0
\$\begingroup\$

So I have a display which has an integrated LED driver for backlight. The driver expects PWM input with frequency between 10 kHz and 100 kHz. The only other information I have are the maximum voltage and current and that there are two LED strings.

Most of the research results I found concern themselves with PWM frequencies in the very low frequency range and there seems to be little information for PWM frequencies above 3 kHz. It seems that the flicker is not an issue above 3 kHz.

So are there any other important factors, other than EMI concerns, which need to be taken into account when selecting the PWM frequency?

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Choose 38kHz and your infra-red remote controls will get a lot less sensitive... \$\endgroup\$
    – user_1818839
    Jan 26, 2018 at 13:26
  • \$\begingroup\$ @Brian Drummond Why don't you post that as an answer? I think it's a potentially good point! \$\endgroup\$
    – AndrejaKo
    Jan 26, 2018 at 13:27

2 Answers 2

Reset to default
3
\$\begingroup\$

So are there any other important factors, other than EMI concerns, which need to be taken into account when selecting the PWM frequency?

Yes.

The higher the switching frequency, the more you have to charge / discharge whatever MOSFET that is being used for switching. So the larger input capacitance it has the more important it is. You're effectively just throwing away charges.

Transistors got rise time and fall time that are both above 0 seconds (not instant). This means that as a transistor is transitioning from conducting to not conducting, it will for a short moment have a high current and a high voltage across it. The higher the frequency the more often those high power losses happen.

Audible noise, I advice you strongly to use a PWM frequency of at least 40 kHz, this way you won't hear the switching and neither will your dogs or cats.

\$\endgroup\$
3
  • \$\begingroup\$ Good point for 40 kHz! I don't expect that this end device will be near animals, but it's a good thing to keep in mind for general case. About MOSFETs, how do you expect that this will manifest itself? I thought it will result in higher current consumption, but after doing measurements, I couldn't see any major differences. I measured current with 25 kHz, 50 kHz, 75 kHz and 100 kHz and the differences between the frequencies were in the 100 μA range, but they did not increase with the increase of frequency. \$\endgroup\$
    – AndrejaKo
    Jan 26, 2018 at 13:19
  • \$\begingroup\$ @AndrejaKo I'm not 100% sure how you measured the current, but I doubt that you were also measuring the RMS gate current. But these charging losses are more important /noticeable for energy harvesting systems... so maybe not super duper relevant to you - And the rise time / fall time shouldn't take more current, just generate slightly more heat. \$\endgroup\$
    – Harry Svensson
    Jan 26, 2018 at 13:23
  • 1
    \$\begingroup\$ Yeah, you're right, I didn't measure the gate current. I'll definitely take a look at it as well. \$\endgroup\$
    – AndrejaKo
    Jan 26, 2018 at 13:25
2
\$\begingroup\$

May or may not be a serious issue in practice, but:

Choose 38kHz and your infra-red remote controls may get a lot less sensitive...

It's certainly true that the first overhead LED room lights (GU10 bulbs) I installed jammed the remote control of anything in the same room.

Whether the same would apply with a LED backlit display would have to be found by experiment, hopefully not too far down the development process.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.