초록 ▼

A transformer-coupled buck-boost DC-DC power converter is disclosed. An active clamp circuit is provided to form a resonant circuit with the transformer to control the slew rate of the secondary current and allow the secondary switch to be turned ON at conditions of zero voltage and relatively low c

A transformer-coupled buck-boost DC-DC power converter is disclosed. An active clamp circuit is provided to form a resonant circuit with the transformer to control the slew rate of the secondary current and allow the secondary switch to be turned ON at conditions of zero voltage and relatively low current. The characteristic resonant period of the active clamp transformer circuit may be less than the minimum converter operating period. A winding of the transformer is shunted during a clamp phase to retain energy in the transformer. ZVS phases are provided to reduce switching losses when switches in the converter are turned ON. An energy-storage phase may be varied to control the amount of energy stored per operating cycle. An input-storage phase may transfer energy to the clamp capacitor during a series of converter operating cycles and transfer said energy to the secondary during different converter operating cycles.

대표청구항 ▼

What is claimed is: 1. Apparatus for converting power from an input source at an input voltage for delivery to a load at an output voltage, the apparatus comprising, a transformer having a primary winding and a secondary winding, secondary circuit elements connected to the secondary winding adapted

What is claimed is: 1. Apparatus for converting power from an input source at an input voltage for delivery to a load at an output voltage, the apparatus comprising, a transformer having a primary winding and a secondary winding, secondary circuit elements connected to the secondary winding adapted to deliver power to the load at the output voltage, active clamp circuitry connected to the primary winding and including a clamp switch and a clamp capacitor, a plurality of primary switches connected to the primary winding, a switch controller adapted to operate the clamp switch and the primary switches in a series of converter operating cycles, each converter operating cycle comprising the following phases: (a) an energy-storage phase during which the primary winding is connected to the input source, the energy-storage phase being characterized by a transfer of energy from the input source to the transformer, wherein an average value of primary current flowing in the primary winding has a polarity and the average is taken over the duration of the energy-storage phase, (b) an energy-transfer phase characterized by a transfer of energy from the transformer to the load, and (c) a clamp phase during which the primary winding of the transformer is clamped, the clamp phase being characterized by essentially zero voltage across said primary winding and an average value of current flowing in the primary winding, the average value of current having a polarity, wherein the average is taken over the duration of the clamp phase and the polarity is opposite of the polarity of the average value of primary current during the energy-storage phase immediately preceding the clamp phase. 2. The apparatus of claim 1 wherein one or more of the converter operating cycles further comprises: (d) an input-storage phase during which the primary winding is connected to the input source and to the clamp capacitor, the input-storage phase being characterized by a transfer of energy between the input source, the transformer, and the clamp capacitor. 3. The apparatus of claim 1 wherein the energy-transfer phase further comprises a transfer of energy from the clamp capacitor to the secondary circuit elements. 4. The apparatus of claim 2 wherein, each converter operating cycle has a period Toc, the energy-transfer phase is further characterized by a resonant circuit formed between a primary-referenced leakage inductance and the clamp capacitor, the resonant circuit has a resonant period TR, and the resonant period, TR, is greater than the operating period, Toc. 5. The apparatus of claim 1 wherein, each converter operating cycle has a period Toc which is greater than or equal to a minimum period Toc-min, the energy-transfer phase is further characterized by a resonant circuit formed between a primary-referenced leakage inductance and the clamp capacitor, the resonant circuit has a resonant period TR, and the resonant period, TR, is less than the minimum period, Toc-min. 6. The apparatus of claim 1 wherein the primary switches comprise a first switch connected between a first terminal of the input source and a first end of the primary winding, a second switch connected between a second terminal of the input source and the first end of the primary winding, and a third switch connected between a second end of the primary winding and the second terminal of the input source. 7. The apparatus of claim 6 wherein the first switch and the third switch are ON during the energy-storage phase. 8. The apparatus of claim 2 wherein the primary switches comprise a first switch connected between a first terminal of the input source and a first end of the primary winding, a second switch connected between a second terminal of the input source and the first end of the primary winding, and a third switch connected between a second end of the primary winding and the second terminal of the input source; wherein the clamp circuitry is connected between the second end of the primary winding and the second terminal of the input source; and wherein the first switch and the clamp switch are ON during the input-storage phase. 9. The apparatus of claim 6 wherein the clamp circuitry is connected between the second end of the primary winding and the second terminal of the input source and the clamp switch and the second switch are ON during the energy-transfer phase. 10. The converter of claim 6 wherein the third switch and the second switch are ON during the clamp phase. 11. The apparatus of claim 1 wherein the secondary circuit elements comprise a secondary switch and wherein the switch controller is further adapted to operate the secondary switch to conduct a secondary current between the secondary winding and the load during the energy-transfer phase. 12. The apparatus of claim 1 wherein the energy-transfer phase begins after the end of the energy-storage phase. 13. The apparatus of claim 2 wherein the energy-transfer phase begins after the end of the input-storage phase. 14. The apparatus of claim 1 wherein the clamp phase begins after the end of the energy-transfer phase. 15. The apparatus of claim 1 wherein each converter operating cycle further comprises a zero-voltage switching phase at the end of the energy-storage phase. 16. The apparatus of claim 2 wherein each converter operating cycle further comprises a zero-voltage switching phase at the end of the input-storage phase. 17. The apparatus of claim 1 wherein each converter operating cycle further comprises a zero-voltage switching phase at the end of the energy-transfer phase. 18. The apparatus of claim 1 wherein each converter operating cycle further comprises a zero-voltage switching phase at the end of the clamp phase. 19. The apparatus of claim 1 wherein the control circuitry senses the output voltage and varies a selected control variable as a means of maintaining the output voltage at a pre-determined value, and the selected control variable(s) is(are) one of the following options: (a) the duration of the energy-storage phase, or (b) the duration of the clamp phase, or (c) the durations of each of the energy-storage phase and of the clamp phase. 20. The apparatus of claim 19 wherein each converter operating cycle comprises a converter operating period, Toc, the converter operating period is essentially constant, and the selected control variable is (a) the duration of the energy-storage phase. 21. The apparatus of claim 19 wherein the duration of the clamp phase is essentially constant and the selected control variable is (a) the duration of the energy-storage phase. 22. The apparatus of claim 19 wherein the selected control variables are (c) the duration of the energy-storage phase and the duration of the clamp phase and the switch controller varies the duration of the energy-storage phase in inverse relation to the magnitude of the input voltage and varies the duration of the clamp phase to control the output voltage. 23. The apparatus of claim 19 wherein the selected control variables are (c) the duration of the energy-storage phase and the duration of the clamp phase and the switch controller controls the output voltage by: (a) varying the duration of the energy-storage phase over a pre-determined range, the range having an upper limit and a lower limit, (b) holding the duration of the clamp phase essentially constant and less than the duration of the energy-storage phase while the duration of the energy-storage phase is less than the upper limit and greater than the lower limit, and (c) varying the duration of the clamp phase while the duration of the energy-storage phase is greater than or equal to the upper limit or less than or equal to the lower limit. 24. The apparatus of claim 21 wherein the duration of the clamp phase varies from a small portion of the converter operating period at high loads to a large portion of the converter operating period at light loads. 25. The apparatus of claim 2 wherein the switch controller is further adapted to provide power factor correction. 26. The apparatus of claim 1 wherein one or more of the converter operating cycles comprise the following phases in sequence: (a) an energy-storage phase; (b) a first zero-voltage switching ("ZVS") phase that begins at the end of the energy-storage phase; (c) an energy-transfer phase that begins at the end of the first ZVS phase; (d) a second ZVS phase that begins at the end of the energy-transfer phase; (e) a clamp phase that begins at the end of the second ZVS phase; (f) a third ZVS phase that begins at the end of the clamp phase. 27. The apparatus of claim 2 wherein one or more of the converter operating cycles comprise the following phases in sequence: (a) an energy-storage phase; (b) a first zero-voltage switching ("ZVS") phase that begins at the end of the energy-storage phase; (c) an input-storage phase that begins at the end of the first ZVS phase; (d) a second ZVS phase that begins at the end of the input-storage phase; (e) an energy-transfer phase that begins at the end of the second ZVS phase; (f) a third ZVS phase that begins at the end of the energy-transfer phase; (g) a clamp phase that begins at the end of the third ZVS phase; (h) a fourth ZVS phase that begins at the end of the clamp phase. 28. The apparatus of claim 2 wherein the control circuitry senses the output voltage and varies one or more selected control variables as a means of maintaining the output voltage at a pre-determined value, wherein the one or more selected control variables comprise one or more of the following: (a) the duration of the energy-storage phase, (b) the duration of the input-storage phase, (c) the duration of the energy-transfer phase, (d) the duration of the clamp phase. 29. The apparatus of claim 6 wherein the clamp circuitry is connected between the second end of the primary winding and the second terminal of the input source; and further comprising: a storage capacitor; a sixth switch adapted to connect the storage capacitor in parallel with the clamp capacitor when the sixth switch is ON and to disconnect the storage capacitor from the clamp capacitor when the sixth switch is OFF. 30. The apparatus of claim 8 further comprising: a storage capacitor; a sixth switch adapted to connect the storage capacitor in parallel with the clamp capacitor when the sixth switch is ON and to disconnect the storage capacitor from the clamp capacitor when the sixth switch is OFF. 31. The apparatus of claim 29 wherein, each converter operating cycle has a period Toc which is greater than or equal to a minimum period Toc-min, the energy-transfer phase is further characterized by a resonant circuit formed between a primary-referenced leakage inductance and the clamp capacitor, the resonant circuit has a resonant period TR, and the resonant period, TR, is less than the minimum period, Toc-min when the sixth switch is OFF. 32. The apparatus of claim 30 wherein, each converter operating cycle has a period Toc which is greater than or equal to a minimum period Toc-min, the energy-transfer phase is further characterized by a resonant circuit formed between a primary-referenced leakage inductance and the clamp capacitor, the resonant circuit has a resonant period TR, and the resonant period, TR, is less than the minimum period, Toc-min when the sixth switch is OFF. 33. The apparatus of claim 29 further comprising a seventh switch adapted to conduct current from the storage capacitor to the converter input when ON and block current between the converter input and the storage capacitor when OFF. 34. The apparatus of claim 30 further comprising a seventh switch adapted to conduct current from the storage capacitor to the converter input when ON and block current between the converter input and the storage capacitor when OFF. 35. The apparatus of claim 1 wherein the output voltage is galvanically isolated from the input source. 36. Apparatus for converting power from an input source at an input voltage for delivery to a load at an output voltage, the apparatus comprising, a transformer having a primary winding, a secondary winding, and a leakage inductance, secondary circuit elements connected to the secondary winding adapted to deliver power to the load at the output voltage, active clamp circuitry connected to the primary winding including a clamp switch and a clamp capacitor, a plurality of primary switches connected to the primary winding, a switch controller adapted to operate the clamp switch and the primary switches in a series of converter operating cycles, each converter operating cycle having a period Toc which is greater than or equal to a minimum period Toc-min and comprising the following phases: (a) an energy-storage phase during which the primary winding is connected to the input source, the energy-storage phase being characterized by a transfer of energy from the input source to the transformer, (b) an energy-transfer phase characterized by a transfer of energy from the transformer to the load and by a transfer of energy between the transformer and the clamp capacitor, wherein the transfer of energy between the transformer and the clamp capacitor comprises a resonant period TR, and Toc-min is greater than TR. 37. A method comprising: converting power, received at an input from an input source at an input voltage, via a transformer for delivery to a load at an output voltage, in a series of converter operating cycles; wherein each converter operating cycle includes the following: (a) transferring energy from the input source to a primary winding of the transformer during an energy-storage phase characterized by an average value of primary current flowing in the primary winding, the average value of primary current having a polarity, and wherein the average is taken over the duration of the energy-storage phase, (b) transferring energy from a secondary winding of the transformer to the load during an energy-transfer phase characterized by connecting a clamp capacitor to the transformer, and (c) clamping the primary winding of the transformer during a clamp phase characterized by essentially zero voltage across the primary winding and an average value of current flowing in the primary winding, the average value of current having a polarity, wherein the average is taken over the duration of the clamp phase and the polarity is opposite of the polarity of the average value of primary current during the energy-storage phase immediately preceding the clamp phase. 38. The method of claim 37 further comprising setting the capacitance of the clamp capacitor to control the slew rate of a current in the secondary winding to enable a switch connected to the secondary winding to be turned ON under conditions of essentially zero voltage and at a current less than a peak value of the current in the secondary winding. 39. The method of claim 37 wherein the clamp capacitor is connected to the primary winding during the energy-transfer phase. 40. The method of claim 37 further comprising charging and discharging parasitic capacitances during a first zero-voltage switching ("ZVS") phase following the end of the energy-storage phase. 41. The method of claim 37 further comprising charging and discharging parasitic capacitances during a second zero-voltage switching ("ZVS") phase following the end of the energy-transfer phase. 42. The method of claim 38 wherein the transferring energy during the energy-transfer phase further comprises forming a resonant circuit between a primary-referenced leakage inductance of value LL and the clamp capacitor of value CC, the resonant circuit having a characteristic time constant, TR=pi*sqrt(LL*CC), and the converter operating cycle comprises a minimum operating period, Toc-min, that is greater than the characteristic time constant TR. 43. The method of claim 37 wherein one or more of the converter operating cycles further comprises: (d) transferring energy between the input source, the transformer, and the clamp capacitor during an input-storage phase. 44. The method of claim 43 wherein one or more of the converter operating cycles further comprises: (b2) transferring energy from the clamp capacitor to the secondary circuit elements during a portion of the energy-transfer phase. 45. The method of claim 43 wherein the transferring energy during the energy-transfer phase further comprises forming a resonant circuit between a primary-referenced leakage inductance of value LL and the clamp capacitor of value CC, the resonant circuit having a characteristic time constant, TR=pi*sqrt(LL*CC), and the converter operating cycle comprises an minimum operating period, Toc-min, wherein TR is greater than Toc-min. 46. The method of claim 37 further comprising: connecting a storage capacitor in parallel with the clamp capacitor to provide a first value of effective clamp capacitance, Ceff=Ceff1, when the input voltage exceeds a first predetermined threshold; disconnecting the storage capacitor from the clamp capacitor to provide a second value of effective clamp capacitance, Ceff=Ceff2, when the input voltage is less than a second predetermined threshold; and wherein one or more of the converter operating cycles further comprises: (d) transferring energy between the input source, the transformer, and the effective clamp capacitance during an input-storage phase; wherein one or more of the converter operating cycles further comprises (b2) transferring energy from the effective clamp capacitance to the secondary circuit elements during a portion of the energy-transfer phase; wherein the transferring energy during the energy-transfer phase further comprises forming a resonant circuit between a primary-referenced leakage inductance of value LL and the effective clamp capacitance, Ceff, the resonant circuit having a characteristic time constant, TR=pi*sqrt(LL*Ceff). 47. The method of claim 46 wherein: the converter operating cycle comprises a minimum operating period, Toc-min; Toc-min is greater than TR when the storage capacitor is connected in parallel with the clamp capacitor; and TR is greater than Toc-min when the storage capacitor is disconnected from the clamp capacitor. 48. The method of claim 46 or 47 further comprising: connecting the storage capacitor to the input and restricting current flow in a direction from the storage capacitor to the input source when the input voltage is less than the second predetermined threshold. 49. The apparatus of claim 36 wherein current in the secondary winding is essentially zero throughout the energy-storage phase. 50. The apparatus of claim 36 wherein the secondary circuit elements comprise rectification circuitry connected between the secondary winding and the load, and wherein the rectification circuitry conducts current during the energy-transfer phase and blocks current throughout the energy-storage phase.