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A custom substrate model is proposed to analyze the noise injected by a DC-DC converter. Even if commercial tools extracting substrate parasitic components already exist, they are based on a capacitive and resistive model that can prove inadequate to simulate noise injected by power switching modules, since unusual voltage variations can activate parasitic vertical bipolar transistors. The proposed model includes these components and shows an important impact on victim circuits in the neighborhood of the noise source. Simulations with the proposed model predict a 6 mV noise in the substrate that is a 67-fold increase compared to the 90 muV obtained with a model produced by SubstrateStorm. These simulations with our custom model show that deep n-well and n-well noise isolations can be much less effective than expected. The proposed model is directly applicable to exploring the tradeoff between power efficiency and substrate noise injection in integrated DC-DC converters.