Stability of Power Converters with EMI Filter using the Extra Element Theorem and the Impedance-Based Stability Criterion

The connection between a power converter and its input electromagnetic interference (EMI) filter can lead to system instability. This is because of feedback interactions that can occur within the power converter, causing the overall system to be unstable. In this thesis the problem of such interconnection is addressed by using the Extra-Element-Theorem (EET), that enables to study how the stability of the system is modified when an extra-element (such as the input filter or an additional power converter) is added in the system. Another way of studying the same problem is by using the Impedance-Based Stability Criterion, which addresses the stability by looking at the interaction between the input impedance of the converter and the output impedance of the input filter. Starting from a non-stable system this dissertation explores some stabilizing techniques, such as the physical damping, that requires the addition of a capacitor and a resistor in the system. Such method enables to modify the frequency behaviour of the system, thus making it stable for the EET and the Impedance-Based Stability Criterion. In the other hand, virtual damping is investigated, allowing to emulate the abovementioned physical damping components. The latter approach effectively makes the system stable, but it decreases the bandwidth of the control and increases the output impedance of the converter, thus losing controllability of the output voltage. Finally, the theoretical analyses are validated with simulations (in the Matlab-Simulink environment) and with experimental prototypes for two power converter topologies, the Dual Active Bridge (DAB) and the LLC Resonant Converter.