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Posted on 03 July 2019

A 10kV HPT IGCT with Improved Switching Capability

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Developing a new platform for high voltage switching

A major issue for the 10 kV IGCT has been the limited turn-off capability. By introducing the new HPT technology a 10 kV IGCT is in development that has a switching capability comparable with the capability of former IGCTs with half the voltage rating.

By Tobias Wikström and Arnost Kopta, ABB Switzerland Ltd, Semiconductors and Iulian Nistor ABB Switzerland Ltd, Corporate Research

 

Numerous modern power electronic applications, e.g. in MV Drives, Flexible AC Transmission Systems (FACTS) are operating at increasing line voltages, thus demanding semiconductors with higher blocking voltages. This allows a reduction of the current and losses in the system, and avoids series connection of semiconductor switches which requires additional snubber components. The 10kV IGCT devices, see figure 1, will allow the operation of drives to voltages up to 7.2kV rms and power of 12MW without series or parallel connection of semiconductor switches (e.g. using a classical 3L-NPC topology). Another possible application field are high voltage applications with multilevel configurations where the low switching frequencies can make the 10 kV IGCT a viable solution compared to lower rated IGBTs.

10kV IGCT module using the HPT

For the design of such high voltage IGCTs, minute attention had to be given to the resilience against cosmic ray events and its impact on device design. In addition, losses and maintaining competitive turn-off current ratings comparable with today’s ratings of 6.5 kV IGCTs were significant technology challenges that had to be addressed. ABB has already presented a 10 kV – 2kA IGCT that demonstrated the feasibility of the 10kV technology on Silicon. However the current ratings were not high enough for the targeted application. This article reports on increased IGCT SOA capability using a novel device design. The High Power Technology or HPT platform is the latest generation of ABB’s high power high current IGCTs with enhanced Safe Operating Area (SOA). These features ensure an increased margin for the IGCT ruggedness, further expanding the power capability per device. The cooling of the semiconductor switch and not the IGCT SOA is now the limiting factor in the power system design. The 10kV HPT IGCT is expected to be first commercially available in 2011.

HPT IGCT Technology

The next generation HPT IGCT platform has been designed to substantially increase the Safe Operating Area of the device. The “High Power Technology” (HPT) IGCT structure is based on a corrugated p-base doping profile as seen in Figure 2.

The HPT GCT wafer structure

The application of the HPT technology platform has enabled ABB to establish a new benchmark in the IGCT technology over the whole voltage range. The concept has been shown to increase the SOA of the previous generation of commercial 4.5kV IGCTs by as much as 40%. A peak power density of 700 kW/cm2 is reached for large area HPT IGCTs, significantly higher than the capability of previous standard devices.

Static 10kV IGCT Characteristics:

The required static blocking capability of a 10kV IGCT has been reached using a wafer termination based on Variation of Lateral Doping. This design of the termination is uniquely suitable for large area power semiconductor devices being very efficient with regards to the ratio active area/ termination area. Figure 3 shows that the voltage blocking capability of these large area Si power semiconductors exceeds 11kV with a leakage current lower than 20mA at 125 °C. Variations in the silicon design to optimise components for lower machine rating, as 6 kV rms, are in consideration.

Forward blocking of the 10kV IGCT Forward blocking of the 10kV IGCT

The on-state characteristic of a 91-mm IGCT at Tj=125°C and after carrier lifetime engineering is shown in Figure 4. The on-state voltage drop is 5.2V at a current density of 50 A/cm2.

On-state characteristics of the 10kV HPT IGCT at Tj=125°C

Dynamic 10kV IGCT Characteristics & SOA Performance:

The switching of the IGCT prototypes was measured in the test circuit showed in Figure 5 in single-pulse & multi-pulse operation. The turn off waveform is shown in Fig. 6. The IGCT can turn off safely a current of at least 3.2kA at both Tj=25°C and 130°C, limited by the capability of the test setup. The peak power in the 10kV HPT IGCT reaches 19.77 MW, see figure 7, corresponding to a power density of 460 kW/cm2. This is significantly higher then standard 10kV IGCT designs which are limited to values of about 12MW at these voltage levels, corresponding to a power density of 300 kW/cm2.

The circuit used for measuring the dynamic performance of 10kV IGCTs Parameters

Turn-off waveform for a 91-mm 10kV HPT IGCT at VDC=6kV, IT=2kA,

Peak power and losses waveforms during turn off of 10kV IGCT at 130°C

However, the power density has decreased from the value of 700 kW/cm2 for the 4500 V device due to the increased voltage ratings. Nonetheless, the power handling capability reached with the 10kV HPT IGCT demonstrates the excellent potential of the HPT platform for high voltage IGCTs.

10kV Soft recovery Diode

The topology of a modern VSI converter requires the use of freewheeling and clamping diodes with similar voltage rating as the main semiconductor switch. Especially the fast recovery freewheeling diodes must meet certain technological criteria among which soft reverse recovery is of main interest. The technological challenges are related to the trade-off between diode recovery losses and snappiness. In general, designing the diode for minimal losses leads to snappy reverse recovery. In a standard HV diode design, the reverse recovery current decreases to zero with a very high di/dt. Taking into account the presence of stray inductances in the circuit, high frequency & high amplitude oscillations can be noticed in the current and voltage waveforms. This can impact significantly the level of electromagnetic noise, negatively affecting the EMI compatibility of the system. In addition, the overvoltage can generate additional electrical stress on the components. As this behaviour is not acceptable in an industrial application, while low losses are of highest importance, the “Field Charge Extraction” (FCE) concept has been applied as a mean to provide the optimal trade-off between the apparently contradictory HV diode design requirements.

Figure 8 shows the reverse recovery of a standard 10kV diode under strong snap-off conditions (current 5% of nominal value). The on-state voltage drop of the standard diode is 5.5V at Ion-state=1.7kA. As expected, high frequency current oscillations as well as the over-voltages are observed. Next, FCE 10kV diode prototypes have been manufactured and the reverse recovery is shown in Figure 9. The on-state voltage drop of the FCE diode is 7.3V at Ion-state=1.7kA, therefore the life time of the minority carriers is further reduced compared to the standard diode. However, the FCE concept can compensate for the reduced level of on-state plasma yielding a soft recovery process.

Snappy reverse recovery waveforms for 91-mm standard 10kV diode in typical snap-off conditions

Soft reverse recovery waveforms for 91-mm FCE 10kV diode in typical snapoff conditions

In order to demonstrate the ruggedness of the FCE concept at higher nominal currents and temperatures, we have measured the reverse recovery at values up to 2000A and 125°C (shown in Figure 11).

Snappy reverse recovery waveforms for 91-mm standard 10kV diode in nominal conditions

Soft reverse recovery waveforms for 91-mm FCE 10kV diode in nominal conditions

In addition, we show that the SOA of the FCE 10kV diode is comparable with standard technology. The peak power during reverse recovery of a standard 10kV diode at Vdc-link=6kV, Inom=1.7kA is 6.8 MW and the losses are 30 J at a temperature of 125°C. For the FCE diode at Vdc-link=5.5kV, Inom=2kA, the peak power during reverse recovery is 5.8 MW, and losses are 20.35J at a temperature of 125°C.

 

 

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