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I have a 25kHz PWM signal to control LED strip. In order to transfer the current (~1.7A) from PSU (24V) to LEDs (<10 meters) I want to add an LC filter to reduce EMI and RFI. Also, I am planning to use shielded cat 5e cable to suppress anything left after LC filtration.

So, at given params, my load is ~14Ohm. After calculations, I got L=1.26mH C=3.22uF values at Q=0.707 and F=2.5kHz.

I've simulated the schematic and it works. But I don't really like the spike at the beginning and non-square edges. There is one more thing with it, the induction is too big.

Schematic

Simulation 1

Question 1. How to fix spikes, get square edges and lower the induction? (I know that it is possible to increase frequency, but I would like to keep it at 25kHz)

I've tried to use the following L=126uH C=32.2uF values at F=2.5kHz and R=1.4Ohm. Which leads to Q=7 and as far as I understood here Ripple reduction with LC filter, is ok. Is it ok to use such technique?

enter image description here

Question 2. Do I need some extra components here? Like flyback diode...

Kvet
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  • If your load is 14 ohms how can it become 1.4 ohms in your 2nd circuit? What spike are you referring to? – Andy aka Feb 04 '22 at 18:50
  • @Andyaka I've just used the different value for filter calculations – Kvet Feb 04 '22 at 19:06
  • How can it still draw 0.8 amps then? – Andy aka Feb 04 '22 at 19:06
  • Well, it's just a hypothetical idea to fix the problems in the first question. It will still be same load ~14A. I want to understand if the "problems" listed can be fixed, how I can do it without frequency rise – Kvet Feb 04 '22 at 19:13
  • Are you saying it is a 14 amp load (`~14A`)? – Andy aka Feb 04 '22 at 19:24
  • That 14Ω is not a good approximation for an LED strip. At 24V, a strip probably starts drawing microamps at 14V and non-linearly. Suggest measuring your strip and developing a more-representative load. – rdtsc Feb 04 '22 at 19:29
  • Sorry, the current is ~1.7A – Kvet Feb 04 '22 at 20:39

1 Answers1

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Assuming that your load does actually need to be 14 Ω and not 1.4 Ω, put a 3.9 μF capacitor in parallel with it and feed the parallel pair from the 24 volts with a 1 mH inductor. Connect the bottom of the load (and capacitor) to your MOSFET drain. That should ensure a smooth delivery of voltage and current to the load but, you'll need a snubber from drain to ground like this maybe: -

enter image description here

With a duty cycle of 87.5% I get these waveforms: -

enter image description here

However, you will need to choose a MOSFET with the capability of dealing with a drain-source voltage of at least 200 volts to be on the safe side. At 97.5% duty the waveforms look like this: -

enter image description here

Andy aka
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