Posted on 05 July 2019

High Currents Under Control

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Compact Vehicle Power Systems

Users of power control equipment have used third-party systems for many years, because they offer the benefits of reduced development costs, shorter time to market and ease of meeting qualification standards. As specifications have become more demanding, it has become necessary to use higher levels of integration, with much tighter control of all the elements in the system.

By Paul Newman, Managing Director, Semikron UK Ltd


To meet the requirements for the broad automotive electronics market – ranging from fork-lift trucks, to hybrid and electric vehicles, to large agricultural and construction vehicles – Semikron has established its Systems Group, to bring all the necessary expertise into one organisation. The Group works closely with customers to develop power systems, which are then marketed under the SKAI brand. The second generation of fully integrated and tested SKAI systems are now available.

The Systems Group has been formed to build on Semikron’s many years of experience in these markets. The company has offered integrated power electronics to its customers for almost 20 years. Initially just combining power silicon and drivers in custom-designed enclosures, these have evolved to become more highly integrated, now including control electronics designed to be ready to accept software to produce a complete system.

The SKAI vehicle power systems boast a high level of integration and offer major advantages over comparable competitor systems. They are developed in line with the latest automotive standards and system qualification standards, allowing short time-to-market and lower development costs. The SKAI systems are supplied as standard platforms with low-voltage MOSFETS or high-voltage IGBTs as the silicon base. SKAI systems can also be developed to meet individual customer specifications. Semikron is a single-source supplier, encompassing everything from feasibility and proof-of-principle studies, to the development of optimum system architecture, to electrical and mechanical simulations, end qualification and complete-system series production.

One system, three types

The high-voltage SKAI 2 is available as a water-cooled 600/1200V IGBT inverter system, and has been optimised for use in applications such as full-electric cars, plug-in hybrid cars and electric buses. This system is based on the established sintered, 100% solder-free SKiM93 IGBT modules, features a polypropylene film DC-link capacitor, driver electronics, a latest-generation DSP controller, EMC filters, and current, voltage and temperature sensors, and is supplied in an IP67 module case. Communication with the vehicle master controller is via a CAN bus. These systems are designed for outputs of up to 150kW (Figures 1 and 2).

A typical SKAI system architecture

The high-voltage SKAI system is available as a liquidcooled 600-1200V IGBT inverter system

The low-voltage version is available as an air-cooled or liquid-cooled 50/100/150/200V MOSFET single and dual inverter system, which is used for material handling and smaller vehicles. These systems are suitable for a motor output power of up to 40kW. They incorporate many of the same features as the IGBT-based systems and therefore offer the benefits to customers that they behave in the same way, use the same core control system and I/O connections, and the same system structure (Figure 3).

The low-voltage version is available as an air-cooled or liquid cooled 50-100-150-200V MOSFETsingle and dual inverter system which is used in material handling and smaller vehicles.

The third type of SKAI 2 platform is a multi-converter box. These systems are also housed in liquid-cooled, IP67-protected cases and communicate with the vehicle master controller via a CAN bus. The signal interface features analogue and digital I/Os to allow for the connection of a wide variety of sensors, such as temperature sensors and resolver inputs. A typical multi-converter system would include a three-phase 40kVA active front-end converter, a three-phase 20kVA drive inverter, a three-phase 10kVA drive inverter, and a 14V/300A or 28V/165A DC/DC converter (Figures 4 and 5).

Topology of a typical multi-converter system

The SKAI 2 highly integrated multi-converter system

All SKAI 2 modules are fully qualified using test techniques such as highly-accelerated life testing (HALT) and end of component-life testing, with full failure-mode effect analysis studies conducted at all critical points of the design cycle, to ensure that they are in line with relevant automotive standards. Thermal and electrical contact of the power semiconductors is established by pressure contact technology, which boasts extended service life and high load cycling capability. The systems and semiconductor components are manufactured in high-tech production processes that include end-of-line function tests and, if required, 100% burn-in tests, ensuring a high degree of quality.

Novel technologies

There are many factors that affect the efficiency and reliability of power systems.

To achieve maximum energy, cost and space efficiency, coupled with high reliability, it is important to combine the best silicon, packaging, layout, thermal performance and control in the design and manufacture of power systems. This can often be difficult if the designer has to depend on off-the-shelf parts. It is very important to be able to optimise the selection of silicon and to be able to connect it as needed for optimum performance in a system.

Many suppliers of systems focus on a single technology, such as MOSFET or IGBT, or may concentrate on applications at a single voltage. However, the wide variation in requirements of today’s systems makes it important to be able to choose from the widest selection of semiconductor technology to achieve the best match to the application. It is also important for the design process to take into account the many issues dependent on the semiconductor technology and the relationships between them, to ensure that the hardware is optimised for the application. As Semikron is a major manufacturer of power semiconductors, it can push boundaries in areas such as temperature and size. For example, the company produces very large volumes of IGBT and MOSFET drivers, and from this expertise has developed optimised application specific integrated circuits (ASICs) to significantly reduce component count and increase reliability, while reducing size dramatically.

Current developments in power electronics aim to achieve higher current densities, system integration and greater reliability. At the same time, there is more call for low-cost, standardized interfaces, as well as flexible and modular product series. Semikron has set trends in this area by the use of spring contacts for the auxiliary and load connections.

The reliability of classical module designs is not sufficient for many developing applications in power electronics. For example, these modules are limited in their capability to withstand passive temperature cycles. It has therefore been necessary to develop new techniques, which are capable of meeting the high-reliability requirements of modern applications.

A major limitation of power module lifetime is the problem of solder fatigue. In traditional constructions, this contributes to the end-of-life failure of power modules, especially in the case of higher temperature swings, which are predominant in most applications. Several new technologies have been developed to eliminate all solder interfaces. Each of these offers advantages compared with traditional constructions, but in combination they offer significant benefits.

The most significant problem caused by higher temperatures and larger swings of temperature is delamination of soldered joints. This problem has been completely overcome in the latest SKAI systems by using sinter technology to join the semiconductor chips to the ceramic substrate instead of solder, meaning that higher operating temperatures are possible with increased reliability. The sinter bond is a thin silver layer that has a superior thermal resistance to a soldered joint and contains far fewer, and smaller, voids. It is not subject to the delamination that affects solder joints, resulting in a low thermal resistance that remains low over many tens of thousands of power cycles. The high melting point of silver also prevents premature material fatigue.

Another issue raised by increased junction temperatures is the fatigue of the wire bonds used to join the chips to the substrate. This has been minimised by a number of changes to production techniques, including changing the geometry of welded joints and the introduction of novel stress-relief techniques.

SKAI 2 systems are offered with or without control software. If required, Semikron will work with its customers to develop software or the customer may prefer to use its own. This gives the customer much more versatility and the capability to produce systems with its own custom features.

SKAI 2 system in tractor application The system is also integrated in tractors. An example:

A tractor manufacturer was developing an electric power supply system for its upper power-class tractors, with the aim of reducing fuel consumption and noise emissions. The development was also intended to introduce the architecture needed for additional electric drive applications in agricultural machinery.

Until now, secondary equipment in tractors has been connected to the main drive mechanically via gears. This does not enable these functions to operate at the optimum operating point, leading to poor overall efficiency and consequently to increased fuel consumption and pollutant emissions. The aim was therefore to disconnect the secondary equipment from the main drive. For this purpose, a modified generator connected to the main drive would be used to generate electric power. This would then be electrically converted to ensure optimum operation of the fan, air compressor, air conditioning equipment and the 14V on-board power-supply.

A highly-integrated power electronics system from the multi-converter system family was developed to meet the customer’s specifications. The system comprises multiple converters used to control electric current flow under harsh ambient conditions. Different operating modes are possible, for example the system can be supplied with electric power by way of a three-phase generator or an HVDC bus. The system communicates with the vehicle master controller via a CAN bus. The integrated semiconductor components come from the tried and tested MiniSKiiP family (2nd generation). The signal interface features analogue and digital I/Os to allow for the connection of a wide variety of sensors, such as temperature sensors and resolver inputs.

The new electric power supply system is the basis for the introduction of ultra-precise, highly efficient electric drives for attachment and trailer equipment, as well in final drive systems. The combination of different technological solutions results in far lower fuel consumption and reduced noise emissions, and ensures that future emissions limits are met. All of the power transmission components were developed or optimised in order to improve overall efficiency. In summary, this constitutes a milestone on the path towards ultra-low consumption in upper power class tractors, which are normally prone to high consumption.

Integrated power systems

Many advances in the drive for higher integration of power systems have been made. However, further advances in reliability, efficiency, size and versatility will be needed to increase the market acceptance of integrated power systems. By offering system-wide expertise in one organisation, Semikron’s Systems Group is well placed to develop higher levels of integration, improved cooling technologies, higher silicon operating temperatures, higher performance from ASICs and novel packaging techniques. New insulation systems, new methods of silicon attach and, in time, the total removal of wire bonds, will all improve efficiency and reliability still further.



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