Improving Solar Inverter Reliability: Techniques For Protecting Output Power Switches

by Jerry Steele, Red Hill Labs, Tucson, Ariz., How2Power Today, Jan 14 2022

Focus:
This article focuses on the reliability of inverters used in off-grid and grid-tied, residential photovoltaic systems at power levels from several hundred watts up to about 20 kW. In such applications, the design of the output power stage (mainly the MOSFET power switches) needs attention to achieve improvements in inverter failure rates. This article discusses the benefits of applying foldback current limiting and other techniques in protecting the MOSFETs. It discusses the differences between inverters with transformer outputs and those that are transformerless, the vulnerability of power MOSFETs in inverters, including the greater stress on MOSFETs in transformerless inverters. It explains the benefits of using foldback current limiting (two possible techniques) to protect the MOSFETs (typically not done in solar inverters). The potential benefits and limitations of using TI’s UCC5870 isolated MOSFET driver to protect the power MOSFETs are also discussed. Finally, the vulnerability of transformerless inverters to delivering high levels of dc in certain failure modes is explained along with a circuit solution to guard against this problem.

What you’ll learn:

• How to improve the reliability of inverters for residential photovoltaic systems
• How to protect the power MOSFETs in solar inverters by applying foldback current limiting
• How to use a dc detector to prevent transformerless inverters from delivering dc under short circuit and other conditions

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