# How do I externally protect a driver FET on a device from load short circuit

I am trying to find a way to protect the output FET in an LED driver. Unfortunately someone else manufactures the LED driver so I don’t have access to the inside of the box or any way to modify the FET drive inside the box. The LED driver is being powered by a 25 Amp , 24 volt DC external power supply. The FET is rated for fairly high current .It is a STP100N8F6 100 Amp N Channel MOSFET . Both the driver and the output load is being driven by the same 24 volt power supply. I want to protect the driver against shorts across the load. I have tried various traditional circuit breakers and PTCs. Unfortunately it appears non are fast enough as the output FET keeps failing. I am not sure if it may be partially because when the load is shorted this effectively shorts the power supply through the FET. This probably causes the drive to drop to the FET so that there is a high current through the FET while a high VDS is forced across the FET putting it outside of it’s SOA (Safe Operating Range) as specified in the datasheet. Does anyone have any clever ideas as to how I can protect this FET from a load short circuit externally without modifying the drive circuit?

• I suppose "stop shorting the output" is out of the question? :/ Dec 14 '16 at 22:55
• I was also wondering why you should be shorting the output at all Dec 14 '16 at 22:57
• It won't be I that shorts it but someone else could short this out when using it. Just building in a fail safe solution. Dec 14 '16 at 23:04
• This is a discretionary design feature called OCP, OTP or PTC and/or fusing. Once defined, this can be made practical depending on features wanted like foldback with hiccup retry. This is not the 1st time this has been done and is integrated in many LED drivers. Add on PTC is possible but not as good as integrated current limiting. It works by slaving a series metal oxide part to run hot faster than the FET with a heatsink so that it is protected as long as the surge current doesn't fuse the wirebond. (NotToExceed current) Dec 14 '16 at 23:08
• This driver is used to drive 24 volt LED strips so the voltage is fixed. Also as I mentioned previously the driver design is fixed. I am looking to design an external circuit to protect the output of this driver not modify the driver itself. Dec 14 '16 at 23:19

Put a second FET in series with the 24V supply, monitor $V_{DS}$ with a fast comparator and use its output (with a latch) to disable the FET. A short typically results in a quite high voltage drop on the MOSFET (>1V) and is thus easy to detect.