The paper is concerned with the stability analysis of a feedback controlled resonant dual channel buck&boost converter. The output voltage is controlled by using constant frequency PWM and PI controller. The analysis is based on the eigenvalues of the Jacobian matrix of the Poincare map function (PMF). After small perturbation determining the deviations of the state variables from the periodic trajectory, the Jacobian matrix is calculated without the need to determine the PMF itself. The introduction of the so-called auxiliary state vector greatly facilitates the calculation. Calculation and simulation results included helps the design of the control loop