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The half-bridge converter is one of the most popular topologies in power electronics, especially in uninterruptible power supplies (UPS), solar inverters, and motor drives.
Fig. 1: The operating modes of a half-bridge circuit
The half-bridge output voltage depends on the switching state and current polarity, shown in Figure 1. If an inductive load draws positive current, Ig>0, it will flow through T1 and supply energy to the load, Vg. On the contrary, if the load current, Ig, is negative, the current flows back through D1 and returns energy to the dc source.
Similarly, if T4 is on, which happens when T1 is off, a voltage equal to half of the bus voltage is applied to the load, and the current decreases. If Ig is positive, the current flows through D4, returning energy to the bus source.
In so-called four-quadrant operation, different aspects of an IGBT’s characteristics are stressed, as shown in Figure 2:
Fig. 2: A half-bridge circuit’s four-quadrant operation
Power at the time intervals 4 and 2 is negative. This negative power is called reactive power. Reactive power is common in motor drives and other applications, and it increases the apparent power of a converter. A converter must be able to accommodate this element of total power to properly drive a reactive load.
Power line networks in most countries have not been upgraded to support the increased number of solar inverters. As a consequence, during peak daylight time, while solar generators feed the line, there is likely to be an over-voltage at sub-nodes. Hence all new solar inverters have to be able to absorb the over-voltage through the generation of reactive power.
Factors affecting IGBTs in motor drives and solar inverters
The main characteristics of a motor drive application using a half-bridge topology are:
By contrast, the main characteristics of inverters suitable for solar and UPS applications are:
Emerging topologies for high-power conversion
The classical half-bridge in these types of applications has some limitations:
To overcome these limitations, new topologies with multi-voltage levels have been developed for use in power electronics. The most common structures are the so-called I-type, shown in Figure 3, and T-type converters, shown in Figure 4. These topologies can operate at higher bus voltages than the half-bridge. Due to the availability of more output states, the voltages across filter components are lower, and result in much lower filter losses and a smaller filter.
Fig. 3: I-type converter
Even switching losses fall significantly, while conduction losses go up slightly, a benefit in higher-frequency applications. These topologies implement unipolar switching by connecting to the neutral point during the off cycles.
Fig. 4: T-type converter
It is worth mentioning that alongside the numerous advantages, these multi-level topologies present some difficulties, such as:
Advances in semiconductors and control technologies are enabling the use of these converter types in mid- to low-power applications consuming less than 10 kW. In fact, each of the I- and T-type topologies has its own advantages and disadvantages depending on operating conditions: the T-type shines at lower frequencies, and has lower switching losses than the half-bridge. The I-type offers better performance at high frequencies.
Semiconductor improvements might shift the balance of benefits and drawbacks between the I- and T-type topologies. In general, it is true that three-level inverters help to improve efficiency and increase operating frequency. In rectifier mode, the T-type is better for mid-frequencies while the I-type offers better high-frequency operation and better thermal balance.
One of the main disadvantages of both lies in the more complex control circuitry and the need for more semiconductor components, though not necessarily a larger silicon area.
Conclusion
Despite the fact that IGBTs have been in the market for a long time, this technology is still very well suited to high-voltage and high-current applications. The use of IGBTs is growing not only in classical applications, but also in new ones. This is because new technologies are able to switch up to 100 kHz. Hence, it is important to better understand the application requirements and choose the right set of IGBT trade-offs.
Figure 5 shows how a given IGBT can produce a different pattern of losses in different topologies operating at the same frequency.
Fig. 5: Distribution of losses for the same IGBT operating in a Vienna topology (top left), half-bridge (top right), and full-bridge (bottom)
Even in the same topology, the pattern can vary with the operating point. Figure 6 shows the pattern of losses in a T−type topology for the outer and the inner IGBT in inverter, A and B, and rectifier, C and D, modes.
Fig. 6: Distribution of losses for the same IGBT operating in a T-type inverter in the outer (left) and inner (right) position in inverter and rectifier modes
Understanding the system requirements and implementing accurate measurement systems are important to the development of reliable designs using IGBTs. It is even more important when operating at the very high efficiencies enabled by modern IGBTs and topologies. This means that adequate analysis and measurement time invested during the design phase can support the selection of the right IGBT for the application.
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