Circuits and techniques for improving power converter efficiencies and controllability are disclosed. For example, a first converter stage includes a push-pull circuit topology and an isolating element including a magnetizing inductance for isolating a primary side of the first converter stage from a secondary side of the first converter stage. The magnetizing inductance may vary as a function of a current flowing in the primary side of the first converter stage. For example, the isolating element can be a transformer with a stepped gap core and have a varying magnetizing inductance profile.