Optimizing high voltage IGBTs for high efficiency solar inverters

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

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

Now, there are different advanced power switches on the market, such as metal oxide semiconductor FET (MOSFET), bipolar junction transistor (BJT), and insulated green gate bipolar transistor (IGBT) to convert power. However, to achieve the highest conversion efficiency and performance requirements for this application, the right power transistor must be selected.

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

Because the inverter topology implemented in this design is a full-bridge, the solar inverter in question uses four 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 has high efficiency, designers must correctly apply a combination of low-side and high-side IGBTs in this DC/AC inverter solution.

Figure 1 Inverter design using 4 IGBTs

Trench and Planar IGBTs

To minimize both harmonics and power losses, the inverter's high-side IGBTs utilize pulse width modulation (PWM), while the low-side power devices are switched at 60Hz. By operating the PWM frequency at or above 20kHz, with 50/60Hz modulation of the high-side IGBTs, the output inductors L1 and L2 can be kept as small as practicable, providing effective harmonic filtering. Furthermore, the audible sound of the inverter can be minimized, since the switching frequency is above the human hearing range.

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

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

In addition, these IGBTs do not require short-circuit ratings because the output inductors L1 and L2 limit the current di/dt when a short circuit occurs at the output of the inverter, giving the controller enough time to respond appropriately. Also, short-circuit rated IGBTs offer higher Vce(on) and ETS compared to non-short-circuit rated IGBTs of the same size. Due to their higher Vce(on) and ETS, short-circuit rated IGBTs result in higher power losses, which reduces the efficiency of the power inverter.

Furthermore, ultra-high-speed trench IGBTs also offer a square reverse bias operating region, a maximum junction temperature of 175°C, and can withstand four times the rated current. To demonstrate their ruggedness, these power devices are also tested with a 100% clamped inductive load. Unlike the high-side, on-state losses dominate the low-side IGBTs. Since the operating frequency of the low-side transistors is only 60Hz, the switching losses are negligible for these devices. Standard speed planar IGBTs are specifically designed for low frequencies and lower on-state losses. Therefore, with the low-side devices switching at 60Hz, these IGBTs achieve the lowest power loss level by using standard speed planar IGBTs. Since the switching losses of these devices are very small, the total dissipation of the standard speed planar IGBT is not affected by their switching losses. Based on these considerations, the standard speed IGBT IRG4BC20SD is therefore the best choice for low power devices. A fourth generation IGBT is co-packaged with an ultra-fast soft recovery anti-parallel diode and optimized for lowest saturation voltage and low operating frequency (<1kHz). Typical Vce(on) at 10A is 1.4V. The co-packaged diode across the low-side IGBT has been optimized for low forward drop and reverse leakage current to minimize losses during freewheeling and reverse recovery.
　　
Inverter Efficiency

Figure 2 shows the full-bridge power inverter circuit at the system level. As shown in the figure, each leg of the H-bridge is driven by a high current, high speed gate driver IC, as well as independent low and high side reference output channels. The floating channel of the driver IRS2106SPBF allows a bootstrap supply to operate for the high side power devices. Therefore, it eliminates the need for an isolated power supply for the high side drive. This helps the overall system to improve inverter efficiency and reduce parts count. The bootstrap capacitors of these drivers are refreshed during each switching cycle (50μs) as the current is freewheeling to the low-side IGBT co-package diode.

Figure 2 Full-bridge power inverter circuit

Since the high-side Q1 and Q2 co-packaged diodes are not affected by the freewheeling current, while the low-side Q3 and Q4 have mainly on-state losses and very little switching losses, the overall system losses are minimized and the system efficiency is maximized. In addition, since at any time, switching is performed on the diagonal device pairs Q1 and Q4, or Q2 and Q3, the possibility of shoot-through is eliminated. At the same time, each output driver IC has a high pulse current buffer stage to minimize the driver's shoot-through. Another outstanding feature of this inverter is that it operates from a single DC bus supply. Therefore, the need for a negative DC bus is eliminated. In simple terms, all of the above arrangements can translate into higher efficiency and fewer parts count for the overall inverter. Fewer parts also means that the design can occupy less space and have a shorter bill of materials.

In this inverter design, the +20V power supply is first used to drive the microprocessor and manage different circuits. Regarding the implementation of the code, the 8-bit microcontroller PIC18F1320 used in this inverter solution generates signals for the IGBT driver, which ultimately provides the signals used to drive the IGBT. The gate driver integrates high voltage conversion and termination techniques using a dedicated advanced high voltage IC process (G5 HVIC) and latch immune CMOS technology, enabling the driver to generate appropriate gate drive signals from the low voltage input of the microcontroller. The associated logic inputs are compatible with standard CMOS or LSTTL outputs, with logic voltages as low as 3.3V.

Ultra-fast diodes D1 and D2 provide paths to charge capacitors C2 and C3 and ensure that the high-side driver receives the correct power. Figure 3 depicts the associated output waveforms. As shown, during the positive output half cycle, the high-side IGBT Q1 is sinusoidally PWM modulated, but the low-side Q4 remains on. Similarly, during the negative output half cycle, the high-side Q2 is sinusoidally PWM modulated, while the low-side Q3 remains on. This switching technique provides a 60Hz AC sine wave across capacitor C4 after the output LC filter.

Figure 3 Capacitor charging waveform

　
The inverter is designed for 500W output, and the measured AC output power is 480.1W, and the power loss is 14.4W. At a frequency of 60Hz, the AC output voltage is 117.8V and the output current is 4.074A. This configuration achieves an efficiency of 97.09%. Using a similar configuration, the inverter is changed to 200W output and the conversion efficiency is re-measured. The results show that under this load, the AC power is 214W, the power loss is 6.0W, and the 60Hz output voltage is 124.6V at an output current of 1.721A. At this power rating, the conversion efficiency obtained is 97.28%. Even at the lower end of the output power (100W), we see similar efficiency performance.

In short, by combining the appropriate high-voltage driver with optimized low-side and high-side high-voltage IGBTs, the solar inverter design we mentioned here can continue to provide high conversion efficiency performance in the power output range of 100 to 500W. Since the conversion efficiency is very high, the low power losses involved do not pose any challenges for temperature management. Therefore, at an output power of up to 500W, the junction temperature of the high-side IGBT (IRGB4062DPBF) is about 80°C, which is less than half of the maximum specific junction temperature of 175°C. Similarly, at the same power level, the low-side IGBT (IRG4BC20SD-PBF) shows a junction temperature of 83°C. At the same time, when the output power reaches around 200W, the temperature will become even lower.