Hysteresis Control MPPT Method with Generic DC/DC Converter
Abstract
For battery powered outdoor equipment, such as video doorbell, to increase the battery working time and easy to install anywhere, sometimes the photovoltaic (PV) panel is used as an auxiliary power supply to charge the battery. Photovoltaic cells are one of the most important parts in PV panel, the output power and output voltage of PV cells varies with sunlight intensity, so a MPPT (maximum power point tracking) method is used to maximize the output energy of PV cells.
To achieve MPPT, it means that the power converter needs to automatically operate within a specific supply voltage range, which is often different from converter default startup and shutdown voltage, so it is hard to find an usual power converter that can be used to different types of PV panel.
This application note introduces a simple hysteresis control method which can provide a flexible design to achieve MPPT with generic DC/DC converter, regardless of the different maximum power point. Take TI's Boost converter TPS61023 as an example, the detailed operation scheme and test results are also given in this documentation.
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1 Introduction of Photovoltaic Cell and MPPT Requirement
As an important component of PV system, photovoltaic cells have a complex relationship about operating voltage and the power produced. The equivalent circuit of the solar cell is shown as Figure 1-1, RS is the series resistance of a single cell connected to others, RP is parallel leakage resistance and large, typically > 100kΩ.
Figure 1-1 Solar Cell Equivalent
CircuitFor any given set of conditions, cells have a single operating point where the values of the current (I) and voltage (V) of the cell allow maximum power output. Figure 1-2 shows the typical I-V curve and P-V curve of a general PV panels at different times in a day, where the light is strongest at noon, moderate in the morning and poor in the evening.
Figure 1-2 Solar Panel I-V and P-V
CurveThe no-load output voltage VOC is 3.3V approximately 3.5V under different light conditions. The output voltage of the solar cells decreases at first as the load current increases. When the output voltage is less than 2.9V, the output current almost unchanged, so 2.9V here is the so called maximum power point (MPP) of this PV panel, and generally engineer needs to make sure that the PV panel operates at this MPP so that the PV panel can always output maximum power.
Therefore, a MPPT (maximum power point tracking) method is used to maximize energy extraction as conditions vary, make sure that the PV system always operate at the maximum output power point.
