Modeling of In-Circuit Impedance Measurement for SMPS Considering Power Noise Source

Obtaining in-circuit impedance of the switched-mode power supply (SMPS) is very important to accurately evaluate the stability of the control system and design an electromagnetic interference (EMI) filter with high insertion losses to suppress EMI. For measuring the in-circuit impedance of SMPS, the coupling approach by capacitance and inductance using high-frequency small signals to stimulate currently is the most convenient and effective way. However, the existing methods are unable to reduce the measurement error caused by the power noise of SMPS on the in-circuit impedance without knowing the SMPS power noise or without extra components. To cope with it, an in-circuit impedance measurement model that considers the power noise source generated by SMPS is proposed in this article. Moreover, an accurate measurement technique, which can reduce the impedance measurement error caused by stationary power noise source of SMPS, is proposed in this article. Finally, experimental results show that the proposed model can accurately describe the effect of the power noise source on measurement impedance and also show that the proposed technique can reduce the measurement error at the power noise source frequency band.