Discussion on IGBT selection method for improving solar inverter efficiency

There are so many types of advanced power components on the market today, and it is a daunting task for engineers to choose the right power components for an application. In the case of solar inverter applications, insulated gate bipolar transistors (IGBTs) offer more benefits than other power components, including high current carrying capacity, control over voltage rather than current, and enable anti-parallel diodes. Cooperate with IGBT. This article will introduce the use of a full-bridge inverter topology and the selection of a suitable 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 typically uses a full-bridge topology of four switches.

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

To meet this requirement, the IGBT can pulse-width modulate the frequency of 50 Hz or 60 Hz at a frequency of 20 kHz or higher, so that the output inductors L1 and L2 can maintain a reasonably small size and effectively suppress harmonics. In addition, because the conversion frequency is higher than the normal auditory spectrum of humans, the design can also minimize the audible noise generated by the inverter.

What is the best way to pulse width modulate these IGBTs? How can we minimize power consumption? One of the methods is to pulse width modulate only the high side IGBT, and the corresponding low side IGBT is commutated at 50 Hz or 60 Hz. Figure 2 shows a typical gate voltage signal. When Q1 is performing pulse width modulation, Q4 maintains a positive half cycle operation. Q2 and Q3 remain off during the positive half cycle. In the negative half cycle, when Q3 performs pulse width modulation, Q2 remains on. Q1 and Q4 will be turned off during the negative half cycle. Figure 2 also shows the AC sinusoidal voltage waveform through the output filter capacitor C1.

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

IGBT technology

The IGBT is basically a bipolar transistor (BJT) with a metal gate oxide gate structure. This design allows the gate of the IGBT to act like a MOSFET, replacing the current with a voltage to control the switch. As a BJT, IGBTs have higher current handling capabilities than MOSFETs. At the same time, IGBTs are a few carrier components like BJT. This means that the speed at which the IGBT is turned off is determined by the speed of a small number of carrier recombinations. 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 volume and technology, an overspeed IGBT has a higher Vce(on) than a standard speed IGBT. However, the speed of the overspeed IGBT is much faster than that of the standard IGBT. This relationship reflected in Figure 3 is achieved by controlling the life cycle of a few carrier recombination rates 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 use the same planar technology, but use different lifetime composite control meters. It can be seen from the table that the standard speed IGBT has the lowest Vce(on), but the standard speed IGBT has the slowest fall time compared to the fast and overspeed planar IGBT. The fourth IGBT is an optimized trench gate IGBT that provides low turn-on and switching losses for high frequency switching applications such as solar inverters. Note that the Vce(on) and total switching loss (Ets) of the trench IGBT are lower than the overspeed planar IGBT.

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