电气工程及其自动化专业英语》课程论文

重庆邮电大学移通学院《电气工程及其自动化专业英语》课程论文

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Insulated-gate Bipolar Transistor Basics 【Abstract】Modern Power Electronics makes generous use of MOSFETs and IGBTs in most applications, and, if the present trend is any indication, the future will see more and more applications making use of MOSFETs and IGBTs. For high-voltage or high-power applications, it may be necessary to realize a logical switch by connecting smaller units in parallel and series to achieve high availability, high-frequency operation, and low cost due to build-in redundancy, reduced dynamic losses, and modular use of standardized units, respectively. IGBTs are very convenient to realize such units, because of quasi-linear controllability via a gate terminal. This thesis investigates control methodologies for power MOS semiconductor switches with focus on combined parallel and series connection of IGBT/diode modules. It is proposed to provide each IGBT with primary local control to monitor and adjust the IGBT's static and dynamic behavior. Secondary (global) control synchronizes the operation of multiple IGBTs. A globally synchronous clock can also be derived locally. This makes it possible to use low-cost low-bandwidth data links between series-connected units. Thereby, a flexible master- slave approach can avoid the need of dedicated global control. That is, the entire system is manageable by the local gate drive circuitry.

Keywords:IGBT applications MOSFET characteristic

Introduction:The IGBT is a semiconductor device with four alternating layers (P-N-P-N) that are controlled by a metal-oxide-semiconductor (MOS) gate structure without regenerative action. This mode of operation was first proposed by Yamagami in his Japanese patent S47-21739, which was filed in 1968. This mode of operation was first experimentally reported in the lateral four layer device (SCR) by B.W. Scharf and J.D. Plummer in 1978.[1] This mode of operation was also experimentally discovered in vertical device in 1979 by B. J. Baliga.[2]The device structure was referred to as a …V-groove MOSFET device with the drain region replaced by a p-type Anode Region? in this paper and subsequently as 'the insulated-gate rectifier' (IGR), the insulated-gate transistor (IGT), the conductivity-modulated field-effect transistor (COMFET) and "bipolar-mode MOSFET".[3]

IGBT Fundamentals:The Insulated Gate Bipolar Transistor (IGBT) is a minority-carrier device with high input impedance and large bipolar current-carrying capability. Many designers view IGBT as a device with MOS input characteristics and bipolar output characteristic that is a voltage-controlled bipolar device. To make use of the advantages of both Power MOSFET and BJT, the IGBT has been introduced. It?s a fun ctional integration of Power MOSFET and BJT devices in monolithic form. It combines the best attributes of both to achieve optimal device characteristics.

1.The main advantages of IGBT over a Power MOSFET and a BJT are:

1. It has a very low on-state voltage drop due to conductivity modulation and has superior on-state current density. So smaller chip size is possible and the cost can be reduced.

2. Low driving power and a simple drive circuit due to the input MOS gate structure. It can be easily controlled as compared to current controlled devices (thyristor, BJT) in high voltage and high current applications.

3. Wide SOA. It has superior current conduction capability compared with the bipolar transistor. It also has excellent forward and reverse blocking capabilities.

2.The main drawbacks are:

1. Switching speed is inferior to that of a Power MOSFET and superior to that of a BJT. The collector current tailing due to the minority carrier causes the turn-off speed to be slow.

2. There is a possibility of latchup due to the internal PNPN thyristor structure. The IGBT is suitable for scaling up the blocking voltage capability. In case of Power MOSFET, the on-resistance increases sharply with the breakdown voltage due to an increase in the resistively and thickness of the drift region required to support the high operating voltage.

Basic Structure:An IGBT cell is constructed similarly to a n-channel vertical construction power MOSFET except the N+ drain is replaced with a P+ collector layer, thus forming a vertical PNP bipolar junction transistor. This additional P+ region creates a cascade connection of a PNP bipolar junction transistor with the surface n-channel MOSFET. Some IGBTs, manufactured without the N+buffer layer, are called non-punch through IGBTs whereas those with this layer are called punch-through IGBTs. The presence of this buffer layer can significantly improve the performance of the device if the doping level and thickness of this layer are chosen appropriately. Despite physical similarities, the operation of an IGBT is closer to that of a power BJT than a power MOSFET. It is due to the P + drain layer (injecting layer) which is responsible for the minority carrier injection into the N-drift region and the resulting conductivity modulation.

IGBT Characteristics：Because the IGBT is a voltage-controlled device, it only requires a small voltage on the Gate to maintain conduction through the device unlike BJT?s which require that the Base current is continuously supplied in a sufficient enough quantity to maintain saturation.

Also the IGBT is a unidirectional device, meaning it can only switch current in the “forward direction”, that is from Collector to Emitter unlike MOSFET?s which have bi-directional current switching capabilities (controlled in the forward direction and uncontrolled in the reverse direction).

The principal of operation and Gate drive circuits for the insulated gate bipolar transistor are very similar to that of the N-channel power MOSFET. The basic difference is that the resistance offered by the main conducting channel when current flows through the device in its “ON” state is very much smaller in the IGBT. Because of this, the current ratings are much higher when compared with an equivalent power MOSFET.[4]

The main advantages of using the Insulated Gate Bipolar Transistor over other types of transistor devices are its high voltage capability, low ON-resistance, ease of drive, relatively fast switching speeds and combined with zero gate drive current makes it a good choice for moderate speed, high voltage applications such as in

pulse-width modulated (PWM), variable speed control, switch-mode power supplies or solar powered DC-AC inverter and frequency converter applications operating in the hundreds of kilohertz range. A general comparison betwe en BJT?s, MOSFET?s and IGBT?s is given in the following table.

IGBT Operating area:The safe operating area is defined as the current-voltage boundary within which a power switching device can be operated without destructive failure. For IGBT, the area is defined by the maximum collector-emitter voltage V CE and collector current I C within which the IGBT operation must be confined to protect it from damage. The IGBT has the following types of SOA operations: forward-biased safe operating area , reverse-biased safe operating area and short-circuit safe operating area .

1.Pulsed Collector Current (I CM ): Within its thermal limits, the IGBT can be used to a peak current well above the rated continuous DC current. The temperature rise during a high current transient can be calculated with the help of the transient thermal impedance curve or simulated in SPICE with the parameters provided in the curve. The test circuit is shown in the data sheet.

2.Collector-to-Emitter Voltage (V CES ): V oltage across the IGBT should never exceed this rating, to prevent breakdown of the collector-emitter junction. The minimum value of the breakdown is stated in the Table of Electrical Characteristics.

3.Maximum Gate-to-Emitter Voltage (V GE): The gate voltage is limited by the thickness and characteristics of the gate oxide layer. Though the gate dielectric rupture is typically around 80 volts, the user is normally limited to 20 or 30V to limit current under fault conditions and to ensure long term reliability.

4.Clamped Inductive Load Current (I LM ):This rating is described in Section 6 and is important in most hard-switching applications. The test circuit can be found in the data sheet (it has changed over the years) and is the same as the switching loss test circuit. This circuit exposes the IGBT to the peak recovery current of the free-wheeling diode, which adds a significant component to the turn-on losses. This rating guarantees that the device can sustain high voltage and high current simultaneously, i.e. a square switching SOA. The test conditions for I LM are specified in the data sheet. This complements the information supplied by the RBSOA.

References:

[1] B.W. Scharf and J.D. Plummer, 1978 IEEE International Solid-State Circuits Conference, SESSION XVI FAM 16.6 "A MOS-Controlled Triac Devices"

[2] B.J. Baliga, "ENHANCEMENT- AND DEPLETION-MODE VERTICAL-CHANNEL M.O.S. GA TED THYRISTORS" Electronics Letters p.645(1979)

[3] A.Nakagawa et al., "High voltage bipolar-mode MOSFETs with high current capability", Ext. Abst. of SSDM, pp. 309–312(1984)

[4] Ralph Locher, “Introduction to Power MOSFETs and their Applications” Fairchild Semiconductor, Application Note 558, October 1998.