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The datasheet states that TPS2400 protects the load from an over-voltage, by disconnecting the load from the power supply (Q1 goes off), without blowing the fuse. Why is the fuse F2 here, if there will be no current flow at over-voltage or under-voltage condition?

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  • \$\begingroup\$ Are you sure there's no current in an over-voltage condition? Where does the energy in Ls go? \$\endgroup\$ – Phil Frost Nov 27 '14 at 18:20
  • \$\begingroup\$ According to the datasheet, U1 will turn Q1 off to isolate the load from supply during the overvoltage condition. \$\endgroup\$ – zud Nov 28 '14 at 9:30
  • \$\begingroup\$ I know, @zud. What happens if you have an inductor with some current flowing in it and you suddenly disconnect it from its load? Does the current just stop flowing? Hint: \$v(t) = L\:\mathrm d i / \mathrm d t\$ \$\endgroup\$ – Phil Frost Nov 28 '14 at 12:09
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Fuse F2 protects against over-current conditions, which can occur when the voltage is within normal limits. Imagine a short in, say, D2. That short would draw excessive current and blow fuse F2.

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The fuse is there as secondary protection. Overvoltage or overstress beyond the rating of the devices could cause semiconductor switches like Q1 to short. In this case the D1 TVS device protects the semiconductors from an overvoltage condition, but relies on the fuse, F1 to blow if the overvoltage lasts long enough. If there were no TVS device and Q1 shorted, F1 would blow preventing further damage and potentially hazardous conditions.

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The function of a fuse is usually to prevent fire - that's the basic protection a fuse offers - between power supply and appliance there may be a few feet or metres of cable - if a short circuit occurs in the appliance, the cable could easily become overheated (due to excess current) and burn or rupture its insulation. Without a fuse to "protect" the cable, this can lead to the direct ignition of material in the home or workplace and suddenly what starts off as a minor incident ends up with a building burning to the ground.

Side effects of course are danger to anyone coming into contact with ruptured cable and receiving an electric shock. A fuse prevents this by protecting the cable.

When it comes to protecting "loads", a zener diode and fuse perform a slightly different job. They are called zener barriers in the petrochem and coal industries because they limit the voltage at the terminals of "otherwise safe" equipment and this significantly reduces the risk of an ignitable gas explosion or dust explosion.

In your case F2 just protects the equipment that is connected to your output and F1 likely protects the cable between power supply and appliance.

See this wiki article

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