Why IGBTs are suitable for chopper applications.

Chopper is a power conversion technique in electric power control, which essentially involves pulse-width modulation of direct current. Its waveform resembles a chopping action, hence the name “chopper.” Chopper holds a crucial position in feedback speed control, as it not only affects the technical performance of speed control but also directly impacts the operational safety and reliability of the equipment. Therefore, it is essential to carefully select chopper circuits and chopper devices.

IGBT (Insulated Gate Bipolar Transistor) is a fully controllable power semiconductor device that has emerged as a recent development in the field. It combines MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) and GTR (Gate Turn-Off Thyristor), with the former serving as the driver. IGBT offers advantages such as low drive power, low on-state voltage drop, and fast switching speed. It has been widely applied in power electronic fields such as variable frequency speed control and switch-mode power supplies.

In terms of full controllability, IGBT is a suitable device for chopper applications, and its technology is relatively simple. In fact, the IGBT device itself almost constitutes the chopper circuit. However, transforming IGBT into a chopper product is not as simple, especially for high-power chopper applications. Without facing the reality, conducting thorough research, discovering, and resolving existing issues, the desired outcomes will not be achieved, and the reliability of chopper equipment will be severely compromised. Recently, it has been observed that certain companies highly favor IGBT transistors while completely disregarding thyristors, whether due to a lack of technical understanding or for commercial purposes. Clearly, this is unscientific. To respect science and clarify the facts, this article aims to analyze and compare the performance and characteristics of thyristors and transistors represented by IGBT. We hope to stimulate discussions and present a scientifically accurate perspective.

I. Rated Current and Overcurrent Capability of IGBT

1. Rated Current of IGBT

Currently, the rated current of IGBT (the current specified by the component) is specified in terms of the device’s direct current. The actual current allowed to pass through the component is reduced due to limitations imposed by the safe operating area, as shown in Figure 1. Factors affecting the current passing through the device include not only the collector-emitter voltage but also the operating frequency. Lower frequencies result in longer conduction times, leading to more severe heat generation and lower conduction current.

Clearly, for safety reasons, it is not possible to operate the component at its current state, and the current usage must be reduced. Therefore, the aforementioned rated current of IGBT actually lowers the current rating of the component, resulting in a fictitiously high rating but inadequate capability. Based on the characteristics shown in Figure 1, when the conduction time of IGBT is long (e.g., 100µs), the UCE voltage will decrease by approximately half of its rated value. If the UCE voltage is kept constant, the collector current of the device will decrease by about two-thirds of its rated value. Therefore, according to the current rating standard of thyristors, the rated current of IGBT is actually only about one-third of an equivalent thyristor. For example, an IGBT rated at 300A is only equivalent to a 100A SCR (thyristor). For instance, in a chopper circuit with a DC operating current of 500A, if a thyristor is chosen according to:

The peak current can reach up to 10 times the rated RMS current, and the overcurrent duration can be as long as 10ms. In contrast, the allowed peak current duration for IGBT, according to available information, is only about 10µs. This indicates that the overcurrent capability of IGBT is too fragile.

The ability to withstand overcurrent is a crucial factor in determining the reliability of chopper operation. It is nearly impossible to prevent overcurrent in a circuit due to load variations and transitions during switching. Overcurrent protection measures are inherently passive and limited. To ensure safe operation of the device, it is ultimately necessary to improve the device’s inherent overcurrent capability. Furthermore, due to limitations imposed by transistor manufacturing processes, it is challenging to produce single-chip IGBTs with high current capacities. Devices with higher current ratings are actually an array of smaller internal elements connected in parallel. For example, an IGBT rated at 600A consists of 8 parallel-connected 75A elements when dissected. The reliability of the device is significantly reduced compared to a single-chip thyristor in terms of reliability due to the less reliable parallel connection process (welding) of the elements.

II. Latch-up Effect of IGBT

The parasitic NPN transistor and the body diode resistance Rbr are parasitic elements formed due to the manufacturing process. As a result, the main PNP transistor and the parasitic NPN transistor form a parasitic thyristor. When the collector current of the device is sufficiently large, a forward bias voltage is generated across the resistance Rbr, leading to the conduction of the parasitic transistor and the thyristor, causing the IGBT to lose control over its gate. The device’s current rapidly increases beyond its rated value, eventually leading to device failure. This phenomenon is known as the latch-up effect. There are static and dynamic latch-up effects in IGBT, caused by excessive current during conduction and excessive voltage during turn-off. It is challenging to completely avoid the latch-up effect in practice, which significantly affects the reliability of IGBT to a certain extent.

III. High Impedance Amplification Region of IGBT

The essential difference between transistors and thyristors is that transistors have amplification functionality, with the device having three operating regions: conduction, cutoff, and amplification. The current carriers in the amplification region are in a non-saturated state, resulting in a much higher resistance compared to the conduction region. Thyristors, on the other hand, are a positive feedback combination of transistors, with only two operating regions: conduction and cutoff, without a high impedance amplification region.

It is well known that power semiconductor devices are used as switches, and the only useful operating states are conduction and cutoff. The amplification state is not only useless but also has a negative impact. The reason is that if current flows through the amplification region, the high resistance in that region will inevitably cause intense heating, leading to device failure. IGBT belongs to the transistor category and also has a high impedance amplification region. When IGBT is used as a switch, it will pass through the amplification region and generate heat, which is the principle behind why transistors, including IGBTs, are inferior to thyristors in switch applications.

IV. Packaging and Heat Dissipation of IGBT

For semiconductor devices, junction temperature is an important reliability criterion. Almost all technical parameter values are valid only under allowed temperatures (typically 120°C to 140°C). If the temperature exceeds the specified limit, the device’s performance rapidly decreases, eventually leading to damage.

The packaging form of semiconductor devices serves the purpose of device installation and heat dissipation. Currently, for devices rated above 200A, the main packaging forms are modular and flat-press types, while bolted types are mostly phased out.

Modular structures are mostly used to integrate several devices into basic power conversion circuits, such as rectifier and inverter modules. They offer advantages such as small size, convenient installation, and simple structure. However, the drawback is that the devices can only dissipate heat on one side, and it requires a baseplate that provides both insulation and good thermal conductivity (which is challenging to achieve). Therefore, modular structures are only suitable for medium to low-power units or devices.

Flat-press structures are primarily used for single high-current devices, where the device and a double-sided heat sink are fastened together, with the heat sink serving both as a heat dissipator and an electrode. The advantages of flat-press structures include excellent heat dissipation performance, ensuring safe and reliable device operation. The downside is that installation is inconvenient, the power unit structure is complex, and maintenance is less convenient compared to modular structures.

Considering the pros and cons, for semiconductor devices with currents exceeding 200A (especially above 500A), the flat-press structure is already an industry consensus. However, due to limitations in the manufacturing principles of IGBT chips, it is currently not feasible to produce high-power chips and adopt the flat-press structure. Therefore, modular structures are used instead, although they offer convenient installation, their inferior heat dissipation performance hampers reliability, which is an undeniable fact.

V. Current Sharing Issues in Parallel IGBTs

Currently, the capacity of single IGBTs abroad is 2000A/2500V, while the actual commercial device capacity is 1200A/2400V. Considering the requirements for high-power chopper applications, the rated operating current is usually between 400A and 1500A. Taking into account device operational safety, a current margin of approximately twice the rated current is necessary. Combined with the aforementioned nominal current deviation issue of IGBTs, a single device cannot meet the requirements, necessitating the use of parallel-connected devices. Current sharing is a critical issue affecting the reliability of parallel semiconductor devices. Due to the limitations of device discreteness, it is impossible to achieve complete parameter consistency among parallel devices, resulting in uneven current distribution. In such cases, the sum of the currents of the parallel devices is less than the expected total, especially in cases of severe current imbalance, where devices with higher current will be damaged. This presents a major challenge in parallel semiconductor devices. To improve the reliability of choppers and other power electronic equipment, it is advisable to avoid device parallel connection and instead use single high-current devices.

In theory, IGBTs have a positive temperature coefficient in high-current states, which can improve current sharing performance. However, this improvement is limited. Additionally, current sharing in controllable semiconductor devices also requires consistent driving, as even if the conduction characteristics are consistent, achieving current sharing is not possible without consistent driving. Therefore, parallel connection of IGBTs poses significant challenges.

VI. IGBT Driving and Isolation Issues

Controlled semiconductor devices, including thyristors and transistors, have a control section. To improve reliability, it is required that the driving or triggering section must be strictly isolated from the main circuit, with no electrical connection between them.

Unlike the pulse-triggering characteristics of thyristors (pulse-driven), transistors such as IGBTs require continuous current or voltage at the gate for conduction (level-driven). Therefore, transistors cannot achieve isolation using pulse transformers like thyristors do. The driving circuit must be active, resulting in a more complex circuit that includes the driving power supply and high-voltage isolation from the main circuit. Practical experience has shown that the driving isolation of transistors is a significant factor contributing to decreased system reliability. According to incomplete statistics, failures caused by driving isolation issues account for more than 15% of total failures.

VII. Conclusion

IGBTs have limitations in their capacity and transistor characteristics, particularly in higher power (above 500 kW) applications, such as regenerative speed control. These limitations primarily manifest in terms of reliability under overcurrent and overvoltage conditions. The shortcomings of IGBTs should not overshadow their advantages in full control. Scientific practice requires a scientific attitude.

In terms of reliability in high-power switching applications, thyristors are superior to transistors, as determined by the principles of semiconductor devices. Currently, there is rapid development of new types of thyristors aimed at addressing the drawbacks of ordinary thyristors, specifically the inability to turn off the gate. The integrated gate-commutated thyristor (IGCT), which combines the advantages of thyristors and MOSFETs, is an ideal thyristor device for high-power chopping applications. However, both IGCTs and IGBTs currently rely on imports and have expensive prices, which pose challenges for regenerative speed control applications in our country. Factors such as high maintenance costs, difficulty in controlling device parameters, and long supply lead times should all be carefully considered during product development.

Despite the difficulty in turning off ordinary thyristors, if this issue can be resolved, they still remain the dominant direction for high-power chopping applications in the near term. The reason is that the other advantages of ordinary thyristors cannot be replaced by transistors.