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TIDM-SOLAR-DCDC: How to control the output voltage while controlling MPPT?


If I use the BOOST circuit as the DCDC converter, which is used as the front end of the grid-tied inverter, how can I achieve MPPT and stabilize the inverter bus voltage? I guess only one of them can be controlled by BOOST. When I do MPPT, so who can stabilize the bus voltage, the grid?

  • You cannot regulate bus voltage because through MPPT you are drawing power from the panel.

    So, now you have to take that power and use the inverter to feed it into the grid. This inverter control can have a input bus voltage control loop. Higher the bus volt the higher will be the grid current fed into the grid.

  • First of all,thank you for your reponse.However,I still have a little confusion that would like to be answered.

    Firstly,what you mean is that in a solar- grid-tied inverter, there are two control systems when entering closed-loop operation. One of the devices for the control system is a DC/DC circuit, and the control target of the control system is MPPT. Another control system device is the DC/AC circuit, which is aimed at the BUS voltage. That's right?

    Secondly,if the case is that, then I'm confused, for example, when BOOST is doing MPPT, it adjusts the PV input voltage, but how can this be adjusted? I know that when the BOOST input voltage is constant, we can change the duty to adjust the output voltage. But when the output voltage is constant, can we adjust the input voltage by changing the duty?Similarly for DC/AC circuits, when the DC voltage is constant, we can change the modulation ratio to adjust the output AC voltage amplitude, but when the AC voltage (grid voltage) is stable, can we adjust the input DC voltage by changing the modulation ratio? Incomprehensible.

    Lastly,if the PV input voltage is greater than the BUS voltage we specified, how does the inverter work? If the voltage passes through the diode of the BOOST circuit is higher than the BUS voltage, should MPPT be abandoned?After all, the inverter will not be turned off at this time.

    Thanks again for your response and I am looking forward to hearing from you.

  • MPPT stage control loop maintains the MPPT input voltage at the level determined by the MPPT algorithm. And yes, the DC/AC stage keep injecting current into the grid in order to maintain its input DC bus voltage. Please read about this. It is available in literature.

    Boost stage controls the inductor current in order to maintain the input at MPPT ref volt. If there is more power from the panel, then it draws more current and vice versa. Again, read about this basic operating principle.

    PV input is higher than bus volt means you are not drawing enough current from the panel. Your boost control loop should prevent this to happen by commanding more current from the panel. This way the panel volt will reduce and bus volt will rise.