IGBT is a newly developed fully-controlled power semiconductor device. It is a combination of MOSFET (Field Effect Transistor) and GTR (High Power Darlington Transistor) and is driven by the former. Therefore, it has: Small driving power, on-state With the advantages of reduced pressure and fast switching speed, 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 chopper applications, and the technology is extremely simple, and almost the IGBT device itself constitutes a chopper circuit. However, the problem of IGBT chopping to form a product is not so simple, especially high-power chopping. If you do not face the reality, serious research, discovery and solution to the existing problems will backfire and the reliability of the chopper will suffer. Serious damage. I do not know whether it is for technological awareness or business purposes. Recently, it has been found that some companies are more respected for IGBT transistors and refusal for thyristors. Obviously, this is unscientific. In order to respect science and clarify the facts, this paper analyzes and compares the performance and characteristics of thyristors and transistors represented by IGBTs. It is hoped that they can be discussed and scientifically taken into consideration.
I. IGBT nominal current and overcurrent capability
1) Rated current of IGBT
At present, the IGBT's rated current (component nominal current) is nominally the maximum DC current of the device. The current that the component actually allows to pass is limited by the safe operating area. The IGBT safe working area shown in Figure 1 is reduced. It can be seen that the factors affecting the passing current are not only the ce voltage but also the operating frequency. The lower the frequency, the longer the conduction time, the more severe the element heating, and the smaller the conduction current.
Figure 1 IGBT safe work area
Obviously, in order to be safe, it is impossible to let the element work in the maximum current state, and the current use must be reduced. Therefore, the above-mentioned current rating of the IGBT actually lowers the current rating of the element to form a nominally imaginary high, and the capacity is insufficient. According to the characteristics of Figure 1, when the IGBT is turned on for a long time (for example, 100us), the UCE voltage will decrease by about 1/2 of the nominal value; if the UCE is kept constant, the maximum collector current of the component will decrease by 2 /3. Therefore, according to the current standard of the thyristor, the nominal current of the IGBT is actually only about 1/3 of the equivalent thyristor. For example, the 300A IGBT is only equivalent to 100A SCR (thyristor). Another example is a chopper circuit with 500A DC operating current. If a thyristor is selected, press:
Ki in the formula is the current margin coefficient, take Ki=2, actually can choose 630A nominal thyristor.
If IGBT is selected, it is:
3000A IGBT components should be selected.
IGBTs, which follow the current guidelines for normal transistors, are reasonable for power switch applications and are worth exploring. But no matter what the result, it is an indisputable fact that the nominal current of the IGBT must be greatly reduced when it is applied.
1) Overcurrent capability of IGBT
The overcurrent capability of semiconductor components is usually measured by the allowable peak current IM. IGBTs do not currently have internationally accepted standards. According to the product parameters of German EUPEC, Mitsubishi and other companies, the peak current of the IGBT is defined as the maximum collector current (nominal). 2 times current)
For example, the peak current for a nominal 300A component is 600A, while the nominal 800A component has a peak current of 1600A.
Compared to thyristors, according to the national standard, the peak current is
The peak current is as high as 10 times the rated rms current, and the overcurrent time is as long as 10ms, while the allowable peak current time of the IGBT is only 10us according to relevant data. It can be seen that the overcurrent capability of the IGBT is too fragile.
The ability to withstand overcurrent is the key to measuring the reliability of the chopper wave. It is almost impossible to make the circuit not overcurrent. The over-current of the load and the transition of the working state will cause over-current and over-voltage. However, over-current protection is a passive and limited measure after all. To make the device work safely, it is ultimately necessary to increase the device's own over-current capability.
In addition, due to the limitations of the transistor manufacturing process, it is difficult for IGBTs to be made into a single die with a large current capacity. The device with a larger current is actually a parallel connection of small internal components. For example, an IGBT with a nominal current of 600 A is disassembled to be 8 Only 75A components are connected in parallel. Due to the poor reliability of the component parallel process (welding), the device is significantly less reliable than a single die thyristor.
II. IGBT's staying effect
The simplified equivalent circuit of IGBT is shown in Figure 3:
Figure 3 Equivalent Circuit and Thyristor Effect of IGBT
The NPN transistor and the body short-circuit resistance Rbr are parasitically formed by the process. Thus, the main PNP transistor and the parasitic NPN transistor form a parasitic thyristor. When the collector current of the device is large enough, a positive result is generated in the resistor Rbr. The bias voltage will cause the parasitic transistor to turn on, cause the parasitic thyristor to turn on, the gate of IGBT loses control, the electric current of the device rises rapidly and exceeds the fixed value, will burn out the device finally, this kind of phenomenon is called the engine hold effect. There are two static and dynamic IGBT holding effects, caused by the current at turn-on and the turn-off voltage is too large. It is very difficult to avoid the hold-up effect at all in practice, which greatly affects The reliability of IGBT.
Three. IGBT high impedance amplification area
"The transistor is an amplifier." ABB's semiconductor expert Carroll gave a fair evaluation of the transistor in Document 1. The essential difference between a transistor and a thyristor is that the transistor has an amplification function, and the device has three working regions of conduction, cut-off, and amplification, and the carrier of the amplifying region is in a non-saturated state, so the resistance of the amplifying region is much higher than the conduction region; The thyristor is a positive feedback combination of transistors. The device only has two working regions, on and off, and no high-resistance amplification region.
As we all know, power semiconductor devices are used as switches, and the useful operating state is only 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 amplifying region, the resistance in the region is large, which inevitably causes severe heat generation and causes the device to be burned. IGBT belongs to the transistor, there is also a high resistance amplification area, the device in the switch application, will inevitably cause amplification through the amplification area, which is the transistor including IGBT in the switching application inferior to the thyristor principle.
Figure 4a PNPN structure and equivalent circuit of thyristor
Four. IGBT package form and heat dissipation
For semiconductor devices, die temperature is the most important reliable condition. Almost all technical parameter values ​​are established under the allowable temperature (usually 120o - 140oC). If the temperature exceeds the limit, the performance of the device is drastic Falling, eventually leading to damage.
Semiconductor device packaging is for device mounting and device cooling. For devices with a rating of more than 200A, the main package types are modular and flat-panel pressure-bonded types. The bolt type has basically been eliminated.
The modular structure is mostly used to integrate several devices into a basic converter circuit, for example, a rectifying and inverting module. The module has the advantages of small size, convenient installation, and simple structure. The disadvantage is that the device can only dissipate heat on one side, and requires that the base plate not only Insulation also requires good thermal conductivity (difficult to achieve), and is only applicable to small to medium power cells or devices.
The flat plate structure is mainly used for a single high-current device, which is to fasten the device and the double-sided heat sink together, and the heat sink is used for both heat dissipation and electrodes. The advantage of the flat type is that the heat dissipation performance is good and the device is safe and reliable. The disadvantage is the inconvenience of installation, the power unit structure is complex, and maintenance is not as convenient as modularity.
Comprehensive advantages and disadvantages, when the current is greater than 200A (especially more than 500A) of the semiconductor device on the preferred plate-type structure, is already a consensus in the industry, but the IGBT is subject to the principle of the production of the core of the tube, can not be manufactured into high-power chips, can not use flat-type structure However, the use of a modular type has been adopted. Although it is easy to install, poor heat dissipation is not conducive to reliability. This is an indisputable fact.
V. IGBT parallel current sharing problem
At present, the single-tube IGBTs in foreign countries have a maximum capacity of 2000A/2500V. The actual commercial device capacity is 1200A/2400V. According to the needs of high-power chopping waves, the rated operating current is usually 400A to 1500A. In view of the safety of the device, it must be Leaving about 2 times the current margin, combined with the aforementioned IGBT maximum current nominal problem, a single device can not meet the requirements, the device must be used in parallel. The current sharing problem of parallel connection of semiconductor devices is the key to reliability. Due to the limitation of discreteness, the parameters of parallel devices cannot be completely consistent, resulting in uneven current of parallel devices. In this case, 1+1 is less than 2, especially When the current is severely uneven, the devices with large on-state currents will be damaged. This is a problem in the parallel connection of semiconductor devices. To improve the reliability of choppers and other power electronic devices, parallel devices should be avoided. Single-tube high-current devices are used.
Theoretically speaking, IGBTs have a positive temperature coefficient in a large current state, which can improve current sharing performance. However, after all, the current sharing performance of controlled-semiconductor devices must be considered in conjunction with the drive current. Otherwise, even if the conduction characteristics are consistent, It is also not possible to achieve current sharing, which makes it extremely difficult to parallel IGBTs.
6. IGBT drive and isolation issues
Controllable semiconductor devices all have control parts, and thyristors and transistors are no exceptions. In order to improve reliability, it is required that the driving or triggering portion must be strictly isolated from the main circuit, and the two cannot have an electric connection.
Unlike thyristor edge-triggered features (along with driving), the turn-on of transistors such as IGBTs requires that the gates have a continuous current or voltage (level drive), so that the transistors cannot be isolated by the use of pulse transformers like thyristors. The driver circuit must be active, the circuit is more complex, and including the drive power, it must have high voltage isolation from the main circuit. Practice has proved that the drive isolation of transistors is a non-negligible factor leading to the reduction of system reliability. According to incomplete statistics, the probability of failure due to drive isolation issues accounts for more than 15% of total failures.
Seventh. Conclusion
Tables 1 and 2 summarize the comparison of the performance of thyristors and IGBTs:
Schedule 1 Comparison of SCR (Thyristor) and IGBT Performance
Devices
Performance parameters
Thyristor
(KP KK)
IGBT
(transistor)
in conclusion
Device equivalent
Bistable trigger
Magnifier
The former is best suited for switching applications
With or without high resistance amplification area
no
Have
Best without amplification
Packaging method
Flat pressure contact
Modular
Cooling
Double-sided
Single side
Double sided heat dissipation
Die mode
Large size
Small point
Internal structure
Single die
Multi-core parallel
Single tube reliable
Rated/nominal current
1.57
0.5-1
Surge/nominal current
10-15
2
On-state pressure drop
1.5-2
2.5-4
Current rise rate control
external
internal
By the interior, easy to heat damage
Schedule 2 Comparison of Trigger and Drive Performance of SCR and IGBT
Devices
performance
Thyristor
IGBT (transistor)
in conclusion
Conduction
Gate trigger
Gate drive
Consistent
Shut down
Anode reverse bias
Grid reverse bias
The latter is excellent, simple and convenient
Apply pulse transformer
Be applicable
Not applicable
Drive isolation
easy
difficult
No isolation, easy to interfere
Pulse drive
cutting edge
Level
IGBT chopping is limited by the device capacity and transistor characteristics, and there are still problems in the application of high-power (above 500 kW) feed-in speed control, which is mainly reflected in the reliability of over-current and over-voltage. The IGBT's full-control advantage cannot be used to mask its shortcomings. Scientific practice requires a scientific attitude.
Thyristors are superior to transistors in the reliability of high-power switching applications, which is determined by the principle of semiconductor devices. At present, the development speed of the new thyristor is very fast, the purpose is to solve the shortcomings of the ordinary thyristor gate can not be shut down, foreign (currently only ABB company) the latest TGO and MOSFET combination - integrated gate commutated thyristor IGCT It is an ideal thyristor device and is most suitable for high power chopper applications.
Currently, IGCT and IGBT have the problems of dependence on import and high price. Due to its impact, China's chopper-wave feed-forward speed control applications cause no small difficulties, high maintenance costs, difficulty in controlling device parameters, and long supply time. All should be carefully considered during productization.
Although common thyristors have the disadvantage of being difficult to turn off, if they can be solved, they are still the dominant direction for the application of high-power choppers in the near future. The reason is that the other advantages of ordinary thyristors are that the transistors cannot be replaced.