A brief discussion on IGBT selection method to improve solar inverter efficiency

　　There are countless types of advanced power components on the market today, and it is indeed a difficult task for engineers to choose the right power component for an application. For solar inverter applications, insulated gate bipolar transistors (IGBTs) can provide more benefits than other power components, including high current carrying capacity, control by voltage rather than current, and the ability to match anti-parallel diodes with IGBTs. This article will introduce how to use a full-bridge inverter topology and select the right IGBT to minimize the power consumption of solar applications.

　　A solar inverter is a power electronic circuit that converts the DC voltage of a solar panel into an AC voltage to drive AC loads such as household appliances, lighting, and motor tools. As shown in Figure 1, the typical architecture of a solar inverter generally uses a full-bridge topology with four switches.

　　In Figure 1, Q1 and Q3 are designated as high-side IGBTs, while Q2 and Q4 are low-side IGBTs. The inverter is designed to generate a single-phase sinusoidal voltage waveform at the frequency and voltage conditions of its target market. Some inverters are used in residential installations connected to the net metering benefit grid, which is one of the target application markets. This application requires the inverter to provide a low-harmonic AC sinusoidal voltage so that power can be injected into the grid.

　　To meet this requirement, the IGBT can pulse width modulate the frequency of 50Hz or 60Hz at a frequency of 20kHz or above, so that the output inductors L1 and L2 can be kept reasonably small and effectively suppress harmonics. In addition, since its switching frequency is higher than the normal human hearing spectrum, this design can also minimize the audible noise generated by the inverter.

　　What is the best way to pulse width modulate these IGBTs? How can the power dissipation be minimized? One approach is to pulse width modulate only the high-side IGBTs, and commutate the corresponding low-side IGBTs at 50Hz or 60Hz. Figure 2 shows a typical gate voltage signal. When Q1 is pulse width modulating, Q4 maintains positive half-cycle operation. Q2 and Q3 remain off during the positive half-cycle. When Q3 is pulse width modulated in the negative half-cycle, Q2 remains on. Q1 and Q4 are turned off during the negative half-cycle. Figure 2 also shows the AC sinusoidal voltage waveform across the output filter capacitor C1.

　　This conversion technology has the following advantages: (1) Current does not flow freely in the high-voltage side anti-parallel diode, so unnecessary losses can be minimized. (2) The low-voltage side IGBT will only switch at a 50Hz or 60Hz power frequency, mainly conduction losses. (3) Since the IGBTs on the same phase will never switch in a complementary manner, bus short-circuit breakdown is impossible. (4) The anti-parallel diode of the low-voltage side IGBT can be optimized to minimize the losses caused by freewheeling and reverse recovery.

　　IGBT Technology

　　IGBT is basically a bipolar junction transistor (BJT) with a metal-gate oxide gate structure. This design allows the gate of the IGBT to control the switch with voltage instead of current, just like a MOSFET. As a BJT, the IGBT has a higher current handling capacity than a MOSFET. At the same time, the IGBT is also a minority carrier component like a BJT. This means that the speed at which the IGBT turns off is determined by the speed at which the minority carriers recombine. In addition, the turn-off time of the IGBT is inversely proportional to its collector-emitter saturation voltage (Vce(on)) (as shown in Figure 3).

　　Taking Figure 3 as an example, if the IGBT has the same size and technology, an ultra-speed IGBT has a higher Vce(on) than a standard-speed IGBT. However, the ultra-speed IGBT turns off much faster than the standard IGBT. This relationship reflected in Figure 3 is achieved by controlling the use cycle of the minority carrier recombination rate of the IGBT to affect the turn-off time.

　　Table 1 shows the parameter values ​​for four IGBTs of the same size. The first three IGBTs are based on the same planar technology but use different lifetime compound control metrics. As can be seen from the table, the standard speed IGBT has the lowest Vce(on) but the slowest fall time compared to the fast and ultra-fast planar IGBTs. The fourth IGBT is an optimized trench gate IGBT that provides low conduction and switching losses for high frequency switching applications such as solar inverters. Note that the trench gate IGBT has a lower Vce(on) and total switching losses (Ets) than the ultra-fast planar IGBT.

　　　　High-voltage side IGBT

　　The previous article discussed that the high-side IGBT switches at a frequency of 20kHz or above. Assuming a 1.5kW solar inverter with a 230V AC output, which IGBT in Table 1 has the lowest power consumption? Figure 4 shows the power consumption analysis of the IGBT switching at 20kHz, which shows that the ultra-fast planar IGBT has lower total power consumption than the other two planar IGBTs.

　　At 20kHz, switching losses become a significant part of the total power consumption. At the same time, although the conduction loss of the standard speed IGBT is the lowest, its switching loss is the largest, making it unsuitable to act as a high-side IGBT.

　　The latest 600V trench gate IGBTs are optimized for 20kHz switching. As shown in Figure 5, this IGBT offers lower total power dissipation than previous planar IGBTs. Therefore, in order to achieve the highest efficiency in solar inverter designs, trench gate IGBTs are the preferred components for high-side IGBTs.

　　Low voltage side IGBT

　　The same question applies to low-side IGBTs. Which IGBT provides the lowest power dissipation? Since these IGBTs only switch at 50Hz or 60Hz, as shown in Figure 5, standard speed IGBTs provide the lowest power dissipation. Although standard IGBTs introduce some switching losses, the value is not enough to affect the total power dissipation of the IGBT. In fact, the latest trench gate IGBTs still have higher power dissipation because this generation of trench gate IGBTs is designed specifically for high frequency applications with the goal of balancing switching and conduction losses. Therefore, for low-side IGBTs, standard speed planar IGBTs are still the inevitable choice.

　　Conclusion

　　This article analyzes the full-bridge topology for solar inverter applications. This topology uses sinusoidal pulse width modulation technology to switch the high-side IGBTs at above 20kHz. The low-side IGBTs of the branch line are switched at 50Hz or 60Hz, depending on the output frequency requirements. If the latest 600V trench-gate IGBTs are selected, the total power consumption will be minimized at 20kHz. For the low-side IGBTs, standard speed planar IGBTs are the best choice. Standard speed IGBTs have the lowest conduction losses at 50Hz or 60Hz, and their switching losses are insignificant to the overall power consumption. Therefore, engineers can minimize the power consumption of solar inverter applications by selecting the right combination of IGBTs.