the primary. In the case when there is little or no primary resistance and little or no switch resistance, the increase of the magnetizing current Im is a "linear ramp" defined by the formula in the first paragraph. In the case when there is significant primary resistance or switch resistance or both (total resistance R, e.g. primary-coil resistance plus a resistor in the emitter, FET channel resistance), the Lp/R time constant causes the magnetizing current to be a rising curve with continually decreasing slope. In either case the magnetizing current Im will come to dominate the total primary (and switch) current Ip. Without a limiter it would increase forever. However, in the first case (low resistance), the switch will eventually be unable to "support" more current meaning that its effective resistance increases so much that the voltage drop across the switch equals the supply voltage; in this condition the switch is said to be "saturated" (e.g. this is determined by a transistor's gain hfe or "beta"). In the second case (e.g. primary and/or emitter resistance dominant) the (decreasing) slope of the current decreases to a point such that the induced voltage into the secondary is no longer adequate to keep the switch closed. In a third case, the magnetic "core" material saturates, meaning it cannot support further increases in its magnetic field; in this condition induction from primary to secondary fails. In all cases, the rate of rise of the primary magnetizing current (and hence the flux), or the rate-of-rise of the flux directly in the case of saturated core material, drops to zero (or close to zero). In the first two cases, although primary current continues to flow, it approaches a steady value equal to the supply voltage Vb divided by the total resistance R in the primary circuit. In this current-limited condition the transformer's flux will be steady. Only changing flux causes induction of voltage into the secondary, so a steady flux represents a failure of induction. The secondary voltage drops to zero. The switch opens. Operation during Topen (time when the switch is open) Now that the switch has opened at Topen, the magnetizing current in the primary is Ipeak,m = Vp×Tclosed/Lp, and the energy Up is stored in this "magnetizing" field as created by Ipeak,m (energy Um = 1/2×Lp×Ipeak,m2). But now there is no primary voltage (Vb) to sustain further increases in the magnetic field, or