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In the buck-boost converter I am finishing, I came out with this question.

At Vin (12V in my circuit) I place a 7A fuse and a P mosfet (IRF5305) to prevent reverse polarity issues.

Because I want to protect everything after these two components (if more than 12V is applied at Vin), I want to place an unidirectional TVS diode (Vishay 1N6376-E3/54) between Vin and GND.

Does it matter if this TVS diode is before the fuse or do I have to place it necessarily after the P mosfet?

This is the actual schematic without the TVS diode. (Dissmis the "-" on +12v)

circuit

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  • \$\begingroup\$ Can you add your schematic instead of or in addition to the PCB routing image with rats-nest lines? \$\endgroup\$ – JYelton Jun 24 '20 at 15:48
  • \$\begingroup\$ Thanks JYelton for the advice. I update image with schematic \$\endgroup\$ – marcosbc Jun 24 '20 at 16:01
  • \$\begingroup\$ Anyone placing a TVS has to understand what type of surge they are trying to protect against. What is your surge? \$\endgroup\$ – Andy aka Jun 24 '20 at 16:26
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TVS devices can and do fail short circuit .This did happen on a client who manufacturers agricultural equipment .You want the fuse to blow ,not the wiring loom on the expensive tractor .So Fuse before TVS or anything else that could fail short circuit.

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  • \$\begingroup\$ Autistic thanks so much!. Many months ago I finish building my DC-DC converter. I put Fuse->PMosfet->Unidirectional TVS in order to avoid what you describe. \$\endgroup\$ – marcosbc Apr 22 at 23:32
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A transient voltage suppressor (TVS) diode will shunt overvoltages to ground and help protect the circuit. If you place it "after" the fuse, then overvoltage events will cause excess current to flow through both the fuse and the TVS. If you place it "before" the fuse, then current will only flow through the TVS. Generally you would want to have it after the fuse so that the fuse can open ("blow") during a prolonged event.

If the TVS is before the fuse, and an overvoltage event occurs which damages the TVS, the fuse may not blow before the circuit is exposed to the now-high voltage in the absence of the working TVS. I think of a TVS as something that's extremely fast acting, but cannot handle sustain high current for very long. The fuse is slower (even a "fast blow"), and I think of it as relying on the TVS to react first so that it then has time to heat up and blow the fuse element as necessary.

Edit:

In case you're wondering whether to handle reverse polarity or overvoltage "first", consider a bidirectional TVS diode which would still be between the fuse and FET, to protect the FET from reverse over-voltage, should you feel that to be necessary.

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  • \$\begingroup\$ Most of this sounds right, but I would disagree that a TVS can't handle much current. They are constructed to handle lots of current, but are limited in the amount of energy they can absorb. If the TVS selected (and placed after the fuse) can handle more energy than the fuse the fuse will blow without damaging the TVS. If the fuse can handle more energy than the TVS then typically the TVS fails shorted and the fuse blows anyway. \$\endgroup\$ – John D Jun 24 '20 at 16:18
  • \$\begingroup\$ @JohnD A fair point - my interpretation is that a TVS doesn't handle sustained current, even though short duration high currents are completely fine. Correct? \$\endgroup\$ – JYelton Jun 24 '20 at 16:35
  • \$\begingroup\$ Based on the datasheet of this tvs diode farnell.com/datasheets/… i look foward IFSM value that is 200A. Based on this can i assume the fuse will blow first if i place this diode after the fuse? Fuse is 7A rated fast acting \$\endgroup\$ – marcosbc Jun 24 '20 at 17:44
  • \$\begingroup\$ @JYelton Yes, a TVS can handle much higher peak then sustained current. \$\endgroup\$ – John D Jun 24 '20 at 20:04
  • \$\begingroup\$ @marcosbc Probably correct, but not necessarily. It's not about peak current but about energy. A fuse has an I2t rating, so you can do the calculation to see if it will blow before the TVS fails. You picked a pretty hefty TVS so I'm guessing it will be OK. \$\endgroup\$ – John D Jun 24 '20 at 20:07

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