Over the last few years, the call for a faster time-to-market for industrial products has become increasingly urgent, causing developers to combine existing subsystems to create a final product. The main aim in doing so is to save time and money in the development stage. To meet the increasing demand for qualified, tried and tested subsystems to be used in product development, SEMIKRON has created a modular IGBT stack platform based on standard 62mm modules.
Daniel Seng, Product Manager, SEMIKRON France
The SEMIKUBE platform is a solution that meets the market demand for a costeffective, flexible, compact and easily maintainable power electronic system. It can be used for drives across a wide power range of 75kW to 1MW by simply paralleling several blocks, depending on the power required (Figures 1 and 2).
Figure 1. Modular and powerful SEMIKUBE arrangement
Figure 2. Power Range of the SEMIKUBE family
The total stack consists of a highly efficient heatsink and comes as an air or water-cooled version. The power electronic modules inside the cube are standard 62mm IGBT modules with two switches per housing. To operate the IGBT switches, a driver developed in-house and based on the features of the Skyper 32 Pro is used. For the DC link, capacitors, either electrolytic or polypropylene, are placed just above the power modules. The number can vary, depending on the customer needs. Finally, a sturdy metal frame protects the complete stack and gives it the cubic shape. To interconnect several cubes for higher power applications, a patented, fast-mount clamp connection is available. In combination with the special busbar design, a very low inductive stack solution can thus be achieved.
To achieve the compact size a highly efficient heatsink is a must. When power electronic modules are used, heat is produced which needs to be transferred from the point of occurrence to the ambient. For this reason, thorough investigations and tests were performed, the purpose of which was to find a suitable compact heatsink that boasts an excellent performance, also in terms of the cost-to-performance ratio. Additional thermal simulations for numerous possible module configurations are performed to ensure that the right choice of heatsink is made.
As an example, a 10-year-old design of eight standard 62mm IGBT modules on one heatsink was compared with a SEMIKUBE size 1 featuring the same power modules. The set-up included the same power loss for each module. The resulting temperature profiles show the difference in the performance of the heatsink (Figure 3).
Figure 3. Thermal Simulation of the heatsink for a SEMIKUBE size 1 with forced air cooling (a), compared to an older design with the same standard 62mm power modules (b)
The hottest point on the heatsink is 108°C, which is still within an acceptable range when compared to 172°C on the old design. The temperature gradient between the hottest point and the coolest spot on the heatsink surface is approx. 130°C compared to around 45°C on the SEMIKUBE heatsink. A closer look also shows that separating the modules by a minimum distance of 20mm is an effective way of increasing the thermal spreading on the heatsink. The further optimisation of the module position on the heatsink (two rows instead of 8 modules in a row) results in a more efficient overall design (SEMIKUBE size 1) as the heatsink needs 75% less surface space.
To ensure heatsink efficiency fans are required, suitable ventilation with the right power to compensate the pressure drop is also available. This fan (230V 50/60Hz) boasts a low-noise design at a maximum of 72dBA. The fan performance curve has to be adapted, both to suit the design of the load to be cooled, and to allow for a little power margin in order to take into account possible additional pressure drops from other devices in the air channel (air filter, choke, mechanical obstacles…). This kind of fan is designed for zero maintenance and includes thermoswitch protection.
For even higher performance, the stack also comes in a water-cooled heatsink version.
Thanks to the heatsink and stack design, compliance with the IP54 standard is ensured, as is the clear separation of the internal atmosphere where the electronics are and external cooling air. This can significantly reduce the pollution around sensitive devices and hence increase reliability.
Low inductive busbar
SEMIKUBE features standard 62mm modules, so up to eight half-bridge IGBT modules can be used in parallel. With paralleled power modules, derating is normally required due to non-homogenous current sharing between modules connected in parallel.
To keep the current derating at a very low level, a special busbar design is used inside the stack. Generally, a good compromise in terms of performance-tolosses ratio can be obtained at a switching speed of around 3kHz, as the difference between the fundamental frequency and the switching frequency results in a relatively low level of losses of typically 1 to 2%.
On the AC side, the busbars are made of tin-plated copper. The optimised structure and identical length of the busbars between the modules result in very low and identical resistance and inductance values. The outcome is excellent paralleling, as well as a reduction in power losses. Likewise, for the DC busbar a purposedeveloped design provides the optimal low-inductance current path, guaranteeing smooth switching behaviour.
Additionally, the design is suitable for interconnecting the DC blocks of several cubes. This allows for very compact arrangements inside a control cabinet. To connect several DC links special patented DC clamps are available. The cubes can be connected on any of the four sides. This provides a maximum of flexibility for the final arrangement on the customer side and allows for easy integration into the customer’s final application. These clamps provide a reliable and very fast connection for the different metal sheets. Only two screws have to be tightened to ensure safe and long-lasting contact between two SEMIKUBES (Figure 4).
Figure 4. Patented DC Clamp for the DC-interconnection of the cubes
Depending on the power between the cubes, several DC clamps can be used in parallel. The inductance of one clamp is in the range of just 10nH. Qualification tests verify the reliability of this connection technology. Dynamic tests with three clamps in parallel show a homogenous current sharing between the clamps. This results in homogenous heat transfer on the DC link level, which in turn improves the overall reliability of the stack (Figure 5).
Figure 5. Homogenous current sharing at the patented interconnection DC clamps
Long-life DC link capacitors
Depending on the power required for the inverter, up to 12 capacitors can be integrated into one cube. The standard capacitors are long-life electrolytic types with screw terminals. The sizing of the capacitors depends mainly on the inverter current and the necessary capacitance; the SEMIKRON standard provides a minimum service life expectancy of 60kHr. For some applications where higher DC-link voltages are needed (>750V), polypropylene capacitors are also available. To boost the reliability of DC-link capacitors, active cooling is the standard cooling method. Even in the event of fan failure, the hot spot temperatures would never reach critical values. Ultimately, this means that fan failure is a non-critical risk. Here again, the positioning and the interconnection method for the capacitors inside the stack are optimised to achieve symmetry, which translates into greater reliability thanks to homogenous current sharing ensuring a balanced current flow in all connected capacitors.
Driver and sensors
The driver is based on the existing Skyper 32 Pro design. Several adjustments carried out on this platform have resulted in the current SEMIKUBE GB (for Size 2 and 3) and SEMIKUBE GD (Size ½ and 1) drivers. Each cube is fitted with its own driver that operates independently; all of the cubes, however, have a common user interface for connection to the customer’s controller. In terms of safety, all of the drivers are fitted with protection and monitoring features. Short-circuit protection, temperature monitoring, galvanic isolation, a safe extra-low voltage interface are just a few of the driver characteristics. Additionally, several LEDs indicate the last fault detected by the driver. This is definitively a big plus in that it provides a quick explanation of unexpected inverter shut down. To ensure a balanced current flow, Hall Effect current sensors are integrated in the stack. The driver will detect a current imbalance or over-current incident, and will ensure safe turn-off.
Normally, the stack features a threephase rectifier part and a three-phase inverter. The rectifier can be designed in uncontrolled (B6U), half-controlled (B6HK) or fully controlled (B6C) versions, depending on the customers’ needs. On the inverter side, the three-phase topology (GD) is the typical version, although H-bridge configurations (GH) or even single-arm versions (GB) can also be used as a buck or boost chopper (Figure 6).
Figure 6. General SEMIKUBE interfacing
The main application for SEMIKUBE stacks can be found in standard industrial variable-speed drives. A new application that has appeared in the last few years is usage in central photovoltaic inverters (Figure 7).
Figure 7. SEMIKUBE in a solar cell inverter application
This application shows the major advantage of the SEMIKUBE: fast time-tomarket is ensured because the inverter is a pre-tested device and is even available in UR (UL recognized) versions. Another major point for the adoption of this platform in photovoltaic inverters is the overall efficiency, which has to be as high as possible in such applications. The platform explained here boasts an efficiency of more than 98% in terms of losses.
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