How to optimize high voltage IGBT design in inverters

　　As the green electricity movement continues to gain momentum, applications including home appliances, lighting, power tools, and other industrial equipment are taking advantage of the benefits of solar energy as much as possible. To effectively meet the needs of these products, power supply designers are converting solar energy into the required AC or DC voltage with high efficiency through a minimum number of components, high reliability and durability.

　　To produce the required AC output voltage and current with high efficiency for these applications, solar inverters require the right combination of controls, drivers and output power devices. To achieve this goal, a DC-to-AC inverter design optimized for 500W power output and having a single-phase sine wave at 120V and 60Hz frequency is presented here. In this design, there is a DC/DC voltage converter connected to the photovoltaic panel to provide 200V DC input to this power converter. However, details of solar panels are not provided here because that aspect is not the focus of our discussion.

　　Nowadays, there are different advanced power switches in the market such as Metal Oxide Semiconductor FET (MOSFET), Bipolar JJT (BJT), and Insulated Green Gate Bipolar Transistor (IGBT) to convert power. However, achieving the highest conversion efficiency and performance requirements for this application requires the selection of the correct power transistor.

　　Years of investigation and analysis have shown that IGBTs have many advantages over other power transistors, including higher current capabilities, the use of voltage rather than current for gate control, and the ability to be co-packaged with an ultra-fast recovery diode for faster turn-off. speed. In addition, fine improvements in process technology and device structure have also significantly improved the switching performance of IGBTs. Other benefits include better through-state performance, as well as high durability and a wide safe operating area. After considering these qualities, this power inverter design will use high voltage IGBTs as the inevitable choice for power switches.

　　Because the inverter topology implemented in this design is a full-bridge, the relevant solar inverter uses 4 high-voltage IGBTs, as shown in Figure 1. In this circuit, Q1 and Q2 transistors are designated as high-side IGBTs, while Q3 and Q4 are low-side power devices. In order to keep the total power loss at a low level, but the power conversion efficiency is high, the designer needs to correctly apply the combination of low-side and high-side IGBTs in this DC/AC inverter solution.

　　Trench and planar IGBTs

　　In order to minimize harmonics and power loss at the same time, the high-side IGBT of the inverter uses pulse width modulation (PWM), while the low-side power device uses 60Hz to change. By operating the PWM frequency at 20kHz or above, the high-side IGBT has 50/60Hz modulation, and the output inductors L1 and L2 can be kept as small as is practical and provide effective harmonic filtering. Furthermore, the audible sound of the inverter can also be minimized because the switching frequency is already above the human hearing range.

　　After studying various switching technologies using different IGBT combinations, we determined that the best combination to achieve the lowest power losses and highest inverter performance is to use ultra-fast channel IGBTs for the high-side transistors and standard speed for the low-side parts. of planar devices. Compared to fast and standard speed planar devices, ultra-fast channel IGBTs with a switching frequency of 20kHz provide the lowest combined on-state and switching power losses. Another advantage of the high-side transistor's switching frequency of 20kHz is that the output inductor is reasonably small and easy to filter. On the low side, we set the switching frequency of standard speed planar IGBT at 60Hz so that power loss can be kept at the lowest level.

　　When we take a closer look at the switching performance of high-voltage (600V) ultra-fast channel IGBTs, we know that these devices are optimized for a switching frequency of 20kHz. This allows the design to maintain minimal switching losses at the relevant frequency, including the collector-to-emitter saturation voltage Vce(on) and the total switching energy ETS. As a result, total on-state and switching power losses are kept to a minimum. Based on this, we selected ultra-high-speed trench IGBT, for example, IRGB4062DPBF as the high-side power device. This ultra-high-speed channel IGBT is co-packaged with an ultra-fast soft-recovery diode to further ensure low switching losses.

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