Fuse for mosfet question - Page 1
Forum section: electronics / general

author: mrcrud5
author_rank: Contributor
timestamp: October 05, 2018, 06:24:20 pm
content: Hey guys,I just have a quick question regarding a fuse I'd like to use for a mosfet.A little bit about my project:I am making something for driving LED strips. However I'd like to protect the mosfets in case someone accidentally shorts out the strips. The mosfet I am using is the BUK7880-55A. The mosfet drain current is rated at 7 amps. I will be using well below that. The power source I am using is limited at 10Amps so if a short does happen, 10amps will be going through the mosfet briefly until the fuse blows.Here is my issue... below is the absolute maximum rating of the moset for a quick pulse. It says 30Amps with a test condition of <10uS.DM peak drain current Tsp = 25 °C; pulsed; tp ≤ 10 µs; Fig. 3 - 30 AIm trying to choose a fuse, and I am looking at the P15131TR-ND on digikey. It says its fusing time is 100uS. Does this mean I can possibly still damage the mosfet? Are there faster fuses out there than can blow under 10uS? Any help is appreciated. Thanks!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 05, 2018, 06:29:04 pm
content: This can be quite a hard problem, blowing teh fuse before the MOSFET, especially for multichannel systems where potential current can be very high.Best solution I've found is automotive protected power switches, e.g.AUIR3315whih switch very fast.For 5V loads USB power switches are excellent, e.g.TPS255x

author: nctnico
author_rank: Super Contributor
timestamp: October 05, 2018, 06:30:12 pm
content: A fuse is never fast enough to protect MOSFETs (or any other semiconductor). A fuse is for preventing fire when something downstream gets shorted.The only way to protect a MOSFETs is to:1) Design the circuit in a way it always stays within the SOA area2) Switch the MOSFET off in an overcurrent situationFor 2 you'll need a current sense resistor somewhere and some circuitry to detect an overcurrent situation and switch the MOSFET off.Edit: I see Mike made some excellent suggestions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2018, 06:31:09 pm
content: Why not simply turn off the MOSFET in the first microseconds of overload? Why leave it on, sweating and grunting through the 10,000+ microseconds of clearing a fuse?Tim

author: mrcrud5
author_rank: Contributor
timestamp: October 05, 2018, 06:45:15 pm
content: Ah of course! That makes way more sense. Thanks!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 05, 2018, 06:46:07 pm
content: If you know your source is current-limited then make use of that. SMPSs generally trip & cycle on/off rather than limit, so only the peak current is a significant issue.

author: VK3DRB
author_rank: Super Contributor
timestamp: October 06, 2018, 01:44:44 am
content: Try using a current sense resistor, op-amp, and a transistor to bias the gate voltage off in the event of a short. You might want to have fun downloading and using LTspice, or try this:https://www.falstad.com/circuit/.It reminds me of a design I had to do for New South Wales trains a few years ago. VERY noisy power rails at high voltage, had to drive multiple N-channel MOSFETs on the high side (there is a trick to doing this), and sense any abnormal condition at start up or during operation. High speed firmware control for user adjustable abnormal conditions, with immediate hardware protection of the MOSFET in the event of potentially damaging current drains. It was quite challenging, as the MOSFETs could not be heatsinked, so their RDSon had to be very low. I modelled the circuit in LTspice. When built - it just worked! Without LTspice, it would have taken a lot longer and and been more painful building and blowing up prototypes.