Electrical – LED modulated @ high voltage -vs- constant @ low voltage

ledpwm

I'm experimenting with LEDs with Arduino and have a question.
For example; I have one basic 20mA LED that runs constant at 3.3v.
I have another LED with that I PWM with a low duty cycle (6%). I up the voltage of this LED by x%~.

I have learned from this experiment that if the second LED passes the 20mA, it burns out.
That's why currently it has to run less bright than the first LED. By my calculations (V/A measurement) it comes at 33%~ of wattage used by LED 1.

My question is: say I replace LED 2 with a LED rated with a higher amperage, will I match the brightness of LED 1 with same wattage used? Or will LED 2 still experience more stress and energy loss to heat?

(I intended the question to be applicable to a broad range of LEDs, not just 20mA LEDs)

Duplicate question

For other readers wondering, this is another post with a similar question:
Does pulsing an LED at higher current yield greater apparent brightness?

Edit

I found out some new things (also thanks to that link); some LEDs are notably more efficient when current is overdriven. Big LEDs are inefficient regarding this.

Also resistors will burn a lot of heat, and limit the rise of current (so a much higher voltage is needed to force the current through the LED than without resistor in front). I noticed one of my COB LEDs has resistors on it, and at roughly the same brightness, the resistors will seriously heat up and overall the COB LED will be inefficient. Perhaps matching the resistor better for the overdriven scenario would work, but at the moment I am relying on my variable voltage supply and no resistor at all.

I seem to have success with a 3W LED, running at 1 W. By eye, the constant and the overdriven PWM'd seem to have the same brightness per watt.
My theory is that using a higher rated LED and running at an average lower wattage increases efficacy per Watt; which is generally true but especially so when overdriven.
I had less success with a few COB LED strings in parallel with a resistor each, I assume because of the earlier mentioned resistors burning a lot of energy.

Best Answer

From the comments:

The peak forward current is what matters in this case. In most documents it is shown with a duty cycle of 10% @ 1KHz. The question remains though; if I drive an LED at peak forward current with PWM, will it be as efficient as a constant LED with same average wattage used?

It's not clear from your original post that that's what you are asking. It shows some confused thinking in your mention of doubling voltage.

enter image description here

Source: Purdue Engineering.

To answer your question you need to look at the relative luminous intensity vs. forward current on the datasheet. If it's linear then using PWM at \$n\$ times continuous reference current for \$ \frac {1}{n} \$ duty cycle will result in the same light output.

For the LED shown in Fig 1. indicates that there will be some reduction in light output if the current is pulsed as described in the previous paragraph.

Check the datasheets for the devices you are using.


Update 1

An article in LEDs Magazine titled Pulse-driven LEDs have higher apparent brightness makes interesting reading.

According to an article by Naoshige Shimizu of Nikkei Electronics, a research group at Ehime University, Japan, has developed a pulse drive control method to make LEDs look twice as bright by leveraging the properties of how people perceive brightness.

When a short-cycle pulse voltage with a frequency of approximately 60Hz is applied to an LED at a duty ratio of about 5%, the LED looks about twice as bright to the human eye in comparison with an LED driven by a direct voltage, the research group said.

And further down ...

There are two principles, the Broca-Sulzer effect and the Talbot-Plateau effect, involved in how human eyes perceive brightness. The Broca-Sulzer effect refers to a phenomenon in which light looks several times brighter to the eye than it actually is when exposed to a spark of light, such as a camera flash.

In addition, the Talbot-Plateau effect is a principle where human eyes repeatedly see flashes and sense the average brightness of the repeated lights. Until now, said Jinno, it has been believed that, due to the Talbot-Plateau effect, the brightness perceived by human eyes would not change even if an LED is pulse driven.