Optimal resetting of the transformer's core in single-ended forward converters

Abstract

The transformer's core in single ended forward converters is reset by a "magnetizing current mirror" consisting of a capacitor in series with an auxiliary switch which, during the OFF period of the primary switch, couples the capacitor to one of the transformer's windings to form a resonant circuit with the transformer's magnetizing inductance. The resonant circuit recycles the transformer's magnetizing energy by creating a mirror image of the magnetic flux between ON periods. This maximizes the flux swing available within a given core. The voltage stress on the primary switch is minimized as the voltage across the switch during the OFF period is approximately constant and automatically tailored to avoid dead time for arbitrary values of the switch duty cycle.

I claim: 
     
       1. In a single ended forward converter in which energy is transferred from a primary winding to a secondary winding of a transformer during the ON period of a primary switch, circuitry for recycling the magnetizing energy stored in said transformer to reset it during the OFF period of said primary switch, comprising: a storage capacitor;   an auxiliary switch connected in series with said storage capacitor;   a switch control circuit operating said auxiliary switch in accordance with a control logic such that (a) said auxiliary switch is opened prior the ON period of said primary switch, (b) said auxiliary switch remains open throughout the ON period of said primary switch, (c) said auxiliary switch is closed after the ON period of said primary switch.   
     
     
       2. The transformer resetting apparatus of claim 1 wherein said circuitry is connected in parallel with said secondary winding. 
     
     
       3. The transformer resetting apparatus of claim 1 wherein said circuitry is connected in parallel with said primary winding. 
     
     
       4. The transformer resetting apparatus of claim 1 wherein said transformer further includes an auxiliary winding, wherein said circuitry is connected in parallel with said auxiliary winding. 
     
     
       5. The transformer resetting apparatus of claim 1 wherein said auxiliary switch is a MOSFET transistor with an integral reverse diode. 
     
     
       6. In a single ended forward converter in which energy is transferred across a transformer during the ON period of a primary switch, an apparatus for recycling the magnetizing energy of said transformer during the OFF period of said primary switch, comprising: a storage capacitor;   auxiliary switching means to selectively couple said storage capacitor to said transformer, wherein said storage capacitor and said transformer form a resonant circuit during the OFF period of said primary switch. .Iadd.   
     
     
       7.  The circuitry of claim 1 wherein said auxiliary switch is always closed to currents in one direction and selectively closed to currents in another direction..Iaddend..Iadd.8. The circuitry of claim 1 wherein said auxiliary switch is adapted to selectively couple said storage capacitor and said transformer to form a resonant circuit during the OFF period of said primary switch..Iaddend..Iadd.9. The circuitry of claim 1 wherein said circuitry is connected to a winding in the transformer..Iaddend..Iadd.10. The circuitry of claim 9 wherein said winding is a primary winding..Iaddend..Iadd.11. The circuitry of claim 9 wherein said winding is a secondary winding..Iaddend..Iadd.12. The circuitry of claim 9 wherein said winding is an auxiliary winding..Iaddend..Iadd.13. The circuitry of claim 9, 10, 11 or 12 wherein said auxiliary switch is adapted to selectively couple said storage capacitor and said transformer to form a resonant circuit during the OFF period of said primary switch..Iaddend..Iadd.14. The circuitry of claim 1 wherein an equivalent circuit of said converter, during the OFF period of said primary switch, comprises a resonant circuit formed by said capacitor and a winding of said transformer..Iaddend..Iadd.15. The circuitry of claim 14 wherein said winding comprises a primary winding..Iaddend..Iadd.16. The circuitry of claim 14 wherein said winding comprises a secondary winding..Iaddend..Iadd.17. The circuitry of claim 14 wherein said winding comprises a auxiliary winding..Iaddend..Iadd.18. The circuitry of claim 2 wherein said auxiliary switch is adapted to selectively couple said storage capacitor and said transformer to form a resonant circuit during the OFF period of said primary 
     
     
        switch..Iaddend..Iadd.19.  The circuitry of claim 3 wherein said auxiliary switch is adapted to selectively couple said storage capacitor and said transformer to form a resonant circuit during the OFF period of said primary switch..Iaddend..Iadd.20. The circuitry of claim 4 wherein said auxiliary switch is adapted to selectively couple said storage capacitor and said transformer to form a resonant circuit during the OFF period of said primary switch..Iaddend..Iadd.21. The circuitry of claim 1, 7, 8, 9, 10, 11, 12, 18, 19 or 20 wherein said auxiliary switch comprises a MOSFET transistor with an integral reverse diode..Iaddend..Iadd.22. The circuitry of claim 1, 7, 8, 9, 10, 11, 12, 18, 19 or 20 wherein the ON period of said primary switch begins a small delay period after opening said auxiliary switch..Iaddend..Iadd.23. The circuitry of claim 22 wherein said small delay period accommodates charging and discharging of capacitances in said converter..Iaddend..Iadd.24. The circuitry of claim 23 wherein said capacitances comprise parasitic capacitances..Iaddend..Iadd.25. The circuitry of claim 23 wherein said capacitances are associated with said auxiliary switch..Iaddend..Iadd.26. The circuitry of claim 23 wherein said capacitances are associated with said primary switch..Iaddend..Iadd.27. The circuitry of claim 23 wherein said capacitances are associated with said primary winding..Iaddend..Iadd.28. The circuitry of claim 23 wherein said parasitic capacitances are associated with said secondary 
     
     
        winding..Iaddend..Iadd.29.  In a single ended forward converter in which energy is transferred across a transformer during the ON period of a primary switch, an apparatus for recycling the magnetizing energy of said transformer during the OFF period of said primary switch, comprising: a storage capacitor;   auxiliary switching means to selectively couple said storage capacitor to said transformer, wherein said storage capacitor and said transformer form a resonant circuit during the OFF period of said primary switch; and   wherein the ON period of said primary switch begins a small delay period after opening said auxiliary switching means..Iaddend..Iadd.30. The apparatus of claim 29 wherein said auxiliary switching means is always closed to currents in one direction and selectively closed to currents in another direction..Iaddend..Iadd.31. The apparatus of claim 29 wherein said storage capacitor is connected to a winding of the transformer..Iaddend..Iadd.32. The apparatus of claim 29 wherein said auxiliary switching means is connected to a winding of the transformer..Iaddend..Iadd.33. The apparatus of claim 29 wherein said storage capacitor and said auxiliary switching means are connected in parallel to a winding of the transformer..Iaddend..Iadd.34. The apparatus of claim 31 wherein said winding comprises a primary winding..Iaddend..Iadd.35. The apparatus of claim 32 wherein said winding comprises a primary winding..Iaddend..Iadd.36. The apparatus of claim 31 wherein said winding comprises a secondary winding..Iaddend..Iadd.37. The apparatus of claim 32 wherein said winding comprises a secondary winding..Iaddend..Iadd.38. The apparatus of claim 31 wherein said winding   
     
     
        comprises an auxiliary winding..Iaddend..Iadd.39.  The apparatus of claim 32 wherein said winding comprises an auxiliary winding..Iaddend..Iadd.40. The apparatus of claim 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, or 39 wherein said small delay period accommodates charging and discharging of capacitances in said converter..Iaddend..Iadd.41. The apparatus of claim 40 wherein said capacitances comprise parasitic capacitances..Iaddend..Iadd.42. The apparatus of claim 41 wherein said capacitances are associated with said primary switch..Iaddend..Iadd.43. The apparatus of claim 41 wherein said capacitances are associated with said auxiliary switching means..Iaddend..Iadd.44. The apparatus of claim 41 wherein said capacitances are associated with windings in the transformer..Iaddend.