Phase Margin Degradation of a Peak Current Controlled Converter at Reduced Duty Cycle

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 problem is related to the need of filtering of the sensed current and was discovered by the authors in the implementation of a 500 W 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 can cause a degradation of the phase margin, especially, if the needed duty cycle is 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 and its delay. The problem becomes worse if we reduce the duty cycle and increase the load current. Therefore, the response to the on time slope of the current waveform is negligible, and we have only the response to a steep rising edge. We will also demonstrate that phase margin loss disappears if current is sensed at the inductor instead of the switch. In both cases, the RC filter results in a different frequency response than the one predicted with the classical peak current mode theory. These differences are analyzed in depth in this paper.