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    Film capacitor failed in flyback smps at high temperature?

    Hello,
    We are using the ECW-FD2J185 (1.8uf, 630v) film capacitor in our offline flyback smps, 240vac input.
    It is for 24v, 10w output.

    We sent it off for extreme temperature testing and it eventually failed with the TNY287 flyback controller showing a short from BP/M pin to Ground.

    The testing was done at 190VAC and 10W load.

    We are a little surprised that it failed. We wonder if actually the 1.8Uf film capacitor may have failed at high temp and become of less capacitance, meaning that the primary current of the flyback would go up.

    Do you think this film capacitor could fail under high temperature such that its capacitance became much lower?

    ECW-FD2J185 Film capacitor
    https://industrial.panasonic.com/cdb...BD0000C255.pdf

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  2. #2
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    Re: Film capacitor failed in flyback smps at high temperature?

    If you read the data sheet, you'll see that the operating frequency of the cap has a large impact on it. What's your frequency? What's your 'extreme' test temperature?


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    Re: Film capacitor failed in flyback smps at high temperature?

    Also, there is a clue on the first sentence on page 1 of the datasheet:

    Non-inductive construction using metalized Polypropylene....

    There are two ways to build a film cap that I know: wound metallzed film, and stacked metalized film.

    The statement above clearly indicates a non-inductive construction, which rules out wound film construction.

    So that leaves a stacked film construction. With this type of construction, a conductive epoxy is smeared at both ends, which serves both as a conductive electrode and adhesive.

    If this process is not done correctly, this joint will fail gradually. Meaning the capacitance will decrease as foil layer after foil layer becomes open circuit.
    Ask me how I know..... a power supply for a remote oil well data transmitter.
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    Re: Film capacitor failed in flyback smps at high temperature?

    hi temp = expansion of either type of cap = end termination failure = cap goes low capacitance ....


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    Re: Film capacitor failed in flyback smps at high temperature?

    Thanks, i wonder how high the temp has to be, and for how long, with the cap of the top post to get this failure?
    Its datasheet doesnt tell of this.



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    Re: Film capacitor failed in flyback smps at high temperature?

    It is really a combination of high ambient temp with localized hot spots due to high current pulses.

    I remember reading in a major capacitor manufacturer's catalog some "pulse rated" film caps.

    EDIT; as a fact, looking at the Digikey selector table for film caps, you will see pulse rated devices. I've highlighted some selections, but have not the time to actually read the individual datasheets
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