LED PWM frequency selection - Page 1
Forum section: electronics / beginners

author: dog80
author_rank: Regular Contributor
timestamp: May 26, 2014, 05:47:59 pm
content: Are there any guidelines for choosing the right PWM frequency?I have one of those chinese flashlights with a single high power Cree led.It has three modes of operation, full power, half power and strobe effect.At half power it is using PWM at 47% duty cycle and 10KHz frequency.Isn't that frequency unnecessarily high?

author: madshaman
author_rank: Frequent Contributor
timestamp: May 26, 2014, 07:21:25 pm
content: Above 100Hz or so, LEDs generally look non-flickery.Frequencies in the audio range can be avoided to ensure nothing will mechanically vibrate and make a sound (e.g. an inductor) or to ensure an electrical signal that might get into an audio amp doesn't make a sound (although there are better ways to prevent this).10KHz is in the audio range still, so 30-50KHz might be better.You can drive a typical LED at frequencies up to 30-100MHz, above that, it's getting intrinsically filtered into a DC voltage.Really, anything between 100Hz and 1MHz is reasonable; keep in mind that the higher the frequency, the more energy is lost (due to many factors).Also note that PWM signals carry heavy odd harmonic content; if you're taking a PWM signal at 1MHz, amplifying it and running it through a string of LEDs you've also built a radio transmitter that broadcasts at 1MHz, 3MHz, 5MHz, ... but at progressively less power.Basically, at reasonable frequencies, it's EMI, eliminating any potential buzz at audio frequencies, efficiency, component cost and size (it requires smaller parts to filter out 10MHz as opposed to 60Hz) which will drive your decision.If it's not for a saleable product, I wouldn't worry.

author: owiecc
author_rank: Frequent Contributor
timestamp: May 26, 2014, 07:52:02 pm
content: Central vision (looking straight at light) is less prone to flicker. There you can get away with 100Hz or so.300Hz is used for bright light sources viewed in peripheral vision. This is a worst-case scenario.Increasing switching frequency above that will not change how you perceive light. However it will reduce LED junction thermal cycling. This will increase the lifetime of the LED. Optimal switching frequency depends on the diode but you need to go above 1kHz to minimize cycling.Anything above 10-15kHz is done most likely either for audio reasons or to reduce input filter caps.

author: madshaman
author_rank: Frequent Contributor
timestamp: May 27, 2014, 12:21:46 pm
content: Quote from: owiecc on May 26, 2014, 07:52:02 pmMainly to the OP: please be aware that increased frequency also means increasing losses in the switching elements because they spend more time in their resistive state between full on and full off, losses inside active elements also increases with frequency.What (I think) owiecc is saying is: if you have, for example, a 50% duty cycle, and the frequency is really slow, the LED turns on, it heats up more and more over a long period, then turns off, then cools down over the same period. Speed up the frequency and the LED has less time to heat up or cool down so the temperature oscillates with less amplitude and causes less stress on the device.