# capacitor for LC filter in DC/AC full bridge

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

Hi

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

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

Man I feel very confused with this part

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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