Method and apparatus for controlling power transfer in a flyback converter by modulating the power switch off time during transient conditions

A method of controlling a flyback DC-DC converter includes using a primary control loop to monitor an auxiliary winding of a transformer for determining the amount of energy being transferred to a load. The voltage in the auxiliary winding is induced by current flowing in the secondary winding of the transformer. The primary control loop disables and enables the turning on of a power switch for driving the primary winding of the transformer, and detects the zero-crossing. The duration that the power switch is turned on is established by a secondary control loop using the output voltage for turning off the power switch for a new off phase. The flyback DC-DC converter further includes a fixed frequency oscillator having a frequency lower than the self-oscillating frequency of the converter. The power transferred from the primary circuit to the secondary circuit of the flyback transformer is controlled by introducing a delay on the turn-on instant of the power switch. This is with respect to a turn-on command generated during a self-oscillating functioning phase regardless of the mode of control of the converter. The turn-on command is based upon a zero crossing, a fixed frequency functioning phase, a rising edge of the signal generated by the oscillator, and as a function of the input variables of the primary and secondary control loops.