Method to operate a resonant converter at a characteristic frequency of the power stage

1. A method of operating a resonant converter, the converter having a controller and power stage and providing an output voltage, the method comprising:

with the power converter at a predetermined operating condition, disabling power stage control signals from the controller;

determining a characteristic frequency of the power stage, the characteristic frequency defined as a natural oscillation frequency of the power stage observed right after the power stage control signals from the controller are disabled;

operating the converter at the determined characteristic frequency during normal operating conditions;

operating the converter at the determined characteristic frequency during ramp-up or ramp-down of the converter output voltage; and

adapting a maximum drive frequency of the controller as a function of the characteristic frequency of the converter, whereby a change in the characteristic frequency of the converter change as a function of output voltage.

2. The method of claim 1 , further comprising operating the converter at the characteristic frequency during overload or input overvoltage conditions.

3. The method of claim 1 , further comprising operating the controller in a burst mode to switch off drive pulses to the power stage if the converter output power is too high to maintain the output voltage regulation.

4. The method of claim 3 , wherein the controller is programmed to provide a time hysteresis to the drive pulse switch-on/switch-off thresholds.

5. The method of claim 1 , further comprising:

adapting controller drive pulse width to maintain substantially ZVS operation of the power stage.

6. The method of claim 1 , wherein the predetermined operating condition comprises a short circuit of the converter at zero voltage output.

7. The method of claim 1 , wherein the predetermined operating condition comprises an open circuit at the converter output.

8. A power converter comprising:

a power stage comprising a plurality of switching elements, one or more of the switching elements controlled for zero volt switching operation;

a resonant stage comprising an inductor and a capacitor;

an isolation transformer; and

a controller configured to

operate the converter at a characteristic frequency of the power stage, wherein the characteristic frequency is defined as a natural oscillation frequency of the power stage observed right after power stage control signals from the controller are disabled for the one or more switching elements having zero volt switching operation,

adapt a maximum drive frequency of the controller as a function of the output voltage level, wherein a change in the maximum drive frequency follows substantially a change of the characteristic frequency of the converter as function of output voltage, and

adapt the drive pulse width to maintain a substantially constant dead time between the pulses.

9. The power converter of claim 8 , wherein the controller is further configured for closed-loop or open loop operation of the converter.

10. The power converter of claim 9 , wherein the controller operates the converter at the determined characteristic frequency during startup and shutdown of the converter to provide linear ramp-up and ramp-down of the converter output voltage, respectively.

11. The power converter of claim 9 , wherein the controller operates the converter in a burst mode to completely switch off drive pulses to the power stage if the converter output power is too high to maintain the output voltage regulation.

12. The power converter of claim 8 , the controller adapting the drive pulse width to maintain substantially ZVS operation of the power stage.

13. A method of operating a resonant converter, the converter having a controller and power stage and providing an output voltage, the method comprising:

with the power converter at a predetermined operating condition, disabling power stage control signals from the controller;

determining a characteristic frequency of the power stage, the characteristic frequency defined as a natural oscillation frequency of the power stage observed right after the power stage control signals from the controller are disabled;

operating the converter at the determined characteristic frequency during normal operating conditions;

operating the converter at the determined characteristic frequency during ramp-up or ramp-down of the converter output voltage;

operating the controller in a burst mode to switch off drive pulses to the power stage if the converter output power is too high to maintain the output voltage regulation; and

providing a time hysteresis to the drive pulses, further providing one or more of: assuring a minimum number of drive pulses with no limitation of off time, assuring a minimum off time with no limitation of drive pulses, and assuring a minimum number of drive pulses and a minimum off time.

14. The method of claim 13 , further comprising:

adapting controller drive pulse width to maintain substantially ZVS operation of the power stage.

15. The method of claim 13 , further comprising:

operating the converter at the characteristic frequency during overload or input overvoltage conditions.

16. A method of operating a resonant converter, the converter having a controller and power stage and providing an output voltage, the method comprising:

with the power converter at a predetermined operating condition, disabling power stage control signals from the controller;

determining a characteristic frequency of the power stage, the characteristic frequency defined as a natural oscillation frequency of the power stage observed right after the power stage control signals from the controller are disabled;

operating the converter at the determined characteristic frequency during normal operating conditions;

operating the converter at the determined characteristic frequency during ramp-up or ramp-down of the converter output voltage;

operating the controller in a burst mode to switch off drive pulses to the power stage if the converter output power is too high to maintain the output voltage regulation;

adapting a maximum drive frequency of the controller as a function of the output voltage level, whereby a change in the maximum drive frequency follows substantially a change of the characteristic frequency of the converter as function of output voltage, and

adapting a drive pulse width to maintain a substantially constant dead time between drive pulses to the power stage.

17. The method of claim 16 , further comprising:

operating the converter at the characteristic frequency during overload or input overvoltage conditions.

18. A power converter comprising:

a power stage comprising a plurality of switching elements, one or more of the switching elements controlled for zero volt switching operation;

a resonant stage comprising an inductor and a capacitor;

an isolation transformer; and

a controller configured to

operate the converter at a characteristic frequency of the power stage, wherein the characteristic frequency is defined as a natural oscillation frequency of the power stage observed right after power stage control signals from the controller are disabled for the one or more switching elements having zero volt switching operation, and

adapt a maximum drive frequency of the controller to follow substantially a change of the characteristic frequency of the converter.

19. The power converter of claim 18 , the controller adapting the drive pulse width to maintain substantially ZVS operation of the power stage.

20. The method of claim 18 , further comprising:

operating the converter at the characteristic frequency during overload or input overvoltage conditions.