Active clamp power converter and method of reducing shoot-through current during soft start

1. A method of controlling soft start in. a power converter including a first transistor and second transistor coupled in a conduction path with the first transistor, comprising:

switching the second transistor based on an oscillator signal having a first initial frequency while attempting to hold the first transistor in. the non-conductive state during a first time period; and

switching the first transistor and second transistor based on the oscillator signal having a second initial frequency during a second time period, wherein the oscillator signal has a first rate of frequency change during the first time period and a second rate of frequency change during the second time period.

2. The method of claim 1 , further including maintaining the first transistor and second. transistor in a non-conductive state during a third time period prior to the first time period.

3. The method of claim 1 , wherein the second rate of frequency change of the oscillator signal is less than the first rate of frequency change of the oscillator signal.

4. The method of claim 1 , further including:

providing a high side driver configured to control the first transistor;

providing a low side driver configured to control the second transistor; and

providing a first operating potential to the low side driver during the first period of time, wherein switching the second transistor during the first time period generates a second operating potential for the high side driver prior to the second time period.

5. The method of claim 4 , further including switching the second transistor based. on the oscillator signal having the first rate of frequency change while attempting to hold the first transistor in the non-conductive state during a fourth time period after the first time period to provide more time for the second operating potential to reach a threshold.

6. The method of claim 1 , wherein the first initial frequency during the first time period is 25 kHz.

7. A semiconductor device including a power converter with a soft start operation, comprising:

a power converter including a first transistor and second transistor coupled in a conduction path with the first transistor;

means for switching the second transistor based on an oscillator al having a first initial frequency while attempting to hold the first transistor in the non-conductive state during a first time period; and

means for switching the first transistor and second transistor based on the oscillator signal having a second initial frequency during a second time period, wherein the oscillator signal has a first rate of frequency change during the first time period and a second rate of frequency change during the second time period.

8. The semiconductor device of claim 7 , further including means for maintaining the first transistor and second transistor in a non-conductive state during a third time period prior to the first time period.

9. The semiconductor device of claim 7 , wherein the second rate of frequency change of the oscillator signal is less than the first rate of frequency change of the oscillator signal.

10. The semiconductor device of claim 7 , further including:

a high side driver configured to control the first transistor;

a low side driver configured to control the second transistor; and

means for providing a first operating potential to the low side driver during the first period of time, wherein switching the second transistor during the first time period generates a second operating potential for the high side driver prior to the second time period.

11. The semiconductor device of claim 10 , further including means for switching the second transistor based on the oscillator signal having the first rate of frequency change while attempting to hold the first transistor in the non-conductive state during a fourth time period after the first time period to provide more time for the second operating potential to reach a threshold.

12. The semiconductor device of claim 7 , wherein the first initial frequency during the first time period is 25 kHz.

13. A method of controlling soft start in a power converter including a first transistor and second transistor coupled in a conduction path with the first transistor, comprising:

switching the second transistor in response to an oscillator signal while holding the first transistor in the non-conductive state during a first time period;

changing a frequency of operation of the oscillator signal; and

switching the first transistor and second transistor in response to the oscillator signal during a second time period, wherein the oscillator signal has a first rate of frequency change during the first time period and a second rate of frequency change during the second time period.

14. The method of claim 13 , wherein the second rate of frequency change of the oscillator signal is less than the first rate of frequency change of the oscillator signal.

15. The method of claim 13 , further including:

providing a high side driver configured to control the first transistor;

providing a low side driver configured to control the second transistor; and

providing a first operating potential to the low side driver dl ring the first period of time, wherein switching the second transistor during the first time period generates a second operating potential for the high side driver prior to the second time period.