immediately before Topen. Np, number of primary turns Ns, number of secondary turns N, the turns ratio defined as Ns/Np, . For an ideal transformer operating under ideal conditions, Is = Ip/N, Vs = N×Vp. Lp, primary (self-)inductance, a value determined by the number of primary turns Np squared, and an "inductance factor" AL. Self-inductance is often written as Lp = AL×Np2×10−9 henries. R, combined switch and primary resistance Up, energy stored in the flux of the magnetic field in the windings, as represented by the magnetizing current Im. A more-detailed analysis would require the following: M = mutual inductance, its value determined by degree to which the magnetic field created by the primary couples to (is shared by) the secondary, and vice versa. coupling. Coupling is never perfect; there is always so-called primary and secondary "leakage flux". Usually calculated from short-circuit secondary and short-circuited primary measurements. Lp,leak = self-inductance that represents the magnetic field created by, and coupled to the primary windings only Ls,leak = self-inductance that represents the magnetic field created by, and coupled to the secondary windings only Cwindings = interwinding capacitance. Values exist for the primary turns only, the secondary turns only, and the primary-to-secondary windings. Usually combined into a single value. Operation during Tclosed (time when the switch is closed) When the switch (transistor, vacuum tube) closes it places the source voltage Vb across the transformer primary. The magnetizing current Im of the transformer is Im = Vprimary×t/Lp; here t (time) is a variable that starts at 0. This magnetizing current Im will "ride upon" any reflected secondary current Is that flows into a secondary load (e.g. into the control terminal of the switch; reflected secondary current in primary = Is/N). The changing primary current causes a changing magnetic field ("flux") through the transformer's windings; this changing field induces a (relatively) steady secondary voltage Vs = N×Vb. In some designs (as shown in the diagrams) the secondary voltage Vs adds to the source voltage Vb; in this case because the voltage across the primary (during the time the switch is closed) is approximately Vb, Vs = (N+1)×Vb. Alternately the switch may get some of its control voltage or current directly from Vb and the rest from the induced Vs. Thus the switch-control voltage or current is "in phase" meaning that it keeps the switch closed, and it (via the switch) maintains the source voltage across