Lighting circuit and method for negative feedback control recovery in overload conditions

1. A power converter, comprising:

first and second switching elements coupled across a direct current (DC) power source;

a resonant circuit coupled between an isolation transformer primary winding and an output node between the first and second switching elements;

a current sensing circuit coupled between an output load and a secondary winding of the isolation transformer, and configured to provide a sensor output signal representative of an output current through the load;

a feedback circuit configured to generate an error signal corresponding to a difference between the sensor output signal and a reference signal;

a controller comprising a frequency control input terminal, and configured to generate drive signals to the first and second switching elements at a determined operating frequency;

a frequency control circuit coupled between the feedback circuit and the frequency control input terminal of the controller, and configured, responsive to the error signal, to determine the operating frequency of the controller with respect to defined minimum and maximum frequencies; and

a current reference control circuit configured to control the error signal of the feedback circuit when the error signal is greater than a reference error signal of the current reference control circuit,

wherein the current reference control circuit includes a slow charge path and a quick discharge path in parallel between the output terminal of the feedback circuit and an integrator capacitor of the current reference control circuit,

wherein the quick discharge path is configured to discharge the integrator capacitor at a first rate when the error signal is greater than the reference error signal, and

wherein the slow charge path is configured to charge the integrator capacitor at a second rate slower than the first rate when the error signal is less than the reference error signal.

2. The power converter of claim 1 , wherein:

the slow charge path includes a first resistor having a first resistance; and

the quick discharge path includes a diode in series with a second resistor having a second resistance smaller than the first resistance.

3. The resonant power converter of claim 1 , wherein:

the integrator capacitor is coupled between an inverting input terminal of an operational amplifier and an output terminal of the operational amplifier; and

the slow charge path and the quick discharge path are coupled to the inverting input terminal of the operational amplifier.

4. The resonant power converter of claim 3 , wherein:

the feedback circuit includes a buffer resistor coupled between the reference signal and the first input terminal, and a buffer capacitor coupled between the first input terminal and a secondary side ground reference; and

the current reference control circuit includes an output control diode coupled in series with an output control resistor between the output terminal of the operational amplifier and the first input terminal of the feedback circuit.

5. The resonant power converter of claim 4 , wherein:

the operational amplifier is configured to generate a reference control signal at the output terminal; and

a voltage across the buffer capacitor is controlled by the reference control signal when the reference control signal is lower than the reference signal.

6. The resonant power converter of claim 5 , wherein:

the voltage across the buffer capacitor is controlled by the reference signal when the reference control signal is greater than the reference signal.

7. The power converter of claim 6 , wherein:

the output control diode is reverse biased when the reference control signal is greater than the reference signal.

8. The power converter of claim 1 , wherein:

the reference error signal is received at a non-inverting input terminal of an operational amplifier of the current reference control circuit.

9. A method to regain negative feedback control from an overload condition in a resonant power converter, the method comprising:

(a) regulating a switch operating frequency of the resonant power converter to provide output power to an LED load;

(b) generating an error signal corresponding to a difference between the provided output power and a reference value;

(c) generating a reference control signal based at least in part on the error signal; and

(d) automatically controlling the reference value when the reference value is greater than the generated reference control signal.

10. The method of claim 9 , wherein step (d) further comprises:

reducing the error signal based on the generated reference control signal.

11. The method of claim 9 , wherein step (d) further comprises:

reducing the generated reference control signal below the reference signal when the sensed error signal is greater than a reference error signal of the current reference control circuit; and

slowly increasing the generated reference control signal when the sensed error signal is less than the reference error signal of the current reference control circuit.

12. The method of claim 11 , further comprising:

repeating each step until the LED load transitions from a cold operational state to a normal operational state.

13. The method of claim 9 , further comprising:

operating the LED load in a cold operational state associated with a maximum cold LED current being less than a maximum normal LED current associated with the LED load in a normal operational state, the cold operational state further associated with the sensed error signal being greater than a reference error signal of the current reference control circuit; and

repeating steps (b) through (d) until the LED load operates in the normal operational state.

14. An LED driver comprising:

a DC-DC converter comprising a plurality of switching elements configured to produce an output voltage across first and second output terminals and an output current through an LED load coupled thereto, based on a DC input and an operating frequency;

a current sensor coupled in series with the LED load;

a control means for regulating the operating frequency based on an error signal derived at least in part from output signals from the current sensor relative to a reference value; and

a reference control means for controlling the error signal in response to the error signal exceeding a reference error signal for regaining negative feedback control of the control means, wherein the reference control means includes:

a quick discharge means for quickly reducing the error signal in response to the error signal exceeding the reference error signal; and

a slow charge means slowly increasing the error signal in response the error signal being reduced.

15. The LED driver of claim 14 , wherein:

wherein the reference control means manipulates the reference value in order to temporarily affect the error signal.

16. The LED driver of claim 14 , wherein:

the slow charge means gives the LED load time to warm up in order to transition from a cold operational state to a normal operational state.

17. A power converter, comprising:

first and second switching elements coupled across a direct current (DC) power source;

a resonant circuit coupled between an isolation transformer primary winding and an output node between the first and second switching elements;

a current sensing circuit coupled between an output load and a secondary winding of the isolation transformer, and configured to provide a sensor output signal representative of an output current through the load;

a feedback circuit configured to generate an error signal corresponding to a difference between the sensor output signal and a reference signal;

a controller comprising a frequency control input terminal, and configured to generate drive signals to the first and second switching elements at a determined operating frequency;

a frequency control circuit coupled between the feedback circuit and the frequency control input terminal of the controller, and configured, responsive to the error signal, to determine the operating frequency of the controller with respect to defined minimum and maximum frequencies;

a current reference control circuit configured to control the error signal of the feedback circuit when the error signal is greater than a reference error signal of the current reference control circuit;

wherein the feedback circuit includes a buffer resistor coupled between the reference signal and the first input terminal, and a buffer capacitor coupled between the first input terminal and a secondary side ground reference; and

wherein the current reference control circuit is configured to control a voltage across the buffer capacitor when a reference control signal generated by the current reference control circuit is less than the reference signal.

18. The power converter of claim 17 , wherein:

the reference error signal is received at a non-inverting input terminal of an operational amplifier of the current reference control circuit.