Reference design for micro-DC/DC inverters
Together with STMicroelectronics, TDK-EPC has developed a reference design for a miniaturized DC inverter for photovoltaic installations. It allows the efficiency of these installations to be significantly increased.
The output of a single solar cell, as well as an entire photovoltaic installation, which as a rule consists of several modules connected in series, is largely determined by the incident light. The greater this is, the more current can be drawn from the modules. Another output-determining variable is the temperature of the solar cell. As it rises, the output voltage drops. Figure 1 shows these dependencies.
|Figure 1: Current and voltage characteristics of a photovoltaic module|
An ideal operating point may be read off each of these characteristics. It is reached when the product of output current and voltage is a maximum. This optimal operating point is also known as the maximum power point (MPP) (Figure 2).
|Figure 2: Ideal operating point|
In state-of-the-art solar inverters, maximum power point trackers (MPPT) continuously determine the ideal operating point via a processor-controlled algorithm. This significantly reduces the dependence of the MPP on changing patterns of incident light and temperature.
This method is acceptable as long as all modules of the installation or 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 cell string. Considerable power losses are incurred as a result.
In order to solve this problem, STMicroelectronics has developed a miniaturized DC inverter with an integrated MPPT jointly with TDK-EPC. These inverters feature dimensions of only 78 × 47 × 13 mm³, so their space-saving design allows them to be accommodated in the terminal boxes of the individual photovoltaic modules. Each cell string inside a module can even be operated by one of these inverters. Figure 3 shows the circuit diagram of the micro inverter.
|Figure 3: Circuit diagram of the micro-inverter with integrated MPPT|
The inverter is built up on the basis of the boost principle so that its output voltage exceeds its input voltage. To keep its input current constant and thus also to increase the efficiency of the module, the inverter 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 × 12.8 × 8.0 mm³. 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 gives the photovoltaic module a highly constant current load. The rated values of the smoothing and buffer capacitors at the input and output (C11 and C12 in Figure 3) of the inverter can simultaneously be kept small. For this reason, TDK MLCCs rated at 1 and 4.7 µF are consequently used here (Table). The ceramic technology of the capacitors enables very long operating lives and long-term stability values to be 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 inverters are accommodated in the terminal boxes of the photovoltaic modules and are thus difficult to access for maintenance or replacement work.
Material list for a DC/DC inverter with MPPT referred to the circuit diagram in Figure 3
|C1, C2, C3, C4, C8||22 nF||TDK||C1608X7R1H223K|
|C6, C12, C13||4.7 µF||TDK||C3225X7R1H475K|
|L1, L2, L3, L4||47 µH||EPCOS||B82477G4473M003|
|R1||2.7 MΩ||VISHAY||D11/CRCW0603 1M 1%|
|R3||4.3 MΩ||VISHAY||D11/CRCW0603 4.3M 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 inverter. 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 inverters. In this case, each of the three cell strings of a module is operated via an inverter. 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 inverter operation are included for comparison (orange and blue).
Overall, the use of an MPPT inverter produces significant increases in the efficiency of individual photovoltaic modules as well as entire installations. For example, an inverter efficiency of more than 97 percent can be achieved in a 24 V module at different outputs (Figure 5).
|Figure 5: Efficiency as a function of output voltage|
50 new reference designs in just one year
TDK-EPC has intensified its support of STMicroelectronics (ST) in the creation of reference designs. ST just recently completed the development of the 50th joint reference design in the past year, predominantly for power supplies. These reference designs combine state-of-the-art ICs from ST and innovative TDK and EPCOS products. A major design challenge for the IC manufacturers is the selection and qualification of the most suitable passive components, which is one determining factor for the energy efficiency of the designs. “Our customers know that reference designs qualified with high-performance TDK and EPCOS products will work reliably without any further design effort,” explains Ulrich Kirchenberger, Senior Market Development Manager, Energy Efficiency, EMEA Region, STMicroelectronics. Customers can thus concentrate on their own core competences, while simultaneously speeding up time to market and reducing development costs.