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Ultrafast Rectifier US1M - Why It Makes Sense to Use a 1000 V Component on a 24 V Smart Building Field Bus System

US1M

Few years ago, the KNX standard for smart building applications was recognized as international standard ISO/IEC 14543-3. It defines information exchange protocols working either via the traditional power line, ethernet, radio frequency or a 24 V field bus system. The latter requires non-isolated, low power DC/DC converters for every device connected to that bus system. It converts the DC bus voltage ranging between 21 V and 30 V to typically 5 V or lower used by the electronics. Since space in typical installation sockets is restricted, DC/DC converters with high switching frequencies are used, in order to minimize magnetic elements like coils and transformers.


Here the US1M having 1 A, 75 ns and 1 000 V comes into play. As a rule of thumb, the free-wheeling diode in switching converters should have minimum a seven times higher V_RRM than maximum DC voltage, so here 7 x 30 V = 210 V. By this, voltage peaks created by high frequency switching will not harm the diode. So a Schottky is out, since maximum available V_RRM is about 200 V only. A 400 V ultrafast rectifier would be the first idea to be used. But smart building systems need to run reliable over many years. Switches and displays, touched by us human beings, are affected by electro static discharges of several 1000 volts. Though not all of that will reach the electronics of the field bus device, it makes sense to spend some extra safety margin on components used. The 1 000 V of the US1M provides sufficient protection for reliable and long lasting smart building functionality.

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