Stability Issues Of The DC-DC Converters With Different Pulse Width Modulations And Its Improvement

The pulse width modulation（PWM）has been widely used in dc-dc converters thanks to its simple and easy implementation.According to the form of the triangular carrier used,the implementation of PWM can be leading-edge modulation and trailing-edge modulation.This thesis analyzes the stability of the dc-dc converters with different PWM methods and the stability improvement.In this thesis,the discrete-time models of the buck,boost and buck-boost converters with leading-edge and trailing-edge modulations are derived,where the instant when the modulation signal intersects with the sawtooth wave is selected as the sampling instant.According to the discrete-time model,the general expressions for the duty-cycle-to-output-voltage transfer function,G_vd,in the discrete domain are obtained.Based on the low-pass characteristic of the dc-dc converter and related properties of matrix functions,the expressions of G_vd in the discrete domain are approximated and the approximate expressions of G_vd are in the frequency domain for the three basic dc-dc converters.This model is quite simple and can be accurate within the half of the switching frequency.With the approximate expressions of G_vd in the frequency domain,the stability of the three basic dc-dc converters with different PWM modulations is analyzed,and it is explained physically.The analysis shows that the stability of buck converter has nothing to do with the PWM modulations,and the leading-edge modulated boost and buck-boost converters have better stability than the trailing-edge modulated ones.Since the trailing-edge modulation is more commonly used in practical applications,the scheme of inductor current feedback is proposed into the trailing-edge modulated boost and buck-boost converters,for the purpose of equivalently achieving the same stability of the leading-edge modulated converters.Finally,three prototypes（buck,boost and buck-boost converters）have been fabricated and tested in the lab.The experimental results are provided to verify the accuracy of the proposed model,and that the trailing-edge modulated boost and buck-boost converters with the inductor current feedback are equivalent to the leading-edge modulated converters.