The Grid 11 BATTERY TECH E-Mobility Engineering | January/February 2024 The smart switch (Image courtesy of Kyocera AVX) Smart switch protects 800V battery packs Kyocera AVX has developed a smart semiconductor switch for EV battery pack designs, writes Nick Flaherty. The switch supports voltage levels up to 930 V for 800 V battery packs and is able to withstand short current events up to several thousand amps. Using a scalable design, the switch can be adapted to different power classes up to 640 kW continuous performance and provides both a safety cutoff and also support for higher charging rates. The high requirements and increased integration of additional safety-relevant features in the vehicle electrical system architecture mean a semiconductorbased design can provide marked advantages when compared to a mechanical relay, including considerably higher switching speeds. Particularly in the event of a short circuit, every microsecond counts in order to guarantee a safe disconnect. The Kyocera AVX Electrical Smart Switch protects the supply circuit during charging and discharging bi-directionally in the event of overcharging or in the event of a short circuit. In addition, the Electrical Smart Switch permits the precharging of the DC network to enhance the service life of system components. In the event of a short circuit, the electrical smart switch turns off in a few microseconds and thereby may prevent severe damage to the supply circuit. Inrush current peaks can be prevented by using the implemented pre-charge function. Using this function the current is limited by pulsing with a frequency up to 100 kHz and thereby is another important gain made whereby overall system costs can be lowered. Thermal efficiency The thermal efficiency of the switch is boosted through a combination of high-level assembly technology and temperature sensing, which monitors the device temperature and provides real time feedback. The electrical smart switch was developed using the specialist ceramic substrate skills at Kyocera AVX in Salzberg, Austria. “There is no such product as the electronic switch on the market replacing the relay in the battery,” said Gerhard Kock, R&D manager at the site. The module is based around an array of 24 1200 V silicon carbide (SiC) MOSFET transistors so that it can control the current flow both in and out of the battery pack, with 12 MOSFETs directing the current in each direction. The array is controlled by a single gate driver and provides a short circuit current protection of 2300 V in under one microsecond. The energy in the system is managed by TVS diodes to handle the short circuit current as there is still energy in the system from the inductance that in a mechanical relay can cause the contacts to fuse. The TVS diodes are mounted in parallel with the MOSFETs so the MOSFETs don’t enter avalanche mode, and it is the speed of these TVS diodes that limits the response of the switch to under 1 microsecond, or 100 kHz, rather than a nanosecond response. The switch has also been tested with 1.5m cycles for endurance for automotive applications. Thermal performance is a major challenge in such a module so the engineers used a silicon nitride ceramic substrate directly soldered to a baseplate with cooling from pin fins to the baseplate. Several NTC temperature sensors are mounted on the substrate to directly monitor the substrate to reduce the current flowing through the switch as an additional safety function. DC-link Using the switch can also eliminate the pre-charge circuit on the DC-link inverter and using a pulsing function can increase the charging rate from C1 to C2 and can be combined with a supercapacitor. The next stage is to integrate a current sensor into the module so it can be autonomous, rather than using a current sensor external to the rest of the battery pack.
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