# Selection of filters for an AC/AC chopper

I am involved in designing an AC/AC chopper circuit as shown in the above image. The details are as follows

1) The switching frequency is 10Khz and the input voltage is 240Vrms serving a load of 22 Ohms

2) On the left hand side, the IGBT1 and IGBT2 are switching on and off simultaneously. The IGBT1 would conduct in the positive half cycle while the IGBT2 would be conducting in the negative half cycle.

3) On the right hand side,the IGBT3 and IGBT4 are to act as a freewheeling path for the current to flow when the IGBT1 and IGBT2 are switched off.

4) Without filtering, we receive chopped voltage and current waveforms at the input and the output

I am required to select input and output filters to be installed for my circuit. How ever my supervisor instructed me that I must install

1) A capacitor at the input for input filtering

2) A two stage RLC filter at the output

3) The resistors in parallel with the inductors are for damping the oscillations from the filter.

My question is

How can I go about selecting the values for RLCs. Are there any equations for selecting these values of R and L and C

• .Hint ...The input cap should have low compared to 22 ohms reactance at 10KHz select this .The RLC output filter should have a cut off well under 10 KHz .Select this for say 2 KHz.This gives you a LC product for this proposed LPF .Now R and the LC ratio will determine Q .You should go for low Q because ringing is bad .Also high Q is more likely to make closing the loop harder .LCR values should now drop out remembering that you still want reasonably low Q when load is open circuit or short circuit .Recalc changing your assumptions if your component values are not practical . – Autistic May 20 '17 at 13:46
• @Autistic The resistor in the RLC filter is the resistive output load itself or the damping resistors on top (parallel) to the inductors? We must remember that our load can change anytime. – Taven May 20 '17 at 14:35