Today, the market for variable speed drives offers a large number of devices which are functionally similar, fulfil all the minimum requirements and are therefore more or less easily interchangeable. Since the applied quality standards are similarly high, differentiation is only possible with application-specific drives or the reduction of cost. An innovative starting point for cost reduction is the integration of additional features into the power module. In the SEMiX 3 Press-Fit series, SEMIKRON offers IGBT modules in half-bridge topology with integrated shunts for current measurement.

Looking at the bill of material of today's variable speed drives in the medium power range, e.g. 75kW to 160kW, it shows that the current sensors, their fastening, supply and interface connection make up a considerable part of the total material costs. In addition to the pure material costs, installation space must also be provided for the sensors. On top, the sensors must be manually assembled and connected, which generates additional costs in the assembly process.

Integrated shunt resistors

This is where the SEMIKRON IGBT module SEMiX 3 Press-Fit Shunt, using integrated shunt resistors for current measurement, has its place. Thus, the use of additional current sensors becomes unnecessary. The SEMiX product family is an IGBT module in half-bridge or chopper topology for 1200V or 1700V and up to 600A. With a height of 17mm, the module is the industry standard for medium to high power applications. Through the use of press-fit contacts for the auxiliary connections the assembly is completely solderless. In conjunction with the SEMiX bridge rectifiers in the same design the system offers maximum performance with optimised system costs. In addition to the significantly lower costs, the current measurement using the integrated shunt resistors also offers considerable advantages in accuracy as well in the bandwidth of the measured values. The shunt resistors consist of a special alloy, which can be used up to an operating temperature of 250°C. Due to the low temperature dependence and high linearity of the shunt resistor, the SEMiX 3 Press-Fit Shunt is ideally suited for demanding control tasks thanks to the very precise feedback signals. Considering the lower component complexity and the simplicity of the components, the SEMiX 3 Press-Fit Shunt also reduces the failure rate compared to conventional inverter systems with standard current transformers. Additionally the shunts clearly surpass the overload capacity inherently limited by current compensated sensors. The intelligent integration of the resistors into the power module does not require any additional space, and the SEMiX 3 PressFit Shunt offers the same performance data as identical modules without integrated measuring resistors.

Integrated signal transmission on driver module

Semix 3 Press-Fit Shunt Evaluation-Kit with Skyper 12

However, the integration of the current measurement also necessitates some challenges: Instead of a galvanically isolated signal from a current transformer, the shunt voltage is now measured at DC-bus potential. Before this can be further processed by a higher-level controller, isolation and digitisation must first take place. This task is carried out by the SKYPER 12 driver in conjunction with a delta-sigma modulator. The SKYPER 12, based on a SEMIKRON developed ASIC, on the one hand takes care of the safe and optimised control in the permitted save operating area (SOA), as well as the evaluation of the DC-bus voltage and the temperature measurement integrated in the module and provides this as a safe galvanically isolated signal to the controller.

SEMIKRON offers an adapter board for the evaluation of SEMiX 3 Press-Fit Shunt systems, which interfaces the SKYPER 12 to the module. By integrating all required supplies and signal and fault management into the ASICs, the number of individual components on the driver can be reduced by 30%, which increases the service life by up to 20%, depending on the load profile. As a result, the SKYPER 12 achieves an MTBF rate of over 8 million hours. The two purely digital ASIC modules are used on both, the primary and the secondary side, and do not use analogue filter modules. As a result, the signal propagation times can be optimised and the time offset of the signals is reduced to below 30ns. The metallic shield of the ASIC housing assures maximum temperature stability and reduces susceptibility to interference.

A serial UART standard interface as well as a differential signal interfaces the driver to the higher-level controller. The IC-based delta-sigma modulator, which is located on the adapter board, converts the voltage signal of the shunt into a digital differential signal at a clock frequency of 20MHz, which emphasises the high accuracy of the current measurement. As a result, there are no deviations in the measured values stemming from converter or component tolerances on the further signal path to the higher-level control. The delta-sigma converter is supplied by the secondary side of the SKYPER 12. Depending on the selected modulator IC the safe separation of the potentials is achieved either capacitively or optically.

By using the package of SEMiX 3 Press-Fit Shunt as well as the optimised SKYPER 12, the development times can be drastically reduced because design loops are avoided and the system development is completed in the shortest possible time.

The press-fit technology also optimises the manufacturing process. Only the driver board has to be pressed onto the module, and thus all current, voltage, and temperature measuring systems are installed. No further boards are required for signal processing, which reduces production costs and improves quality and FIT rate.

Pre-applied phase change material

The final stage of the integration is provided by the pre-applied thermal interface material. Phase change material (PCM) is used which, at room temperature, is solid and thus smudge-free. Compared to viscous thermal pastes, this allows the maximum process reliability during assembly of the modules. The print pattern of the PCM is perfectly matched to the module baseplate and is applied by means of a controlled screen printing process, thus guaranteeing optimum distribution of the thermal conducting material. This results in a 10% lower thermal resistance which, in turn, leads to lower junction temperatures during operation. This means either increased lifetime of the module or increased output current, which in turn increases the power density.

Optimised overall concept through integration

The connection of SEMiX 3 Press-Fit Shunt with SKYPER 12 and pre-applied phase change material offers the maximum integration of components, which results in significant cost savings, space savings as well as reduced development times and increased reliability for the entire system. This makes SEMIKRON a reliable partner for inverter manufacturers, to develop competitive equipment for a demanding market.