22 SOLAR POWER www.panasonic.com/global/industry/ Issue 3 2023 Power Electronics Europe www.power-mag.com company. Maximum operating temperatures is now 175ºC) with rated operating range of 70ºC to 105ºC. The company also claims that solder crack resistance has been improved at high temperature. In the motor drive control unit, resistors are required in the amplifier and feedback circuits. Key requirements are high reliability, long life and stable resistance. Resistors with an additonal resin layer on the underside can reduce solder joint cracking, while improvements to material and construction design can improve precision and extend life expectancy. Devices which meet anti-sulphuration specifications enable them to survive the harsh operating conditions that micro inverter applications routinely encounter. Inductor design Solar inverters need inductors that are capable of handling high voltages and large currents in the main circuit. These components are typically a custom design to meet both current handling and inductance requirements of the system. In addition to the inductor’s role in the primary circuit, power inductors are also used in the auxiliary circuit for the controller and gate drivers, where digital logic provides critical controlling and monitoring functions for solar energy harvesting systems. The high switching speeds and harsh operating conditions can require high performance, metal composite power inductors to be specified. One of the most important requirements of a power inductor for a DC/DC converter is high power efficiency. Inverter suppliers are facing tough demands for reduced inverter system size and higher efficiency, so the challenge for the inductor supplier is to provide an inductor at small size with high current capability and minimal heat dissipation. A general trend in the electronic industry is the standardisation and modularisation of systems but this means that passive components suppliers need to provide products which scale over a broad range of requirements in regards to electrical power capability. A high current capability of an inductor series can support the standardisation of solar inverter systems, supporting a wider range of current flow, depending of the requirement of the individual system. Exhibiting stability Metal composite power choke coils exhibit stable inductance over current, but are also stable with temperature, in comparison to ferrite inductors, where the inductance value is influenced by the temperature of the inductor. This means design engineers need to qualify the component for different temperature ranges when used in solar inverters. With stable temperature behaviour, much less time is required for qualification, which reduces development cost and time. Another area within a solar power inverter that requires a power inductor is the gate driver of the FET that transforms the DC current of the battery to the threephase sine wave, which is fed in the power grid. While the inductance of a ferrite inductor will vary with age, the metal composite material is free from any effects of ageing, helping the inverter manufacturer to guarantee a system that functions over its entire specified lifetime. As is the case with the DC/DC converter, the power inductor of the gate driver circuit benefits from a stable temperature behaviour of the inductance value. This attribute also helps to reduce development and qualification time of the gate driver circuit, as engineers do not need to consider a fluctuation of the inductance value of the inductor, which is not only influenced by the power loss of the inductor itself, but also by the heat dissipated by surrounding components. Additionally, designers of inverter systems are under constant pressure to make systems smaller and more efficient. Compared to ferrite inductors, metal composite inductors have a much higher energy density, which leads to a size reduction of 30% to 50% for comparable current specifications. PCB space can be saved, or higher currents used, simply by replacing ferrite inductors with metal composite ones. Figure 6: The use of resistors in solar inverters. Figure 7: Exploded view of Panasonic’s metal composite power inductors.
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