Adaptively controlled soft start-up scheme for switching power converters

1. A switching power converter comprising:

a transformer including a primary winding coupled to an input voltage and a secondary winding coupled to an output of the switching power converter;

a switch coupled to the primary winding of the transformer, current through the primary winding being generated while the switch is turned on and not being generated while the switch is turned off; and

a switch controller configured to generate a control signal to turn on or turn off the switch, the switch being turned on responsive to the control signal being in a first state and the switch being turned off responsive to the control signal being in a second state,

the switch controller monitoring a sensed voltage representing an output voltage of the switching power converter, and the switch controller configured to control switching of the switch to operate the switching power converter in a continuous conduction mode while the sensed voltage indicates that the output voltage is less than a first threshold voltage, the switch controller further configured to control switching of the switch to operate the switching power converter in an open loop discontinuous conduction mode while the sensed voltage indicates that the output voltage is above the first threshold voltage and below a second threshold voltage, wherein the switching controller controls switching of the switch independently of the sensed voltage when in the open loop discontinuous conduction mode, and the switch controller configured to control switching of the switch to operate the switching power converter in a closed loop discontinuous conduction mode while the sensed voltage indicates that the output voltage is above the second threshold voltage, wherein the first threshold voltage and the second threshold voltage are less than a desired regulation voltage of the switching power converter.

2. The switching power converter of claim 1 , wherein the switching controller controls switching of the switch in the continuous conduction mode such that a transformer magnetizing current remains above zero.

3. The switching power converter of claim 1 , wherein the switching controller controls switching of the switch in the open loop discontinuous conduction mode and the closed loop discontinuous conduction mode such that a transformer magnetizing current drops to zero before the switch is turned on in each switching cycle of the switch.

4. The switching power converter of claim 1 , wherein the switching controller controls switching of the switch in the open loop discontinuous conduction mode such that a switching frequency and peak primary side current are maintained at predefined levels regardless of the sensed voltage.

5. The switching power converter of claim 1 , wherein the switch controller controls switching of the switch in the closed loop discontinuous conduction mode such that the output voltage is maintained within an allowable error range of the desired regulation voltage.

6. The switching power converter of claim 1 , wherein the second threshold voltage comprises a predefined percentage of a desired regulation voltage, wherein the predefined percentage is in a range of 60%-80%.

7. The switching power converter of claim 1 , wherein the first threshold voltage comprises a predefined percentage of a desired regulation voltage, wherein the predefined percentage is in a range of 10%-25%.

8. The switch power converter of claim 1 , wherein the switch controller is further configured to activate a timer upon start up of the switching power converter, and wherein the switch controller is configured to detect a fault condition if the sensed voltage indicates that the output voltage fails to rise above a first fault detection voltage prior to the timer reaching a first fault detection time.

9. The switching power converter of claim 8 , wherein the switch controller is further configured to detect the fault condition if the sensed voltage indicates that the output voltage fails to rise above a second fault detection voltage prior to the timer reaching a second fault detection time, wherein the second fault detection voltage is greater than the first fault detection voltage, and wherein the second fault detection time is greater than the first fault detection time.

10. In a controller, a method of controlling a switching power converter, the switching power converter including a transformer with a primary winding coupled to an input voltage and a secondary winding coupled to an output of the switching power converter, and a switch coupled to the primary winding of the transformer, current through the primary winding being generated while the switch is turned on and not being generated while the switch is turned off, the method comprising:

generating a pulse signal for turning the switch on and off;

monitoring a sensed voltage representing an output voltage of the switching power converter;

controlling switching of the switch to operate the switching power converter in a continuous conduction mode while the sensed voltage indicates that the output voltage is less than a first threshold voltage;

controlling switching of the switch to operate the switch controller in an open loop discontinuous conduction mode while the sensed voltage indicates that the output voltage is above the first threshold voltage and below a second threshold voltage, wherein the switching controller controls switching of the switch independently of the sensed voltage when in the open loop discontinuous conduction mode, and wherein the first threshold voltage and the second threshold voltage are less than a desired regulation voltage of the switching power converter; and

controlling switching of the switch to operate the switching power converter in a closed loop discontinuous conduction mode while the sensed voltage indicates that the output voltage is above the second threshold voltage.

11. The method of claim 10 , wherein controlling switching in the continuous conduction mode comprises:

controlling switching of the switch such that a transformer magnetizing current remains above zero.

12. The method of claim 10 , wherein controlling switching in the open loop discontinuous conduction mode and the closed loop discontinuous conduction mode comprises:

controlling switching of the switch such that a transformer magnetizing current drops to zero before the switch is turned on in each switching cycle of the switch.

13. The method of claim 10 , wherein controlling switching in the open loop discontinuous conduction mode comprises:

controlling switching of the switch such that a switching frequency and peak primary side current are maintained at predefined levels regardless of the sensed voltage.

14. The method of claim 10 , wherein controlling switching in the closed loop discontinuous conduction mode comprises:

controlling switching of the switch such that the output voltage is maintained within an allowable error range of the desired regulation voltage.

15. The method of claim 10 , wherein the second threshold voltage comprises a predefined percentage of a desired regulation voltage, wherein the predefined percentage is in a range of 60%-80%.

16. The method of claim 10 , wherein the first threshold voltage comprises a predefined percentage of a desired regulation voltage, wherein the predefined percentage is in a range of 10%-25%.

17. The method of claim 10 , further comprising:

activating a timer upon start up of the switching power converter; and

detecting a fault condition if the sensed voltage indicates that the output voltage fails to rise above a first fault detection voltage prior to the timer reaching a first fault detection time.

18. The method of claim 17 , further comprising:

detecting the fault condition if the sensed voltage indicates that the output voltage fails to rise above a second fault detection voltage prior to the timer reaching a second fault detection time, wherein the second fault detection voltage is greater than the first fault detection voltage, and wherein the second fault detection time is greater than the first fault detection time.