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To control the brightness of a LED often pwm is directly used as input to the LED. Does this on/off turning by the pwm has any negative effect on the live expectation of the LED?
Would it be better for the MTTF (mean time to failure) to smoothen the current by adding a capacitor?
One potential problem with using a capacitor to 'smooth' a PWM voltage for the LED, is the LED has a minimum forward voltage before it turns on sufficient to be visible.
Its brightness is not controlled by voltage, it is controlled by current, and the amount of time it is switched on (i.e. the PWM duty cycle).
The capacitor might reduce the 'smoothed' PWM voltage below the minimum forward voltage, so the LED would no longer be visible, even though it would be visible using exactly the same PWM signal directly (without the capacitor).
So it would reduce the brightness range over which the LED can be controlled.
AFAIK, the bigger killer of LEDs is heat leading to a significant temperature rise, and not switching.
Typically we want to drive an LED with a constant current (or something near, e.g. a resistor), so that it is protected from too much heat leading to temperature rise and permanent damage. Edit: Depending on how the capacitor is connected, a capacitor may actually reduce the effectiveness of the constant current circuitry.
Would it be better for the MTTF (mean time to failure) to smoothen the current by adding a capacitor?
This is usually done with an inductor: -
The inductor will smooth the current into the LED. Also note that average current is maintained by the sense resistor attached to the chip in the diagram. A capacitor will not smooth the current into an LED unless there is a series component like a resistor or inductor.
If you put raw PWM voltages across an LED it is likely to destroy it. Observe the LED spec regards current. Here's other examples: -
This one doesn't use an inductor but relies on the transistor arrrangement to regulate max current into the LED: -
LED forward conduction characteristics: -
At 2V across this "typical" LED the forward current is 20 mA. With only 1.7 volts applied there's hardly any current and the LED will be very dim. If you applied 2.5 volts, the current is off the scale and the LED is damaged. But some LEDs are designed for 1A I hear someone say and that is true but applying a few tens of millivolts more than the recommended value will kill it nonetheless. The LED brightness should always be controlled by current.
It is unnecessary to smooth the current.
If you are PWMming at a rate faster than the eye can follow, any heat cycling effects will be negligible. Smoothing the current might give a very small (but unnoticeable) increase in brightness because an LEDs emissivity vs. current is not perfectly linear, especially at higher currents. If you are adjusting the PWM to control brightness, then it's best NOT to filter it because a) effective brightness will not be linear with PWM duty cycle, and b) LED colour changes slightly with current, so you would get a colour shift also. This would be most noticeable in white LEDs.
TVs and displays that use LEDs all PWM them without filtering at 50/60 Hz or faster.
LEDs have some internal series resistance, and overall power efficiency would be higher by filtering the current, but if you are regulating current with an external resistor or current source, there is no difference (same average current consumed from supply)
If you are controlling a single LED (or single string) from a DC/DC converter, the DC/DC will smooth the current anyway. This is optimal for efficiency. TV screens use DC/DC converters to generate a sufficient voltage, but then PWM the current to maintain colour and brightness accuracy.
