Power Factor Correction (PFC) converters belong to the variable structure piecewise linear systems due to the switching action. Their complex behaviour is intensively studied. Present paper represents a stability analysis method using the socalled auxiliary state vector to determine the Jacobian matrix of the Poincare Map Function (PMF) to calculate the critical angle where oscillations developes in the inductor current. The effect caused by the Zero-Order Hold, the digital computation delays and the nonideal circuit elements are taken into consideration. Furthermore by adding a stabilizing signal the stable range can be extended. The parameters of the stabilizing signal is also determined by the auxiliary state vector. The theoretic results are verified by simulation and the experiment results.