Lowpass Filter (LC) resonance problem

mikemillernow · 2013-10-28T03:28:11+00:00

Hello everyone! I am looking to design a low pass filter that has a cutoff frequency of 20 kHz. The filter needs to be able to handle about 20A of current so the dir ection I have taken is an LC filter. (Inductor with a high current rating). I simulated the low pass filter tonight (LTspice) using the values of 7.9 u H and 15 uF suggested from some of the online lowpass filter calculators. From the simulation I see 0 dB in most of the passband but then I see a hug e resonance point of +32 dB at 14.49 kHz! Here is a link to the simulation result: 1.) Is there a way to eliminate this resonance? 2.) Is there a different filter that would have less peaking but still able to handle the high current for this application (20A)? 3.) There is an H-brige that will be modulated at 200 kHz (pwm) and this wi ll going through the 20kHz lowpass filter to given a DC voltage without all the pulses. If the resonance point is kept as it is what type of issues w ould this cause? Is this even a problem? 4.) From the simulation I see a cutoff frequency of 22.7 kHz instead of 20 kHz. What is the best way to tune this? Maybe to keep the capacitor fixed ( 15 uF) and then sweep the inductance value in simulation? Thank you you so much for your thoughts. -Mike

Oct 28, 2013 22 Replies

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John Fields 12 years ago

??? http://www.parts-express.com/cat/solid-core-inductors/299 http://www.ieee.li/pdf/viewgraphs/fundamentals_of_magnetics_design.pdf

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Phil Allison 12 years ago

"John Fields"

** Nothing like a fair contest, at all.

Be like punching a ripe tomato .....

..... Phil

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Mike Miller 12 years ago

Thanks for the comments on the resonance. The response is much better with the load! :)

-Mike

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