Abstract: A d-c to d-c forward converter power supply has a magnetic modulator in the timing circuit which operates between its residual magnetic flux density and its saturation flux density to deliver a fixed number of volt seconds during sequential clock interval periods. The clock period is fixed so that the output voltage averaged over each clock period will be constant and independent of input voltage over a given design range. A bias current is applied to the magnetic modulating transformer to controllably modify volt second capability of the device. In one embodiment, two transformers are employed, one of which is non-saturating and the other of which is a control transformer and saturates during its operation. In a second embodiment of the invention, only a single transformer, which is a saturating transformer, is employed.