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What is the IGBT ?
Introduction
Device structure
Static and Dynamic Thermal Behavior of IGBT Power Modules
An equivalent circuit for the IGBT
Switching behavior
Heat Transfer Modeling
Conductive Heat Transfer
Initial and boundary conditions
MOSFE
A Short History
How does a MOSFET Amplify Electrical Signals?
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What is the IGBT ?
Since its first commercial introduction in 1983, the IGBT has successfully replaced the power bipolar transistor in most applications. At the extremely high power levels, applications lie in traction (electric streetcars and locomotives) and power distribution (high voltage DC transmission). These capabilities make the IGBT suitable device for the applications such as motor drives, power supplies and inverters that require devices rated for 600-1200V.

The IGBT is a ``spinoff'' from power MOSFET technology. By combining the low conduction loss of a Bipolar Junction Transistor (BJT) with high switching speed of a power MOSFET, the IGBT offers the combination of these attributes. The IGBT has high input impedance and fast turn-on speed like the MOSFET. Furthermore, it exhibits an on-voltage and current density comparable to the BJT while switching much faster. The IGBTs are, therefore, replacing the MOSFETs in the high voltage applications where conduction losses must be kept low. With zero current switching or resonant switching techniques, the IGBT can be operated in the hundreds of kHz range [Bal87]. At the operating frequencies between one and 50kHz, the IGBTs offer an attractive solution over the traditional bipolar transistors, MOSFETs and thyristors.

Compared to the thyristor, the IGBT is faster. It has also better dv/dtimmunity and gate turnoff capability . While some thyristors such as GTOs can be turned off at the gate, substantial reverse gate current is required. However, turning off the IGBT only requires that the gate capacitance is being discharged. Nevertheless, thyristor has a lower forward on-voltage and a higher surge capability than the IGBT [Mot95]. The MOSFETs are often utilized because of their simple gate drive requirements. Since the structure of both devices is so similar, the change from MOSFETs to IGBTs is possible without having to redesign the gate drive circuit. The IGBTs, like MOSFETs, are transconductance devices and can remain fully on-state by keeping the gate voltage above a certain threshold level.
When compared to the BJTs, the IGBTs have similar ratings in voltage and current. However, the presence of an isolated gate in the IGBT makes it simpler to drive than the BJT. The BJTs require that base current should be kept constant to prevent desaturation under high current loads and excessive base drives under low load conditions. This additional base current increases the power dissipation of the drive circuits. Consequently, the BJT drive circuits must be sensitive to variable load conditions
Introduction
Power electronics has been generally defined as a branch of electrical engineering that concerns itself with controlling or modifying the flow of an electrical energy. Here, electrical energy belongs to a wide range of voltages and currents. It has many different characteristics including DC, pulsed DC, continuous-wave AC, burst AC, and so on. Some application examples of power electronics are illustrated in Fig. 1.1 [Bal95]. Power supplies require technology operating at low voltage but at high current level. The other applications, however, demand both a high current capability and rising operating voltages. Over the last thirty years, the driving forces behind power electronics have been laid in the cost-effective application of control either to stabilize voltages (power supplies) or to control motor speed, acceleration and torque (industrial processes or transportation).
Figure 1.1: Application map for power electronics.
Figure 1.2: Power capacity of an IGBT and a GTO. (a) Shows the advance in power handling capacity of the IGBT switches between 1983 and 1998. (b) Shows improvement in the GTO thyristors over the past decade.
For the past several years, power electronics for the middle and lower power applications has been widespread because of advanced silicon technology. Below several hundred volts, it has become straightforward to integrate very large scale MOSFETs monolithically and progressed towards better and cheaper products. This remarkable progress could be realized by new ideas not only on the device and circuit design but also on the process and innovative manufacturing technology.
Between approximately 500V and 1kV, silicon electronics has had great impact because of rapid advances in an Insulated Gate Bipolar Transistor (IGBT) and modular packaging. The IGBT device combines the high current density and low loss of a bipolar transistor with the speed and high input impedance of the MOSFET. This allows the IGBTs to be easily interconnected with control circuitry in low cost modules, which has been a successful approach in driving small machines for factory automation. If the load current is high, current scaling is readily achieved by connecting many devices in parallel.
For voltages above 1kV, silicon technology has made impact through remarkable advances in the electrical performance of both the IGBTs and Gate Turn Off (GTO) thyristors . Figure 1.2a shows the progress in power handling capacity of the IGBTs switched between 1983 and 1998 years, and Fig. 1.2b shows improvement in the GTO thyristors over the past decade [Bro98]. Initially, the power handling capacity of the IGBTs increased at a rate of twenty times over roughly five year period [Bal95]. However, the growth rate diminished to approximately six times per five years around 1988, leading to one demonstration of a 2500Vand 1000A device by Toshiba [Kap96]. Not to be surpassed, the GTO thyristors have been following a trend of approximately four times per five years, achieving a maximum power handling of 36MW at Mitsubishi [Kap97]. In both cases, the improvements have occurred through current scaling.
The IGBTs have been frequently connected in parallel to create large modules. Single GTO thyristors are manufactured up to 150mm in diameter. Now, the GTO technology has found interesting applications in the power range of 0.5-20MW mainly in adjustable speed drives and railway inverters . Replacing bipolar semiconductors, e.g., frequency thyristors or GTOs means a cost reduction up to 50% for a whole converter. This also means weight reduction and improved performance due to higher switching frequency. On the other hand, a faster increase in the rate of the IGBTs has being achieved by enlarging both the chip size and the blocking voltage capability. Increased blocking voltage leads to more compact inverter designs because of higher switching power density of the semiconductors. Therefore, the IGBT will replace the GTO in more applications [Hie95]. Recently introduced systems that use IGBT modules cover the frequencies from 200Hz to 50kHz. It is in the power ratings of 5-4000kW[Kap97]. The IGBTs are now beginning to have a major impact on the power electronic system designed for industrial, consumer, and military applications.
In this thesis, the thermal behavior of a 1200A, 3.3kV IGBT module has been analyzed by using the numerical tools in the static and dynamic conditions. Measurement system was also built to verify the numerical results. Before the thermal analysis, the IGBT device is briefly introduced in this chapter.
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