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Posted on 01 November 2019

The Internally Commutated Thyristor (ICT)

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A new GCT with integrated turn-off unit

Today, Gate Commutated Thyristors (GCTs) and Insulated Gate Bipolar Transistors (IGBTs) dominate the high-power semiconductor market. While IGBTs are mainly offered in modules, GCTs are only available in press-pack housings. Since the operation of GCTs requires a very low inductive and low resistive connection between the GCT device and the gate drive unit (GDU), the GCT housing is directly connected to the GDU (Figure 1), resulting in an Integrated Gate Commutated Thyristor (IGCT). This design requires close attention to creepage distances and thermal stressing of the GDU. In addition, the gate drive of IGCTs is usually much larger than an IGBT driver and consumes considerably more power.

By Peter Köllensperger and Rik W. De Doncker; Institute for Power Electronics and Electrical Drives, RWTH Aachen University

 

ICT Concept

Thyristor-type turn-off devices like the GCT are controlled by the gate current [2]. During turn-off, the complete anode current has to be commutated from the cathode to the gate connection before the anode-cathode voltage starts to rise. To ensure this unity gain turnoff, the gate-cathode voltage has to be kept below approximately 0.5 V. Using MOSFETs to shortcircuit this path, i.e. the MTO concept [3], a safe turn-off can only be guaranteed if the impedance in the commutation path is very low. This is not easy to realize in a hybrid fashion, but becomes feasible by directly connecting the MOSFETs to the wafer [4].

Figure 1

Alternatively, an additional voltage source can be implemented in the gate-cathode path. Therefore, the influence of the parasitic impedances is minimized and the unity-gain implementation is simplified. Since the voltage source is stressed with very high current pulses, precharged capacitors are used. Not only do the parameters of the selected devices influence the impedance, but also the geometrical layout. Since a long current loop area yields a high inductive impedance, commercial drivers for GCTs minimize the impedance by integrating the GCT power semiconductor into the gate drive unit.

Integrating critical parts of the GDU into the GCT housing can improve the overall performance significantly. In the proposed ICT [5], the turn-off unit, consisting of MOSFETs and capacitors, is placed next to the GCT wafer inside the presspack housing (Figure 2). Consequently, the remaining GDU is simplified and can be connected via cable to the ICT (Figure 3). In addition, the coupling between GCT wafer and turn-off unit benefits from the shorter connection and improves the commutation process during turn-off.

Principle of the Internally Commutated Thyristor

Integrated Turn-Off Unit

To enable the integration of the turn-off unit into the presspack housing, two important aspects have to be considered: limited space and high temperatures (up to 125°C) inside the housing.

First ICT prototype

The size of the standard turn-off unit, that can be seen in the red box in Figure 4, is much too large for an integration into the press-pack housing. This requires a drastic volume reduction, which can only be achieved by a complete redesign of all components.

Comparison between commercial and new turn-off unit

The standard electrolytic capacitors, which are very sensitive to high temperatures, are replaced by Multi-Layer-Ceramic-Capacitors (MLCCs) using X7R material.

The limited ac-current capability of the electrolytic capacitors in the standard GDU requires the parallel connection of many capacitors, resulting in a relatively high capacitance. In contrast, the necessary capacitance of the new turn-off unit can be calculated by defining a maximum voltage drop during turn-off of the GCT. In addition, a series of fast switching cycles is usually required without the possibility of recharging in between. In an ICT, these two aspects are the main design criterion for the capacitors of the turn-off unit. Consequently, it is considerably smaller, e.g. 470 μF instead of 44.7 mF for a 520 A device. In addition, the MLCCs have lower parasitic impedances, which is also beneficial during turn-off.

Concerning the MOSFET switches of the conventional GDU, only the volumetric ratio between silicon and packaging has to be improved in order to minimize the required volume. This can be accomplished by the use of DirectFETs from IR [6]. They basically consist of the MOSFET chip with a small metal shielding at the drain contact that enables a direct soldering to the PCB and double sided cooling. Furthermore, this package is able to sustain pressure, which allows for pressure contacting.

The resulting turn-off unit for the first ICT prototype can be seen in Figure 5 in comparison to an one Euro coin. Further details can be found in [5].

Internal turn-off unit

Measurement Results

Several tests were carried out to verify the function of the first prototype ICT. Figure 6 shows a successful Safe Operating Area (SOA) test at 3.3 kV, 520 A and 125°C with the ICT, which equals the performance of the commercial version. The current commutation from cathode to gate starts at t = 0 μs and is finished at t = 0.3 μs well before the anode-cathode voltage begins to rise (t = 0.8 μs), which approves the hard-drive of the GCT.

SOA test with ICT device

The capacitors were precharged to -10 V, compared to the -20 V of commercial GDUs. This leads to a significantly reduced power consumption of the ICT GDU. Since the final GDU was not yet available for this test, the capacitors were not recharged quickly after the turnoff.

The gate current IG was measured with a small custom-made Rogowski-coil between turn-off unit and GCT. The gate current also shows a distinctive reverse-recovery of the gate-cathode diode part of the GCT, proving the hard turn-off and indicating potential for switching off even higher currents [7].

Remaining Gate Drive Unit

As stated in II, the GCT is a current controlled device. In order to turn-on both the IGCT and the ICT, a positive current peak is needed, followed by a continuous dc-current to be fed into the gate contact. However, the demands on the respective GDUs during turn-off switching differ significantly. An IGCT driver has to carry the full anode current of the device, whereas an ICT driver just has to issue the turn-on signal for the internal MOSFETs of the ICT. In contrast to the IGCT driver, the ICT GDU has no need for a low inductive, low resistive current path to the GCT, enabling a simple cable connection.

Since the turn-off unit, which scales with the ICT current rating, is integrated, the remaining GDU can be used for different ICT ratings. Consequently, it is no longer necessary to build specific GDUs for each wafer size and therefore possible to build application specific drivers [8].

Reliability

If the reliability of the total ICT switch is investigated, three different factors can be distinguished. First, the reliability of the GCT wafer and the presspack housing, secondly the reliability of the internal turn-off unit and finally the reliability of the GDU. Thyristor-type highpower devices in press-pack housings, e.g. GTOs and GCTs, usually have excellent reliability records. Therefore, the GCT wafer should not be a concern, since it is a proven design. In contrast, no work experience with the internal turn-off unit, consisting of DirectFETs and MLC capacitors, has been gathered yet. The DirectFETs do not contain any bond wires, produce only very low losses and are thermally coupled closer to the heatsink than to the wafer, since the thermal conductivity of the MLCCs is very high. The DirectFET housing can sustain pressures of several hundred Newton without fatigue effects [9].

MLC capacitors show no electrical wear out mechanism, e.g. they do not dry out like electrolytic capacitors and can operate over the required temperature range without derating. The ceramic capacitors are mounted vertically underneath the PCB and the complete turn-off unit is spring mounted into the press-pack housing. Thus, mechanical stresses due to different thermal expansion coefficients between copper and X7R are absorbed and cannot cause cracks in the MLCCs.

The GDU can be located outside the press-pack stack with considerable distance to heatsinks and ICT. In addition, no electrolytic capacitors are needed that could limit the allowable ambient temperature. Considering the total switch, consisting of ICT and GDU, an excellent reliability is expected.

Conclusion

The ICT concept with optimized gate drive possesses several advantages over the standard IGCT in three major areas. The turn-off capability of the GCT wafer is improved by the lower stray inductance and the lower ESR of the MLC capacitors compared to electrolytic capacitors. Ease of use of the complete switch is better due to the flexible low power cable connection, the reduced number of different GDUs, the lower power consumption and the larger ambient temperature range. Finally, the reliability is improved by absence of electrolytic capacitors and a lower temperature at the GDU (cable connection enables relocation).

Acknowledgement

The authors would like to thank Eric Carroll, from ABB Semiconductors Ltd., for supporting this work.

 

References:

1) “5SHX 06F6004 datasheet,” ABB Switzerland Ltd, 2002, www.abb.com.
2) H. Grüning, B. Ødegard, J. Rees, A. Weber, E. Carroll and S. Eicher, “High-power hard-driven GTO module for 4.5 kV/3 kA snubberless operation”, PCIM 1996, Nueremberg, Germany.
3) D. Piccone, R. W. De Doncker, J. Barrow and W. Tobin, “The MTO thyristor-a new high power bipolar MOS thyristor”, IAS 1996, San Diego, USA.
4) Dirk Detjen, Stefan Schröder and Rik W. De Doncker, “ New High-Power BIMOS-Devices Based on Silicon-Silicon Bonding“, IAS 2002, Pittsburgh, USA.
5) P. Köllensperger and R. W. De Doncker, “The Internally Commutated Thyristor - a new GCT with integrated turn-off unit”, CIPS 2006, Naples, Italy.
6) A. Sawle, C. Blake and D. Maric, “Novel power MOSFET packaging technology doubles power density in synchronous buck converters for next generation microprocessors”, APEC 2002, Dallas, 2002, USA.
7) H. Grüning and K. Koyanagi, “A new compact high dI/dt gate drive unit for 6-inch GCTs”, ISPSD 2004, Kitakyushu, Japan.
8) Peter Köllensperger and Rik W. De Doncker, “Optimized Gate Drivers for Internally Commutated Thyristors (ICTs)”, IAS 2006, Tampa, USA.
9) “Application note an-1035”, International Rectifier, 2004, www.irf.com.

 

 

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