Pulse transformers, being typically unipolar (D.C.) applications, require the primary switch to be opened ( thereby removing the voltage source ) before saturation occurs, whereas A.C. applications reversed the applied voltage before saturation occurs. Unipolar applications require that sufficient time be allowed to pass to re-set the core before starting the next pulse. This time permits the magnetic field to collapse ( reset ). The field does not completely collapse to zero value ( unless forced to zero, or lower ) because of core material remanence. A slight air gap may be used to bring remanence closer to zero value. The gap lowers the pulse transformer inductance. The flux range between remanence and the maximum flux is referred to as dB, the maximum change in flux density during the pulse duration, dt. The dB of the typical pulse transformer is less than half for that of an A.C. application because flux in A.C. applications can go from positive Bmax to negative Bmax. Operating frequency and maximum expected temperature affect the choice of maximum usable flux density value, Bmax. Saturation can be avoided by applying the following equation; dB x Np x Ac x Sf = V x dt x 100000000, where dt is the maximum time duration of the pulse, Ac is the cores cross-sectional area and Sf is the core stacking factor ratio. Units are gausses, turns, square centimeters, volts and seconds. Be aware that dt does not include reset time, tr. Maximum operating frequency equals 1 / ( dt + tr ). The voltage-time product, V x dt is quite useful. The size and cost of a pulse transformer is roughly proportional to this product.