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Posted on 01 December 2019

High-Power Technology (HPT)

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IGCTs with outstanding current handling capability

To improve performance, reduce the size and cost of power electronic systems and allow more flexibility in designing power electronic applications, the development trend in high power semiconductors continues to be one of increasing current and voltage capabilities. In this article, the high Safe Operating Area (SOA) of a new range of High-Power Technology (HPT) IGCTs with voltage ratings from 4.5kV to 6.5kV is presented.

By T. Wikström, M. Rahimo and E. Carroll, ABB Switzerland Ltd., Semiconductors

 

The Integrated Gate Commutated Thyristor (IGCT) has been developed to a reliable and efficient device and has gained wide market acceptance since its introduction, a decade ago. In many high power applications, such as Medium Voltage Drives (MVD), Power Quality, Static Breakers and others, the IGCT is well established. IGCTs are normally employed in voltage or current source inverters in two or three level topologies as one device per function although there are also examples of series connections for higher voltage. Thanks to its low losses, it has been regarded as the preferred technology for very high power conversion (typically above 2 MW). The main strengths of the IGCT can be summarised as follows:

• low losses
• voltage scalability
• compact package
• high power capability
• compact integration of the FWD (reverse-conducting devices)

The IGCT, unlike the IGBT, has a thyristor structure and generates low on-state losses. Furthermore, since it turns off like an IGBT, it requires no voltage snubber, unlike the GTO - a combination that has proven unbeatable for very high power applications. Today, the IGCT is available in current and voltage ratings starting at 4500 V, at a few hundred amps, to 6500 V and 4000 A. The product range includes asymmetric, reverse-conducting (integrated anti-parallel diode) and reverse-blocking (symmetric) types.

Power electronic equipment manufacturers are constantly seeking to improve the cost and performance of their systems. By increasing the power capability of the semiconductor switches, the cost and size may be reduced while improving performance and allowing more flexibility in system design. SOA determines the current and voltage boundaries within which the IGCT may safely be operated and as such, is an important and limiting parameter for power devices and their applications. The possibility of scaling the current capability of the IGCT linearly with device area has been limited by the uniformity of the gate current distribution over large areas. Therefore, the maximal permissible power density for large area devices has been restricted to 200 – 300 kW per square centimetre of IGCT active area, while the performance of small-area devices has been shown to reach 1.5 MW/cm2. In this article, results are shown from ABB’s new HPT product line for 4.5, 5.5 and 6.5 kV devices with 5.5, 5.0 and 4.0 kA turn-off ratings, respectively. HPT IGCTs attain twice the power density of present devices – over 600 kW/cm2 – without trading off any of the other desirable IGCT features, such as low losses.

The IGCT wafer consists of a massive parallel connection of thyristor segments, each surrounded by the gate metallization that contacts the p-base of the thyristor. At turn-off, the anode current is commutated from the cathode segments to the gate thereby interrupting the regenerative pnp-npn thyristor action. The process must be fast in order to operate the IGCT safely because the commutation has to be completed before anode voltage appears. This is commonly referred to as the “hard drive limit”. To achieve this, a very high di/dt is necessary which imposes high demands on impedance minimization in the circuit connecting gate and cathode – i.e. the housing and gate unit. For ABB’s 91 mm HPT wafer, shown in Figure 1, the 2700 thyristor segments are laid out concentrically in ten segment rings. The gate contact divides the segment rings into two regions – five inside and five outside the gate contact. In Figure 2, a cross-sectional drawing of a wafer in its housing shows the path of lateral current flow during turn off. It can be seen that the gate impedance (resistance and inductance) is ring-position dependent, resulting in an uneven SOA loading of the segment rings, the load being larger in the centre and at the periphery of the wafer. As the thyristor goes through its meta-stable state during turn-off, electron emission from the cathode segments is completely cancelled, initially in rings close to the gate and finally in those at the wafer periphery. Current redistribution to segments in gate-remote locations can, therefore, not be completely avoided. The distances across the wafer surface and hence the remoteness of the segments from the gate contact, scale with the square root of the wafer area. This is approximately also the scaling in current handling capability that can be achieved by simply increasing the device area.

A 91 mm HPT IGCT wafer with approximately 2700 cathode segments organized into ten segment rings

Unlike SOA, cooling and surge-current capabilities do increase with device area in a linear fashion. As a result, SOA has traditionally been the limiting parameter in high-power applications, which is the motivation for ABB’s current efforts to extend it through the HPT development.

Cross-sectional view of a GCT. The white arrows mark the anode current flow at turn-off

In developing the previous (intermediate) generation (“Gen II”) of asymmetric IGCTs, work was focussed on improving the gate circuit and enhancing local SOA (equivalent to increasing the SOA of a single segment). Local SOA improvement for Gen II essentially consisted of reinforcing the p-base conductivity, which lowered the gain of the npn transistor of the classical pnp-npn transistor pair (thyristor) making it easier to turn off (but harder to turn on). The ultimate limit of this strategy was determined by degradation of the classical thyristor properties.

For the current HPT generation, the gate drive improvements from Gen II were matched to innovative improvements in the vertical wafer design. As a result, the trade-off involving reduction of npn gain was no longer required, which led to even better turn-on properties than those of the first generation (Figure 3) while dramatically increasing SOA.

Waveforms comparing the turn-on under very high di-dt (6kA-µs) for Gen I

A consequence of the SOA being limited by the intra-wafer distances is that a lower capability at 25°C, compared to 125°C, is normally observed. This contrasts with other devices not subject to current redistribution at turn-off and is the result of higher charge-carrier concentration at elevated temperature which ensures that the npn transistor blocks before charge can be swept from the pnp transistor thus increasing the margin for safe operation with increasing temperature (i.e. it is a pnp transistor and not a thyristor, which turns off and sustains voltage). In Figure 4, the maximum controllable current for three 4.5 kV IGCT generations is compared. The second generation achieved good results at 125°C but since the large-area effects still dominated the SOA, the overall rating was reduced at 25°C and the device was, hence, only marginally better than Gen I. With the new HPT wafer technology, the “cold limit” was eliminated. Consequently, the overall capability was increased by 3 kA compared to Gen I and by 2 kA compared to Gen II. In Figure 5, SOA waveforms from 25°C and 125°C measurements are shown.

The SOA capability of the last three generations of 91 mm 4.5kV IGCTs at 25°C and 125°C

The SOA waveforms at 125°C (red) and 25°C (blue) for the 4.5kV HPT IGCT

For a circuit designer it is, of course, desirable that the components of his system not be destroyed should his equipment inadvertently experience an overload or fault condition whereby the SOA limits of the components are exceeded. With the new HPT IGCT, we are able to present a self-clamping feature that allows precisely that at turnoff. As shown in Figures 6 and 7, when the IGCT is stressed beyond its specified limits, the inductive voltage overshoot is clamped by the HPT IGCT itself without failure. Although it is not advisable to rely on overstressing the device, having this capability represents an extra level of safety should such operating conditions arise. Moreover, an increased margin between normal operation and SOA failure limit increases the overall reliability of the system.

Current and voltage waveforms at turn-off for high voltage and stray inductance

Locus of the current and voltage during the SSCM event shown in Figure 5.

Comparing the 6.5 and 5.5 kV HPT IGCTs, the HPT platform maintains excellent SOA with increasing voltage (historically, higher voltage devices suffered from lower SOA in kW/cm2 than their lower voltage counterparts). In Figures 8 and 9, SOA waveforms are shown. In the 6.5kV IGCT SOA measurement of Figure 9, the peak power slightly exceeds that of the 5.5 kV devices with a power density of 700 kW/cm2, an excellent benchmark for the HPT IGCT.

SOA Waveforms for the 5.5kV HPT IGCT. IT=6800A

SOA Waveforms for the 6.5 kV HPT IGCT. IT=6500A

HPT, the next generation of IGCTs scheduled for a 2007 market introduction, has achieved a number of significant improvements. Firstly, it now allows a 50% increase in SOA with respect to the presently commercialised products. Secondly, it allows SSCM which has been extensively described in connection with IGBTs over the past two years but is presented here for the first time in relation to IGCTs. SSCM is the definition of ruggedness for Turn-off Devices and is ultimately the guarantor of reliability. Thirdly, the HPT IGCT now demonstrates a negative temperature coefficient of ITGQ which means that the turn-off current at rated maximal junction temperature is the lowest current which the device can turn-off for a given set of conditions. Finally, the turn-on waveforms have now been improved to allow still lower turn-on losses and higher di/dt. The object of HPT development was primarily one of increased turn-off current for increased inverter ratings and this has been met, meaning that this latest generation of IGCTs has now moved its limitations from SOA to thermal management – which must become the next area of focus.

 

 

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