Multiple PTC fuse configurations

[Ross]

Hi,

I hope this is in the correct section of the forum, apologies is it is not.

I have an application where I need a 60V 2A PTC, but space constants mean I cannot use a single through hole part (nothing in SMD at this rating). I think I have two options:

• Two or more, lower voltage, same trip current in series
• Two or more, same voltage, lower trip current in parallel

In either case multiple PTCs means I can use SMD parts which I  think I have space for.

I am sure I saw on one of the EEVBlog tear down videos of a measurement instrument (not sure what it was) that they used about 5 PTC in series to obtain high voltage protection. But when I try to search "PTC's in series", good old google thinks I mean PTC range, so I got links to DigiKey etc. There must be something else in this circuit, because when one ptc becomes open circuit then it is exposed to the voltage of the whole string which would exceed its maximum.

When I search for "PTC's in parallel", I find a number of forum posts that say it is a bad idea but none of them say why in depth. I would have though that one would start to open, transferring fault current to the other which would hasten it to go. Is it a bad idea because it is unsafe or is it just an inaccurate method of breaking fault current.

What do people think?

Thanks Ross

[T3sl4co1l]

When I search for "PTC's in parallel", I find a number of forum posts that say it is a bad idea but none of them say why in depth. I would have though that one would start to open, transferring fault current to the other which would hasten it to go. Is it a bad idea because it is unsafe or is it just an inaccurate method of breaking fault current.

That's exactly the problem (or at least I should imagine it to be so).

The resistance doesn't increase very fast (it's quite slow, on the order of 100ms!), and during that time, the remaining fuses in turn experience far more than their ratings.

For series parts, overvoltage would lead to internal sparking and breakdown.

For parallel parts, overcurrent would lead to hot spots and damage.

I would think parallel would be less bad than series, but still not recommended.  (FWIW, conventional fuses are okay in parallel.  Ideally, they should be matched resistances.  Each fuse must be rated for the full fault current capability of the system, whatever that happens to be.  For automotive, this is around 1kA; residential low voltage (120/240V), under 10kA; for industrial 400/480V, 100kA or more; and so on.)

Have you considered using a protected switch, or a hot-swap controller, something like that?

Tim

[Ross]

Hi Tim,

Thanks for taking the time to reply to my message. So far I agree with you and am tending toward a parallel solution. Could you elaborate on the parallel option failure mode? The maximum current rating of the PTC's is in the order of 10 to 50 times the hold current. I can find two smaller PTC with similar maximum current, same max voltage but half the hold current and are smaller.

I am not fimular with the term hot swap controller, could please provide more detail?

Thanks Ross

[T3sl4co1l]

Like these:
http://www.digikey.com/product-search/en/integrated-circuits-ics/pmic-hot-swap-controllers/2556351?k=hot%20swap
Protected switches are in a different category.  They're MOSFETs with added protection (powered by leakage from the gate).

Tim

[Falcon69]

Tim,

Are you saying I could use one of those 'Hot Swap Controllers' instead of the three combination circuit protection I'm currently using? Fuse, Overvoltage Protection, Reverse Polarity Protection

[T3sl4co1l]

Maybe?  Not usually so good at OV or reverse, unless they're specifically made to do that (there's an LT chip that does... if you don't mind paying $5 for it).  Which, if that's what you need -- it can be worth it!

Tim

[Falcon69]

hmm, interesting, I'll have to check them out. DId not know they existed.

[Zero999]

No mention has been made of whether it's for AC or DC.

I don't see any problem with connecting the PTCs in parallel, as long as the maximum short circuit current of either PTC is not exceeded.  There will be some current sharing. The worst case can be predicted using the numbers on the datasheet, therefore the maximum fault current trough either PTC can be calculated.

[Ross]

@Tim, I too have not seen these types of components before. Always wounded what underlied hot swappable drives etc, I guess these components are part of the package.

@Hero, I agree, perhaps a couple of emails to PTC manufacturers would be useful... will post any useful replies.