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I have been asked to create a dimming system (Along with other features) that interfaces to the cheap LED strip lights. A lot of people seem to use standard N-FETs with PWM on the low side. My concern is with a long strip (3M) wouldn't this produce a large amount of EMC/EMI due to a hard edged voltage and current wave?

enter image description here Should the design instead incorporate more of a buck style control with an LC filter to smooth the output waveform? And effectively make it analog dimming if EMC/EMI or radiated emissions is of concern to for instance pass FCC or CE tester? Note values in schematic are for reference only, not calculated.

If so, is there an easy way to do it without a high side switch, or an easy way to efficiently drive it from 0 to 100% PWM?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I don't think 10 kHz switching noise will be of concern. Highly unlikely that 10 kHz noise is going to interfere with any communication and appliance. \$\endgroup\$ – Whiskeyjack Jun 28 '19 at 1:54
  • \$\begingroup\$ @Whiskeyjack, EMC testing starts at 9khz I believe, and that squarewave would have harmonics into the 100khz I would assume? \$\endgroup\$ – MadHatter Jun 28 '19 at 2:11
  • \$\begingroup\$ Thanks for correcting me. I totally forgot about the harmonics. \$\endgroup\$ – Whiskeyjack Jun 28 '19 at 2:24
  • \$\begingroup\$ The harmonics on the fundamental go up to the 1/rise time of the switch which includes the AM band but what is SRF resonance and Q of your filter? You can reduce the slew rate by increasing the series gate resistor with Rs * Ciss *gm controlling the input rate. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 28 '19 at 5:31
  • \$\begingroup\$ This design has some peaking gain at 50kHz for large strings that draw say 12V/6A or Req=2 ohm when the switch opens. WHat happens if your SMPS has ripple at 50kHz? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 28 '19 at 11:54
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The idea of PWM controlling LEDs is to drive them with their correct drive current for a proportion of the time and let the eye do the averaging as its response is in the 25Hz range.

Using a lower than recommended drive voltage produces a very non linear response and reducing the drive current can produce some unexpected results like spectral shift particularly if the leds are the multiple die types.

If you want to improve EMI then the first thing to do would be to reduce your PWM frequency by a factor of 10. Then you can think about softening the edges. A very simple RC filter would be sufficient.

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