The simplified Quasi-Resonant (QR) controller emulates a free-running system. In these topologies, the line / load conditions dictate the peak current setpoint and therefore the power switch turn-off event. As a result, Ton depends upon the primary inductance (Lp), the input voltage (Vin) and the wanted peak current Ip: . At the end of Ton, the primary inductance is fully magnetized and the energy is released to the secondary side. The core is then reset by a level applied across Lp, the Flyback voltage: Vreset = N x ( Vout + Vf ) where N is Np:Ns turn ratio, Vout the output voltage and Vf the secondary rectifier drop. The off time duration is thus: . These systems thus exhibit a highly variable switching frequency depending on their operating conditions. The simplified QR model runs fast since it does not account for any operating internal parameters typical of a real integrated circuit (quiescent current,Vcc range etc.). There are two models, FreeRunDT which includes an adjustable minimum Toff (the switch cannot be re-activated until min. Toff has elapsed, this brings natural leakage inductance blanking but also a frequency clamp) and FreeRunNDT which does not feature any blanking. The core reset detection is classically made by observing a voltage image of the flux via an auxiliary winding brought to a demagnetization pin. If a delay is introduced, e.g. through a simple RC network, the re-start of the switch can be made right in the minimum of the drain-source wave. This is the so-called valley switching operation.