Inverter LC Filter Calculator

Let’s say we have a push-pull or full bridge inverter and its output goes to the primary of a transformer, and the secondary gives you high voltage AC like 220V square wave.

Now to make this AC waveform smooth and more like a sine wave we connect an LC low-pass filter just after this secondary, like this:

Connections Details of the LC Filter:

• Take the two output wires from the transformer secondary — this is your 220V AC (square wave).
• Now we connect a series inductor (L) in one of the wires — just one wire, like in series, not both wires.
• Then after the inductor, we put a high voltage capacitor (C) across the output, like between the two wires, that is parallel to the load.

Now to caalculate the values of the L and C, you can use the following calculator:

How to Decide this fc Value?

We do not want to block 50Hz because that is our main signal. We want to block the switching frequency harmonics like:

• 300Hz
• 600Hz
• 1kHz
• And higher junk frequencies

So we choose fc just above 50Hz but much below the unwanted high frequencies.

Rule of Thumb:

You can select cut-off (fc) between:

3 × main frequency......... upto 10 × main frequency

Example:

If your inverter PWM has big amount of noise like around 1kHz to 3kHz, then you pick fc = 300Hz, so that:

• 50Hz passes cleanly.
• Above 300Hz (like 1kHz, 2kHz) get blocked fast.
• Gives smooth sine output.

Final Tip:

So just take 5 or 6 times the 50Hz, then:

fc ≈ 50 × 6 = 300Hz

It is a safe and effective choice for LC filter to clean inverter waveform.