Digital Control of switching converters in renewable sources applications 

Keywords: digital control, sliding mode control, switching converters, inverters, FPGA

Background: Switching converters transfer electrical power from energy sources to their loads, while regulating the output voltage and/or current according to the loads’ needs: dc converters regulate the output voltage to a constant value provided that the current requirement is within a specified range; inverters produce a sinusoidal voltage at the output. In addition to their control over the output, switching converters are sometimes required to regulate input voltages and/or currents, such as the voltage drop across a photovoltaic panel or the input current of a PFC, which must be sinusoidal and synchronized with the phase voltage. Similarly, the output current of a grid-connected inverter must be sinusoidal and synchronized with the phase voltage.
Switching converters ensure protection against fault conditions, power management in systems comprising multiple energy sources and loads, bidirectional power transfer. The above goals are all met with the help of control algorithms, which can be assessed in terms of their steady-state and transient performance.

The goals of the thesis are:
– The control of switching regulators using an FPGA: the design of the DAC, the control circuit and the generation of the DPWM signal;
– The control of a grid-connected inverter in a photovoltaic system.

Contributions:
• An analog-to-digital converter was designed using an FPGA;
• The ADC was implemented in Vivado and it was written in Verilog;
• Frequency-domain system identification was implemented on an FPGA and it comprises: the generation of a sequence at the input of the identified circuit; the measurement of the system’s output voltage; the inference of the system coefficients;
• The sliding mode control of a buck circuit was modeled in Mathcad, simulated in LTSpice and implemented on an FPGA;
• The control of a grid-connected inverter was simulated and implemented practically;
• The output current of the inverter was regulated to be a sinusoidal by attenuating the higher harmonics, which arise because of the photovoltaic panel at the input; the control methods used are dq control, proportional-resonant control and repetitive control.