In modern photovoltaic (PV) system design, the maximum power point tracking (MPPT) circuit and algorithms are implemented digitally (in a microcontroller unit or FPGA), so that those advanced algorithms can be used efficiently to improve tracking performance.
The biggest challenges in building an analog MPPT controller, as described in a recent IEEE paper1 , are implementing the algorithm and storing necessary information (i.e., PV panel voltage VPV and output power PPV ) to find the MPP. By creating a truth table from the perturbation and observation (P&O) algorithm, the algorithm can be implemented with just several logic gates. Simplicity is key to the cost, size, and ease of implementation.
The other alternative is to implement the MPPT with analog circuitry. The benefit of such a solution is that the MPPT can be integrated with a DC-to-DC controller, so that plug-and-play capability can be expected for many low-power PV applications — such as an AC module for residential applications. The IEEE paper includes a demonstration that shows the integration of certain functions into a normal PWM controller chip is the best technique for special applications to reduce complexity and system cost. One of the best examples is the single-phase power factor correction (PFC) chip. This integrated solution has led to simpler system designs and lower costs.
However, finding the real MPP with this approach presents problems. Consequently, only three types of MPPT techniques can find the real MPP and be implemented by analog circuitry: hill-climbing/P&O, ripple correlation control (RCC), and state-based MPPT.
Considering the complexity of state-based MPPT and some drawbacks of the RCC method1 , neither of them are good candidates. The most desirable approach for universal analog MPPT is the P&O method. But implementing the algorithm with simple circuits and storing the values of VPV and PPV in the last perturbation cycle remain challenging for analog MPPT.
Thanks to capacitor-based storage cells, information like VPV and PPV can be stored easily. Each value is stored in its own cell. In one of these cells, as the IEEE paper shows, the storage capacitor voltage is proportional to the PV panel voltage and can be adjusted in each perturbation cycle. The minimum voltage step of perturbation can be set by the amplitude of charge (discharge) current and the time duration of charge (discharge) action.
Compared with analog MPPT solutions, this type of controller can find the real MPP dynamically, and its operation will be independent of converter topologies or their control.
The tracking performance of the controller is validated by both the simulation and experimental results using a single-phase PV inverter prototype with 200W PV panels under different sun irradiation levels and temperatures.
(Source: See Reference 1)
The next step is to create the truth table and simple algorithm1 on which the MPPT operation is based. Only a few logic gates are required to implement the MPPT algorithm.
The P&O method will continue injecting perturbation into the panel voltage and examine the change of output power. Then the values of VPV and PPV in the last perturbation cycle must be stored.
For this application, the multiplier works only in the first quadrant. This keeps the complexity and cost of this solution down.
The final system diagram
(Source: See Reference 1)
This is the circuit diagram of the analog MPPT controller. The PV panel voltage and current need to be sensed and filtered by a low pass filter to remove the switching frequency ripples. An analog multiplier calculates the instantaneous output power of the panel.
As the IEEE paper tells us, this controller can be divided into four blocks.
- Block I executes the MPPT algorithm and decides the next perturbation direction.
- Blocks II and IV generate the control signals for the storage cells. In block II, the control signal CMD, combined with CMD-S, will determine a period during which the voltage of the capacitor C_S in the cell “PPV storage” will be updated. Consequently, PPV (k-1) will be equal to the latest PPV after this period.
- Block III determines the charge (discharge) time duration for the storage cell “VPV _storage.”
The high time duration of the signal “Timer,” as well as the charge (discharge) current in the storage cell, will determine the minimum perturbation step of VPV .
Voila! We have a cost-effective, highly integrated analog MPPT solution.
- A New Cost-Effective Analog Maximum Power Point Tracker for PV Systems, Zhigang Liang, IEEE Student Member; Rong Guo, IEEE Student Member; Alex Huang, IEEE Fellow, 2010.