Transformer-based light-source driving systems

1 . A light-source driving system comprising:

a first controller configured to control power transferred from a primary winding of a transformer to a first secondary winding and a second secondary winding of said transformer according to a load of a system circuit powered by said second secondary winding;

a switch circuit configured to enable said first secondary winding to provide a portion of said power to a light source when said switch circuit is turned on; and

a second controller, coupled to said switch circuit and said first controller, and configured to monitor a status of said light source and control said switch circuit according to said status,

wherein said second controller comprises:

a driving terminal configured to provide a driving signal to control said switch circuit; and

a synchronizing terminal configured to detect an electrical polarity of an output terminal of said second secondary winding, wherein said second controller is configured to generate said driving signal according to said status when a positive polarity is detected at said synchronizing terminal, and configured to pause generating said driving signal when said positive polarity is not detected at said synchronizing terminal.

2 . The light-source driving system of claim 1 , wherein said second controller further comprises:

an error amplifier configured to generate a compensation signal according to a difference between a signal indicative of said status of said light source and a reference signal;

a ramp generator, coupled to said synchronizing terminal, and configured to be enabled by said positive polarity, wherein when said ramp generator is enabled, said ramp generator is configured to generate a ramp signal; and

a comparison circuit, coupled to said error amplifier and said ramp generator, and configured to generate said driving signal by comparing said compensation signal with said ramp signal.

3 . The light-source driving system of claim 1 , wherein said first controller increases said power if said load increases and reduces said power if said load decreases.

4 . A light-source driving system comprising:

a first controller configured to control power transferred from a primary winding of a transformer to a first secondary winding and a second secondary winding of said transformer according to a load of a system circuit powered by said second secondary winding;

a switch circuit configured to enable said first secondary winding to provide a portion of said power to a light source when said switch circuit is turned on; and

a second controller, coupled to said switch circuit and said first controller, and configured to monitor a status of said light source and control said switch circuit according to said status,

wherein said second controller comprises:

a monitoring terminal configured to monitor said status of said light source; and

an adjusting terminal configured to receive an adjusting signal indicating a target level of a current through said light source, wherein said second controller is configured to adjust said current to said target level when said status of said light source indicates that said light source is in a normal-power state.

5 . The light-source driving system of claim 4 , wherein said second controller is configured to control said light source to be in said normal-power state by: increasing a duty cycle of said switch circuit if a voltage level at said monitoring terminal is less than a reference level, and reducing said duty cycle of said switch circuit if said voltage level is greater than said reference level.

6 . The light-source driving system of claim 4 , wherein said light source comprises an LED (light-emitting diode) string comprising at least one LED, wherein a positive terminal of said LED string is configured to receive said portion of said power, wherein a negative terminal of said LED string is coupled to a reference ground through a transistor and a resistor, and wherein said monitoring terminal is configured to monitor said status of said light source by sensing a voltage level at said negative terminal.

7 . The light-source driving system of claim 6 , wherein said adjusting signal comprises a PWM (pulse with modulation) signal, and wherein said second controller comprises:

a converter configured to convert a duty cycle of said PWM signal to an adjusting voltage; and

an adjusting circuit, coupled to said converter, and configured to apply said adjusting voltage to said resistor by controlling said transistor such that a current through said LED string is adjusted to said target level.

8 . A method for controlling a light source, comprising:

controlling power transferred from a primary winding of a transformer to a first secondary winding and a second secondary winding of said transformer according to a load of a system circuit powered by said second secondary winding;

enabling said first secondary winding to provide a portion of said power to a light source by turning on a switch circuit coupled to said first secondary winding and said light source;

monitoring a status of said light source;

controlling, using a controller coupled to said switch circuit and said light source, said switch circuit according to said status;

receive an adjusting signal, at an adjusting terminal of said controller, indicating a target level of a current through said light source;

adjusting said current through said light source to said target level when said status of said light source indicates that said light source is in a normal-power state;

monitoring said status of said light source by sensing a voltage level at a negative terminal of an LED (light-emitting diode) string in said light source, wherein said negative terminal is coupled to a reference ground through a transistor and a resistor;

receiving a PWM (pulse with modulation) signal at said adjusting terminal;

converting a duty cycle of said PWM signal to an adjusting voltage; and

applying said adjusting voltage to said resistor by controlling said transistor such that a current through said LED string is adjusted to said target level.

9 . The method of claim 8 , further comprising:

detecting an electrical polarity of an output terminal of said second secondary winding;

generating a driving signal to control said switch circuit according to said status when a positive polarity is detected at said output terminal; and

pausing said generating of said driving signal when said positive polarity is not detected at said output terminal.

10 . The method of claim 8 , further comprising:

increasing said power if said load increases; and

reducing said power if said load decreases.

11 . The method of claim 8 , further comprising:

controlling said light source to be in said normal-power state by:

increasing a duty cycle of said switch circuit if said voltage level at said negative terminal is less than a reference level; and

reducing said duty cycle of said switch circuit if said voltage level is greater than said reference level.

12 . A light source controller comprising:

a monitoring terminal configured to monitor a status of a light source that is powered by a first secondary winding of a transformer comprising a primary winding, said first secondary winding, and a second secondary winding;

a driving terminal configured to provide a driving signal to control a switch circuit that, when said switch circuit is turned on, enables said first secondary winding to provide said light source with a portion of power transferred from said primary winding to said first and second secondary windings;

a synchronizing terminal configured to detect an electrical polarity of an output terminal of said second secondary winding; and

control circuitry, coupled to said monitoring terminal, said driving terminal, and said synchronizing terminal, configured to generate said driving signal according to said status of said light source when a positive polarity is detected at said synchronizing terminal, and configured to pause generating said driving signal when said positive polarity is not detected at said synchronizing.

13 . The light source controller of claim 12 , wherein said control circuitry comprises:

an error amplifier configured to generate a compensation signal according to a difference between a signal indicative of said status of said light source and a reference signal;

a ramp generator, coupled to said synchronizing terminal, and configured to be enabled by said positive polarity, wherein when said ramp generator is enabled, said ramp generator is configured to generate a ramp signal; and

a comparison circuit, coupled to said error amplifier and said ramp generator, and configured to generate said driving signal by comparing said compensation signal with said ramp signal.

14 . The light source controller of claim 12 , further comprising:

an adjusting terminal configured to receive an adjusting signal indicating a target level of a current through said light source, wherein said control circuitry is further configured to adjust said current to said target level when said status of said light source indicates that said light source is in a normal-power state.

15 . The light source controller of claim 14 , wherein said control circuitry is further configured to control said light source to be in said normal-power state by increasing a duty cycle of said switch circuit if a voltage level at said monitoring terminal is less than a reference level and reducing said duty cycle of said switch circuit if said voltage level is greater than said reference level.

16 . The light source controller of claim 14 , wherein said monitoring terminal is configured to monitor said status of said light source by sensing a voltage level at a negative terminal of an LED (light-emitting diode) string in said light source, wherein said negative terminal is coupled to a reference ground through a transistor and a resistor.

17 . The light source controller of claim 16 , wherein said adjusting signal comprises a PWM (pulse with modulation) signal, and wherein said control circuitry comprises:

a converter configured to convert a duty cycle of said PWM signal to an adjusting voltage; and

an adjusting circuit, coupled to said converter, and configured to apply said adjusting voltage to said resistor by controlling said transistor such that a current through said LED string is adjusted to said target level.