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I want to protect an IC output (MOSFET driver) that the vendor says can survive V_cc +0.3V on the output pin from an accidentially connected overvoltage of about 35V on the plug. Problem: my V_cc is only 5V!

One approach I found here at StackExchange EE was to use a combination of TVS and a PTC.

My assumption is, that the 35V will create a current trough the highside switch to V_cc of the MOSFET Driver or through the lowside switch to GND.

1) How will I know if the IC can survive the time that the PTC needs to become highly resistive? Can I look for hints in the IC datasheet?

2) I always thought TVS are meant for ESD or surge protection (short time and high voltages) and not for longer durations of moderate (35V) Voltages. Can they also be used for these things?

3) Whats a good starting point to understand these time related questions concerning PTC and TVS? Are there other solutions you can point me to? I have a bad market overview on what's available.

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  • \$\begingroup\$ Usually the MOSFET driver has to be very close to the transistor, adding PTC to the output of the driver makes no sense since it will degrade the basic function of the driver. \$\endgroup\$ – Marko Buršič Jun 8 '16 at 10:03
  • \$\begingroup\$ @MarkoBuršič you are of course right. I need to mention, that I do not use the IC as a MOSFET driver. I use it only because of its capability to drive some capacitive load. \$\endgroup\$ – Villotron Jun 8 '16 at 11:18
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    \$\begingroup\$ If you describe what is your intention, maybe some better solution could be found, like a galvanic separation, other mosfet switch totem pole with OC OV protection,... \$\endgroup\$ – Marko Buršič Jun 8 '16 at 11:31
  • \$\begingroup\$ How about reading the data sheet? \$\endgroup\$ – Andy aka Jun 8 '16 at 11:46
  • \$\begingroup\$ @MarkoBuršič My intention is to create an 5V trigger signal for a input that capacitive load. To get a decent edge (in the oder of 200ns) For this task, I thought this IC type was fitting. \$\endgroup\$ – Villotron Jun 8 '16 at 13:27
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I don't think using normal PTC will give us a well predictable behavior. The thermal time constant given by datasheet is referred to zero-power conditions. Is behavior is determined by too much factors (ambient temperature, voltage applied, etc).

Why dont use a fuse? Maybe a resettable fuse (PPTC, I think you means this device in the question), that generally is little slow compared to fast-acting fuse, according to Littlefuse specification (see "Time-Current Characteristic").

Or better, why not an overvoltage crowbar?

3) Whats a good starting point to understand [...]

Some useful links about thermistors:

  1. http://www.vishay.com/docs/33016/engnote.pdf
  2. http://www.littelfuse.com/products/resettable-ptcs.aspx
  3. http://www.avx.com/resources/technical-info-papers/circuit-protection-emi-filtering/
  4. AVX about PTC (interesting for current-time characteristic)

And about TVS, Littlefuse and Vishay sites are a good sources of informations.

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  • \$\begingroup\$ I deleted overvoltage crowbar because if I'm right the crowbar it is not so fast... There's anyone experienced about this that could confirm or deny? \$\endgroup\$ – Antonio Jun 8 '16 at 10:31
  • \$\begingroup\$ thank you @Antonio for this information. I think I am getting closer to understand. So will the TVS need to clamp my error voltage down to V_cc +0.3V for the duration until the PPTC catches up and becomes high impedant? I still have some work to do until I get behind it ^^ \$\endgroup\$ – Villotron Jun 8 '16 at 13:30
  • \$\begingroup\$ They even seem to make the 'resattable PTC with TVS'-solution as one part called PolyZen (by TE). digikey.com/en/product-highlight/t/te-connectivity/polyzen-ce I'll try to figure out if this is what will solve my application. Still a lot of thermal considerations I have to catch up to. Thanks again for the hints! \$\endgroup\$ – Villotron Jun 8 '16 at 15:18
  • \$\begingroup\$ @Villotron Yes. The only problem is the TVS act more fast than PPTC. Polyzen-ce? I'll go to read abuot this device. :-) \$\endgroup\$ – Antonio Jun 8 '16 at 15:36

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