Printed circuit heat exchangers (PCHEs) are ideal for sCO2 power cycles due to their compactness, effectiveness, and high-pressure capability. However, their unique architecture complicates the modeling of their dynamic behavior in power cycles, which experience rapid transients. Both the significant computational resources required and the high investment costs of experiments limit their widespread application. To model the component- and system-level transients of sCO2-PCHEs, this study presents two 1D modeling approaches for different purposes: one for component-level simulation based on local properties of sCO2, and the other for system-level simulation based on transfer functions. Given the significant discrepancies observed when using a fixed time constant in the latter approach, this paper introduces the concept of an optimal time constant to model the transient behavior of PCHEs as a first-order system with minimal prediction error. This optimal time constant varies with operating conditions, contrary to what the name might suggest. These results demonstrate the potential of PCHEs in advanced power cycles and provide valuable insights for accurate system-level sCO2 power cycle control studies.