IGBT Module Optimizes Inverter Solutions

New power modules with solderable pin terminals have changed the structure of inverters in applications up to 20kW. The new EconoPACK+ package was designed to fill the gap between 20kW and 100kW applications. This new design has several advantages compared with standard 62 mm modules. These advantages include a reduction of inverter volume and system cost, due to easy mounting of the module; driver board and power connectors; and a dc-bus structure.

The EconoPACK+ is a six-pack IGBT module that you can configure as a half-bridge through paralleling (Fig. 1). Therefore, you can consider each of the three segments of one module as a basic cell. High power levels are possible by paralleling these basic cells, even over the physical boundaries of one module. This configuration meets the requirements of the industrial inverter applications for drives and UPS applications from 30kW up to the megawatt range.

This module features a so-called flow through concept shown in Fig. 2, on page 42. One side contains the connections for the dc bus and the opposite side houses the output terminals. The top of the module leaves space for mounting a driver board. The new module has the same function height of 17 mm as the rest of the Econo family, allowing it to combine different modules. This new concept is the best way to minimize the inverter volume while keeping the creepage and clearance distance required by UL and EN Pollution Degree 2 standards.

Omitting any unnecessary components for the module's function, the new module doesn't use hard epoxy for mechanical stabilization. This improves its behavior in case of an explosion caused by a malfunction of the inverter. Wire bonds make all internal connections to auxiliary and power terminals as well as between ceramics, further increasing the module's reliability.

The module minimizes the variety of internal components. All ceramics have the same structure. The housing is the same for currents from 150A to 450A. This increases the economy of scale for the module as well as its peripheries. Heat sinks, bus bars, and drivers can all be scaled and produced in large quantities.

Fig. 3, on page 42, demonstrates the paralleling concept. You can use one module as a one-module-six-pack-inverter. For more power, parallel as many segments as necessary to a high current half-bridge. You can also connect three modules to configure three half-bridges, each capable of a nominal 2 × 1350A. A simple conductor structure can connect the dc-bus and output terminals of different segments.

Applications

At the beginning of any new design process is a target application selection, which in this case are drives and UPSs. Designers created these modules with the unique requirements of these applications in mind. The result is a module that can replace a standard 62-mm module for a more compact inverter design. However, only the use as a scalable solution in high-power inverters fully exploits the potential of the new module. It's so easy to parallel that many different frame sizes are possible with just one module size.

With the increasingly competitive inverter market, the challenge at the outset of the design process was to create a module with a unique combination of design features and advantages. To ensure that the module would fulfill the application requirements, designers developed it in close cooperation with key customers. Its objectives were a 30% inverter volume reduction and a significantly lower system cost. This reduction required expertise in module design and an excellent knowledge of the entire inverter structure. The price of the new module had to remain at a level acceptable for the next generation of inverters, while simultaneously facilitating the inverter structure and reducing assembly costs. At the same time, the module needed to meet the state-of-the-art quality and reliability requirements for industrial applications. Its high volume production concept was selected to best meet these demands.

IGBT Chipset

All 1200V devices feature the new IGBT³ chip technology. Module and chip development occurred simultaneously, to perfectly match the module design. It employs a new trench IGBT concept, based on NPT process technology and vertical optimization ^{[1], [4]}. Its major advantages include:

At the chip level, 20% less conduction losses compared with the second generation low-loss IGBTs.
Positive temperature coefficient of saturation voltage.
20% less switching losses, as compared with second generation low loss IGBTs.
Limited tail current.
Small gate charge with trench cell design.
NPT robustness.
Short circuit capability of 10 µsec without logic in the module.

Figs. 4 (a) and (b) show the turn-on and turn-off behavior of a 450A EconoPACK+. The curves look similar to former IGBT generations. The gate charge is similar to the last generation NPT IGBTs, so that the same driver dimensioning is sufficient for its trench IGBT. Its robustness can be best seen in short-circuit and overload conditions, such as in Fig. 5, which shows the FS450R12KE3 can withstand an unprotected short circuit.

A low internal inductance of less than 20 nH per segment helps minimize voltage peaks during turn-off, so the device can also be used at high dc voltage levels. This technology, combined with an improved high efficiency EMCON diode (named EMCON HE), is used in the new module.

Compact Inverter

The module, whose topology is shown in Fig. 6, on page 47, has a high ratio of integrated silicon in the required space, making a compact inverter design. A module height of 17 mm reduces volume requirements significantly, while allowing an easy mounting procedure. This results in a flat inverter structure. The capacitor bank is the height limitation factor to save additional space. This is the best way to use a liquid cooling system.

A module height of 17 mm for power and auxiliary connections allows just one p.c. board for power connection, dc-link capacitors, and driver electronics. The smallest module, the FS150R12KE3G, only requires 800W heat dissipation for optimized utilization. You can achieve this with a relatively small heat sink with two inexpensive axial fans. Therefore, it is practical to obtain a cost-effective inverter with that housing, while using only one p. c. board instead of a dc bus bar and driver p. c. board separately, as shown in Figs. 7 and 8.

Designers normally develop the modules with solder pins, to be soldered on a p. c. board with extra thick copper layers. This is state-of-the-art technology for module currents up to 150A. Above this, p. c. board-based solutions do not find much acceptance. That's why the EconoPACK+ features M6 screw-type power terminals that you can connect using copper traces.

The plus and minus terminals as well as the output terminals are in line, enabling a bus bar structure for easy paralleling of the output terminals. The number of output terminals was doubled for a secure conduction of even the high rms currents and to increase the design's mechanical strength. A current sensor has to be positioned externally on the bus bar, which makes the design more flexible for paralleling. An inverter family without gaps is possible by paralleling segment and modules, as shown in Fig. 9, on page 48.

Smart Driver

After intensive discussion, designers decided not to integrate a driver into the module. An integrated driver function would be a useful feature, but would mean some practical drawbacks. System costs would be higher for an intelligent module, especially if paralleling leads to over-engineering the driver stage. There would also be dramatically reduced flexibility, losing the ability to adapt the driver circuit to application requirements.

Another drawback is the relatively high temperature in the power module. This would lead to an increased failure rate and lifetime limitation for the logic in the module, which might cause reliability problems.

Therefore, keeping the driver separately, while allowing easy assembly on top of the module, was the best decision. The module has the same pins for driver connection on the top, as all EconoPACK+ modules have, which is the most reliable solution for the driver connection. From today's point of view, two techniques are acceptable for mounting the driver:

Soldering the pins by a selective soldering process or even hand-soldered for smaller quantities, and
A press fit connection of the entire driver board.

After mounting a driver, the unit could be called a “customer specific intelligent power module (IPM).” These power electronic building block (PEBB) modules, with a connected driver, can be pretested and mounted to heat sink, power input and output as best suited for production requirements. This is also an advantage when servicing the inverter, where it is common to change the driver and module at once.

To achieve flexibility for the driver stage it's best if you place only IGBT-near components on the first layer of the driver p. c. board. This could be a gate resistor, gate Zener diode, diode for realization of different turn-on and turn-off gate resistor values, active clamping diodes, and boost transistors.

On a second layer, that can be connected by standard plugs, the entire logic and interface functions with opto-coupler and dc-dc SMPS can be concentrated so a driver is possible with a minimum of devices. There is also an opportunity to position a current sensor on that second layer for measurement of output currents. The second layer can be mounted by stand-off with a 2.5-mm metric thread. For measurement of baseplate temperature, a thermistor (NTC) is integrated in the module. This isolated sensor is useful for a direct connection to the driver. An external safe isolation to the surrounding is necessary, e.g. by opto-coupler. A variety of ready-to-use drivers are available for these modules. The drivers solve the safe isolation issue with their integrated dc-dc converter and signal transformers.

Heat Dissipation

The module has a 380µm aluminum oxide ceramic material for isolation. It can withstand up to 3.4kVac, has a good thermal performance, and is mechanically rugged. The ceramics are soldered on a 3-mm copper baseplate that:

Spreads the heat.
Guarantees a good static and dynamic thermal behavior.
Improves mechanical robustness.
Provides a good thermal contact with its reproducible shape.

Fig. 10 shows the three segments of the baseplate. Each segment has a convex shape that provides a low-cavity thermal interface between the baseplate and heat sink surface. The module has a special internal design that improves the thermal behavior. At outputs of about 0 Hz, only one switch conducts the peak current for a relatively long period of time, which increases the temperature on that chip position. Here, the hot IGBTs are not concentrated in one edge of the module — which significantly improves heat distribution.

A realistic value for the heat dissipation with radial fan forced air-cooling systems is 10W/cm² for the baseplate surface that implies 2kW. These values provide a good utilization of the FS450R12KE3, which has the highest current rating. For modules with less silicon, heat dissipation is easier. For FS225R12KE3 and FS300R12KE3, a heat-dissipation of 1200W to 1400W would meet the requirements, which implies less cooling effort. The same calculations are possible with 1700V modules.

The center position IGBTs are up to 10K higher than the IGBTs at the edges of the module. Heat sink limitations cause this temperature difference because of less thermal spreading capability for the middle positioned IGBTs compared to the edges. For optimized performance, the introduction of an improved cooling system is essential. Therefore, a liquid cooling system is recommended for even higher power dissipation and better utilization of compact modules.

Reliability

Design experience from previous modules helped to make EconoPACK+ more reliable with minimum expense ^[3]. The choice of the ceramic layout and sizes was important to ensure the thermal cycling capability be best practiced with solder connections between ceramic and copper. The utilization of aluminum silicon carbide (AlSiC) wasn't necessary to reach the thermal cycling capability required in industrial applications and would have potentially led to a significant increase in module costs. In addition, bond connections were used instead of a solder connection for the power terminals, as well as for ceramic-to-ceramic connections.

References

M. Hierholzer, Th. Laska, M. Munzer, F. Pfirsch, C. Schaffer, Th. Schmidt, “Third Generation of 1200V IGBT Modules,” PCIM proceedings 1999, Nuremberg.
Heat sink catalogue and application manual Fischer elektronik, 1999, Germany.
Thomas Schutze, Hermann Berg, Martin Hierholzer, “Further Improvements in the Reliability of IGBT Modules,” PCIM proceedings 1999, Nuremberg.
L. Lorenz, A. Mauder, Thomas Laska, “The Field Stop IGBT Concept with an Optimized Diode,” PCIM proceedings 2000.
Concept Technology, Swiss presented an adapted driver for EconoPACK+, PCIM Nuremberg 2000.