How to choose right PWM frequency for LED?

Question

I want to control LED's brightness with PWM (via BJT transistor). What frequency of PWM should I choose?

Answer 1

For a question like this, you will probably get as many answers as there are people interested in answering. Here is my answer: It depends.

Here are some of the limiting factors, first the lower limits:

• Persistence of vision:
  • Different people are differently sensitive to flicker in a light source. Some would notice flicker even at 100 Hz, others perhaps not even at as low as 10 Hz.
  • Motion of light source relative to the eye makes flicker more discernible, scaling up with speed of the motion.
  • Human vision sensitivity at low intensity of light - both ambient and source intensity. At very low intensity, the eye is much more sensitive to any change in intensity. So an LED operated at low duty cycle / low current and in a dark environment would require a higher minimum PWM frequency.

Now the upper limits:

• LED turn-on characteristics: An LED cannot be toggled at arbitrarily high frequency, once the pulse duration approaches the turn-on time, the LED never really turns on fully, hence linearity of PWM control is lost to begin with, and at higher frequency / shorter pulses, eventually the LED just stays dim or off.
• PWM provider capabilities: Your microcontroller would have its own maximum PWM rate, which sets a hard limit.
• Switching losses: Any switching system, MOSFET based, BJT based, or other, suffers switching losses of power as switching rate increases. At one point this become significant both in terms of heating of switching device, and efficiency of illumination.

Thus, depending on these parameters, and any others affecting your specific requirement, the correct answer could be anywhere in the 50 Hz to few dozen KHz range.

Answer 2

It depends entirely on the application and human eye natural sensitivity to flicker when the light is moving or when your eye is moving with peripheral sensitivity improved to flicker, while steady-state peripheral vision declines.

Cadillac was one of the, if not THE first car to introduce LED brake lights and we as Engineers wonder how this missed this major detail.

People would say why is that car brake light flickering as it was driving past our view? It was so irritating to some with Seizure sensitive to flicker, that it could trigger an "episode".

Since we have house LEDs stationary we, don't often care yet we know something is irritating to some people. This is often because Engineers don't know much about Biomedical responses and think 150 Hz is OK, because they can't tell anything staring at it. That's because it requires motion artifact to detect the flicker or moving images across the retina peripheral.

The range of frequency I would suggest is 300 Hz minimum for stationary and 1kHz minimum for moving flicker-free. Although the industry has been slack with 300 Hz and peripheral eye motion flicker-sensitivity still exists at this flicker rate in moving tail lights.

White LED phosphor responds much much faster than TV tube phosphor.

If you don't care about my concerns, feel free to follow the others' advice at lower frequencies.

I looked for references to back up my experience. This is just one example. http://www.thenakedscientists.com/forum/index.php?topic=45126.0

Answer 3

As I've seen, almost all of the PWM dimmer diagrams with LM555, have the oscillating circuit formed by a 0u1 capacitor and a 1k resistor. That means something about 2 kHz with slight variations at low and high widths. I think this frequency offers enough switching speed in order to not see bad flickers on moving and, on the other hand, slowly enough to turn on the LED al low width. I wanna try higher frequencies to see what happens (5...10 kHz)

Answer 4

Agree with those above on the low end, but on the higher end of frequency spectrum, you can sometimes hear an audible switching if you are in the 1k-15k range. You'll want to stay out of the audible range or you can potentially hear a high pitched noise at your PWM frequency.

Answer 5

There is research that suggest the majority cannot distinguish past 500Hz, and some other research that suggest a minority can see flicker up to 1500Hz. I have a housemate who is very sensitive to flicker - 1Khz caused them problems but had no further problems once my lighting system as set with a 4kHz PWM frequency.

(apologies I read it some time ago and cannot find the reference now)

Note my application was soft lighting for the whole room, so peripheral visions would definitely be an issue.

The limiting factor is really the rise and fall time/resistance of your mosfets as this will dictate switching losses in relatively high current applications. Fast low resistance fets are worthwhile in this application.

Answer 6

If you put a capacitor parallel to the led you should be able to avoid the flickering and instead get a slight declining wave, or ripple. The higher the frequency, as well as longer 'on' pulse width will give the most consistent light.

Choice of frequency now depends more on how many different widths you wish be able to squeeze into a cycle, or intended dimming resolution so to speak.