Application of IGBT in high power chopping

Chopping is a current conversion technology in power electronic control. Its essence is pulse width modulation of DC control. It is named chopping because its waveform is neat and symmetrical like chopping. Chopping plays an extremely important role in internal feedback speed control. It is not only related to the technical performance of speed regulation, but also directly affects the operation safety and reliability of equipment. Therefore, how to choose chopping circuit and chopping device is very important.

IGBT is a fully controlled power semiconductor device newly developed in modern times. It is a combination of MOSFET (field effect transistor) and GTR (high power Darlington transistor), and is driven by the former. Therefore, it has the advantages of small driving power, low on-state voltage, fast switching speed, etc. It has been widely used in power electronics fields such as variable frequency speed regulation and switching power supply.

In terms of full control performance, IGBT is the most suitable device for chopping applications, and the technology is extremely simple. Almost the IGBT device itself constitutes the chopping circuit. However, it is not so simple to turn IGBT chopping into a product, especially for high-power chopping. If we do not face the reality, seriously study, discover and solve the existing problems, it will be counterproductive and the reliability of the chopping equipment will be seriously damaged. I don’t know whether it is due to technical understanding or business purposes. Recently, it has been found that some companies highly praise IGBT transistors and completely deny thyristors. Obviously, this is unscientific. In order to respect science and clarify the facts, this article analyzes and compares the performance and characteristics of thyristors and transistors represented by IGBTs, hoping to arouse discussion and restore science to its original appearance.

1. IGBT nominal current and overcurrent capability

1) Rated current of IGBT

At present, the rated current of IGBT (nominal current of the component) is nominally based on the maximum DC current of the device. The current actually allowed to pass through the component is reduced by the limitation of the safe working area. It can be seen from the IGBT safe working area shown in Figure 1 that the factors affecting the current passing through are not only the CE voltage, but also the operating frequency. The lower the frequency, the longer the conduction time, the more serious the component heating, and the smaller the conduction current.

Figure 1 IGBT safe operating area

Obviously, for safety reasons, it is impossible to make the components work at the maximum current state, and the current must be reduced. Therefore, the above-mentioned current rating of the IGBT actually reduces the current rating of the component, resulting in a falsely high nominal value and insufficient capacity. According to the characteristics of Figure 1, when the IGBT conduction time is long (for example, 100us), the UCE voltage will be reduced by about 1/2 of the nominal value; if the UCE remains unchanged, the maximum collector current of the component will be reduced by 2/3 of the rated value. Therefore, according to the current nominal standard of the thyristor, the nominal current of the IGBT is actually only about 1/3 of that of the equivalent thyristor. For example, an IGBT rated at 300A is only equivalent to an SCR (thyristor) of 100A. For another example, in a chopper circuit with a DC operating current of 500A, if a thyristor is selected, when:

Ki in the formula is the current margin coefficient. If Ki=2, a thyristor with a nominal value of 630A can actually be selected.

If IGBT is selected, then:

A 3000A IGBT component should be selected.

Whether the current rating of IGBT, which follows the current rating of ordinary transistors, is reasonable in power switch applications is worth discussing. However, no matter what the result is, it is an indisputable fact that the rated current of IGBT must be greatly discounted when it is used.

1) IGBT overcurrent capability

The overcurrent capacity of semiconductor components is usually measured by the allowable peak current IM. There is currently no internationally accepted standard for IGBTs. According to the product parameters of companies such as EUPEC in Germany and Mitsubishi in Japan, the peak current of IGBTs is set at twice the maximum collector current (nominal current).

For example, a component with a nominal current of 300A has a peak current of 600A; while a component with a nominal current of 800A has a peak current of 1600A.

Compared with thyristors, according to national standards, the peak current is

The peak current is as high as 10 times the rated effective current, and the overcurrent time is as long as 10ms. However, according to relevant information, the allowable peak current time of IGBT is only 10us. It can be seen that the overcurrent capability of IGBT is too fragile.

The ability to withstand overcurrent is the key to measuring the reliability of chopper operation. It is almost impossible to prevent overcurrent from occurring in the circuit. Changes in load and the transition process of working state switching will cause overcurrent and overvoltage. Overcurrent protection is, after all, a passive and limited measure. To ensure the safe operation of the device, it is ultimately necessary to improve the device's own overcurrent capability.

In addition, due to the limitations of the transistor manufacturing process, it is difficult to make an IGBT into a single die with a large current capacity. Devices with larger currents are actually the parallel connection of small internal components. For example, an IGBT with a nominal current of 600A is actually eight 75A components connected in parallel. Due to the poor reliability of the component parallel connection process (welding), the device is significantly less reliable than a single-die thyristor.

2. IGBT's Holding Effect

The simplified equivalent circuit of IGBT is shown in Figure 3:

Figure 3 IGBT equivalent circuit and thyristor effect

The NPN transistor and the body short-circuit resistor Rbr are both parasitic due to the process. In this way, the main PNP transistor and the parasitic NPN transistor form a parasitic thyristor. When the collector current of the device is large enough, the positive bias voltage generated on the resistor Rbr will cause the parasitic transistor to turn on, causing the parasitic thyristor to turn on, the gate of the IGBT to lose control, the current of the device to rise rapidly and exceed the rated value, and finally burn the device. This phenomenon is called the holding effect. There are two kinds of holding effects in IGBT, static and dynamic, which are caused by excessive current when turned on and voltage when turned off. It is very difficult to avoid the holding effect in practice, which greatly affects the reliability of IGBT to some extent.

3. High resistance amplification area of ​​IGBT

"The transistor is an amplifier," Carroll, a semiconductor expert at ABB, gave a fair evaluation of the transistor in the literature 1. The essential difference between transistors and thyristors is that transistors have an amplification function, and the device has three working areas: on, off, and amplification. The carriers in the amplification area are in a non-saturated state, so the resistance of the amplification area is much higher than that of the conduction area; thyristors are a positive feedback combination of transistors, and the device has only two working areas: on and off, and no high-resistance amplification area.

As we all know, power semiconductor devices are used as switches. The only useful working states are on and off. The amplified state is not only useless, but also has a negative effect. The reason is that if the current passes through the amplified area, due to the large resistance of this area, it will inevitably cause intense heating and cause the device to burn out. IGBT is subordinate to transistors and also has a high-resistance amplified area. When the device is used as a switch, it will inevitably pass through the amplified area and cause heating. This is the principle why transistors including IGBT are inferior to thyristors in switching applications.

Figure 4a PNPN structure and equivalent circuit of thyristor

4. IGBT packaging and heat dissipation

For semiconductor devices, die temperature is the most important reliability condition. Almost all technical parameter values ​​are valid only under the allowable temperature (usually 120℃-140℃). If the temperature exceeds the limit, the performance of the device will drop sharply and eventually lead to damage.

The packaging form of semiconductor devices is for device installation and device heat dissipation. For devices with a rated current of more than 200A, the main packaging forms are module type and flat plate press-fit type, and the bolt type has been basically eliminated.

Modular structures are mostly used to integrate several devices into basic power conversion circuits, such as rectifier and inverter modules. They have the advantages of small size, easy installation, and simple structure. The disadvantage is that the device can only dissipate heat on one side, and the base plate must be both insulated and have good thermal conductivity (which is difficult to achieve). It is only suitable for small and medium power units or devices.

The flat plate structure is mainly used for single high current devices. It is to fasten the device and the double-sided heat sink together. The heat sink is used for both heat dissipation and electrode. The advantages of the flat plate type are good heat dissipation performance and safe and reliable device operation. The disadvantages are inconvenient installation, complex power unit structure, and less convenient maintenance than the modular type.

Considering the pros and cons, it is a consensus in the industry that a flat-plate structure is preferred for semiconductor devices with a current greater than 200A (especially above 500A). However, due to the limitations of the core manufacturing principle, IGBT cannot be manufactured into a high-power chip and a flat-plate structure cannot be used. Therefore, a modular structure has to be used. Although it is easy to install, it is an indisputable fact that the poor heat dissipation performance is not conducive to reliability.

5. IGBT parallel current sharing problem

At present, the maximum capacity of a single-tube IGBT abroad is 2000A/2500V, and the actual commercial device capacity is 1200A/2400V. According to the needs of high-power chopping, the rated working current is usually 400A-1500A. Considering the safety of the device, a current margin of about 2 times must be reserved. Combined with the maximum current nominal problem of IGBT mentioned above, a single device cannot meet the requirements, and devices must be connected in parallel. The current sharing problem in parallel semiconductor devices is the key to affecting reliability. Due to the limitation of discreteness, the parameters of parallel devices cannot be completely consistent, which leads to uneven current in parallel devices. At this time, 1+1 is less than 2. Especially when the current is seriously uneven, the device with large on-state current will be damaged. This is a long-standing problem in parallel semiconductor devices. Therefore, in order to improve the reliability of chopping and other power electronic equipment, devices should be avoided in parallel as much as possible, and single-tube high-current devices should be used.

Theoretically, IGBT has a positive temperature coefficient under high current conditions, which can improve the current sharing performance, but it is limited after all. In addition, the current sharing of controllable semiconductor devices must also consider the driving consistency. Otherwise, even if the conduction characteristics are consistent, current sharing cannot be achieved. This creates great difficulties for parallel connection of IGBTs.

6. IGBT drive and isolation issues

Controllable semiconductor devices all have control parts, and thyristors and transistors are no exception. In order to improve reliability, the drive or trigger part must be strictly isolated from the main circuit, and the two cannot have electrical connection.

Different from the pulse edge triggering characteristics of thyristors (edge ​​drive), the conduction of transistors such as IGBTs requires the gate to have a continuous current or voltage (level drive). In this way, transistors cannot be isolated by using pulse transformers like thyristors. The drive circuit must be active, the circuit is relatively complex, and the drive power supply must be isolated from the main circuit with a high withstand voltage. Practice has proved that the drive isolation of transistors is a factor that cannot be ignored in reducing system reliability. According to incomplete statistics, the probability of failures caused by drive isolation problems accounts for more than 15% of the total failures.

VII. Conclusion

Appendix 1 and 2 summarize the comparison of some performances of thyristors and IGBTs:

Appendix 1 Partial performance comparison between SCR (thyristor) and IGBT

IGBT chopper is limited by device capacity and transistor characteristics, and there are still problems in the application of internal feedback speed regulation with higher power (above 500KW), which is mainly manifested in the reliability of withstanding overcurrent and overvoltage. The full control advantages of IGBT cannot cover up its shortcomings. Scientific practice requires a scientific attitude.

In terms of reliability in high-power switch applications, thyristors are superior to transistors, which is determined by the principle of semiconductor devices. At present, the development of new thyristors is very fast, with the aim of solving the disadvantage of ordinary thyristors that they cannot turn off the gate. The latest combination of TGO and MOSFET launched by foreign countries (currently only ABB) - integrated gate commutation thyristor IGCT is an ideal thyristor device and is most suitable for high-power chopping applications.

Currently, both IGCT and IGBT have the problem of dependence on imports and high prices. As a result, the application of chopper internal feedback speed regulation in my country has caused considerable difficulties. Factors such as high maintenance costs, difficulty in controlling device parameters, and long delivery time should be carefully considered during productization.

Although ordinary thyristors have the disadvantage of difficult turn-off, if this disadvantage can be solved, they will still be the dominant direction of high-power chopper applications in the near future. The reason is that ordinary thyristors have other advantages that cannot be replaced by transistors.