Industry Article

Optimal IGBT Performance in Diverse Applications

March 31, 2024 by José Padilla, Littelfuse

Selecting the appropriate discrete IGBT that aligns with the application-specific requirements is crucial to optimal performance.

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

 

Since the advent of Insulated Gate Bipolar Transistors (IGBT) in the 1980s, tremendous progress has been made in terms of power density, switching frequency capability, on-state voltage drop, and ruggedness. The consistent demand for efficient power semiconductors from the industry has fueled the advancements in IGBT technology and optimization of both, dynamic losses (Esw), and static losses (Econd). Achieving high power density in applications such as inverters, converters, and power supplies has been a major challenge and the key reason modern IGBTs are optimized to have high switching frequency capability. Like any other power semiconductor device, IGBTs also have limitations. There is a trade-off between switching frequency capability and on-state voltage drop. Higher switching frequency capability leads to increased forward voltage. Most applications benefit from higher switching frequencies to reach higher efficiency and power density. However, not all applications require high switching frequencies. There are numerous applications like safety switches, lamp ballast, capacitor discharge circuits, or grid-frequency switched transistors in transformer-less solar inverter topologies. For these applications, the key requirement is the use of power semiconductor devices with low conduction losses. IGBTs with low voltage drops are most favorable for these applications. Using an IGBT, which is optimized for higher switching frequency operation but featuring higher static losses, would reduce the system efficiency of those applications.

 

Image used courtesy of Adobe Stock

 

With so-called Collector Engineering, IGBTs can be designed to feature a combination of switching losses and forward voltage drop according to the physical limits given by the trade-off correlation. 

 

Figure 1. Trade-off curve between switching losses Psw and on-state voltage drop VCE(sat). Image used courtesy of Bodo’s Power Systems [PDF]

 

IGBTs for Application-Specific Requirements

An IGBT’s on-state voltage drop and its switching losses are correlated to each other, as illustrated in Figure 1.

Littelfuse offers three different classes of discrete IGBTs in the 600 V – 1200 V voltage range. Separated in classes A, B, and C, they are optimized to support low, medium, and high switching frequencies. A-Class IGBTs are optimized to have low on-state voltage drops. These IGBTs are suitable for applications operating at switching frequencies from DC to 5 kHz. Similarly, B and C class IGBTs are optimized for 5 – 20  kHz and greater than 20  kHz, respectively.

Littelfuse offers one of the widest ranges of discrete IGBTs, in single and co-pack, in different current ratings and packages in the market, as shown in Figure 2.

 

Figure 2. XPT 650 V Gen5 and 1200 V Gen4 A- Class. Image used courtesy of Bodo’s Power Systems [PDF]

 

The XPT 650 V Gen5 and 1200 V Gen4 A-Class Trench discrete IGBT series offer a significant reduction in on-state voltage drop, high surge current carrying capability, low gate charge, low thermal resistance, and high power density in a discrete package. The low forward voltage drop is particularly important for applications where a high switching frequency is not needed. High surge current capability is extremely helpful for protection applications such as hybrid DC breakers. The low gate charge QG results in a low power requirement for the gate drive circuitry. With low thermal resistance, thermal-related design challenges are easier to overcome.

 

Figure Of Merit

Figure 3 represents the figure of merit (FOM) of the new 1200 V A4 class IGBT compared with selected competitor devices. The first figure of merit is QG x VCE(sat), this FOM combines two critical parameters of IGBTs in low switching frequency applications: The required power of the gate driver to turn on the IGBT by charging the gate and how much the conduction losses will be based on the collector current flowing through the IGBT, expressed by VCE(sat). Another figure of merit is Rth(j-c) x VCE(sat), this FOM points to another important parameter. In low switching frequency applications, thermal burden is dominated by conduction losses. Rth(j-c) x VCE(sat) stands for the ease of taking out heat losses produced during the IGBT’s low switching frequency operation. The smaller this factor is, the easier it is to extract the heat out of the die and remain within the given thermal limits.

 

Figure 3. The figure of merit, Littelfuse’s IXYH55N120A4, and Competition. Image used courtesy of Bodo’s Power Systems [PDF]

 

The 1200 V, 55 A, A4 IGBT has 40% lower Rth(j-c) x VCE(sat) and 50% lower QG x VCE(sat) compared to a similar competitor’s device.

 

Applications

Automatic Transfer Switch

For critical infrastructure such as hospitals and airports, there are multiple energy sources to prevent loss of power in case of grid failure. Normally, the preferred AC source is the grid connection. In case of an interruption in the AC supply from the grid, an alternative energy source is used to ensure an uninterrupted supply of the critical load as shown in Figure 4a. Here, IGBTs in common emitter configuration are used to form a bidirectional switch. Since these IGBTs conduct continuously, the critical parameter is low forward voltage drop to achieve low conduction losses. A-Class IGBTs are the best fit for this application.

 

DC Load Switch

DC load switches are used as a protection switch for DC load. An IGBT is connected in series with the load being supplied by a DC source. An RCD snubber circuit can be used to protect the IGBT in case the load is inductive. A typical circuit is shown in Figure 4b. In this application, the IGBT is conducted as long as the DC load is being supplied by the DC source. Hence, low conduction losses are desired in this application and A-Class IGBTs are perfectly suited for this application.

 

Lamp Discharge/Laser Generator

Lamp discharge is a typical capacitor discharge application, as shown in Figure 5a. Once the user triggers the button, capacitor ‘C’ is charged by the charging circuit, and a control circuit controls the gate driver of the IGBT. Once the trigger circuit generates a few kV of voltage, the lamp discharge occurs. The laser pulse generator works on a similar principle. The DC bus voltage is stepped up, and the discharge is controlled through an IGBT connected in series on the primary side of the step-up transformer, as depicted in Figure 5b. These applications require high surge current capability of the IGBT and low conduction losses to conserve the battery energy. Therefore, A-Class IGBTs are suitable for this application.

 

Figure 4. a) Static transfer switch, b) DC load switch. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 5. a) Generic Lamp discharge circuit, b) Generic Laser pulse generation circuit. Image used courtesy of Bodo’s Power Systems [PDF]

 

Hybrid DC Breaker

DC circuits are getting more popular not only for photovoltaic plants but also for industrial, marine, and data center applications. The basic working principle of a hybrid DC breaker involves three components in parallel as depicted in Figure 6. An MOV, an IGBT, and a mechanical switch are connected in parallel. In normal operation, the current flows through the mechanical switch. In case a fault occurs, the IGBT is turned on to take over the load current from the mechanical switch. While the load current is bypassed through the IGBT, the mechanical switch can be opened without arcing. After the mechanical switch has opened, the IGBT can turn off and the MOV prevents the IGBT and the mechanical to suffer from an overvoltage breakdown. A very high surge current capability of the IGBT is required. The high current passing through the IGBT contributes to high conduction losses and the resulting heat needs to be dissipated out of the IGBT. Therefore, low static losses of the IGBT are favorable as well as a low thermal resistance supports safe operation. The A-Class IGBTs of Littelfuse are perfectly suitable for this application.

 

Figure 6. Hybrid DC breaker basic circuit topology. Image used courtesy of Bodo’s Power Systems [PDF]

 

Takeaways of Optimal IGBT Performance 

Optimal performance in diverse applications depends significantly on the careful selection of discrete IGBTs that precisely match the application-specific requirements. The decision-making process involves a crucial trade-off between managing the on-state voltage drop, commonly known as VCE(sat), and the switching losses of the IGBT. Many applications demand IGBTs to have low VCE(sat) where switching losses are of less concern. Littelfuse introduces the latest XPT™ 650  V Gen5 and 1200 V Gen4 A-Class IGBTs, which have been specifically optimized for low VCE(sat). high surge current capability, low gate charge QG, and reduced thermal resistance Rth(j-c).

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine and is co-authored by Faheem Zahid, Product Marketing Manager of Discrete IGBT, and José Padilla, Director of Product Marketing at Littelfuse.