The concept of a dynamical profile of a switched-mode converter was introduced in the thesis. It was composed of six g-parameters at open and/or closed loop and two special admittances fully characterizing the dynamical properties of a single switched-mode converter. The previous works presented e.g. in [44], [82], [48] and [43] have set the starting point for the thesis. However, the true internal dynamical profile has not been clearly defined earlier. It was found that the internal or nominal dynamics could be obtained for the voltage-output converters by using a pure voltage
source at the supply side and a constant-current sink as a load. The two-port representation and the supply and load interaction formalisms were reviewed. It was claimed that the stability and performance of a single converter as a part of a larger system could be studied by means of the internal dynamical profile and interaction formalisms. Many existing converter topologies and control principles have different dynamical properties, and hence, dynamical profiles. By studying the dynamical profiles it is easy to evaluate the sensitivities for different interactions and the choosing of a proper topology and control mode becomes simple. It was found out that the open-loop parameters predominantly reflected the interactions. The open-loop output and input impedances were shown to mainly reflect the interactions e.g. onto the loop gain. However, the two special admittances had to also be studied in the supply interaction analysis. It was also discovered that the load or supply impedance could cause certain double interactions that change the internal parameters, and hence, the dynamical properties. The closed-loop parameters could not be effectively used in the interaction analysis, because the information of the dynamical properties was typically hidden. On the other hand, the closed-loop input and output impedances could be used to derive safe load and supply profiles in order to guarantee the stability. This was based on the input-to-output and internal stability formalism that resulted in an impedance ratio of the closed-loop input or output impedance and the input or output impedance of the interconnected subsystem. This impedance ratio, which is also known as the minor-loop gain, is sometimes the only way to check the stability of the converter (i.e. the loop gain can be unchanged even if the converter is unstable). As examples, the dynamical profiles of the VMC-CCM, VMC-DCM, PCMC and PCMC-OCF buck converters were analyzed analytically and the experimental evidence was provided by obtaining the profiles for the VMC-CCM, VMC-DCM and PCMC buck converters. Even if the thesis concentrated on the buck converters the proposed methods are naturally applicable to other topologies and control modes as well.
The derivation of the dynamical profile can be carried out analytically or it can be measured. The analytical profile derived by modeling the small-signal behavior of the switched-mode converter. The selected modeling method is irrelevant as long as the model accuracy can be verified. It is also important that the model reveals the true internal dynamics, not the dynamics e.g. with a resistive load. The dynamical profile can be measured by using a frequency response analyzer. Again, it is important to be sure that the internal dynamics are measured. However, sometimes it is not possible to use the constant-current-sink load (i.e. PCMC at open loop) and a resistive load
method combined the analytical and experimental data. Specific software could be used to compute the internal profile. The mixed-data method was also useful e.g. in the controller design, studying the non-idealities and recovering the internal profile from the supply/load-affected profile. It was obvious that if the internal dynamical profile was derived both analytically and experimentally and they were in a good agreement, the accuracy of the profile could be maximized.
On the other hand, it is not always possible or reasonable to compute the converter model due to its complexity. Therefore, the only way to obtain the dynamical profile is to use the experimental measurements. It is noteworthy that it is not always necessary or even physically possible to measure all the parameters in the dynamical profile. Typically, the open-loop input and output impedances, open-loop forward transfer function, control-to-output transfer function and the loop gain provide enough information to analyze the performance and stability of the converter.
The prevailing method to use a resistive load in modeling and analyzing switched-mode converters was argued for being incorrect. Several examples were shown to verify that. Typically, the resistive load damps the magnitude and resonances, if present, making the converter to look more insensitive to the interactions than the internal dynamical profile would be. The resistive load can also change the phase of the transfer function. This may cause severe problems if e.g. the control-to-output transfer function changes the phase 90° between resistive and nominal load.
The basic ideas of the dynamical profile were first applied to the voltage-output converters. However, the current-output converters, where the output current is regulated, are used in numerous applications. The dynamical issues of the current-output converters have been previously analyzed only in a few papers [74], [75] and [76], but they have not provided the true internal dynamical profile due to the use of the resistive load. In this research, the true internal or nominal profile of the current-output converter was derived by applying conventional modeling methods and from the two-port representation of the corresponding voltage-output converter by applying duality. The parameter set was composed of modified y-parameters and two special admittance parameters. The nominal load for the current-output converters was found out to be a pure voltage source. The load and supply interaction formalisms were derived and it was found that a low impedance load recovers the nominal features.
The dynamical review of the CC-VMC and CC-PCMC converters was carried out.
The previously observed peculiar phenomenon of the increasing crossover frequency in the loop gain, when having the low impedance load (i.e. typically a back-up
battery), was discovered to be the result of a wrong initial modeling with the resistive load. Both analytical and experimental analyses were provided to confirm this.
The main contributions of the thesis can be summarized as follows:
x The concept of dynamical profile was proposed and shown to be an efficient tool and framework for analyzing stability, performance and external interactions of a switched-mode converter independent of the topology or control mode.
x A buck-converter under three control modes was used as a source of evidence supporting the existence of the dynamical profile.
x It was shown that the open-loop parameters fully characterize the defined dynamical profile. The closed-loop output and input impedances can be used to stability assessment under external interactions.
x The use of a resistive load as the nominal load was shown to be a wrong approach. The nominal loads of the voltage- and current-output converters were stated to be a constant-current sink and a pure voltage source, respectively.
x The dynamical profile of the current-output converter was presented for the first time. The profile was obtained from the two-port representation of the voltage-output converter by applying duality. The a priori information from the voltage-output converter makes this approach extremely useful.
x It was noticed that the most suitable nominal practical load for the current-output converter was a RC-circuit, where the large capacitor behaved as a dynamical short-circuit at the higher frequencies exposing the internal dynamics.
The title of this thesis includes an open question. Many examples and aspects presented in the thesis provided the answer and explicitly proved that the unique dynamical profile of any given converter is a fact not a fiction.