Critical Thinking

Introduction the system of the IGBT has to

Introduction

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.

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 Currently IGBT module is applicable in a range
of applications ranging from wind power production, motor drives, industrial
inverters and the HVDC converters (Byron, 2015). 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 (Christou,
2013).

 

 

 

 

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 (Claeys, 2013). 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 (Colin, 2015).

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 (Cressler, 2012).

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 (Flandre, 2014).

HiPak Technology.

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  (Jason, 2015). 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.

Chipset

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 (Jones, 2013). 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 (Kolawa, 2015).

Package

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 (Krozer, 2014).

 

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 (Lucian, 2012).

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 (Mantooth,
2015).

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 (McCormick,
2011).
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 (Parsons, 2013). 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 (Willander,
2011).

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 (Podlesak, 2016). 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.

Conclusion

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. (Claeys, 2013).

 

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 (Flandre, 2014).

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 (Claeys, 2013).

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