Peak current control instabilities at narrow duty cycles

An inner current loop is frequently used in many switching power supplies to achieve higher stability and a good current sharing. Nevertheless, some problems derived from its practical implementation can be encountered. One of these is related with the need of filtering of the sensed current and was suffered for the authors in the implementation of a 500 W power converter when peak current control was applied by sensing the switch current. This paper will demonstrate mathematically that an RC filter not only filters out the noise but also brings a lot of stability problems especially if the needed duty cycle is very close to zero. The main reason for it is the severe distortion of the current waveform that results in an excessive phase loss. This phase loss is much larger than the expected from the filter itself. The problem becomes worse if we reduce the duty cycle and if we increase the load current. Then the response to the ramp of the current waveform is negligible and we have only response to a rising steep edge. This effect can lead to instabilities. This problem does not appear when current is sensed at the inductor instead of the switch. We will also demonstrate that sensing the switch current is worse than sensing the inductor current from the stability point of view.