Analysis of an unstable phenomenon related to the different turn-on time between the subsystems in cascaded systems

In this paper, we present an unstable phenomenon in cascaded systems that the source converter might become unstable after connected with the load converter though it can work stably individually. However, by staggering the switches´ turn-on time between the source converter and load converter, it can become stable. Analysis in this paper shows that the essential reason for this phenomenon is that the model of PWM comparator will change with different turn-on time interval, which might significantly influence the loop gain of the source converter and reduce its stability margin, or even worse, make it unstable. The model of the source converter should be revised first before using the Middlebrook criterion to check the system´s stability margin. A cascaded buck-buck converter is built to verify the correctness of the analysis.