulse pauses can occasionally be very long, the semiconductor light source has enough time to cool down before the next pulse, with the result that the average loading over time does not turn out to be excessively high.
However, the semiconductor light source heats up during such a pulse to such a great extent that the emitted quantity of light does not remain constant during the pulse, but rather decreases continuously. In display applications, this can lead to impairment of the backlighting quality, and hence of the picture quality, and is therefore undesirable.
Therefore, the invention is based on the object of specifying a control method in which the emitted quantity of light of a semiconductor light source remains constant over the duration of a drive pulse. This object is achieved by means of the features of patent claim 1.
A further object is to specify a circuit arrangement which has implemented the control method according to the invention and operates a semiconductor light source in such a way that the emitted quantity of light of the semiconductor light source substantially remains constant over the pulse duration. This object is achieved by means of the features of claim 4.
Particularly advantageous embodiments of the invention are described in the independent claims.
In order to solve the problem, a control method for semiconductor light sources is proposed in which the characteristic light decrease of a semiconductor light source depending on the power introduced into the semiconductor light source is stored in the operating device, and the operating device varies the power introduced into the semiconductor light source during the pulse duration in such a way that the emitted quantity of light of the semiconductor light source substantially remains constant over the pulse duration.
a application circuit of an implementation of the control method according to the invention.
b alternative application circuit of an implementation of the control method according to the invention.
The light emission over time follows an exponential function and can be represented by the following equation:
In the present case of a light emitting diode that emits red light, the light emission is represented by the following values:
These values produce the curve 27 in
In order to comply with different current intensities and different initial temperatures, the characteristic light decrease can be stored as a family of characteristic curves in the control circuit.
In order to save storage space in the control circuit, however, it is also conceivable for the characteristic light decrease to be stored as a mathematical relationship in the control circuit, and for the predefined current value to be respectively calculated from the mathematical relationship with the aid of the relevant measurement variables. A mixed form is also conceivable, however, in which auxiliary values derived from the relevant measurement variables are stored in a table and the predefined current value is calculated with the auxiliary values and a stored mathematical relationship.
The result of this procedure is shown once again by way of example for a red light emitting diode in
It goes without saying that the control method according to the invention can also be used for any other semiconductor light source, such as an OLED for example.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2007/056232 | 6/22/2007 | WO | 00 | 12/22/2009 |