The present invention is directed to an improved flyback converter in which energy is re-circulated through the converter's flyback transformer windings in a controlled manner to eliminate the need for pre-load or bleeder resistors on the output(s) and to provide enhanced cross regulation on lightly loaded multiple output windings.
Conventional dc-to-dc power flyback converters designed for discontinuous mode operation subject the input power bus to a demand for current that ramps up from zero to a controlled peak level into the primary of a coupled inductor and is then interrupted. The input current then remains zero for an interval typically greater than one half of the total power period. At the instant the input current is interrupted, the stored energy in the coupled inductor forces current to immediately start flowing in the secondary or output winding. The wave shapes of the input current to the primary and the output current from the output winding are the same as that of a right triangle in each case. The current wave shapes thus produced are therefore the least desirable from the standpoint of stress management. In addition, during that period that follows the reset of the core as the current in the secondary falls to zero, and before the start of the next power cycle, the coupled inductor is performing no function.
Conventionally designed dc-to-dc power converters are typically provided with bleeder resistors that provide a minimum guaranteed load to prevent the output voltage from increasing at light load and to reduce the likelihood of a temporary spike on the output in the event of normal load interruption. The control loop bandwidth is limited and if a converter were operating at 50-150 kHz, a number of power cycles might be produced after the interruption of the load before the duty cycle on-time could be reduced, resulting in a jump in voltage on an unloaded output. Although necessary, a bleeder contributes to loss of efficiency. Flyback dc-to-dc power converters having multiple output windings typically have bleeder resistors on all outputs for the same reason as above and also because energy stored in the leakage inductance of the primary is coupled to the secondary windings and voltage on an isolated unloaded secondary output will continue to rise with the interruption of a normal load if there is no bleeder. Bleeders are also used on multiple outputs to enhance cross regulation.