I am making an inverter to convert 12 DC to AC(~8.5Vrms) @50Hz. Later i will do the same for 400V DC to 250Vrms.
I am using IRF540N and IRF9540 mosfets as switch. And IR2110 mosfer driver for gate control. I get SinePWM from arduino with carrier frequency of 15kHz. And the whole project work successfully till this stage.
But now i am confuse about how to filter this PWM to pure sine wave. I know i can use low pass filter but how to choose value of L and C?
And is there more than one topologies to make filter?(i read on internet but i am not sure)
Also does this L and C's value depends on how much load i have connected?
Best Answer
Normally, a good starting point for the LC filter is choosing the cut-off frequency and that can be found by finding the logarithmic half-point between 50 Hz and 15 kHz (in your example). Numerically it is \$\sqrt{50 \cdot 15000}\$ = 866 Hz. It's not a hard rule but something to get you started.
So this constrains L and C somewhat but loading effects can reduce the Q of the LC circuit and this is probably best done using a simulator like micro-cap 11 or LTSpice (both free). Just model the circuit with with highest and lowest loads and see what happens to the shape of the sine wave and its amplitude.
I'd consider starting with L = 10 mH and C = 3.3 uF (876 Hz) with a little series resistance with the inductor to avoid Q factor rising too high on low output currents.