Differential measurement of a transformer magnetizing current and a delta-modulation technique is used to provide compensation for dc saturation of the transformer core, with faster response times, low losses, and with immunity to dc drift in the measuring electronics. During the half-cycle in which the output voltage transitions from its positive maximum to its negative maximum, the positive peak value of the magnetizing current is determined, and during the half-cycle in which the output voltage transitions from its negative maximum to its positive maximum, the negative peak value of the magnetizing current is determined. The positive peak magnetizing current value is compared to the similar value from the previous cycle, and the onset of core saturation in the positive direction is then determined. The negative peak magnetizing current value is compared to the similar value from the previous cycle, and the onset of core saturation in the negative direction is then determined. Based on the results of the core saturation determinations, a control action is determined in accordance with a truth table.