LTspice is a powerful simulation language that is specifically optimized for modeling Switch Mode Power conversion. It is not limited to small numbers of nodes and freely available . We are presenting several examples of simulations for popular electronic power factor correction circuits that improve the input power factor of AC Power Supplies by active wave-shaping of the AC input current and the associated avoidance of harmonics. The simulations are performed using a cycle-by cycle switching approach as well as using a time averaged PWM model. A fast inner current loop is controlling the shape of the input current such that it matches the sinusoidal shape of the AC input voltage. A time averaged PWM model is useful for fast simulations covering many cycles to design and study secondary feedback control that regulates the output voltage of the converter.