Making IGBT Modules Flexible and User-Friendly

Development engineers working on the design of cost-efficient power electronic converters used in electrical drives, UPSs, welding machines or inductive heating systems depend on reliable and robust insulated gate bipolar transistor (IGBT) modules. Key requirements for these IGBT modules include:

Optimized static and dynamic power losses due to the latest IGBT chip technology.
Compact IGBT module dimensions leading to compact converter sizes.
Low module case height to reduce internal stray inductance.
User-friendly dc plus/minus and ac terminals for a low-inductive dc link layout.
Module availability in various switch topologies, such as half bridge/chopper/six pack.
Simple but flexible driver interface on top of module.
Integration of a temperature sensor for a basic protective function.
Possibility to easy parallel modules for higher output power.

However, comparing the above requirements to the existing IGBT module technologies reveals some drawbacks. In the case of IGBT half-bridge packages of 34-mm/62-mm width, they are:

Position of ac and dc terminals is impeding with an easy dc link construction.
Position of ac and dc terminals is limiting the space for on-top driver.
Minimum terminal height is 30 mm because of a common standard.
Different pitch between main terminals for different module sizes.
Automated production is difficult because of internal construction.
Main terminals have limited maximum current.

Shortcomings in the present six-pack modules include:

The high-power (heat) density is too high for an effective use of silicon.
Constraints for manufacturers and users due to large mechanical size and on-top driver.
Six-pack modules show reduced flexibility and a limited product range.

Overcoming Module Weaknesses

In addition to the above limitations, new chip generation developments, such as IGBT³ Trench and Soft-Punch-Through, require an adapted packaging layout. Moreover, the internal design of present power modules needs further optimization to match the IGBT characteristics, such as low saturation voltage V_CE(sat) of Trench devices, and best performance tradeoff of E_sw vs. V_CE(sat) for SPT devices. Also, the internal module construction has to show low ohmic resistance for low conduction losses and low stray inductance to minimize overvoltages during commutation.

Table. Comparison of generations of 1200-V IGBT technologies (123, 126, 128).1200-V IGBT Generations (V_CC=600 V; I_N=100 A, R_G=8 Ω)ParameterNPT (123)Trench (126)SPT (128)UnitV_CE(sat) 25°C 2,5 1,7 2,0 V 125°C 3,1 2,0 2,3 V E_ON + E_OFF 125°C 26 25 21 mJ Chip size factor 100 70 100 % Positive temperature coeff. V_CE(sat) Yes Yes Yes R_{th(junction-case)} 100 143 100 %

Consequently, a new IGBT module technology called SEMiX^™ has been developed to overcome the apparent drawbacks of present-generation modules and provides perfectly matched latest IGBT semiconductor technologies. In fact, it brings platform concept to this arena, thereby forming the basis of this cost-efficient and expandable IGBT module family. Applying the new technology, an extensive power range of SEMiX modules have been developed to meet the requirements of various applications.

Internal Construction

SEMiX modules incorporate the very latest IGBT and diode technologies. Currently, the IGBTs have 1200-V and 1700-V blocking voltage. However, the 600-V Trench series is under development. The voltage classes available in IGBT3 Trench technology are used in combination with optimized high-density CAL HD diodes. These freewheeling diodes show reduced forward-voltage drop, increased current density and improved temperature dependence above nominal current. The 1200-V module also comes with SPT IGBTs in combination with CAL diodes — particularly useful for applications with higher switching rates (f_sw > 6 kHz) for which the IGBT³ Trench modules are not so well suited.

The internal construction of SEMiX follows the established tradition of power modules in copper base plate technology. As shown in Fig.1, aluminum oxide ceramic substrates (direct bonded copper or DBC) are soldered onto the Cu base plate. Each DBC is arranged as a half-bridge circuit with a TOP and BOTTOM IGBT switch plus inverse FWD. Identical DBCs produced in high quantities are paralleled inside the module to assure the required output power. The same DBC design is used for all SEMiX types.

The main terminals of the SEMiX are soldered directly on to the ceramic DBC substrates. This feature provides excellent cooling for the main terminals and guarantees extremely low package conduction losses. Even the inner connection between DBCs is realized by soldered bridges — again leading to low ohmic internal losses. This approach fully exploits the advantages of Trench IGBTs with low saturation voltages and increased current density.

Internally, the new module construction uses spiral springs for the IGBT auxiliary connections to gate, emitter, collector sense and temperature sensor. The spiral springs are placed vertically between the respective DBC copper area and the interface PCB integrated into the module's lid. With pressure contact, the springs ensure the connection of auxiliary contacts and the control terminal strip on the module top side. The company's experience with spring contact technology, employed in its inverter systems and modules, has proven extremely reliable in harsh environments. Every half-bridge module contains an integrated temperature sensor with an NTC thermistor characteristic, which monitors the module temperature.

The new IGBT platform's terminal height of 17 mm favors the compact power electronics system's requirements of low overall height. The layout of properly separated dc plus/minus and ac main terminals on the two narrow sides of the module provides a user-friendly connection to the dc link and to the ac load. It allows clear isolation of the dc intermediate circuit from the ac output terminal to the load. A low-inductive laminated bus bar can be fixed to one side of the SEMiX module. The ac terminals enable the connection of two load cables. Due to the terminal separation, the power converter layout becomes simple and compact.

The positioning of the main terminals allows a driver to be fitted to the top of a half-bridge module. The user can place its driver board directly on top of the IGBT module. Three separate versions are offered for the driver interface: solderable/plug pin connectors, spring-loaded (snap-on) interface for custom-specific solutions, and an optimized IGBT driver (IPM solution).

IGBT Platform Concept

One fundamental advantage of the SEMiX platform is the scalability of the rated output current. Based on the new platform, three half-bridge IGBT modules have been developed: SEMiX 2 (190 A to 480 A), SEMiX 3 (250 A to 700 A) and SEMiX 4 (300 A to 1000 A). All three modules consist of two, three or four DBC substrates in parallel, each of which functions as a half-bridge circuit (Fig. 2).

The flexibility of these IGBT modules is useful for applications such as ac motor drives, UPS, electronic welding and power supplies. The SEMiX 2 and SEMiX 3 modules, which are suitable for medium-power applications (22 kW to 110 kW), are identical in width (50 mm), but differ in length (77 mm and 110 mm, respectively). This means that one single mechanical design can be used within a broad power range. SEMiX 4 is suitable for high-power applications (up to 150 kW) and measures 57 mm × 147 mm.

The platform is scalable and offers a choice of identical-sized modules for applications with different power requirements (Fig. 3). It allows up to six parallel connections with negligent derating due to symmetry. These new IGBT modules can be connected easily in parallel due to their optimum geometric design and the exclusive and consistent use of semiconductor technology in IGBTs and diodes, whose on-state voltage (V_CE(sat), V_f) has a positive temperature coefficient in the main power range.

The most commonly used circuit in power electronics today is the 3-phase inverter bridge, which features six IGBT circuits and freewheeling diodes. With the SEMiX platform, this topology is possible in a six-pack version or with three half-bridge modules. For applications in which dynamic energy recovery in the dc link is expected, brake chopper modules are also needed. In the SEMiX platform, chopper modules for buck converters (step-down converters) and boost converters (step-up converters or brake choppers) are integrated into the same enclosure as the half-bridges, which are ideal for use in combination with the six-pack modules. As a result, simple mechanical converter designs are feasible.

The option of designing a 3-phase converter with a six-pack or with three half-bridges is a unique feature of the SEMiX platform. Unlike the six-pack SEMiX 33, the positioning of the three SEMiX 3 half-bridge modules 42 mm apart brings about a 20% reduction in heatsink-to-ambient thermal resistance due to the better thermal spreading inside the heat sink (Fig. 4). This reduction can be used either to boost the output current or, with a steady current, to reduce cooling costs (heat sink, fan) or to increase lifetime by using the approximately 10°C lower junction temperature.

The IGBT modules available today are normally fitted with a soldered terminal block or a standard snap-on terminal strip for the driver. The pinout of this interface is determined mainly by the internal design requirements for the module and is not necessarily optimum in terms of converter design. The flexible pinout in the SEMiX structure alleviates this problem to provide an optimum match between module and converter designs. Fig. 5 shows an example of a compact 3-phase converter using SEMiX 3 half-bridge modules.

A choice of three driver interface versions is available for SEMiX half-bridge modules:

Solderable/plug-pin connectors for the standard version.
Spring-loaded (snap-on) interface for the spring version.

The internal spiral springs, showing more height than in standard versions, protrude through suitable openings in the top cover. As a counterpart, the user can place its own driver PCB on top of the spring areas for a reliable electrical connection via solder pads.

Optimized IGBT driver solution for the IPM version.

The IPM version provides the complete driver functionality for an IGBT half-bridge: galvanical isolation primary-secondary side, generation of turn-on/-off pulse signals for TOP and BOTTOM switch, interlock and dead time, de-saturation monitoring, and undervoltage lockout for auxiliary voltage levels.

Conclusion

The new IGBT platform overcomes many drawbacks of existing power module designs. Employing new-generation IGBTs and freewheeling diodes, its internal construction is optimized to exploit the latest characteristics of IGBTs. In addition, the SEMiX modules offer several possibilities for driver interfaces, as well as basic protective features such as temperature sensors and auxiliary collector terminal for overvoltage protection. The mechanical design of the modules allows compact and cost-efficient converter designs with high-output power: current and voltage range (190 A to 1000 A; 600 V, 1200 V and 1700 V).

With the new construction and layout approach of the IGBT module family, different module versions can be defined as late as possible during the production process. Its range of product variants is produced quickly and flexibly to meet the requirements of various applications.

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