The proposed inner control loop for the inductor current is
shown in Fig. 2. Capital letters are used for the variables in
the block diagram. The system to be controlled is defined by
(1) and (3). In this system the term vO1+vIN behaves as a
variable gain, and vO1 as an external disturbance. The input
to the system is the duty cycle d1. If this variable is chosen to
be the control variable, the variable gain vO1+vIN makes
very difficult, almost impossible, the design of a controller
that stabilises the system under a variable operating
condition. The proposal is to choose the inductor voltage
vL1 as the control variable instead of the duty cycle d1, as
shown in Fig. 2. From the inductor voltage reference
generated by the controller vL1ref, the duty cycle d1 can be
obtained by means of the following expression, derived
from (1):
d