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Posted on 29 June 2019

Boosting the Efficiency of Photovoltaic Installations

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Converter efficiency of more than 97 percent was achieved

TDK-EPC has developed a reference design for a miniaturized DC converter for photovoltaic installations jointly with STMicroelectronics. It allows the efficiency of these installations to be significantly increased.

By Francesco Pulvirenti, Amedeo La Scala, Domenico Ragonese, STMicroelectronics and Davide Giavarini, Sales IC Reference Design, TDK-EPC

 

The output of a single solar cell, which are commonly connected together in series within a large photovoltaic installation, is determined largely by the incident light. The greater this is, the more current can be drawn from the modules. Another output-determining variable is the temperature. As it rises, the output voltage drops. Figure 1 shows this relationship.

Current and voltage characteristics of a photovoltaic module. The output of a module varies strongly as a result of different levels of light irradiation and temperature.

Each of these characteristics yields an ideal operating point. It is reached when the product of output current and voltage is a maximum. It is also designated as the Maximum Power Point (MPP) (Figure 2).

Ideal operating point. The current-voltage characteristic and the power characteristic of a photovoltaic module

Because the MPP changes as a function of light irradiation and temperature, state-of-the-art solar inverters, which prepare the output of the modules for feeding into the power network, are equipped with Maximum Power Point Trackers (MPPT). These use a processorcontrolled algorithm to continuously determine the optimal operating point. This method is acceptable as long as all modules of the installation or phase cell string operate under identical conditions. If the installation or some of its modules are partially in shadow, however, this method no longer works, as the MPPT always evaluates only the entire installation or phase element cell string. Considerable power losses are incurred as a result.

In order to solve this problem, STMicroelectronics has developed a miniaturized DC converter with an integrated MPPT jointly with TDKEPC. These converters have small dimensions, so their space-saving design allows them to be accommodated in the terminal boxes of the individual photovoltaic modules. Each cell string element inside a module can even be operated by one of these converters. Figure 3 shows the converter and its circuit diagram.

Micro-converter with integrated MPPT Micro-converter and circuit diagram

Material list for a DC/DC converter with MPPT referred to the circuit diagram in Figure 3

The converter is built up on the basis of the boost principle so that its output voltage exceeds its input voltage. To keep the input current of the converter constant and thus also to boost the efficiency of the module, the converter operates internally with four boost strings, each of which is implemented with a MOSFET switch and an SMT power inductor from EPCOS (B82477G4473M000). These storage chokes have an inductance of 47 µH and are designed for a constant rated current of 2.5 A. Despite their high performance, they have dimensions of only 12.8 x 12.8 x 8.0 mm. In order to improve the EMC of the circuit, the storage chokes are equipped with magnetic shielding. These components can be clearly seen in the implemented circuit (Figure 3). The MPPT microcontroller drives the four boost strings. They operate with a phase offset of 90°. This division results in a highly constant current load of the photovoltaic module. The capacitance values of the smoothing and buffer capacitors at the input and output (C11 and C12 in Figure 3) of the converter can simultaneously be kept small. TDK multilayer ceramic capacitors rated at 1 and 4.7 µF are consequently used here (see also the material list). Thanks to the ceramic technology of the capacitors, very high operating lives and long-term stability values are attained at small case sizes compared with polar components such as tantalum or aluminum-electrolytic capacitors. A long operating life is a decisive criterion for selecting the components, as the converters are accom- modated in the terminal boxes of the photovoltaic modules and are thus difficult to access for maintenance or replacement work.

Illustration 1

The single-chip solutions from STMicroelectronics incorporate not only the power MOSFETs and the MPPT controller but also three analog-digital converters (ADC). Two of them record the voltage and current at the input of the converter. The microcontroller calculates the MPP on the basis of these values. As the boost topology can be used to obtain very high output voltages, the output is monitored by a third ADC. If the output voltage exceeds a defined value, driving of the MOSFET switches is interrupted. The module current then flows directly through the four storage chokes and the decoupling diode to the load.

Figure 4 shows the potential available for boosting the efficiency of photovoltaic modules by the use of converters. In this case, each of the three phase cell strings of a module is operated via a converter. This yields the red characteristic for current and voltage as well as the green one for power and voltage. The characteristics of the same module without converter operation are included for comparison (brown and blue).

Characteristic with and without converter operation. The use of converters produces a significantly wider field of the module.

Overall, the use of an MPPT converter produces significant increases in the efficiency of individual photovoltaic modules as well as entire installations. For example, a converter efficiency of more than 97 percent was achieved in a 24 V module at different outputs (Figure 5).

Efficiency as a function of output voltage. The efficiency of the combined module and converter is better than 97 percent at a range of powers and output voltages.

 

 

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