Synchronous rectifier for high currents in resistance spot welding systems

Title of the research project L2-7556

Background

The European directive on energy efficiency predicts twenty percent (20 %) reduction of consumed energy up to year 2020. For Slovenia, acceptance of the directive represents commitment, which alongside with energy rehabilitation of buildings and an energy infrastructure encompasses development and design of new energy efficient products in the area of electromagnetic devices. The RSW represents a process of joining two or more layers of metal sheets without adding material. It is commonly used in automotive industry for automatized welding of vehicle bodies. Due to its low cost, high quality and possibility of automatization, the RSW is widely applied, including in other branches of industry. Due to its popularity, the welding is a significant consumer of electric energy. Hence, improvement of efficiency of the RSW process represents a notable potential within the accepted binding requirement of the reduction of the consumption of the electric energy. It has been found in to-date research that the efficiency in the RSW system can be improved in various ways. However, the most gains can be achieved with improvement of efficiencies of two subsystems, where the losses are the highest. The first and second subsystems represent the transformer for RSW and the output rectifier, respectively. In existing implementations, the rectifier consists of two high current diodes. The RSW system is a complex device containing a large number of components.

From a system point of view this nonlinear Time dependent system cannot be controlled without simplifications in analytical modeling and calculation. The simplifications are needed largely due to complexity of materials and principles of electromagnetic and electric conversions utilized, as well as time dependency of the system. It makes sense to carry out designing improvements with combination of analytical calculation on particular subsystems, numerical calculation by means of appropriate models and with real time tests and confirmation of the both types of calculation with laboratory measurements. The models should be further improved and upgraded with novel ones. The measurements should confirm effectiveness and viability of the proposed solutions before final design of a prototype of a new RSW system. Every attempt to reduce losses in the RSW system by means of partial improvements (quick fixes, e. g. only replacement of a classical diode rectifier with a synchronous one) on certain subsystems can lead only to local optimums, which can lay astray from a global optimum.

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Presentation of problem

In the existing RSW systems, given a nominal value of the welding current, absolute values of losses in TR and in the output rectifier are approximately equal. They can be significantly reduced with optimal design of RWS TR and implementation of a different output rectifier. In an optimistic case, the losses in TR could be reduced for additional 25%, while losses in the output rectifier could be reduced with use of output SR even for 75%. This can result in 50% reduction of losses in total, which can, due to high power of the RSW systems and a continuous regime of use cause enormous energy savings. Design solutions, which will constitute the planed research project, mostly influence of power density, operation reliability and efficiency of the overall R W S system.

Accounting for a fact that only in automotive industry with yearly production rate of 60 million of personal automotive vehicles and around 6000 welds incorporated into an average car body, we can see that huge energy is required for welding of only automotive bodies, e. g. for welding, automatized moving of robotic arms and last but not least for cooling of the welding systems. Increasing efficiency of welding systems for 10-20% would result in huge energy savings, which is an important ecological motive. With increased effectiveness in exploitation of materials utilized in power DC-DC converters, additional savings in material and therefore energy for its production and treatment as well as production of components of devices would be generated. The described issue is not limited only to welding in automotive and other industries.

An idea of replacing the diode output rectifier with the synchronous one in low power systems is not entirely new and original. However, this idea was tested up until now under entirely different conditions, such as within ten times lesser extent of power and most of all at incomparable, i.e. significantly lower currents, as they are required by the considered, i.e. planned RSW system. For information, the currents in such RSW systems can reach also up to 25 kA. A fact that complexity of solving the problem of reduction of the losses requires a scientific approach is obvious. According to available data to our best knowledge, this scientific approach has not been applied on this issue until now. After literature review it became clear that for implementation of SR suitable for high welding currents, a parallel circuit of at least 50 to 100 MOSFET transistors is required. This represents a novelty with a great potential, which cannot be spotted in literature. Below, various attempts of several authors who tried to reduce the losses will be presented. They worked on the RSW systems or broader on the DC-DC converters involving particular partial measures without accounting of all the elements of the system. Hence, their particular solutions have been later on revealed as not optimal and too limited for transfer of applications into an industrial environment.

The objectives

The first objective represents the reduction of the losses of the output rectifier of the RSW system. We plan to achieve this goal with the development of output SR. In the case of utilization of output SR, suitable MOSFET transistors take a role of rectifying diodes. Since the transistors have very low conducting resistance, by their utilization, the rectifying losses can be significantly reduced compared to the classical diodes. Very rarely recorded quick attempts of replacing the classical diode's output rectifier with SR are presented in details in a review of the past research with the relevant literature. They all fall short on a question of technical realization of proposed implementations. The authors of this proposed project are convinced that in the previously documented research, all the research opportunities offered by the complex scientific approach were not exhausted in this case, i.e. solving the technically and technologically exceptionally complicated problem of the anticipated reduction of the losses in the RSW systems.

The second objective represents the integration of output SR module into the existing RSW systems, which will allow significant improvement in efficiency of the entire RSW system and in parallel reduce a needed quantity of a liquid medium used for cooling of the system. Beside this, quantity of the needed energy for cooling of the cooling medium will be reduced. The solutions known to-date, which are based on data from literature and are at least laboratory confirmed were a result of partial interventions on a particular segment of the output rectifier or the proposed solutions were entirely outside the required frameworks due to low power. In design of output SR, a complete approach is needed with taking into account the characteristics of the whole RSW system using suitable scientific methods. In addition, vast acquired technological knowledge of participating applicants on the project needs to be incorporated into the solutions.

The third objective represents the selection of a suitable structure of the output SR, which will ensure required physical and electromagnetic compatibility of developed output SR with the welding TR, which will in a modified form remain an integral part of the RSW system. An important objective of the project will by all means be a test of feasibility of technological and practical implementation of the proposed solution (output SR) as well as exploring a possibility of more complete implementation of the integrated module of RSW TR with output SR. The applicants of the proposed project possess numerous innovative solutions in the area of design and construction of technically complex devices. Some of them were already protected with patents across the world.

The fourth objective represents the selection of such a structure of output SR that will, with improved symmetry in the electric subsystem, also ensure improved symmetry in the magnetic subsystem. This improvement will also reflect into improved control of the entire RSW system.

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