Closed loop frequency control method for a self-oscillating circuit

1. A resonant 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 voltage sensing circuit coupled to a secondary winding of the isolation transformer and configured to provide a sensor output signal representative of an output voltage across a load; and

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

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

a current control circuit coupled between the feedback circuit and the frequency control input terminal of the controller, and configured to

apply a first resistance between the frequency control input terminal and a primary side ground, thereby defining a minimum operating frequency associated with a maximum output current to the load, and

apply a second resistance between the frequency control input terminal and the primary side ground, the combination of the first resistance and the second resistance thereby defining a maximum operating frequency associated with a minimum output current to the load,

wherein the controller is configured to regulate the operating frequency of the first and second switching elements between the minimum and maximum operating frequencies based at least on the error signal.

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

the minimum operating frequency is greater than a resonant frequency of the resonant power converter.

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

the second resistance includes a first diode having an anode of the first diode coupled to the frequency control input terminal; and

the second resistance further includes first and second resistors coupled in series between a cathode of the first diode and the primary side ground.

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

the minimum operating frequency is defined when the first diode is reverse biased.

5. The resonant power converter of claim 3 , wherein the current control circuit includes:

an optocoupler having an input stage and an output stage, the input stage coupled to receive the error signal, the output stage having a variable impedance between a first output terminal and a second output terminal responsive to the error signal; and

a second diode coupled between the second output terminal and a node defined between the first and second resistors of the second resistance.

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

the maximum operating frequency is defined when the second diode is reverse biased.

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

the error signal of the feedback circuit increases when the sensor output signal is greater than the reference signal; and

the error signal of the feedback circuit decreases when the sensor output signal is less than the reference signal.

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

the controller adjusts the operating frequency of the first and second switching elements for maintaining a substantially constant output voltage across the load when a resistance of the load changes.

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

the operating frequency of the controller is directly proportional to a current output of the frequency control input terminal; and

the frequency control input terminal is coupled to the reference signal.

10. A method of controlling an output power of a resonant power converter having a substantially constant output voltage, the method comprising:

sensing a voltage signal across a load of the resonant power converter;

generating a feedback error signal based at least on the voltage signal relative to a reference voltage;

determining an operating frequency of the resonant power converter based at least on the feedback error signal;

setting a minimum operating frequency of the resonant power converter based at least in part on a first current signal at a frequency control pin of a controller of the resonant power converter; and

regulating the operating frequency of the resonant power converter above the set minimum operating frequency based on a current at the frequency control pin.

11. The method of claim 10 , further comprising:

setting the minimum operating frequency greater than a resonant frequency of the resonant power converter.

12. The method of claim 10 , further comprising:

controlling an output current of the resonant power converter based on the operating frequency.

13. The method of claim 10 , further comprising:

defining the first current signal based on a first resistance coupled between the frequency control pin and a primary side ground.

14. The method of claim 10 , further comprising:

setting a maximum operating frequency of the resonant power converter based at least in part on a second current signal at the frequency control pin of the controller; and

regulating the operating frequency of the resonant power converter between the set minimum operating frequency and the set maximum operating frequency based on a combination of the first and second current signals at the frequency control pin.

15. The method of claim 14 , further comprising:

defining the first current signal based on a first resistance coupled between the frequency control pin and a primary side ground; and

defining the second current signal based on a second resistance coupled between the frequency control pin and the primary side ground, the second resistance coupled in parallel with the first resistance.