In the late 1970s, before the rise of MOSFETs, thyristors and bipolar junction transistors (BJTs) were the primary power switches. BJTs are current-controlled devices, while MOSFETs are voltage-controlled, offering better performance in certain applications. The introduction of IGBTs in the 1980s brought another option—still a voltage-controlled device. However, MOSFETs have a positive temperature coefficient, making them more stable under varying conditions. They are also majority carrier devices, which makes them ideal for high-frequency applications.
High-frequency inverters that convert DC to AC can operate at ultrasonic frequencies, avoiding audible noise. Compared to IGBTs, MOSFETs exhibit higher avalanche resistance. The operating frequency is a crucial factor when selecting a MOSFET, as it affects efficiency and thermal performance. While IGBTs are often used in high-voltage applications (200V and above), MOSFETs are suitable for a broader range, from 20V up to 1000V. Even though some manufacturers offer 300V IGBTs, MOSFETs typically operate at much higher switching frequencies.
Recent advancements in MOSFET technology have led to lower conduction and switching losses, enabling their use in medium-voltage applications up to 600V. Engineers working on renewable energy systems, UPS units, and industrial power supplies are always looking for ways to improve efficiency, power density, and reliability. For example, wind turbines rely heavily on MOSFETs for blade control, contributing to system performance improvements.
Newer applications, such as electric vehicle (EV) charging stations, are also driving demand for specialized MOSFET solutions. These systems, whether installed in homes or commercial areas, often integrate with solar PV systems and utility grids. Fast-charging capabilities are essential for wall-mounted EV chargers, and PV battery charging stations play an important role in this ecosystem.
While three-phase motor drives and UPS inverters may use similar MOSFETs, solar inverters often require different types, like Ultra FRFETs or conventional body diode MOSFETs. The growth of the photovoltaic industry has been significant, starting with residential projects and expanding into larger commercial ventures. Drops in polysilicon prices over the years have further accelerated market expansion.
Grid-connected inverters are essential in converting DC power from renewable sources like wind turbines or solar panels into AC for the utility grid. These inverters, also known as synchronous inverters, only function when connected to the grid. Market demand for solar systems continues to grow due to cost savings, reduced fuel volatility, and environmental benefits.
The U.S. government has set ambitious goals for green energy, aiming for 80% of electricity to come from clean sources. This push has increased the demand for MOSFETs, especially in optimized topologies that enhance system efficiency. High-frequency applications prioritize parasitic capacitance, while low-frequency designs focus on RDSON. In single-ended configurations, body diode recovery isn't critical, but it's essential in double-ended systems where fast and soft recovery is needed for reliability.
MOSFETs support both zero-voltage and zero-current switching, unlike IGBTs, which only support ZCS. Advances in silicon and trench technology have improved performance, reducing on-resistance and enhancing body diode recovery. Packaging innovations also play a key role in specific applications.
Inverter systems are widely used in motor drives, UPS, and green energy setups. For low to medium voltage applications (12V to 400V), MOSFETs are commonly used, especially in solar and UPS inverters. Their ability to operate at high frequencies reduces RFI and allows for smaller, less costly filters.
Key requirements for MOSFETs in inverter applications include low RSP to minimize conduction losses, minimal RDSON variation for better output stability, and good UIS and SOA ratings. A low CGD/CGS ratio helps reduce EMI, while a high VTH improves noise immunity and parallel operation.
Body diode recovery is critical, with low QRR and tRR being essential to avoid voltage spikes and improve efficiency. In some cases, high pulsed drain current capability is required for short-circuit protection and motor startup. Controlling dv/dt and di/dt during switching helps manage EMI, and minimizing common source inductance through wire bonds improves performance.
Fast body diode MOSFETs with reduced tRR and QRR are ideal for high-frequency inverters, including solar applications. Conventional body diodes can cause inefficiencies if forced to conduct during load transitions, leading to higher losses. External SiC or Schottky diodes can be used to bypass the MOSFET body diode, improving efficiency at the cost of added complexity.
Fairchild’s UniFET II MOSFETs with FRFET technology are designed for these demanding applications. They offer improved body diode recovery and reduced wafer size, with two versions available: F-type for better body diodes and U-type for the lowest QRR and tRR. The U-type eliminates the need for SiC in inverter arms, achieving similar efficiency at a lower cost. With QRR reduced from 3100nC to 260nC, these devices are ideal for modern inverter designs.
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