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# Harmonics reduction in Sine PWM Full bridge INVERTERS?

For triggering circuit, I have frequency of sine wave as 50 Hz and frequency of carrier (triangular wave) as 5000 Hz. I have a 100 volts VDC applied to it and I get a +100 to -100 square voltage.

Now How do i make the output voltage as pure sinosoidal and reduce the harmonics. How to figure the frequency at which these harmoncis are happening? can i use LC filter?

### 2 Answers

- HankmLv 71 decade agoFavorite Answer
you have to feed the triangle and a sinewave into a comparator to generate the pwm rectangular signal

that will be output. The frequencies in the output will be integer (1,2,3,4,5,6...) multiples of 5000 hz

The exact composition of the waveform changes with the duty cycle. You could put a single LC filter

section on the output lines.

You don't want too low a break frequency because it saps energy. Too high a frequency doesn't clean up the output. The inductor has to be able to carry the full output current and have low I*R losses.

The capacitor has to be non-electrolytic and be able to handle the ripple current.

If for example you set it for a break frequency of 1000Hz and an impedance of 10 ohms.

2*PI*1000*L = 10................L = 1.59mhy

2*PI*1000*C=1/10 .............C = 15.91uf

Johanson has a switch mode capacitor that might handle this but it's about 1/2" dia 1" long

several coils are available depending on current needed. You might have to parallel components to get the right currents for your application.

the filter would present a shunt impedance of 318 ohms at 50 hz (1/3 amp reactive)

and a series impedance of 0.5 ohms

at 5kHz, it would present a series impedance of 50 ohms and a shunt impedance of 2 ohms

and would attenuate the fundamental by at least a factor of 25 (28db) and add 40 db/decade for the harmonics

- 4 years ago
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