Lamp driver circuit and method for driving a discharge lamp
A lamp driver circuit ( 400 ) comprises a feedback circuit for controlling stable operation of a discharge lamp (La), e.g. an inductively coupled discharge lamp such as a molecular radiation lamp, and for controlling a light output level of the discharge lamp (La). In particular, if the discharge lamp (La) is operated at a dimmed light output level, the light output is sensitive to changes in the lamp voltage (V La ), possibly resulting in flickering. In order to control stable lamp operation and prevent flickering, a high-sp feedback circuit is provided for controlling an operating frequency. In order to provide a relatively large dimming range for controlling the light output level, a low-speed feedback circuit is provided for controlling a DC supply voltage level (V DC ).
1. Lamp driver circuit ( 400 ) for operating a discharge lamp (La) at a set lamp power, the lamp driver circuit comprising:
a DC supply voltage circuit ( 408 ) for generating a DC supply voltage (V DC );
an output circuit for supplying an AC current to the discharge lamp (La), the output circuit comprising an inverter circuit for generating an AC current at an operating frequency from the DC supply voltage;
a feedback circuit comprising:
an actual lamp power sensing circuit for determining an actual lamp power;
a high-speed feedback circuit coupled to the inverter circuit for controlling the operating frequency of the AC current in response to a power difference between the determined actual lamp power and the set lamp power in order to maintain stable lamp operation; and
a low-speed feedback circuit coupled to the DC supply voltage circuit for controlling the DC supply voltage in response to the power difference between the determined actual lamp power and the set lamp power in order to control the actual lamp power.
2. Lamp driver circuit according to claim 1 , wherein the actual lamp power sensing circuit comprises a resistor (R 1 ) in series coupled to the inverter circuit for determining an inverter current flowing through the inverter circuit, the inverter current being substantially equal to the actual lamp power divided by the DC supply voltage.
3. Lamp driver circuit according to claim 1 , wherein the high-speed feedback circuit comprises a voltage controlled oscillator, VCO ( 104 ), configured to receive a voltage signal representing the power difference in order to convert the power difference in a suitable operating frequency.
4. Lamp driver circuit according to claim 3 , wherein the inverter circuit comprises at least two switching elements (S 1 , S 2 ) in a bridged topology, the lamp driver circuit further comprising an inverter driver circuit ( 106 , 108 ) for controlling switching of the switching elements, the inverter driver circuit being coupled to an output of the VCO.
5. Lamp driver circuit according to claim 4 , wherein the low-speed feedback circuit is:
configured to receive a set frequency;
coupled to an output of the VCO for receiving the operating frequency; and
configured to control the DC supply voltage in response to a frequency difference between the operating frequency and the set frequency, the high-speed feedback circuit being configured to, in response, adjust the operating frequency towards the set frequency.
6. Lamp driver circuit according to claim 1 , wherein the low-speed feedback circuit is configured to receive a set frequency, to determine the operating frequency and to control the DC supply voltage in response to a difference between the operating frequency and the set frequency, the high-speed feedback circuit being configured to, in response, adjust the operating frequency towards the set frequency.
7. Method for operating a discharge lamp at a set lamp power, the method comprising:
generating a DC voltage;
generating an AC current at an operating frequency from the DC voltage;
supplying an AC current to the discharge lamp;
determining an actual lamp power;
controlling the frequency of the AC current in response to a difference between the determined actual lamp power and the set lamp power in order to maintain stable operation; and
controlling the DC voltage in response to the determined actual lamp power and the set lamp power in order to control the actual lamp power.
8. Method according to claim 7 , wherein the DC voltage is controlled in response to a difference between the operating frequency and a predetermined frequency.