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  1. #1
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    capacitor for LC filter in DC/AC full bridge

    Hi
    I am designed a DC/AC inverter. I already made a prototype
    Click image for larger version. 

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    The output voltage is 120Vrms and current of 8 Arms. DC link is 250vdc, Fsw = 25khz.
    Now I'm designing a new version that is coupled to the electrical network, and with corrections of the first version .
    The filter is an LC filter with an inductance value of 2mH, and I plan to change the value of the capacitor, by one of greater value (6uF or more).
    I had used a capacitor PHE426JF7220JR06L2. The capacitor is film capacitor, and some of its specifications are:
    Tolerace: 5%
    Capacitance: 2.2uF
    Voltage AC: 160Vac
    Volatage DC: 300Vdc
    When looking for another capacitor of the same series, I see a graphic that I had not seen before
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    1. What does the previous graphic mean? Before the filter there is a square signal that switches between 250V and -250V@ 25Khz. so what the capacitor derating to 30Vrms for 25Khz ?.
    2. is film capacitor the best type for this aplication?. What about ceramic capacitors or other material?

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  2. #2
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    Re: capacitor for LC filter in DC/AC full bridge

    Derating is due to capacitor ESR losses. Did you ever calculate the AC voltage across the capacitor? It should be below 10 V at 25 kHz.

    30V @25 kHz for a 2.2 µF capacitor means more than 10 A.



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  3. #3
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    Re: capacitor for LC filter in DC/AC full bridge

    Man I feel very confused with this part
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    In the simulation the signal that enters to the filter is the green signal in the graph below(V(out1,out2)), and out, the sinusoidal signal with 170 Vpk@60hz.
    Which signal should I calculate AC voltage across the capacitor, with the sinusoidal 170vpk@60hz(red) or PWM 200Vpk@25khz(green) ?



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