About the placement of thermistor and varistor,please help,thanks.

I am confusing that why so many SMPS manufacturer's application notes recommend placing thermistor after fuse but before varistor.As there is a likelihood that the thermistor would explode in a surge test.Just change the placement of thermistor and varistor can solve the problem simply.

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L_jack_xing said:

...there is a likelihood that the thermistor would explode...

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pradhan.rachit said:

NTC Thermistors (Negative Temperature Coefficient) are used to limit the inrush current flowing through the SMPS when it is powered on.
A NTC Thermistor's resistance decreases when it's temperature increases.

At room temperature, the initial resistance of the thermistor absorbs the power of peak inrush current. Due the current load and gradual heating of the thermistor,it's resistance drops. Their power consumption is negligible during continuous operation...

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FvM said:

Is it so? Why do you expect that the energy which can be absorbed safely by the varistor will "explode" the NTC? Did you oberve the problem in your tests? I rather guess that the series connection will result in a certain share of surge energy and probably less total surge energy delivered to the DUT.[/QUOAS

As the test result show the phenomenon that the NTC was explode.Some sustained a few number of surge.Some can only sustained one!What problems that the series connection can make? Thanks for help.

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As the test result show the phenomenon that the NTC was explode.Some sustained a few number of surge.Some can only sustained one!What problems that the series connection can make? Thanks for help.

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andre_teprom said:

What I have noticed in some products which have repaired recently have been the opposite. The Varistors are the ones who explode, but the reason is precisely because they were not properly sized to electrical condition to which they are subjected.



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Really confusing.I am thinking that is it the NTC's problem?I will find some time test it again.Thank you.

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As the test result show the phenomenon that the NTC was explode.Some sustained a few number of surge.Some can only sustained one!What problems that the series connection can make? Thanks for help.

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What I have noticed in some products which have repaired recently have been the opposite. The Varistors are the ones who explode, but the reason is precisely because they were not properly sized to electrical condition to which they are subjected.

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FvM said:

Thanks for reporting the details. I take it as a fact, at least for the NTC type you have tested.


This had been my first guess, too. I believe, the VDR and NTC failure are happenning in different situations, both observations can be true.

In surge tests, you have probably a small NTC volume absorbing much of the surge energy due to inhomogenous initial heating and respective conductivity distribution.

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mtwieg said:

Was the NTC actually hot when applying the surge? Did you try the test under max load?

In general I don't think it makes sense to put an ICL like a NTC before a surge protection device. They're generally not meant to withstand surges (which are quite different from normal inrush events).

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L_jack_xing said:

I am sorry,what is ICL?

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I had no chance to touch the NTC,because it exploded only after a few of surge wave.I think NTC is useless in withstanding surge too,but,a lot of company's application note put the NTC before VDR indeed.Why they do not put NTC after VDR instead?

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mtwieg said:

Inrush current limiter, like a NTC.

The NTC can likely withstand much greater surges when hot, though they're not usually rated for surges in that state.

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FvM said:

You should draw your own conclusions. At the end of the day, your design has to pass the tests.

I'm not sure if the "exploding NTC" problem is common to all fabricates or might be a specific problem of the type you are using. As I already mentioned, I can imagine reasons why NTC are specifically sensitive to high power surges (more than VDR). But presently it's just a guess.

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