On the switching performance of semiconductor devices in a cascode switch

The use of an insulated-gate bipolar transistor (IGBT) to improve the switching performance of a gate turn-off (GTO) thyristor in a cascode switch is documented. The IGBT-gated GTO-cascode switch features simple drive requirement, fast switching, robustness, and overcurrent protection. The cascode switch was applied in a quasi-resonant buck power converter and simulated on a computer using PSpice. The IGBT-gated GTO-cascode switch is suitable for high-power (1200 V, 140 A) and high-frequency operation up to 30 kHz. The turn-off time of the main switch, GTO, is reduced significantly due to the reverse current gain of unity in cascode configuration.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Chapter 2 - Power Semiconductors

New devices that are challenging the silicon-controlled rectifier are considered. Among them are the high-power bipolar-junction transistor (HPBT), the power MOS field-effect transistor (MOSFET), the gate turnoff (GTO) thyristor, and the insulated-gate bipolar transistor (IGBT), sometimes called a conductivity-modulated field-effect transistor (COMFET). Also important are the static-induction or short-channel junction-effect transistor, and its cousin, the static-induction thyristor. Applications for these devices are outlined, and their characteristics are briefly sketched. The use of MOSFETs in automobiles and IGBTs for motor drives is examined in more detail. Packaging issues are discussed.

Power semiconductor devices have been widely used in the fields of daily consumer goods, industrial manufacturing and transportation. Power semiconductor devices can generally be divided into two categories: controllable power semiconductors and uncontrollable power semiconductors. The first category mainly includes bipolar transistors, power MOSFET and IGBT (insulated gate bipolar transistors). The second category includes various types of power diodes. IGBT is widely used in various fields because of its advantages such as high input impedance, fast switching speed and low on-state voltage drop. However, because IGBT operation has a very high voltage and current change rate in the process of turning on and off, the electromagnetic compatibility problem caused by the module becomes more and more prominent. In this paper, the finite element simulation is used to analyze the internal structure damage caused by the repeated action of thermal stress on IGBT module, so as to study the quality and reliability of IGBT module more intuitively. IGBT device failures include total or partial functional failures (failures that occur upon commencement of operation can be excluded from failures). In addition, according to the IGBT module bonding line state (whether all failure), determination of failure boundary, so as to better show the electromagnetic interference characteristics of IGBT module failure state. Based on the basic mechanism of IGBT accelerated aging failure, finite element modeling of IGBT is carried out with Ansys software, and the simulation results are analyzed to obtain the electromagnetic interference characteristics of IGBT module during the opening and closing process.

The switching performance of both the bipolar transistor and gate turnoff thyristor is improved when used in a cascode switch configuration. ‘Snubberless’ turnoff occurs without second breakdown and the technique results in shorter saturation delay times, faster current fall and higher operational sustaining voltages than obtained with conventional switching techniques. Improved switching performance is traded for increased drive circuit complexity and an increased on-state power loss associated with two series connected power semiconductor switches. The circuit techniques features and performance of two 720 V DC, 320 A cascode switches are presented. The bipolar transistor cascode switch is tested up to 100 kHz, whereas tail current power loss limits the GTO thyristor cascode switch to 16 kHz.

Over the last few years high power semi-conductor devices with intrinsic turn-off capability have become available. These devices, called gate turn-off (GTO) thyristors, consist of several layers of silicon with appropriate dotations; they are able to turn off currents of 1000 Amperes at thousands of volts within microseconds. In circuits used in power electronics the usual resistive, inductive and capacitive circuit elements are combined with thyristors which may simply be considered as switches. This technology is still an active field of research, and it has many important applications such as AC/DC conversion (both ways), speed control of locomotives and electric cars, control of power stations and power networks, etc.

A large-capacity inverter using high-power gate turn-off (GTO) thyristors that can be used instead of a cycloconverter in AC drives is presented. With the techniques of multiple configuration and GTO series connection, the inverter capacity is increased effectively. High conversion efficiency is obtained by using low-loss snubber circuits for GTOs. It is confirmed that the low-loss snubber circuits using energy recovery units (ERUs) have good performance in pulse-width modulated (PWM) operation. High-energy recovery efficiency is obtained, especially in leading operation (turn-on operation when freewheeling diodes conduct). These snubber circuits have superior ability for clamping overvoltage at GTO turn-off. A 2750 kVA inverter with low-loss snubber circuits using high-power GTOs has been fabricated and evaluated. >

Fundamental equations describing carrier transport, current continuity, and potential distribution are used in a finite-difference scheme to model switching phenomena in semiconductor power devices. An irregular and variable mesh approach is introduced that can reduce computation time by 40%. If the device structure satisfies certain conditions, the method works even more effectively. The program is used to analyze gate-turn-off (GTO) thyristor and static-induction thyristor (SIT) switching. Carrier concentration, potential, and current distribution transients during switching of the devices are shown. The current squeeze and concentration during turn-off in the GTO are related to the gate structure. The effective channel width of the SIT is also determined through numerical analysis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ultra-high voltage (>10 kV) power devices based on SiC are gaining significant attentions since Si power devices are typically at lower voltage levels. In this paper, a world record 22kV Silicon Carbide (SiC) p-type ETO thyristor is developed and reported as a promising candidate for ultra-high voltage applications. The device is based on a 2cm2 22kV p type gate turn off thyristor (p-GTO) structure. Its static as well as dynamic performances are analyzed, including the anode to cathode blocking characteristics, forward conduction characteristics at different temperatures, turn-on and turn-off dynamic performances. The turn-off energy at 6kV, 7kV and 8kV respectively is also presented. In addition, theoretical boundary of the reverse biased safe operation area (RBSOA) of the 22kV SiC ETO is obtained by simulations and the experimental test also demonstrated a wide RBSOA.

Highly integrated field emitter arrays (FEAs) have been fabricated by the transfer mold technique to obtain highly uniform and stable field emission for highly efficient and reliable vacuum microelectronic devices. Transfer mold Mo FEAs containing 9,400,000 emitter tips with high emitter density of 7,840,000 tips/cm2 have demonstrated a low value of less than 0.7%. Highly uniform, and flickerless fluorescence has been observed in the entire area of the fluorescent screen corresponding to the FEAs. The principle of the vacuum microelectronic switching devices has been proposed and experimentally verified by applying a high anode voltage of ±1 kV, which will be capable of decreasing the size and the electric power loss of the high voltage electric power converters using conventional semiconductor devices such as light-triggered thyristors (LTT) and gate turn off thyristors (GTO) to less than 1/100 and 1/10, respectively.

The authors review the fundamental mechanism by which rotating electrical machines develop torque and draw conclusions as to which configuration offers inherent advantages in this regard. Development of a DC machine with a continuous armature winding where gate turn-off (GTO) thyristors replace the traditional copper-segment-carbon-brush commutator is described. Operation of a separately excited machine with 12 armature tapping points and 18 GTO thyristors is demonstrated, and waveforms are presented to illustrate the switching duties involved. >

A system for measuring the subthyristor parameters of GTO (gate-turn-off) thyristors is presented. With the aid of a personal computer the gate breakdown voltage and leakage current, the turn-on and turn-off times, and the forward voltage drop of each subthyristor can be automatically measured, stored, and statistically evaluated. The cumulative frequency and distribution of the gate breakdown voltages on an ordinary pellet are discussed. A further example shows the distributions of gate breakdown voltage and turn-off times on a pellet with a doping inhomogeneity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Due to the low current ratings of the currently available silicon carbide (SiC) switches they cannot be employed in high-power converters. Thus, it is necessary to parallel-connect several switches in order to reach higher current ratings. This paper presents an investigation of parallel-connected normally-on SiC junction field effect transistors. There are four crucial parameters affecting the effectiveness of the parallel-connected switches. However, the pinch-off voltage and the reverse breakdown voltage of the gates seem to be the most important parameters which affect the switching performance of the devices. In particular, the spread in these two parameters might affect the stable off-state operation of the switches. The switching performance and the switching losses of a pair of parallel-connected devices having different reverse breakdown voltages of the gates is investigated by employing three different gate-driver configurations. It is experimentally shown that using a single gate-driver circuit the switching performance of the parallel-connected devices is almost identical, while the total switching losses are lower compared to the other two configurations.

Gate turn off thyristor (GTO) gating circuits, especially off-gating circuits, are the most important for reliable operation of GTO equipment. This paper describes a new gating circuit for high power GTO of 600A class. The off-gating circuit can provide a negative pulse of 200A with its rate of rise of 30A/us. Its power dissipation decreases to 20 percent of a previous type. The on-gating and negative bias circuits are also described.

In view of the many system planning uncertainties faced by the UK National Grid Company (NGC), there is a clear need for readily relocatable reactive power compensation systems with enhanced system performance. To date 12 relocatable SVCs (RSVCs) have been installed as part of a planned programme of work to meet these changing system needs. Developments in the rating capabilities of gate turn-off (GTO) thyristors have enabled power electronic converters using GTOs to be proposed as serious competitive alternatives to the well proven thyristor controlled reactor (TCR) and thyristor switched capacitor (TSC). GTO-based STATCOM convertors have a number of benefits which are of value to NGC, such as reduction in required site area and ease of relocation. The authors describe the East Claydon project, which is viewed as an important step towards long-term objectives which could see the deployment of a range of compact power electronic devices on NGC's system.