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Posted on 01 March 2019

High Voltage Semiconductor Switches

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Introducing the 10 kV IGCT chip set for 7.2 kV (RMS) VSI application

To allow more simplicity and cost reduction in the design of modern power electronic systems, significant efforts are made to continuously increase the voltage and current capabilities of high power semiconductors. In this article, the improved Safe Operating Area (SOA) of a new IGCT chip set based on ABB’s High Power Technology (HPT) platform with a rated voltage of 10kV is presented. A matching 10kV freewheeling diode is also reported. Combined, these developments open the door to new applications of silicon IGCTs reaching voltage levels of 7.2kV RMS or more.

By Iulian Nistor, Tobias Wikström, and Maxi Scheinert, ABB Switzerland Ltd.

 

Since their introduction in 1996, the Integrated Gate Commutated Thyristor (IGCT) has gained market importance as a semiconductor switch characterized by low on-state, fast switching capability, and the possibility to fit a single device with a current capability of thousands of Amperes. The IGCT has a thyristor structure and therefore generates low conduction losses. The device is very compact, having the gate drive unit incorporated to minimize the gate inductance in the circuit connecting gate and cathode (Figure 1). This allows the IGCT to be “hard driven”, meaning that very high di/dt’s (thousands of amps/us) can be used during the fast turn-off process. Due to all these advantages, IGCTs have become a top choice today in numerous applications such as converters (industrial medium-voltage drives or MVDs), as well as railway interties and other energy management systems (typically above 2MW). At the same time, the high ratio between the active silicon area and the junction termination area has practically made the IGCTs a very attractive choice in high voltage applications (above 6.5 kV).

ABB GCT power semiconductor with integrated gate drive

Generally MVDs today are offered in phase-to-phase voltage classes of 2.3, 3.3, and 4.16 kV. IGCTs are normally used in two or three level topologies as one device per function. Topologies involving series connection of IGCTs for higher voltages have also been reported, however voltage sharing limitations prevent such applications from gaining significant importance.

IGCTs are available in current and voltage ratings starting at 4500V and a few hundred amps to 6500V and 4000A. An IGCT with a blocking capability of at least 10kV or a series connection of two 4.5- and 5.5-kV IGCTs, respectively have to be applied per switch position of a three-level neutral-point-clamp voltage-source converter in order to increase the converter voltage to the phase-to-phase RMS voltage of 6.0–7.2 kV. The development of turn-off devices operating at 7kV DC is thus of paramount interest because it enables a three-level converter topology for up to 7.2 kV RMS, covering a large part of installed industrial drives, without requiring series connection of power semiconductors. This introduces significant advantages for the manufacturer as well as for the customer through less snubbering effort leading to fewer components, lower costs, and subsequently increased system reliability[1].

The 7kV DC rating requires blocking capability beyond 9 kV. The current development builds on previously demonstrated High Power Technology (HPT) with a corrugated p-base design that has been shown to facilitate further SOA expansion of the IGCT, scaling the voltage rating to 11kV [2]. The conception of an IGCT circuit with 7.2kV output requires freewheeling and clamp diodes of similar voltage rating. Diode loss-to-snappiness trade-off is of critical importance. This was addressed through the use of the Field Charge Extraction (FCE) concept. [3] Simulations are used to demonstrate the effectiveness of the proposed diode design against diode snapoff. Furthermore, minute attention has been given to the resilience against cosmic ray events and its implications on device design.

Requirements for 10kV IGCT chip set

The IGCT and diode for 7.2kV RMS VSI applications were designed to switch under a DC voltage of typically 6 kV, maximally 7 kV. Of critical importance were also the cosmic ray withstand capability of 100 FIT and the long-term dc stability at the nominal dc voltage of VDC = 5.9 kV. In order to safely block the dynamic overvoltage in the converter, the maximum repetitive forward blocking voltage VDRM was set to be higher than 9kV. Other critical requirements include: a maximum junction temperature of Tj = 125 °C; small leakage currents at the blocking voltages VDC NOM, and VDRM; a wide SOA; as well as on-state and switching losses according to the calculated on-state voltage drop vs turn-off losses trade-off. In addition a diode with softturn off recovery at low currents and high voltage is required for optimal converter performance.

10 kV IGCT Electrical Characteristics

The IGCT wafer consists of a large number of thyristor segments (approx. 2700 for a 91mm wafer) connected in parallel. Each segment is surrounded by the gate metallization. During the turn-off process, the anode current is taken over by the gate interrupting the regenerative pnp-npn thyristor action. Three different designs have been used to manufacture the 10kV IGCTs. The “standard” planar p-base junction design had an active area of approximately 20 cm2. The same wafer size was used for devices with a fortified p-base, i.e. with a deeper and more heavily doped p-base. The larger IGCTs (approx. 40 cm2) were all fabricated with a fortified HPT design.

In addition to the active area, the junction termination is critical to ensure good voltage blocking capability. This is accomplished through the use of a negative bevel. We have optimized this bevel for a blocking voltage above 10kV by decoupling the depth of the p-base from the voltage capability of the edge.

The measured forward-blocking characteristic of a Ø91-mm IGCT is depicted in Figure 2. The devices avalanched at 125 °C at about 11.2 kV. For all devices, the leakage current IDR was smaller than 15 mA at a device voltage of VAK = 7 kV and a junction temperature of Tj = 125 °C.

Forward blocking characteristics of manufactured 10kV HPT IGCTs at 125°C

The use of the HPT concept was recently demonstrated to enhance the SOA capability of 4.5kV, 5.5kV and 6.5kV HPT IGCT devices [2]. To follow up on those promising developments, the HPT design was included in our moderate junction depth design for a 10kV IGCT. To understand the effect on SOA we have compared a standard Ø68 mm IGCT with Ø91 mm IGCT with an HPT design having a similar p-base depth and voltage rating. The active area of the Ø91 mm device is twice as large as the Ø68 mm device, however for standard devices the maximum turn-off current does not scale linearly with the device area. This is caused by the difficulty to distribute the gate signal uniformly across the wafer area. The current is redistributed to segments in gate-remote locations during turn-off. Under these conditions, the device can ultimately fail either by violation of the hard-drive criterion, or by locally exceeding the maximum permissible power density.

The controllable current for the standard Ø68mm device was 300 A at a DC-link voltage of 6kV at 25°C. This corresponds to a peak power of 96.2 kW/cm2 of active GCT area. The device failed at 400A and 6kV. Based on the above discussion, the SOA of a standard Ø91 mm IGCT would then be limited to less than 800 A at 6kV.

Using lifetime-control techniques, the onstate of the new 10kV IGCTs was tuned to a value between 4 and 6 V at a nominal current of 1700A. Afterwards, the switching behavior of the IGCT was investigated in a buck test circuit in single-shot operation (Figure 3). The SOA turn-off waveforms of the IGCT at VDC=6kV (anticipated nominal voltage) are shown in Figure 4. The IGCTs with a fortified HPT design turned-off safely more than 2000A at 6kV, equivalent to a peak power density of 300 kW/cm2 of active IGCT area. This represents a significant increase in peak power handling capability of 10kV HPT devices compared to standard lower voltage IGCT (200-300kW/cm2 for currently existing standard large area IGCTs up to 6.5kV). However, the power density has decreased from the value of 600kW/cm2 reported in [2] due to the increased voltage ratings (from 6.5kV to 10kV). The fortification of the p-base is not unlimited. A deeper and more heavily doped p-base will inevitably slow down the thyristor turn-on as shown in Figure 5.

The circuit used for measuring the dynamic performance of IGCTs

The SOA waveforms for large-area (91mm) 10kV HPT IGCT at 130°C VDC=6kV

Turn-on waveforms of the different 10kV IGCT designs

10 kV Diode electrical characteristics

Manufacturing a robust diode for applications at 7kV DC voltage is a challenging task due to the trade-off between diode losses and hardness against cosmic rays. To reach a low cosmic ray failure rate a design based on a low n-base doping and thick wafers is recommended. On the other hand, to minimize the on-state losses, a design based on high n-base doping and thin wafers is preferred. This trade-off means that for high voltage designs, the diodes will have a snappy recovery process by having a punch through voltage that is much lower than the DC-link voltage.

Figure 6 shows the SOA waveforms for the reverse recovery of a 10kV diode at a DC link voltage of 6kV. The diode can handle more than 2000A at a DC link voltage of 6kV at 125°C. This SOA matches the improved SOA obtained for the 10kV IGCT. However, snap off of the diode during the reverse recovery phase remains an issue with standard high power diode designs. As this behavior is not desirable in an industrial application, while low losses are of highest importance, two concepts for reducing the snappiness of the diode are currently considered. In simulations, a 10kV diode with FCE design shows snap-free recovery even under hard switching conditions, i.e. low temperature, high DC link voltage and low on-state currents (Figure 7). The work to manufacture 10kV diodes with these improved designs is currently carried out on silicon.

SOA reverse recovery waveforms for large-area (91mm) 10kV diode at 115°C VDC=6kV

Device level simulations showing reduced diode snap-off effect by using the FCE design

With an excellent blocking capability of more than 11kV, and significantly enhanced SOA capability over standard designs, the 10kV IGCT represents the next step towards the next generation of high power semiconductor switches. Together with the development of a high voltage soft switching diode, future high power electronic applications will continue to fully benefit from the versatility of the IGCT technology.

Acknowledgment

The authors would like to thank M. Rahimo, J. Vobecky, A. Kopta, E. Nanser, and M. Kunow from ABB Switzerland Ltd, Semiconductors, for their essential contributions to this work.

 

References:

1) S. Bernet, E. Carroll, P. Streit, O. Apeldoorn, P. Steimer, and S. Tschirley, “Design, test and characteristics of 10kV IGCTs”, Industry Applications Society, 38th IAS Annual General Meeting, October 12-16, 2003, p.1012.
2) T. Wikström, T. Stiasny, M. Rahimo, D. Cottet, and P. Streit, “The corrugated p-base - A new benchmark for large area SOA scaling”, in Proc. ISPSD 2007, Jeju, Korea, p.29.
3) A. Kopta, and M. Rahimo, “The Field Charge Extraction (FCE) Diode – A novel technology for soft recovery high voltage diodes”, in Proc. ISPSD 2005, Santa Barbara, California, USA, p.83.

 

 

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