An insulated-gate bipolar transistor (IGBT) refers to a power semiconductor device usually with three terminals which is primarily used as an electrical switch. IGBTs are gaining great importance in the power electronics from both the served application and installed devices.
Currently IGBT module is applicable in a range of applications ranging from wind power production, motor drives, industrial inverters and the HVDC converters. A Lot of focus is being put in place for the IGBT module to ensure higher power densities. The higher power densities requires the IGBT to operate under high temperatures. Due to that the system of the IGBT has to be improved to meet the requirements which can allow it to operate efficiently under such conditions of high temperatures.
The development of the IGBT power modules has been in the recent past been characterized by the frequent increase in power density with the main aim of reduction of costs of power. The demand for a high power density is directly associated with the current per chip. Increasing the current per chip results in an increase of temperature during operations as shown in the figure below.
Analyses of the Problem
The generator of a wind turbine is usually controlled by a power converter which consists insulated gate bipolar transistors and other components. Increase in the wind speed results to proportional increase of the turbine speed which directly leads to production of high power density. There is a proportional increases in junction temperature with the increase in the power density which is being produced with the IGBT. The temperature which is produced is used to determine the output current which can be achieved by the generator. A power converter which is configured as an H-bridge is usually used for validation. The converter is usually equipped with the Infineon adapter board which is used for monitoring of the thermal behaviour of the components which are usually under the real field conditions.
State of the Art literature review
There are many benefits which are associated with the use of module components which has high junction temperature capabilities such as; the possibility of sink’s thermal resistance increasing to ambient, this results in lower cost of the heat sink which has a lower performance such as the case of the windmill generator where higher liquid cooling temperatures are accepted.
Due to the increase in temperature all the components which are surrounding the module needs to be adjusted so as to work effectively without the reduction in lifetime. Once there is an increase in the temperature of the module components it call for high attention on thermal management to avoid destruction of the components.
For thermal management, the lifetime IGBT module has to be estimate to determine the power cycling capability of the IGBT. Once the junction temperature is high, it results in high stress levels which the device has to undergo thus reduction in the cycle number of the device. The lifetime of the IGBT device in most cases is limited by the package technologies which include the soft soldering and wire bonding. There are new technologies which have been introduced to increase the number of cycles such as the XT technology which has overcome the limitation of the current technologies such as the wire soldering.
HiPak technology refers to the high power IGBTs which covers a wide range of voltage from 1700V to 6500 V and current such as 400A to 3600 A. This HiPak module exists in different forms such as the single IGBT, dual diode, dual IGBT and also in a chopper configuration.
Any IGBT module is made of IGBTs and diodes which are built on the basis substrates that are soldered to a base plate. At the terminals are conductor leads which are mainly used to provide an electrical connection from the electrical circuit to the outside of the module. Under high temperatures and current . For the module to work effectively there are improvements which have to be made as discussed below.
Solution explanation and evaluation
There are many possibilities which I came up with to ensure that the IGBT module is capable of operating under high temperatures of 230oc.I proposed a lot of modification and adjustments to the module components, joining technology and thermal management as discussed below.
The design of the IGBT and the diode chip require a lot of improvement to be able to operate at high temperatures. Controllable and soft switching is very essential when the chipsets are used in the modules with high temperature. This is due to the combination of the large stray inductance and high currents which will normally result in the snappy behaviour and a very high voltage during the turning off. For high current using the same technology, the platform has to be upgraded from the initial SPT to SPT+. The technology of the SPT+ works more efficiently as compared to the initial SPT, this is because it offers up to 15 % lower losses while it ensures to maintain the turn-off losses. As shown in figure 2 below.
The high temperature which is expected requires reliable and stable operations of all the devices which are beyond the limit. This requires a well-optimized termination design for the diode to reduce the leakages of high temperatures. The figure below shows a range of cool temperatures where by both the diode and the IGBT have been found out to be stable with no thermal runway which is under the direct application of a DC of about 1400V and 1700V which takes not less than 300 sec.
There are four main functions of the packaging technology. They include: provision of a current path directly from Bus bar to the chip and back, cooling down the heat which is generated by the module, isolating the electrical contacts from one another and ensuring that the package has mechanical robustness. Considering the improvements which were done on the Gel, terminal, module soldering a new robust product with high voltage and the current were developed.
High-temperature capable Gel.
The silicone Gels to be applied in the prevention of the partial discharge and also seal the atmospheric contaminants and moisture from getting into the system. Moreover, where the system has to remain operational there are environmental rules which require the junction temperature to be stored at -55oc.
The current material which to be used for insulation is the silicone gel with the specification of operating between the range of -40 and 230oc.The new operational temperature and the new requirement of the chips called for verification of the characteristics of the material of two alternatives which are Gel E and Gel S. For the selection of alternative gels, dielectric properties together the extended temperature range are the most crucial requirements. The selected potential alternatives gel it had to undergo many investigations and test.
Differential scanning calorimetry and thermos gravimetric analysis have to be carried out to be able to determine the thermal stability of the silicone gels which are to be selected. Thermal gravimetric indicated that both samples Gel R and Gel S dah lost the same amount of weight at the same temperature of 230oc.
Physical characterisation focused mainly on the thickness and hardness of the isolation of the materials and also to the components of the system. The main objective is to have an insulating material which is soft and has a good sealing. Carrying out a comparison of the different Gels it is very clear that Gel E had the highest adhesion force.
Module Soldering with Spacer
The requirements of the packaging technologies increase due to the increase in the operational temperatures; this is aimed at ensuring a long lifetime and high reliability of the IGBT module. Some of the lifetime failures which are identified included; wire bond contact, large area solder joints and terminal solder joints.AS a result of that additional step which was not there initial has to be included in the process of soldering the substrates to the base plate. Where substrate edges are attached a flat aluminium are soldered to the base plate. Mechanically and the reproducible stable spacer is given as a result of these bond, which guarantees a small thickness of the solder. Therefore reducing the tilting of the substrate.
Modules without and with spacers have to undergone temperature swings to determine the importance of reliability. Some substrate corners can be observed in all modules after they undergo cycling cracks in the substrate solder. Relating the solder thickness with the crack growth rate at their location it is clear that the locations which had the solder which was the thinnest had the highest crack growth rate as shown in figure 6 below. Therefore the application of spaces to better the cycling capability.
High Current Terminals
The contribution of resistive power losses of the module is increasing due to the increase in the semiconductor current ratings. Unwanted power impassions are caused by high currents from the bus bar to the power terminals. Moreover, they can cause reliability challenges as a result of the overheating of the internal conductor. This call for investigations of the current path.
Besides switching losses and dominant conduction, resistive losses happen at many points. On this kind of the module the losses which occur contribute greatly to the overall losses that are witnessed. The terminal contributes a lot to the resistive losses. The chip metallization, the bond wires and the wire bonds are few contributors.
To lower the losses that are generated in the terminal, it compulsory that the electrical resistance has to be lowered. Because there is no other good conductor which is affordable like copper it is important to change the geometry of the conductor which is being used. The terminals which are currently used in the HiPak module are shown in figure 7 below.
By the use of the current terminals which are used in the HiPak module, there is a significant reduction in the electrical resistance which is mainly achieved by making the current path shorter and balancing the current density in the conductor. At the same time maintaining the mechanical reliability. With this new designs of the terminals, the wind turbine generator can be able to work at even very high temperatures.
Materials used and their temperature limits.
The following materials were proposed to be used to enable the wind turbine generator operate at very high temperatures;
Aerogel Material; this material is used for insulation and has properties which allows it to perform under in high temperature environments. Aerogel is capable of withstanding high temperatures of up to 2000 degrees centigrade with very little or no transfer of heat to other components of the wind turbine generators. In that way it can be able to insulate the components of the generator effectively.
Nickel alloys; Due to the advancement in technology. Nickel alloys can be used in the wind turbine generators.
Niobium Alloys; is very dense when alloyed together with tungsten it can withstand high temperature of up 900 degree Celsius in that way it is much possible to be used in the manufacture of components of the wind turbine generator.
Molybdenum; this material shows very unique creeps and strength resistance and the ability to withstand very high temperatures molybdenum can be able to withstand up to a temperature of 12oo degrees centigrade
In conclusion, in most cases, the IGBT is used as an electronic switch in many electrical appliances. It has a wide application in electric power, such as; wind power generation, trains, electric cars, lamp ballasts, refrigerators, stereo systems and even in the air conditioning..
With the increase of the operating temperatures of the IGBT of the wind turbine generator. The user has the choice of utilising the operating temperatures to raise the output current or to increase the cooling cost. The IGBT module can increase its current output up to 12.5% if the operating temperatures are raised between 175oc to 230oc.For that reason good thermal management is very important considering the area in which the module is located.
There are many improvements which can be done to the components of the IGBT module to ensure that it is capable of operating at 230oc.The improvement which are to be done include:
The introduction of the HiPak technology, which can operate at very high temperatures and a wide range of voltage and current.
Use of high Current terminals to reduce the unwanted power impassions which are caused by high currents from the bus bar to the power terminals. Other adjustments which were to be done included the Module Soldering with Spacer to ensure a long lifetime and high reliability of the IGBT module. Application of high temperature capable Gel which are used in the prevention of the partial discharge and also seal the atmospheric contaminants and moisture from getting into the system of the module.