Forward power converter controllers

1. A method of controlling a resonant discontinuous forward converter (RDFC), said converter including a transformer with primary and secondary matched polarity windings and a switch to, in operation, cyclically switch DC power to said primary winding of said transformer, said converter further having a DC output coupled to said secondary winding of said converter, said method comprising:

sensing a transformer signal, said transformer signal representing a voltage across a winding of said transformer or a resonant current in a winding of said transformer;

calculating a resonance period of increasing and reducing of a resonant voltage of said primary winding of said RDFC from said sensed transformer signal; and

controlling an off duration of said switch in response to said calculated resonance period such that a sub-harmonic oscillation in said resonant voltage across said primary winding is reduced.

2. A method according to claim 1 , wherein calculating a resonance period of said RDFC comprises:

setting a resonance period value;

measuring an initial resonance period value using one or more previous converter cycles, each of said converter cycles representing a complete on and off period of said switch; and

updating said resonance period value: in response to said measurement.

3. A method according to claim 2 , wherein said updating comprises increasing said resonance period value in response to a resonant period in said one or more previous converter cycles that is greater than said resonant period value, and decreased in response to a resonant period in said one or more previous converter cycles that is less than said resonant period value.

4. A method according to claim 2 wherein, during start-up of said RDFC, said period is calculated using said sensed transformer signal from a first number of converter cycles following startup of said RDFC.

5. A method according to claim 4 , wherein said first number of cycles is less than 50.

6. A method according to claim 5 , wherein said first number of cycles is the first cycle following startup.

7. A method according to claim 1 , wherein said calculated resonance period is offset by an amount from one converter cycle to the next converter cycle.

8. A method according to claim 1 , wherein said off duration of said switch is further controlled in response to a value of said sensed transformer signal, said off duration of said switch being increased in response to an increasing sensed transformer signal.

9. A method of reducing sub-harmonic oscillations in a resonant voltage across a primary winding of a resonant discontinuous forward converter (RDFC), said converter including a transformer with primary and secondary matched polarity windings and a switch to, in operation, cyclically switch DC power to said primary winding of said transformer, said converter further having a DC output coupled to said secondary winding of said converter, said method comprising:

sensing a transformer signal, said transformer signal representing a voltage across a winding of said transformer or a resonant current in a winding of said transformer;

calculating a resonance period of increasing and reducing of a resonant voltage of said primary winding of said RDFC from said sensed transformer signal; and

controlling an off duration of said switch in response to said calculated resonance period such that sub-harmonic oscillations in said resonant voltage across said primary windings are reduced.

10. A method according to claim 9 , wherein calculating a resonance period of said RDFC comprises:

setting a resonance period value;

measuring an said initial resonance period value using one or more previous converter cycles, each of said converter cycles representing a complete on and off period of said switch; and

updating said resonance period value in response to said measurement.

11. A method according to claim 10 , wherein said updating comprises increasing said resonance period value in response to a resonant period in said one or more previous converter cycles that is greater than said resonant period value, and decreased in response to a resonant period in said one or more previous converter cycles that is less than said resonant period value.

12. A method according to claim 10 wherein, during start-up of said RDFC, said period is calculated using said sensed transformer signal from a first number of converter cycles.

13. A method according to claim 12 , wherein said first number of cycles is less than 50.

14. A method according to claim 13 , wherein said first number of cycles is the first cycle following startup.

15. A method according to claim 9 , wherein said calculated resonance period is offset by an amount from one converter cycle to the next converter cycle.

16. A method according to claim 9 , wherein said off duration of said switch is further controlled in response to a value of said sensed transformer signal, said off duration of said switch being increased in response to an increasing sensed transformer signal.

17. A controller for controlling a resonant discontinuous forward converter (RDFC), said converter including a transformer with primary and secondary matched polarity windings and a switch to, in operation, cyclically switch DC power to said primary winding of said transformer, said converter further having a DC output coupled to said secondary winding of said converter, comprising:

an input to sense a transformer signal, said transformer signal representing a voltage across a winding of said transformer or a resonant current in a winding of said transformer;

a system to calculate a resonance period of increasing and reducing of a resonant voltage of said primary winding of said RDFC from said sensed transformer signal; and

an output to control an off duration of said switch in response to said calculated resonance period such that a sub-harmonic oscillation in said resonant voltage across said primary winding is reduced.

18. A controller configured to reduce sub-harmonic oscillations in a resonant voltage across a primary winding of a resonant discontinuous forward converter (RDFC), said converter including a transformer with primary and secondary matched polarity windings and a switch to, in operation, cyclically switch DC power to said primary winding of said transformer, said converter further having a DC output coupled to said secondary winding of said converter, the controller comprising:

means for sensing a transformer signal, said transformer signal representing a resonant voltage across a winding of said transformer or a resonant current in a winding of said transformer;

means for calculating a resonance period of increasing and reducing of a resonant voltage of said primary winding of said RDFC from said sensed transformer signal; and

means for controlling an off duration of said switch in response to said calculated resonance period such that sub-harmonic oscillations in said resonant voltage across said primary windings are reduced.

19. A resonant discontinuous forward converter (RDFC) including a controller according to claim 17 .

20. A resonant discontinuous forward converter (RDFC) including a controller according to claim 18 .