Power converter with output inductance
In a power converter, the duty cycle of a primary winding circuit causes near continuous flow of power through the primary and secondary winding circuits during normal operation. By providing no regulation during normal operation, a very efficient circuit is obtained with a synchronous rectifier in the secondary operating at all times. However, during certain conditions such as start up or a short-circuit, the duty cycle of the primary may be reduced to cause freewheeling periods. To simplify the gate drive, the synchronous rectifiers may be allowed to turn off during the freewheeling periods, resulting in large ripple. A filter inductance of the secondary winding circuit reduces that ripple, and is sufficient to minimize ripple during normal operation, but still allows large ripple during the freewheeling periods. By accepting large ripple during other than normal operation, a smaller filter inductance can be used.
1. A power converter comprising:
a primary winding circuit;
a secondary winding circuit coupled to the primary winding circuit, a secondary winding circuit comprising a secondary transformer winding in series with a controlled rectifier having a parallel uncontrolled rectifier; and
a control circuit which controls duty cycle of the primary winding circuit, the duty cycle causing near continuous flow of power through the primary and secondary winding circuits during normal operation but being reduced to cause freewheeling periods in other than normal operation, the duty cycle being reduced as a function of sensed current, the control circuit comparing the duty cycle to a threshold and shutting down operation when the duty cycle drops below the threshold.
2. A power converter as claimed in claim 1 wherein the controlled rectifier is caused to be off during a portion of each cycle when the duty cycle is reduced.
3. A power converter as claimed in claim 1 wherein the duty cycle is reduced to limit current.
4. A power converter as claimed in claim 1 wherein the secondary winding circuit comprises plural secondary transformer windings.
5. A power converter as claimed in claim 1 wherein the primary winding circuit includes a single primary winding and the secondary winding circuit includes two secondary windings coupled to the single primary winding.
6. A power converter as claimed in claim 5 wherein the primary winding is in a full bridge circuit.
7. A power converter as claimed in claim 6 , further comprising a capacitor in series with the primary winding.
8. A power converter as claimed in claim 1 wherein the converter provides no regulation during normal operation.
9. A method of converting power comprising:
providing power to primary and secondary transformer windings, there being a controlled rectifier having a parallel uncontrolled rectifier in series with the secondary transformer winding;
controlling duty cycle of the power to the primary winding, the duty cycle causing near continuous flow of power through the primary and secondary winding circuits during normal operation but being reduced to cause freewheeling periods in other than normal operation, the duty cycle being reduced as a function of sensed current; and
comparing the duty cycle to a threshold and shutting down operation when the duty cycle drops below the threshold.
10. A method as claimed in claim 9 wherein the controlled rectifiers are caused to be off during a portion of each cycle when the duty cycle is reduced.
11. A method as claimed in claim 9 wherein the duty cycle is reduced to limit current.
12. A method as claimed in claim 9 wherein the power is applied to plural secondary windings.
13. A method as claimed in claim 9 wherein power is applied through a single primary winding and two secondary windings coupled to the single primary winding.
14. A method as claimed in claim 13 wherein the primary winding is in a full-bridge circuit.
15. A method as claimed in claim 14 wherein a capacitor is in series with the primary winding.
16. A method as claimed in claim 9 wherein the converter provides no regulation during normal operation.
17. A power converter comprising:
primary and secondary transformer windings, there being a controlled rectifier having a parallel uncontrolled rectifier in series with the secondary transformer winding;
means for controlling duty cycle of the power to the primary winding, the duty cycle causing near continuous flow at power through the primary and secondary winding circuits during normal operation but being reduced to cause freewheeling periods in other than normal operation, the duty cycle being reduced as a function of sensed current; and
means for comparing the duty cycle to a threshold and shutting down operation when the duty cycle drops below the threshold.