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Electronic fuse up to 400V

     I decided to design electronic fuse with tripping current of approximately 10A and working voltage up to 400V. As usual, I found only "children's" circuits in the internet, that did not limit dI / dt and work with low voltages, no more than 30V. They are good as overload protection, but in short circuit condition those circuits would be probably destroyed. Therefore, I designed this electronic fuse.
     Electronic fuse circuit:
The diagram below shows the electronic fuse circuit schematic and different ways to connect it between the supply and load. Fuse circuit utilizes the integrated circuit UC3843. From the variety of its functions only UVLO and current sensor (pin 3) are used. The current is sensed at the voltaeg drop of MOSFET D-S on state resistance. The threshold voltage of pin 3 is 1V. Voltage drop at which the circuit switches off is affected by resistors R1 and R2. Pin 3 has a certain bias current (2-10uA) and it must be taken into account during design of R1, R2 divider. R1 must be able to withstand the full supply voltage. Capacitor 10n 1kV serve as dV / dt limiting and thus protects the MOSFET. It also helps to accelerate the response of the electronic fuse in case of very rapid growth in the current. The threshold current is dependent on the choice of MOSFET (the Rds (on) resistace). More MOSFETs can be connected in parallel. MOSFET must have at least about 25% higher D-S voltage than the operating voltage. The circuit can of course be adapted for a wide range of currents and voltages. The circuit requires an auxiliary 12-18V power supply with a current of about 20 mA. MOSFET has adequate heat sink.
     Connection:
Circuit breakers, of course, can not be simply connected between supply and load, because in the event of a short circuit there was not limit of the slope of the current rise (di / dt). This must be ensured by adding inductor L1. It is necessary to add protection against voltage overshoots that arise on L1 and the parasitic inductances. The fuse can be used as protection of switching power supplies. At the output, use only a small blocking capacity to prevent damage to the protected circuit. Big filtration capacity is placed at the input of the fuse. The diagram below shows the various ways to connect the fuse into the circuit. It can be used in negative or positive rail. There are versions with blocked choke and unblocked choke. For most cases, you can use the second version. Choke determines the current rate of rise (di / dt) in the event of a hard short circuit. It must be designed with regard to the maximum working voltage and current and delay of the electrinoc fuse circuit. For operation with 320 - 400V at least 160uH inductance seems the best. It limits the dI / dt to 2 - 2.5 A / us, wherein electronic fuse circuit manages to safely shut down. Choke must of course be rated to the current with some reserve to prevent saturation ‚Äč‚Äčeven in short-circuit conditions.
     Control:
The fuse has a reset button, which turns on (resets) it. Turning on must be done before connecting the main supply voltage. Who ever designed the "real" electronic fuse, certainly knows that turning it on when it is under voltage is a big problem :). This fuse wont turn on when energized. When the button is pressed during operation, it turns the fuse off.

     Warning! If you want to use this fuse for hazardous voltage, be careful. No part of the circuit (including the button and auxiliary voltage input) are not galvanically isolated.


The schematic of electronic fuse up to 400V with dI/dt limiting.


Electronic fuse circuit. The MOSFET transistor is STB25NM50N-1 (SMD equivalent of STP25NM50N).


Electronic fuse circuit together with a prototype of L1 (air core version).



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