I need an output filter for UPS inverter. I am using 9.6kHz PWM and 60Hz frequency. Can anyone help me with the right filter as I am new with filter design?

  • \$\begingroup\$ can you tell us what you have tried. \$\endgroup\$
    – Damon
    Dec 18, 2014 at 16:20
  • \$\begingroup\$ i need output filter i am trying but with no load i get the sine wave but when i connect load then its change the wave. \$\endgroup\$ Dec 26, 2014 at 19:06

1 Answer 1


You ought to consider an RLC low pass filter so that the switching frequency is significantly reduced at your output. You need to set the output filter cut-off frequency at least 30% greater than 60Hz also - this is to avoid catastrophic resonance if the filter frequency starts to coincide with the 60Hz.

You also need to "govern" the Q of the circuit when an output load is not connected - this is what the resistor does: -

enter image description here

The value of R needs to be chosen so that you don't get significant voltage peaking at 60Hz. With a 25mH inductor and a 150uF non-polarized capacitance you'll get a cut-off frequency of 82Hz. To prevent significant peaking you'll need a resistance of no-more-than 18 ohms but don't go too low or the peaking could cause you problems.

Given all this information you can decide whether this method is suitable. Here is the site I used for the RLC calculation with a damping ratio of 0.707.

  • \$\begingroup\$ Couldn't R be placed between the cap and GND? It would provide the same open circuit damping, but not be directly in series with the load. \$\endgroup\$
    – user57037
    Dec 18, 2014 at 18:24
  • 1
    \$\begingroup\$ @mkeith: It may not matter here, but doing that will half the db/decade roll-off at high frequencies. \$\endgroup\$
    – copper.hat
    Dec 18, 2014 at 19:14
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    \$\begingroup\$ @copper.hat - yeah thanks for that. Also, given the output impedance of the driving source before the filter may be a few ohms this will help reduce R to maybe 12 ohms and, I'm thinking that if a inrush current resistor is used (higher impedance at lower currents where it is needed) this might also work when the real load gets low and takes over the job of damping the filter. Response time may be a problem though. \$\endgroup\$
    – Andy aka
    Dec 18, 2014 at 19:28

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