IGBT versus IGCT power semiconductors - A brief comparison
Semiconductor is the heart of any power electronic equipment. In medium voltage range of variable frequency drives two types of power semiconductors emerged: Insulated Gate Bipolar Transistor (IGBT) and Integrated Gate-Commutated Thyristor (IGCT). Naturally, there are many (white)papers, articles and presentations discussing advantages and drawbacks of those two semiconductor technologies. In this article we also provide a brief comparison of IGBT versus IGCT.
History
IGBT historically evolved from a MOSFET transistor. The power device has been used since approx. mid 1970s. For a long time the main domain were low voltage applications. Until early 1990s the blocking voltage was limited to 1200 V class. In late 1990s the development accelerated towards medium voltage range up to 6500 V class as known today. Companies such as ABB, GE, Mitsubishi or Toshiba contributed significantly to the IGBT evolution.
IGCT was introduced by ABB in 1996. This power semiconductor is basically an evolution of the Gate Turn-Off (GTO) thyristor. Integration of the gate unit next to the thyristor allowed minimization of parasitic inductance, higher switching frequency, significantly reduced losses and overall better performance of the device. For argument sake, the turn off time of IGCT semiconductor was reduced by a factor of 30 compared to GTO.
Function principle and structure
Both IGBT and IGCT are fully controllable power switches that can be used in self-commutated power electronic modules.
IGBT is a monolithic integration of a bipolar transistor controlled through a MOSFET gate structure. it combines a good switching behavior of MOSFET and the on-state performance of a bipolar transistor. The gate unit requires relatively low power.
The structure of IGCT evolves from GTO thyristor. IGCT is an integration of a high-power semiconductor and a powerful gate unit. The device combines the strengths of both GTO (high power density, low conduction losses) and IGBT (simple gate circuit, high switching frequency). Conduction and commutation of current corresponds to thyristor behaviour whereas the turn-off and blocking behaviour is like a transistor.
Construction and packaging
IGBT is available in a variety of configurations: from single device, phase leg up to complete inverter bridge. That is however applicable for low voltage applications. In medium voltage range single IGBT elements inside the module are state of the art. Plastic housing is common in lower power applications. In medium voltage and high power range the packaging is usually flat pack or press pack. The latter solution is preferred in high power application due to its double sided cooling for more efective heat removal.
IGCT is always realized as a robust press pack device of monolithic design. The ceramic housing is hermetically sealed and ideally protects the power semiconductor against environmental influence. An anti-parallel (freewheeling) diode can be integrated into the same device (→ reverse conducting IGCT) or being mounted externally (→ asymmetric IGCT). Third variant is a reverse blocking IGCT used e.g. in current source inverters.
Device rating
IGBT and IGCT are continuously developed and their device rating is being gradually extended. High power rating translates into more compact design, lower parts count and better reliability (KISS principle – see our previous post [3]).
The medium voltage IGBT and IGCT nowadays reach up to 6.5 kV blocking voltage. IGCTs with up to 10 kV rating have been reported by the manufacturer.
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Capability
There is a relationship between rated voltage, rated current and effective switching frequency [4]. Hence the semiconductor capability shall be presented as “3-D figure” linking these three quantities. Selection of the right device is usually a compromise between them. Optimal design of the power semiconductor utilizes the peak turn-off current and the thermal capability of the device (i.e. operating close to the corner of the safe operating area, SOA).
Overloadability
As known, semiconductors generally have limited overloadability due to short thermal time constant. However, certain overloadability is of clear advantage, be it for the application requirements or (mainly) for the purpose of protection.
IGCT with its thyristor heritage has excellent short time overloadability that paves the way for a fuseless protection concept. IGBT withstands considerably lower maximum surge current.
Scalability
In medium and high power range the rating of an individual device (as presented above) may be insufficient. With scalability we mean the suitability of series and parallel connection. Series connection is used to increase the voltage while parallel connection increases the total current rating.
Series and parallel connection of IGCT devices is complex and may require a balancing network. The preferred solution is to connect entire modules (building blocks) in parallel and in series [5].
In contrast, series and parallel connection of IGBTs is simple and supported by the gate unit that can shape the switching behaviour (e.g. active clamping).
Failure mode
For availability considerations, protection concept and safety measures the failure mode of the device is very important. The basic two states are open circuit or short circuit.
The press pack design is preferred due to its non-rupture failure behavior.
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Power losses
Semiconductor losses shall be minimized. One reason is to achieve a power conversion with as high efficiency as possible. The other reason is to keep the cooling equipment small and generally achieve a compact VFD design with high power density.
Semiconductor losses are typically the dominant losses of VFD. They make up to 70-80% of total losses of the VFD. There are three components of semiconductor losses:
— Conduction losses
— Switching losses
— Losses in blocking state
What is the loss distribution?
Losses in blocking state are caused by very small leakage current. These losses represent the smallest portion of semiconductor losses and can sometimes be completely neglected without making any notable error.
Conduction losses are product of the on-state voltage and actual load current. Semiconductors with low on-state voltage achieve lower conduction losses.
Switching losses obviously depend on the switching frequency. As the switching frequency goes up the switching losses increase.
How do the losses of IGBT and IGCT compare?
In general, IGCT has inherently lower conduction losses due to its lower on-state voltage (approx 1.8 to 2.2 V for IGCT, approx. 2.8 to 3.5 V for IGBT with comparable voltage rating). The switching losses are comparable or eventually marginally lower in case of latest IGBT (higher turn-on losses and lower turn-off losses). The overall comparison therefore depends on the specific switching frequency of the application.
A look inside
Before we conclude let’s look inside the semiconductor housing to see the internal structure. Figure 3 shows a typical IGCT wafer normally hermetically sealed inside a ceramic housing (left) and the structure of a conventional IGBT when removing the plastic housing (right).
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Aging
For lifetime considerations aging is certainly a topic. Research, test results and analysis of used semiconductors from the field showed that IGCT power part practically does not age. The device has a disk structure (so called wafer) and the electrical contact is created by applying a pressure through the clamping device. That eliminates the need for wire bonding. Moreover, the materials inside the IGCT have some “sliding” room when being heated up and cooled down. One particular analysis was performed on a VFD serving in a mine hoist application during construction of Gotthard Basis Tunnel in Switzerland. After finishing the construction, the drive was decommissioned and the IGCTs were sent back to the manufacturer for a detailed analysis. The investigations showed that there are practically no signs of aging.
On the other hand, IGBT is subject to high thermal loads. The device often consists of a considerable quantity of tiny wires (wire bonds) inside that connect the individual chips. The soldered wire bonding is subject to a mechanical stress when experiencing temperature fluctuation due to duty cycles (expansion when heated up and compression when cooled down). The cyclic heating and cooling down may lead to material fatigue and failure of IGBT module. This is obviously more critical for applications with larger cyclic load variation within a time span of few seconds.
There are high power press pack IGBTs that do not have wire bonds and eliminate above mentioned drawback (e.g. StakPack IGBT from Hitachi Energy [2] with unform chip pressure).
Verdict
IGBT and IGCT are the two most common power semiconductor switches used in medium voltage variable frequency drives, Statcom, grid interties, wind converters etc. Both technologies are well proven. Moreover, the development is certainly not over yet. Both types of devices are continuously being improved in terms of their capability, further loss optimization, robustness etc. Based on above information, eventually combined with additional sources, you can make your own judgement.
Remark: Comparing IGBT and IGCT based on their datasheet figures is difficult and may be misleading and confusing at the same time. A better way is to apply an application oriented approach, i.e. having a specific drive topology, power rating and operating mode in mind. That is certainly more transparent and more meaningful.
References
[1] IGCTs – megawatt power switches for medium-voltage applications, ABB Review 3/97
[2] Hitachi Energy – The most comprehensive power semiconductor portfolio, https://www.hitachienergy.com/offering/product-and-system/semiconductors
[3] Keep it simple stupid (KISS), https://mb-drive-services.com/keep-it-simple-stupid/
[4] VFD switching frequency, https://mb-drive-services.com/vfd-switching-frequency/
[5] Power scaling of VFD, https://mb-drive-services.com/power-scaling-of-vfd/
[6] Experience with IGCT based VFDs, https://mb-drive-services.com/experience-with-igct-based-vfd/
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