Model Orders and Numerical Issues in LLC Resonant DC-DC Converters

Ryosuke Higashiya and Noriyuki Hori


Power and Energy Systems, LLC Resonant Converter, Condition Number


The condition number of a Sylvester matrix composed of the denominator and numerator polynomial of a model is investigated numerically for an LLC resonant dc-dc converter. The order of the model is changed from six to one in a certain manner and they are discretized using the step-invariant and matched-pole-zero methods expressed in the delta and shift operators. Their condition numbers show that in both operator forms, the higher the order is, the larger the condition number becomes, implying poorer numerical conditioning. It was also found that the step-invariant model is more ill-conditioned than the matched-pole-zero models in general. While the number becomes constant as the sampling frequency increases for the delta cases, it keeps increasing for the shift cases, which is well known. However, in the range of sampling frequencies expected in recent practices, the models expressed in shift-form have smaller condition numbers and are better-conditioned than those in delta-form of respective orders.

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