Pressure contact IGBT, the ideal switch for high power applications.

Transcription

1 Pressure contact IGBT, the ideal switch for high power applications. F. Wakeman, G. Lockwood, M. Davies & K. Billett. Westcode Semiconductors Ltd., P.O. Box 57,Chippenham, Wilts, SN15 1JL, U.K. Abstract Models and experimental data are used to predict the performance of large area pressure contact IGBTs, offering ratings equivalent to the largest power conventional technology devices. Differences in the electromechanical characteristics of pressure contact devices, when compared to substrate mounted devices, are reviewed and how these influence the potential performance of devices with higher power ratings. Employing high power pressure contact IGBTs in practical applications is considered and some possible advantages over alternative technology devices is indicated. I. Introduction The IGBT has made much progress in filling many of the requirements of power system designers, however, further expansion of applications is limited by the maximum ratings of individual switches. Lower powers are adequately supported by the extensive range of module devices, produced by a wide range of manufacturers, but aspects of reliability bring into question their suitability for very high power applications [1 7]. Several manufactures have now introduced high reliability pressure contact devices, which offer the possibility of IGBT switches suitable for much higher powers [8-2]. Combining the chip technology of pressure contact IGBTs with the packaging technology of high power thyristors and GTO thyristors [21-26] may in the future offer suitable pressure contact IGBT switches for all power applications. It is the intention of this paper to explore what is available and what with the combination of different available technologies is possible, how this can be achieved and at what cost. II. Nominal current rating (Amperes) Present technology A vast range of different power switches are available to meet the needs of industry and each offering its own advantages in particular applications. Arguably among these the most versatile is the IGBT, due to its simple gating requirements and fast switching speed. Presently, however, IGBTs are limited to lower power applications due to the limitation of available ratings, unless used in complex arrays of series parallel combinations which introduce unacceptable levels of system reliability. A. Available power switches. It is not a simple matter to directly compare the performance of different types of power switches. While it is easy to compare IGBT module IGBT capsule GTO thyristor 1mm Thyristor Light fired thyristor the maximum voltage ratings, finding a common basis on which to compare maximum operating current is not so simple due to the limitations imposed by controllable current in both IGBTs and GTO thyristors/igct. Equally, it is difficult to use a common definition of controllable power, as this is so dependent on the specifics of the application. However, to give some indication of the present state of power switch technology fig. 1, compares the average current rating of conventional technology thyristors and the maximum controllable current of GTO thyristor/igct and IGBTs, to their respective available voltage ratings. Fig. 1 should not be considered as exclusive as new devices are continually in development and ever higher ratings are being achieved for all device types except GTO thyristors, which appear to have passed their zenith. It is clear from fig. 1, that at present, conventional thyristors and GTO thyristors/igcts offer the only single device solutions for high power applications. GTO thyristors can be seen to be at their most effective for higher voltage grades due to the direct relationship between silicon thickness and current rating. The conventional thyristor ratings are based on 1mm devices, which represent the largest devices in general usage, although larger devices are known to be used in some power conditioning applications, here being represented by the 8kV light fired device. The relatively low controllable current of both the pressure contact IGBT and their substrate mounted module equivalents, must however be put in to context. If the influence of the switching frequency is brought into account then the effective limits to operating currents converge with those of the thyristors. This however is not generally an issue when the highest power applications are considered. B. Present IGBT ratings. The nominal controllable current of the pressure contact IGBT and the equivalent substrate mounted module device can be seen from fig. 1 to be quite similar. This however, may not be the true case, in all applications, if some of the issues discussed in this paper are considered. Pressure contact IGBTs with current ratings from 4A to 18A and voltage ratings of 1.2kV to 4.5kV have been introduced [8-2]. These devices are available in both standard pressure contact packages as illustrated in fig. 2, [8-1, 16-2] as well as some radical new formats, arguably more suited to the square chips the packaging must contain [11-15]. Fig. 1, Comparing controllable current. Copyright 22 IXYS Corporation

2 III. What can be achieved A. Packaging Pressure contact devices are available with round housing of total diameter 12mm, with an 85mm boss [1] and square packages with overall dimensions of 14mm, and boss edge lengths of 92.3mm [15]. To achieve operating current equivalent to a conventional thyristor much larger outlines will be required. The possibilities to produce IGBT pressure contact devices in packages to the maximum dimensions offered in conventional thyristors and GTO thyristors is primarily a question of acceptability, as the technologies required are already available [21-26]. Ceramic packaging of the form seen in fig. 2 are used to house devices manufactured on silicon of >15mm diameter, requiring a package with an overall diameter >2 mm and a boss diameter of >14mm. The very large numbers of individual IGBT chips, will also add complexity to the internal structure required to interconnect the individual gates and a much larger peak gate current can be anticipated. Structures such as the planar system of fig. 5, which was introduced in [16], offer a simple solution to this potential problem, without the need for large numbers of parallel wire connections. L E L C Fig. 3, Distribution of gate inductance, Pressure contact device. L E L E L E L T L W L W L W L C L C L C Fig. 4, Distribution of gate inductance, substrate mounted device. L T Fig. 2, Outline of a 14A pressure contact device. Some questions do need to be answered regards the mechanical handling of such devices, but as with the packaging, the fundamentals of assembly and clamping are already in place for the large power thyristors. Connections to gates of individual IGBT chips External gate connection To achieve high current ratings, in a single package, it is necessary to parallel large numbers of individual IGBT chips. This raises a number of mechanical and electrical issues, as to how best to interconnect the individual chips. The mechanical design described in [16], for a 4A/18V device with a 47mm boss diameter, offered a sound base for the development of a larger device [17] with a 75mm boss diameter and is eminently suited to even larger devices. The pre-assembled chip carrier offers two options for the development of a larger device, with either a larger single unit, or multiple sub units, being employed. The electromechanical properties of a pressure contact IGBT have been shown [16] to have some important differences to those of substrate mounted devices, which make the paralleling of large numbers of chips possible. The uniform distribution of inductance between individual chips, fig. 3, prevents the unequal transient voltages of the more complex substrate mounted device, fig 4, an issue which is bound to become more dominant as the overall package dimensions increase. Fig. 5, Planar gate contact. B. Optimising chip design for voltage. It has already been indicated that, unlike the conventional thyristor or GTO thyristor/igct, the IGBT comprises of a number of smaller discrete devices operating in parallel, rather than a large single device. These individual devices or chips are square and cut from a single slice of silicon to ensure near identical characteristics, when operated in parallel to form a complete device. The complexity of the MOS gate structure limits the practical size of the active area of the individual IGBT chips, typically 1cm 2 [27], if reasonable yields are to be obtained during processing. The overall dimensions of an individual IGBT chip is dictated by the active area and the additional surface area around the chip required for the junction termination. The voltage termination

3 is essentially of a planar nature, with several methods being deployed and is a function of the junction depletion depth [28-3]. From these basic principles it is possible to determine that the optimum chip size for an IGBT is a function of its required voltage grade and that the greater the proposed voltage the larger the chip size. Fig. 6 shows the approximated relationship between the chip edge length and the IGBT voltage, scaled from the dimensions used in the 18V chip developed for the devices in [16-2]. Fig. 6 assumes a common active area for the device regardless of voltage grade, in practice it may be beneficial to use a larger active area for the higher voltage devices to reduce the on-state losses. However, to limit the number of variables, in the comparisons made later in the text, this is not considered further here. 18 voltage range 1.2kV to 6.5kV. The actual housing sizes used in the predictions, which have finite size defined by the chip dimensions, are represented by the symbols in the graphs with the lines only being present to aid clarity. Normalised current per chip Blocking voltage (volts) Fig. 7, Normalised current rating per chip. 6 Nominal chip edge length (mm) V 18V 25V 33V 45V 65V Forward blocking voltage (volts). Fig. 6, Chip size against voltage. C. Prospective IGBT ratings. Based on the known performance of available devices it is possible to predict the performance of larger area devices, two possible package concepts were considered, firstly with a conventional round pole piece of the form given in fig. 2 and secondly with the more radical square pole face. Before considering the performance of chips optimised for active area, as discussed in the previous section, results are presented for devices with a common chip size of 12mm square. To allow the predicted operating current of devices with different voltage grades to be compared, current per chip is proportioned to the active area. A graphical representation of the current per chip, normalised against the 18V chip considered in the previous section, is included in fig. 7. Scaling the current in this manner does not fully take into account the on-state voltage of a device, with a given voltage grade, as this does not change proportionally. It is difficult to include this factor due to the large number of additional variables that would need to be taken into account and that its significance will be application related, however this point will be taken up again later in the text. Fig. 8 to 11, present the predicted operating currents for devices based around the common chip size of 12mm and for package sizes up to the maximum presently used for all types of power semiconductors, as introduced earlier in the text. Graphs are presented for devices in both round and square packages, with and without fully rated anti-parallel diodes. Package dimensions are based on either the boss diameter for round housings, or the boss edge length for square housings. In all the examples given, the anti-parallel diode occupies ¼ of the available chip locations. In each of the graphs operating currents are predicted for devices in the Fig. 8, 12mm chips in a round housing, with diode V 18V 25V 33V 45V 65V Fig. 9, 12mm chips in a round housing, without diode V 18V 25V 33V 45V 65V Fig. 1, 12mm chips in a square housing, with diode.

4 V 18V 25V 33V 45V 65V V 18V 25V 33V 45V 65V Fig. 11, 12mm chips in a square housing, without diode. The results of fig 8. & 9 indicate how the current rating is influenced by the square chips in a round housing. The number of chips which can be housed in a 75mm and 9mm boss device are not significantly different and this is reflected in the predicted current ratings, which are very similar. As the housing diameter becomes greater this effect is observed to be less significant. It is also clear from fig. 8 to 11 that the square package offers a significant advantage in current ratings over a similarly dimensioned round package. Obviously a square package of a given edge length has a greater boss area than a round package of the same diameter, but as both would require the same clamping assembly, this seems an appropriate comparison. Extending the same approach to the chips with optimised area produced fig. 12 & 13, presented only for the square package option. In these a significant improvement in operating current is seen in the higher voltage grade devices. The same would not be the case for round housings, where the larger the chip the less efficiently they can be accommodated in the package as illustrated by the example for a 4.5kV device given in fig. 14, which shows no significant advantage in using a larger optimised chip V 18V 25V 33V 45V 65V Fig. 13, Chips optimised for size in a square housing (without diode), V CE not taken into consideration mm 14.7mm Fig. 14, Current rating of 12mm chips compared to 14.7mm chips, for a round housing without diode. IV. Why a pressure contact IGBT? As well as the issues regarding the inductance of the package, several other aspects of the pressure contact IGBT may offer significant advantages in some applications, when compared to other technology devices. A. Thermal resistance The differences seen in the thermal resistance paths of pressure contact and substrate mounted IGBT [16] are potentially more significant in larger power devices, fig 17 & 18. The uniform junction temperature of the IGBT chips, in a pressure contact device, ensures equal sharing of the current, which is essential when such a large number of chips are operated in parallel R ThE Fig. 12, Chips optimised for size in a square housing (with diode), V CE not taken into consideration. These results indicate that for devices with dimensions less than the largest package size used for conventional thyristors, it is possible to develop IGBTs with current ratings equivalent to those available in all alternative technologies. Naturally this does not imply their suitability for all applications, or that the predicted current can always be exploited. Particularly in the case of the higher voltage devices, it may be that the relatively high on-state losses, when compared to a conventional thyristor, preclude the operation at the predicted levels of current. R ThC Fig. 17, Distribution of thermal resistance, Pressure contact device.

5 smaller devices described in [16-2], which have been confirmed by measurement. B. Power rating R ThA ;;;;;;;;;;;;; Baseplate R ThB R ThC Sink Fig. 18 Distribution of thermal resistance, substrate mounted device. Regardless of the size of package, individual chips in a pressure contact IGBT should be maintained at essentially the same junction temperature, due to the negligible thermal resistance between adjacent chips. This is not always the case where chips are substrate mounted, due to the thermal restraints of the materials used. It is not possible to bond all the chips to a single substrate in large module devices, hence there may be a relatively high lateral thermal resistance between groups of chips. When this is combined with the very large area of the base plate, required in high current module devices, it is not difficult to see how temperature differentials on the external cooling system can easily be transferred to the chips inside the device As well as the steady state thermal resistance, considered above, the transient thermal characteristic of a pressure contact IGBT has quite different properties from a module. The pressure contact device has a high thermal capacity, due to the large mass of the pole pieces, which typically results in a tenfold difference in thermal resistance at short time intervals, when compared to a typical substrate mounted device. The transient thermal resistance for a 14A pressure contact IGBT is included in fig 19 and it can be determined from this curve, that under short pulse conditions, the junction temperature of the chips will be subject to much smaller transient temperatures. Therm al im pedance junction to sink R q, (K/W att) Westcode WTC14AAC18 Em itter Collector Double side Tim e (Seconds) Fig. 19, Transient thermal resistance of a 14A pressure contact device. To give an indication of the steady state thermal resistance of large area pressure contact IGBTs the thermal resistance of the 12mm chip devices of figs. 8 to 11 were modelled, the results of which are included in figs. 15 & 16. These curves were generated by expanding the results obtained with the From the thermal resistance it is possible to determine the maximum permissible power loss and this is represented in fig 17 & 18 for the 12mm chips in a square package. As well as the power rating for the IGBTs, data points are included for other power semiconductors to allow a comparison of power per unit by size. Both figures assume double side cooled and a sink temperature of 5 C. Fig. 17 is for a maximum IGBT junction temperature of 125 C and fig 18, for maximum junction of 15 C. These curves show clearly how the large area IGBT may offer size advantage for a given power requirement. This is particularly noticeable when the higher maximum permitted junction temperature of 15 C is taken into account, as in fig. 18. Thermal resistance (K/W) With diode Without diode Boss diameter (mm) Fig. 15, Thermal resistance of a round device. Thermal resistance (K/W).1.1 With diode Without diode Fig. 16, Thermal resistance of a square device. Maximum power (kw) IGBT with diode IGBT without diode 1mm thyristor GTO thyristor Light fired thyristor Fig. 17, Maximum permitted power loss, 12mm chips in a square housing, IGBT maximum junction 125 C.

6 Maximum power (kw) IGBT with diode IGBT without diode 1mm thyristor GTO thyristor Light fired thyristor Fig. 18, Maximum permitted power loss, 12mm chips in a square housing, IGBT maximum junction 15 C. The extent to which the higher permitted power per unit can be translated in to higher operating current, is determined by the on-state losses and the switching losses. This will have most effect for the higher voltage IGBTs and this can be seen from the on-state voltage normalised against an 18V device in fig. 19. A similar relationship would be seen for the switching losses, the ultimate significance of these effects will be determined by application and requires consideration on an individual basis. Normalised on-state voltage Forward bloking voltage(volts) Fig. 19, Normalised on-state voltage. Number of cycles, Nx Westcode WTC4AAC Junction temperature excursion Fig. 2, Thermal cycling performance. C. Thermal cycling performance. T J (C). Typical module data Pressure contact test data (no failures) Thermal cycling performance of the pressure contact device is a principle factor in its perceived high reliability and this is illustrated by fig. 2. Accelerated life-testing of pressure contact devices, with DT j up to 15 C, has produced very encouraging results. Sample devices have completed >9k cycles, without failure. One sample was determined to have failed when the on-state voltage increased by 5% after 96k cycles and eventually suffered catastrophic failure at 116k cycles. Further evaluation is on going, but from these results it is clear that the pressure contact IGBT has the potential to out perform most other power devices in this aspect of reliability. D. Series parallel operation. The pressure contact device is ideally suited to applications which require devices to be operated in series. Both electrical and mechanical advantages are seen over some alternative technologies. In particular the isolation of gate firing boards is greatly simplified by the IGBTs small gate power requirement, when compared to the GTO thyristors or IGCT and even some conventional thyristors. Similarly the relatively high threshold voltage greatly reduces the possibilities of miss-fire due to noise and the ability to turn the device off at the gate as well as on, may offer advantages over conventional thyristors in some applications. In all cases the pressure contact device offers a more practical solution, in terms of the mechanical design for series operation than module technology. In general the assembly will be both more compact and easier to cool. As with all pressure contact devices failure is always to a short-circuit, unlike modules which generally fail to opencircuit. With guaranteed failure to short-circuit, pressure contact devices operated in series in a system incorporating device redundancy, will also continue to function after redundant device failure. Where as the substrate mounted IGBT has been paralleled to achieve high currents, the pressure contact device may be more appropriately operated in series to obtain the required voltage. As the required current is potentially available in a single switch, several lower voltage devices in series could be operate at a higher frequency, due to the lower switching losses per device. As well as performance advantages, the use of several lower voltage devices in series may have other advantages in some applications. It is predicted that a 3A/1.8kV IGBT with diode, could be housed in a square package of <1mm edge length, were as a 4.5kV device would require >12mm just to achieve the same current, and larger to achieve the same operating performance. There may be advantages in both system cost and size in using two or three smaller, lower voltage devices in series. V. Commercial Considerations The opportunity to use single large area IGBT devices may have significant advantages in terms of both cost and system size in many applications. The IGBT market is well established and continues to move to ever-higher current and voltage ratings making them viable components for equipment ratings up to approximately 2 MVA. For these applications multiple modules are used in parallel and series to achieve these required power levels. At a ceiling of around 5MVA the commercial benefits are minimised and the technical demands of the equipment exceed the capability of the module. Parallel and series arrangements become extremely complex and expensive, reliability becomes an even greater concern. Above 5MVA, module technology is rarely used.

7 Pressure contact technology is the proven solution with GTO thyristor/igct technology being used in medium voltage drives and pressure contact IGBT technology in HVDC systems to 15MVA. Transportation, which has made a significant move towards the IGBT module, is also having to deal with the technology limitations. With higher reliability and unit power the pressure contact IGBT could be considered for the replacement of the GTO thyristors still being used in specific applications i.e. Freight Locomotives. Pressure contact technology has enhanced the benefits of the IGBT to provide longer-term reliability comparable to the conventional and GTO thyristor. With the introduction by several manufacturers of high reliability pressure contact IGBTs the opportunity to use them for higher power levels is now possible, hence replacing multiple module arrangements whilst retaining reliability and being commercially more acceptable than the modular or GTO thyristor approach. Equipment relying upon GTO thyristors can now move to IGBT technology whilst retaining this reliability, realising cost benefits without sacrificing quality. Pressure contact devices of >12mm are achievable for the IGBT, thus realising switching currents in line with GTO thyristor/igct technology whilst retaining all the cost benefits of an IGBT design. It has been shown that current ratings of >4A can be achieved for a 12V IGBT in a 12mm diameter package, with this increasing to >6A if the package is square. VI. Conclusion The technology is available to develop large area pressure contact IGBTs with ratings comparable to conventional technology devices. Large area IGBTs offer characteristics, which allow the full use of the design power rating and may offer significant advantages in the design of some power systems, in particular where line voltages require devices to be operated in series. The future development of large area pressure contact IGBTs requires careful consideration of the most appropriate approach. It is clear that a square package design offers the best performance for a given assemblies dimensions, but its acceptability is unproven. The advantages in terms of both device size and performance of using several lower voltage devices in series are quite clear, but may be difficult to encourage this approach, even though it equates to the equivalent practice of parallel operation of modules to achieve the required current. Acknowledgement The authors wish to acknowledge the contribution of R.Iorns and M.Evans in developing the 18V IGBT chips used as the base reference devices in this paper. Also W.Alderson for development work associated with the thermal resistance models. 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