Spice Simulation of Ouasi-resonant Zero-current -switching Dc-dc Convertors

A simple, topology-independent method is proposed for simulating the responses of open and closed-loop quasiresonant DC-DC convertors operating in the zero-currentswitching mode (ZCS-QRC). The method hinges on the substitution of the resonant network and switch assembly, fundamental in the realisation of such systems, by an equivalent circuit which represents its average behaviour. This permits simulation by a general-purpose electronic circuit simulator such as SPICE. The proposed approach is demonstrated by presenting the simulation results of a buck-boost convertor. ant capacitor is AC grounded which is the most common situation. Applying eqns. 1-3, an average, topologyindependent behaviour of the RSI ofZCS-QRC (Fig. I) can be represented by the average model of Fig. 2. As in the case of the PWM model,1 the dependent voltage and current sources (la, Ib, ~) are nonlinear. Here we have an added computational complexity resulting from the fact that the relationships involve trigonometric functions which cannot be emulated by linear networks. Examination of the nonlinear functions F (x. n) for fulland half-wave operation, reveals that they are smooth and monotonic and can therefore conceivably be approximated by high-order polynomials. The advantage of such an approach is that polynomial dependent sources are compatible with most modern versions of SPICE.1.s Applying the least-square-fitting algorithm using the symbolic software Introduction: Recently, a topology independent model for SPICE simulation of a voltage feedback, continuous mode PWM DC-DC convertor was introduced.1 We propose an extension of this model to include quasi-resonant convertors operating in the zero-current-switching mode (ZCS-QRC) which have been gaining popularity. As has been shown by others,2-4 the analytical expressions for the voltage and current waveforms, involve trigonometric functions, and the so-called 'DC' transfer functions (steady-state DC ratio) are topology-dependent. Consequently, the analytical expressions for the frequency domain small-signal transfer functions are complex. This computational complexity is partly to blame for the scarcity of published studies on the transient response and on procedures for the design of the control loop for ZCSQRC.