A prototype has been developed to experimentally verify the
performance of the proposed modular architecture and control
approach. The circuit has separable converters and load stages
to employ the modularity of the architecture. Each cell is controlled
via an onboard floating field-programmable gate array
(FPGA), corresponding sensing circuitry and A/D converters
(ADCs). Communication of the digital duty cycle command is
implemented through the FPGA and an isolator chip. A fourswitch
buck-boost topology, as shown in Fig. 7, is implemented
for the experiments described next. The devices used are shown
in Table II. The values of input capacitor, inductor, and output
capacitor are 0.4 μF, 10 μH, and 1 μF, respectively. The switching
frequency is about 800 kHz. Three cells were used, with two,
three, and four LEDs as load, respectively. FPGAs are utilized
only for experimental verification of the principle. The simple
control algorithm can be realized with a small number of logic
gates in a customer IC. Four-switch buck-boost converters are used for convenience in experimental development to operate
under various conditions including buck, boost, or buck-boost
modes with a wide range of input voltages, number of cells, and
LEDs per substring. In practice, only a single topology is required
based on the specific system requirements and operating
conditions, resulting in fewer switches and lower cost.