Parallel converter systems are an interesting solution to the problem of providing a tightly regulated output 
voltage at high current levels. Paralleling of converters allows for equal sharing of the load current between
modules. In this way, the stress in the semiconductor switches is reduced and reliability is improved.

In multimodule parallel converter systems, a current mode control scheme, like average current mode control (ACC), is 
usually needed in order to share the current between  modules. In this scheme the control loop dynamic characteristics
depend strongly on the line and load conditions, and also on the number of modules on stream.

In this thesis two different robust model-following (RMF) control techniques have been applied to parallel converter systems 
in order to improve the robustness of the ACC control. This work has been carried out in three different steps:

In first place, a high-pass RMF control scheme has been presented and applied to the voltage loop of a parallel Buck DC-DC 
converter. The proposed scheme adds a inner loop to the conventional current and voltage ACC loops, reducing the sensitivity
of the outer voltage loop to the changing power stage parameters: number of modules, input voltage, load and component 
tolerances. Also, the loop improves significantly the disturbance rejection of the converter, i.e. the closed loop output 
impedance and audiosusceptibility, at low frequencies in comparison with conventional ACC.

The main limitation of this control scheme is that its performance is limited by the switching ripple at the output
voltage. The reason for that is the use of a model-based high-pass first order auxiliary regulator in the inner robust loop,
which amplifies the switching frequency noise. Hints for the design of the robust loop with minimization of the noise 
problems have been given, resulting in a compromise between noise and robustness.

Next, a low-pass RMF control scheme has been presented and applied to the voltage loop of the same parallel Buck DC-DC 
converter. The practical implementation of this control scheme consists of adding an inner loop based on a low-pass 
first-order reference model and a conventional PI regulator besides the outer voltage loop. 

With this method, the same as with high-pass RMF control, the small signal bandwidth and stability of the outer voltage loop 
is preserved  over important variations of the input voltage and the load, independent of the number of connected modules. 
It has been also demonstrated  that the disturbance rejection characteristics of the system improve appreciably in spite
of having slower inner and outer loops than conventional ACC. 

An advantage when compared with other methods based on reference models, such as high-pass RMF control, is the low-pass
nature of the reference model, which doesn't add noise limitations to the design.

In last place, the low-pass RMF control scheme has been applied to a modular online UPS with two single-phase inverters
connected in parallel. Both the current and voltage loops have been designed by means of the RMF technique. With a
simple design of three individual controllers a high order equivalent controller is obtained by means of the proposed 
technique. The equivalent controller leads to an open loop gain with higher low frequency gain than that of a conventional 
PI controller, without the need of increasing the crossover frequency, so that robustness to noise, parameter uncertainties 
and operation point changes is preserved. The design of an RMF controller has more degrees of freedom than those of a simple
PI controller.

On one hand, the RMF controller applied to the current loops achieves a fine current equalization between inverter modules, 
even in the case of high CF nonlinear loads. On the other hand, the RMF controller in the voltage loop reduces the UPS 
closed loop output impedance at the frequencies of the load current fundamental and of its harmonics, so that a low 
distortion of the output voltage both with linear and with high CF nonlinear loads is achieved, lower than 3%. The low 
frequency reduction of the output impedance also leads to a good transient response of the output voltage to fast 
transients in the load.