Stabilitiy analysis of constant-frequency current-mode controlled switching regulators operating above 50% duty ratio

Extension of the analysis of the constant-frequency current-mode controlled switching voltage regulators to operation above 50% duty ratio is presented. As it is known, stable behavior above 50% can be achieved at open voltage regulating loop by employing an external ramp for compensation but the possibility of instabilities at closed voltage regulating loop still exists. calculation of the maximum usable ac loop gain is accomplished by taking into account the amplified and fed-back output ripple voltage. Both the continuous and hysteretic instabilities are discussed and diagrams to facilitate the design of the optimum error processor are given. Experiments carried out on the buck and boost converters support the derived results.