Power loss-oriented evaluation of high voltage IGBTs and multilevel converters in transformerless traction applications

The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and a substantial weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern high-voltage insulated gate bipolar transistors (HV-IGBTs). In a first design step, the switching characteristics and losses of 6.5-kV IGBTs are compared to 3.3-kV and 4.5-kV IGBTs which are already commercially used in traction applications. Based on the considered HV-IGBTs, the properties of multilevel converters are analyzed and their applicability to the transformerless system is evaluated. The paper focusses on a loss analysis of the converters. Reliability aspects and harmonic spectra are briefly discussed. Taking these design aspects into account, the three-level neutral point clamped converter turns out to be a reasonable solution to realize line and motor converter modules in a transformerless traction system.     

4.5-kV injection-enhanced gate transistors (IEGTs) with high turn-off ruggedness

Although high blocking voltage insulated gate bipolar transistors (IGBTs) have wider safe operating areas (SOAs) than do gate turn-off thyristors, a failure problem remains at turn-off transient. The purpose of this paper is to clarify the mechanism of failure at turn-off transient and to develop a high-voltage injection-enhanced gate transistors (IEGTs) with wide SOA at turn-off transient [wide reverse-biased SOA (RBSOA)]. We discuss this destruction mechanism in detail on the basis of comparison of experimental results with calculated results obtained by an analytical model considering dynamic avalanche generation. These results lead to the conclusion that the design of the n-emitter and the control of avalanche generation onset are key parameters for realizing high ruggedness of high-voltage IEGT. Based on the proper design of the n-emitter and the gate driving condition, a high-voltage and high-current 4.5-kV IEGT with wide RBSOA, keeping low saturation voltage and low turn-off switching loss, has been successfully developed.     

Snubberless voltage sharing of series-connected insulated-gatedevices by a novel gate control strategy

Insulated gate devices, such as metal oxide semiconductor field effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs), are increasingly used in high-voltage power converters where a request for fast power switches is growing. Series connection of devices is a viable approach to manage voltages higher than the blocking voltage of the single device. The main problem in such an application is to guarantee the voltage balance across the devices both in steady-state and during switching transients. In this paper, a novel approach is presented, which is used to equalize the voltage sharing during the switching transients. The main advantages of the proposed method consist in avoiding the traditional use of the snubber capacitors, in the output power side, and in working on the gate side. The application of the proposed gate drive technique is firstly discussed and compared with different solutions, hence, validated by experimental tests applied to the control of series connected devices. Finally, a comparison is performed between the transient behaviors of two different configurations: a single switch with high-voltage blocking capability, and in alternative a series of two devices which together ensure the voltage blocking capability of the single switch. The better performances of the latter configuration, working with the proposed control circuit, over the former have been experimentally demonstrated     

New zero voltage switching DC converter with flying capacitors

A new soft switching converter is presented for medium power applications. Two full-bridge converters are connected in series at high voltage side in order to limit the voltage stress of power switches at V_in/2. Therefore, power metal–oxide–semiconductor field-effect transistors (MOSFETs) with 600 V voltage rating can be adopted for 1200 V input voltage applications. In order to balance two input split capacitor voltages in every switching cycle, two flying capacitors are connected on the AC side of two full-bridge converters. Phase-shift pulse-width modulation (PS-PWM) is adopted to regulate the output voltage. Based on the resonant behaviour by the output capacitance of MOSFETs and the resonant inductance, active MOSFETs can be turned on under zero voltage switching (ZVS) during the transition interval. Thus, the switching losses of power MOSFETs are reduced. Two full-bridge converters are used in the proposed circuit to share load current and reduce the current stress of passive and active components. The circuit analysis and design example of the prototype circuit are provided in detail and the performance of the proposed converter is verified by the experiments.     

A 6.6-kV Transformerless Motor Drive Using a Five-Level Diode-Clamped PWM Inverter for Energy Savings of Pumps and Blowers

This paper describes a 6.6-kV adjustable-speed motor drive for pumps and blowers without transformer. The power conversion system consists of a front-end diode rectifier, a five-level diode-clamped pulsewidth modulation (PWM) inverter with a voltage balancing circuit, and a hybrid active filter for harmonic-current mitigation of the diode rectifier. The control of the inverter is characterized by superimposing a third-harmonic zero-sequence voltage on each of the three-phase reference voltages to achieve the so-called overmodulation and reduce the switching stress of insulated gate bipolar transistors (IGBTs). A 200-V 5.5-kW downscale model is designed, constructed, and tested with focus on the five-level PWM inverter and the voltage balancing circuit. Experimental results obtained from the 200-V downscale model verify the viability and effectiveness of the 6.6-kV adjustable-speed motor drive, showing that the four split dc capacitor voltages are well balanced in all the operating conditions and that the switching stress of the IGBTs is reduced at low modulation indexes.     

Low stress switching for efficiency

Power modules that turn on or off “softly,” at near zero voltage or current, promise marked gains in performance by cutting switching losses and allowing faster controls-to the likely benefit of power converters. Medium power inverters have undergone significant changes. One improvement stems from new power transistors such as insulated-gate bipolar transistors (IGBTs) with their increasing voltage- and current-carrying capabilities and increasing switching frequencies. Others derive from the use of digital signal processors and such modern control techniques as fuzzy logic and neural networks, as well as from advances in the applications of power converters that employ soft switching of the power devices. Soft-switching technologies promise marked gains in performance-lower losses and higher switching frequencies than those in the prevailing hard-switching technology. The idea is to switch a device only when the voltage across it, or the current through it, is zero. Representative soft switching converters include DC-to-DC converters rated at up to several hundred watts, as well as inductive chargers for electric vehicle batteries rated at up to 120 kW. Technologically speaking, advances in soft-switching inverters have arisen chiefly from enhancements to semiconductor power devices. Today, IGBTs lead the market for medium-power applications     

A Bidirectional Isolated DC–DC Converter as a Core Circuit of the Next-Generation Medium-Voltage Power Conversion System

This paper describes a bidirectional isolated dc-dc converter considered as a core circuit of 3.3-kV/6.6-kV high-power-density power conversion systems in the next generation. The dc-dc converter is intended to use power switching devices based on silicon carbide (SiC) and/or gallium nitride, which will be available on the market in the near future. A 350-V, 10-kW and 20 kHz dc-dc converter is designed, constructed and tested. It consists of two single-phase full-bridge converters with the latest trench-gate insulated gate bipolar transistors and a 20-kHz transformer with a nano-crystalline soft-magnetic material core and litz wires. The transformer plays an essential role in achieving galvanic isolation between the two full-bridge converters. The overall efficiency from the dc-input to dc-output terminals is accurately measured to be as high as 97%, excluding gate drive and control circuit losses from the whole loss. Moreover, loss analysis is carried out to estimate effectiveness in using SiC-based power switching devices. Loss analysis clarifies that the use of SiC-based power devices may bring a significant reduction in conducting and switching losses to the dc-dc converter. As a result, the overall efficiency may reach 99% or higher     

Large static converters for industry and utility applications

The emergence of high-power semiconductor devices such as insulated-gate bipolar transistors (IGBTs) or injection-enhanced gate transistors (IEGTs) and gate-commutated turn-off (GCT) thyristors or integrated gate-commutated thyristors (IGCTs) enables large static converters to expand into utility and industry applications. For instance, a ±80 kV 50 MW HVDC transmission system based on a string of many IGBTs connected in series was commissioned in 1999. This paper describes the present status of large static converters, with focus on their applications to utility and industry. The applications discussed are: HVDC transmission system, UPFC, flywheel energy storage system, pumped hydro plant adjustable speed generator, active filters for power conditioning, and steel mill drives. The paper also describes their future prospects and directions in the 21st century, including the personal views and expectations of the author     

Suitability and optimization of high-voltage IGBTs for series connection with active voltage clamping

The successful series combination of 5.2-kV high-voltage integrated gate bipolar transistors (HV IGBTs) is reported in this paper. The tail current cut-off encountered in punchthrough type HV IGBTs can represent a particularly severe handicap for the full control of the inductive voltage overshoot when connecting two devices in series. Advanced voltage clamping techniques are demonstrated, which can also limit the second voltage spike originating from the tail current cut off. It is shown in this paper, that IGBTs with a carrier lifetime profile localized at the anode side are particularly well suited for this application; the shorter the tail current interval, the lower the turn-off losses can be kept. The discussion focuses on the optimum on-state plasma distribution in punchthrough-type HV IGBTs with respect to series connection of these devices. The most recent trends in the development of HV IGBTs seem to be in line with the conclusions drawn in the discussion. Advanced future HV IGBT concepts may substantially ease the difficulties encountered in the series connection of first generation HV IGBTs as used experimentally in this paper.     

Design of a Soft-Switched 6-kW Battery Charger for Traction Applications

Auxiliary power converters for traction rolling stock applications have to operate under difficult conditions, including high-input voltages which are subject to wide fluctuations, high temperatures, and harsh environmental constraints. Additionally there is often a need for silent operation, which implies switching frequencies above 20 kHz. Increasingly, high-frequency DC-DC converters are being used for these applications, with their advantages of reduced size and weight. However, the requirement to accommodate high-input voltages and switch at high frequencies is challenging for a conventional hard-switched converter based on IGBTs, which makes soft-switching topologies an attractive alternative. This paper presents the design strategy for a zero-voltage switched (ZVS) 6-kW battery charger switching at 20 kHz using IGBTs. This paper illustrates how the design is a tradeoff between managing the hard-switch turn-on losses at light load, minimizing the duty cycle loss caused by soft-switching delays, and minimizing the effects of tail current-switching losses. These tradeoffs affect the selection of the ZVS capacitors, the determination of the series inductance value, the transformer turns ratio, and the selection of the IGBTs to be used. Design details, theoretical predictions, and experimental results are presented in this paper for the conversion system that was developed.     

High-Voltage Self-Aligned p-Channel DMOS-IGBTs in 4H-SiC

SiC power MOSFETs designed for blocking voltages of 10 kV and higher face the problem of high drift layer resistance that gives rise to a high internal power dissipation in the ON -state. For this reason, the ON-state current density must be severely restricted to keep the power dissipation below the package limit. We have designed, optimized, and fabricated high-voltage SiC p-channel doubly-implanted metal-oxide-semiconductor insulated gate bipolar transistors (IGBTs) on 20-kV blocking layers for use as the next generation of power switches. These IGBTs exhibit significant conductivity modulation in the drift layer, which reduces the ON-state resistance. Assuming a 300 W/cm² power package limit, the maximum currents of the experimental IGBTs are 1.2x and 2.1x higher than the theoretical maximum current of a 20-kV MOSFET at room temperature and 177 degC, respectively.     

New family of zero-current-switching PWM converters using a new zero-current-switching PWM auxiliary circuit

A new family of zero-current-switching (ZCS) pulsewidth-modulation (PWM) converters using a new ZCS-PWM auxiliary circuit is presented in this paper. The main switch and auxiliary switch operate at ZCS turn-on and turn-off, and the all-passive semiconductor devices in the ZCS-PWM converters operate at zero-voltage-switching (ZVS) turn-on and turn-off. Besides operating at constant frequency and reducing commutation losses, these new converters have no additional current stress and conduction loss in the main switch in comparison to the hard-switching converter counterpart. The PWM switch model and state-space averaging approach is used to estimate and examine the steady-state and dynamic character of the system. The new family of ZCS-PWM converters is suitable for high-power applications using insulated gate bipolar transistors (IGBTs). The principle of operation, theoretical analysis, and experimental results of the new ZCS-PWM boost converter, rated 1.6 kW and operating at 30 kHz, are provided in this paper to verify the performance of this new family of converters.     

A New Family of Zero-Current-Switching (ZCS) PWM Converters

A new family of zero-current-switching (ZCS) pulsewidth-modulated (PWM) converters which uses a new ZCS-PWM switch cell is presented in this paper. The main switch and auxiliary switch operate at ZCS turn-on and turn-off, and all the passive semiconductor devices in the ZCS-PWM converter operate at zero-voltage-switching (ZVS) turn-on and turn-off. Besides operating at constant frequency and with reduced commutation losses, these new converters have no additional current stress in comparison to the hard-switching converter counterpart. The new family of ZCS-PWM converters is suitable for high-power applications using insulated gate bipolar transistors (IGBTs). The PWM switch model and state-space averaging approach is used to estimate and examine the steady-state and dynamic character of the system. The principle of operation, theoretical analysis, and experimental results of the new ZCS-PWM boost converter, rated 1 kW and operating at 30 kHz, are provided in this paper to verify the performance of this new family of converters.     

12-kV p-Channel IGBTs With Low On-Resistance in 4H-SiC

SiC bipolar devices are favored over SiC unipolar devices for applications requiring breakdown voltage in excess of 10 kV. We have designed and fabricated p-channel insulated-gate bipolar transistors (IGBTs) in 4H-SiC with 12-kV blocking voltage for high-power applications. A differential on-resistance of 18.6 $hbox{m}Omegacdothbox{cm}^{2}$ was achieved with a gate bias of 16 V, corresponding to a forward voltage drop of 5.3 V at 100 $ hbox{A/cm}^{2}$, indicating strong conductivity modulation in the p-type drift region. A moderately doped current enhancement layer grown on the lightly doped drift layer effectively reduces the JFET resistance while maintaining a high carrier lifetime for conductivity modulation. The p-channel IGBT (p-IGBT) exhibits a transconductance that is $hbox{3}times$ higher than that of the 12-kV n-channel SiC IGBTs. An inductive switching test was done at 1.5 kV and 0.55 A $(sim !!hbox{140} hbox{A/cm}^{2})$ for the p-IGBTs, and a turn-on time of 40 ns and a turn-off time of $sim !!hbox{2.8} muhbox{s}$ were measured.      

Real-time system for monitoring the electro-thermal behaviour of power electronic devices used in boost converters

Reliability of power electronic devices (PEDs) is a key issue to secure power supplies in modern word, especially, those generated from renewable energy sources. Thermal stress due to switching frequency and environmental conditions are commonest cause of currently unsatisfactory PEDs reliability scores.In this paper, the electro thermal performance of PEDs and related parameters are critically investigated using three types of differently manufactured insulated gate bipolar transistors (IGBTs). Namely, punch through (PT), non-punch through (NPT) and field stop (FS) silicon trench gate technologies.First, currents and voltages of the examined IGBTs were measured under different operating temperatures, switching frequencies and electrical loading conditions.Second, power losses of the examined devices were calculated, in real time, based on their measured currents and voltages using realistic mathematical model embedded in a dSPACE system. Subsequently, the power losses for each device were used as an input to a finite element model to graphically predict heat distributions for each of the monitored devices.Compared to expensive measurements taken by high-resolution thermal imaging cameras, the accuracy of the developed system achieved 97%. The obtained results demonstrate the developed model would serve as an inexpensive and powerful tool for monitoring PEDs thermal conditions.     

Actively clamped zero-current-switching quasi-resonant convertersusing IGBTs

Conventional zero-current-switching quasi-resonant power converters (ZCS-QRCs) suffer from the disadvantages of high switch current stress and variable switching frequency. This paper proposes the use of a “current-clamping circuit” to overcome these disadvantages. By incorporating such a circuit into the family of ZCS-QRCs, a new family of actively clamped ZCS-QRCs using insulated gate bipolar transistors (IGBTs) is derived. These power converters feature high (and constant) switching frequency and zero-current turn-off (without increased current stress), which are particularly useful for high-power applications where minority-carrier semiconductor devices (such as IGBTs and bipolar junction transistors) are used as power switches. The design criteria, simulation and experimental results are reported     

Development of a Scalable Power Semiconductor Switch (SPSS)

This paper presents the design and development of a 4800-V, 300-A, 10-kHz scalable power semiconductor switch (SPSS) based on series connecting low voltage insulated gate bipolar transistors (IGBTs). The static and dynamic voltage balance among IGBTs is achieved using a hybrid approach of active clamp circuit and an active gate control that is also effective during tail current phase. The developed SPSS derives its control power directly from the main power bus. Control, packaging, and thermal characteristics are an integral part of the SPSS design. From a user's standpoint, the SPSS is a three-terminal optically controlled high-power switch. Experimental evaluation of the prototype SPSS shows it fully achieved the design objectives. In principle, the approach can be extended to building switches with higher voltages, currents, and switching frequencies, or even with other types of devices than IGBTs     

High-voltage junction-gate field-effect transistor with recessed gates

A new recessed-gate structure for vertical-channel junction field-effect transistors (JFET's) is described together with a self-aligned gate-source process developed to fabricate these devices. Using this technology, devices with groove depths ranging from 8 to 18 µm have been fabricated. The characteristics of these devices is described as a function of the groove depth. It has been found that the devices display pentode-like characteristics at low gate voltages and triode-like characteristics at high gate voltages. The blocking gain has been found to increase with groove depth. However, this is accompanied by an increase in the on-resistance and a decrease in the saturated drain current. Devices with gate breakdown voltages of up to 600 V have been fabricated with the recessed-gate structure. These high-voltage field-effect transistors (FET's) have a unity power gain cutoff frequency of 600 MHz and gate turn-off times of less than 25 ns.     

High-voltage bipolar mode JFET with normally off characteristics

The first realization of a power vertical JFET operated in the bipolar mode (BJFET) with normally off behavior is reported. The structure combines minority carrier injection from the gate region in the on-state, and lateral pinch-off of the channel, due to the built-in voltage, in the off-state. The realized devices show high blocking voltages, up to 900 V, with zero gate bias, and have extremely low on-resistance. Fast switching speeds with forced gate turn-off times as low as 100 ns for devices of 600-V blocking voltages have been obtained.     

Recent developments of high power converters for industry andtraction applications

The introduction of new high power devices like integrated gate commutated thyristors (IGCTs) and high voltage insulated gate bipolar transistors (IGBTs) accelerates the broad use of pulse width modulation (PWM) voltage source converters in industrial and traction applications. This paper summarizes the state-of-the-art of power semiconductors. The characteristics of IGCTs and high voltage IGBTs are described in detail. Both the design and loss simulations of a two level 1.14 MVA voltage source inverter and a 6 MVA three-level neutral point clamped voltage source converter with active front end enable a detailed comparison of both power semiconductors for high power PWM converters. The design and the characteristics of a commercially available IGCT neutral point clamped PWM voltage source converter for medium voltage drives are discussed. Recent developments and trends of traction converters at DC mains and AC mains are summarized