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I have simulated a buck converter in LTSPICE, and below is the switching signal (red )that goes into an RLC filter( vout as green).

Simulation with L=2.2, C=1000p

The filter does not average the switching pulses, So I believe the value for L and C are not correct. I increased the inductor to L=10u, and I was able to have a sharper increase to 28 V: L=10u,C=1000p I was thinking that if I decrease the discharging time by increasing capacitor, I will be able to get a smooth output, like this:

enter image description here However, It does not matter what values I change the capacitor to, the vout still reaches to zero. Could you please explain how I can control the rising and falling portion of the VOUT?

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  • \$\begingroup\$ How did you choose impedance ratios for cutoff? For TBD dB @ f Choose your attenuation 1st then impedances for some Q<=1, determine what frequency response you need for passband and bandstop, but there are easier ways. Your driver impedance is nonlinear. Fast attack, slow decay. What response do you need? Sine output? \$\endgroup\$
    – Tony Stewart EE75
    Dec 21, 2021 at 1:17
  • \$\begingroup\$ Here's a higher Q>1 LPF tinyurl.com/yyhx54p6 \$\endgroup\$
    – Tony Stewart EE75
    Dec 21, 2021 at 1:25
  • \$\begingroup\$ @Tony Stewart EE75 I am trying to get a smooth output signal that does not oscillate. I will back to you after searching what means "impedance ratio for cutoff". I simply put my resistor load as 15 ohm. \$\endgroup\$
    – chami
    Dec 21, 2021 at 1:27
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    \$\begingroup\$ @ jp314 I followed your instructions and the signal stabilized after 180us. Thank you. \$\endgroup\$
    – chami
    Dec 21, 2021 at 1:44
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    \$\begingroup\$ @chami Don't forget that what you see there is (probably) the open loop response. Once you add a stabilizing loop, things will change (for the better, if done right). \$\endgroup\$
    – a concerned citizen
    Dec 21, 2021 at 7:58

1 Answer 1

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Typically a DCDC will have 10's or 100's of uF in the output capacitor; the L.C resonance frequency \$1/(2\pi\sqrt{LC})\$ should be 10-100x lower than the switching frequency (you are about 200 kHz here).

If you simulate, you will find it takes a long time for the output voltage to stabilize; if you know how, set an Initial Condition (.IC) on the capacitor at around the output voltage you expect; however this may generate a very large (reverse) current in your inductor -- it depends on how the MOSFETs are controlled at t=0.

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