EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Mad ID on January 03, 2015, 04:32:36 pm
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Hi,
I'm designing an AC/DC Power supply based on LNK302 high voltage buck converter (12V @ 100mA).
They recommend that for the input stage I should use, and I quote "The fusible resistor should be chosen as flameproof and, depending on the differential line input surge requirements, a wire-wound type may be required".
In their designs they always use a 1W wirewound 8.2R THT resistor from VitroOhm (CRF253-4 8R2). I know that 1W is only for the surge immunity.
I cannot find a suitable fusible flameproof resistor in SMD package. Why is that? I'm missing something or there aren't any?
I want my design to be well designed, i.e. from functional and safety prospective.
Thanks guys!
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What is wrong with having 2 holes and using a through hole version, which is pretty common.
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What is wrong with having 2 holes and using a through hole version, which is pretty common.
For manufacturing reasons I was hoping to find a SMD version.
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Use a normal 1W resistor and put a suitable fuse in series with it.
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Chip resistors are not rated for fusible duty, at least not normally as far as I recall seeing in datasheets.
You can connect multiple in series to achieve the peak voltage rating, but I don't know that the result will be self-fusing in the same way.
I would suggest not using SMT resistors at all, as they may fail before the fuse proper is able to open.
You can use an NTC for surge reduction instead of the fusible resistor, but I don't know that these are normally available in SMT formats for this purpose, either. Probably for the same reason -- power, voltage and energy handling are necessarily all inferior.
If this is a low quantity thing, you can do the half-ass solution and mount the part horizontally, soldered sideways to SMT pads. Lower profile I guess, but only more trouble than a proper THT solution.
Tim
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At a glance, it seems Vitrohm at least has series such as RWS5020 which they state will clear when AC line voltage is applied. I do not see any mention of safety agencies in the datasheet, however.
Tim