Efficient thermal management is critical for high-power electronics, with microchannel cooling providing enhanced heat dissipation in compact designs. Conventional topology optimisation (TO) methods often fail to capture fine-scale features without relying on extremely fine meshes, leading to high computational costs. This study proposes a homogenisation-based TO framework integrated with an on-the-fly phasor-based dehomogenisation technique. The phasor-based method efficiently maps homogenised designs to fine-scale microchannel networks, maintaining continuous variations in channel properties and improving robustness while reducing computational expense compared to the conventional projection-based method. An idealised two-dimensional chip with heterogeneous heat generation is used to demonstrate the integrated design workflow. The homogenisation-based TO is performed on a coarse 50 by 50 mesh, with the phasor-based dehomogenisation reconstructing fine-scale microchannel networks on a 3213 by 3213 mesh. The results show that the framework achieves a three orders-of-magnitude reduction in computational cost compared to hypothetical brute-force full-scale optimisation and a 35.3% improvement in thermal performance over traditional pin-fin designs (the initial guess). Limitations of the current study, such as refining the homogenisation model and incorporating irregular meshes, point to promising directions for future investigations.