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An innovative solid state relay, as a result of cooperation between scientists and industry

A collaborative project of the Barcelona Microelectronics Institute (IMB-CNM) of the CSIC and BSH, a company, has produced an innovative solid state relay for induction cookers. It is made of gallium nitride, a new and promising material for power electronics. This is an example of collaboration between a research centre and a company.

A prototype of the relay and the cooker where tests have been performed.They can be unnoticed but power electronics systems are everywhere. These circuits, also called power converters, are aimed at taking the electric energy from a source (a battery, a generator, a plug, a solar cell…) and adapting it for a load (a LED bulb, a motor, a computer, a heater…).  

Their main component is a power semiconductor device (diodes, transistors…), which works as a switch, turning on and off at relatively high frecuencies (tens of kHz). Most of these devices are made of silicon, the semiconductor material linked to the big technological revolution of microelectronics.

Nevertheless, in some applications the semiconductor devices are not used and the lower cost option is the electromechanical relay, a device created at the end of the XIX century, which has kept  virtually the same structure since then.

The electromechanical relay is a switch that blocks or carries electricity by a small control signal. Contrarily to the semiconductor devices, it has mobile mechanical elements which imply some limitations and worse performances: it switches the current more than 1000 times slower than semiconductor devices.

Besides, it makes acoustic noise when switching, its mobile pieces suffer mechanical wear and it can produce a small electric arc, which  can be a risk when inflamable gases are present.

To overcome these limitations, solid state relays were developed 40 years ago, showing the same function than their electromechanical counterpart but where mobile pieces have been replaced with transistors.

Solid state relays are quite common as a comercial alternative to electromechanical relays but they have two inconvenients: they are more expensive and less robust. That’s why electronics industry keep widely using electromechanical relays.

The problem comes when converters have to accomplish functionalities and  performances which are impossible to get due to the electromechanical relay limitations.   

This is the case of the induction cookers manufactured by BSH. This company has been developing for 30 years the most advanced technology in induction cooktops in its Montañana factory (Zaragoza, Spain). To avoid the noise of the electromechanical relays used in their converters, as well as gaining in switching speed and relaiability, BSH aimed at exploring new possibilities to replace them .

For 18 months, scientists of the Power Devices and Systems Group of the Barcelona Microelectronics Institute (IMB-CNM) of the CSIC have been collaborationg with BSH. They have built and tested more than 20 prototypes of solid state relays and  a similar amount of advanced power semiconductor devices.

For 18 months, scientists collaborating with the company have built and tested more than 20 prototypes of solid state relays, and a similar amount of advanced power semiconductor devices

“A shocking result of the research is that we detected for first time unexpected failure mechanisms in two transistors (RC-IGBT and Super-Junction MOSFETs), which were initially among the most promising options concerning performance and economical cost”, explain the scientists.

Scientists also tested devices based on silicon carbide, a new and promising material. These devices showed an outstanding performance but show a relatively high economical cost. This could be overcome in the future if these devices become widely used in applicattions such as the electric vehicle and, therefore, its massive production could imply a reduction in the prize per unit.

An emergent material 

Eventually, the scientists tested several prototypes based on HEMT transistors (HEMT, “high electron mobility transistors”). These devices are made of gallium nitride, another new and promising semiconductor material, which can efficiently work at high temperatures and frecuencies, showing lower costs than silicon carbide.

Another characteristic that makes them a good option is its monolithic integration cabability, which means that several devices can be combinaed together in a single chip implementing, for example, the bidirectional switch function. This would simplify the static relay manufacturing and would improve the integration level (compactness).

The research team and BSH have applied for an European patent dealing with static relays based on galium nitride HEMTs. The results have been published in the IEEE Transactions on Industrial Electronics journal and presented in an international conference. The work is included in the doctoral research of a young scientist, who after finishing  the project has been recruited by the BSH company.

As the principal investigator explains, “this collaboration betwen companies and research centres is very positive because it helps to improve the industrial network, their products and competitiveness by training and recruiting highly qualified scientists and technicians”.