Resolution requirements to avoid limit cycling in LLC resonant converter

This paper introduces a general method for finding minimum resolution of digital pulse frequency modulators (DPFM) of LLC converters to avoid limit cycling oscillation (LCO) in the output voltage under worst operating conditions. This work is built on the prior art [1], [2] where detailed analysis of limit cycling has been investigated for different hard switching converters and for constant load current controlled soft switching converters. The analysis introduced here takes into account that both the switching frequency and the gain characteristic of the converter change with operating conditions and that the resolution of the DPFM can be selected from a constrained set of the combinations of those two values. Experimental results obtained from a 350 W, 400 V to 12 V LLC converter prove the effectiveness of the introduced analysis by varying DPFM resolution and comparison of LCO with the theoretically predicted results for different loads.