Pwm on power led with more current than needed

[Rimbaldo]

Hi!!

I have a 3w common anode RGB power led. Each color needs 300-350mA max to work.
I've got a few 600mA tiny pwm-able drivers (based on PT4115 chip). So I could use three of them with arduino to generate PWM for each color.

But as each color needs half the current these drivers provide, if I PWM each of them at 50% max, It would be the same effect as having 3 drivers of 300 mA each, and my leds would be at 100%?

If positive, would I have the same color (in 0-255 pwm values) that would be for instance (255,255,255) for a RGB color in the led, would be the same effect in my case that the pwm is cut by half as (128,128,128)? I mean instead of full pwm to 255, half pwm to 128 would provide the max current in my led (300mA for each color, in a 600 mA driver)?

Thanks!

 

[DIWdiver]

With ideal diodes, that would be correct. With real diodes, it's close to correct, but not quite.

There are some differences between a diode at 300 mA 100% DC (duty cycle) and at 600 mA 50% DC.

1. The Vf at 600 mA is higher, so the power consumption is higher.

2. Efficacy/efficiency is (generally) lower at 600 mA, so the average output is less.

3. Both 1 and 2 mean higher heat generation.

4. Since efficacy changes are probably not the same in each color, combined color will likely not be the same.

Also, I don't know if colored LEDs have this issue, but pumped phosphor white LEDs show color changes with current changes and also with temperature changes.

How significant these differences are depends a lot on your LED and how it is cooled, not to mention how critical your color balances are.

That said, if you color balance at 600 mA/50%, then change the DC, the only variable affecting color is die temperature. This would yield much better results than balancing at 300mA/100% then changing to 600mA/PWM.

 

[Rimbaldo]

With ideal diodes, that would be correct. With real diodes, it's close to correct, but not quite.

There are some differences between a diode at 300 mA 100% DC (duty cycle) and at 600 mA 50% DC.

1. The Vf at 600 mA is higher, so the power consumption is higher.

2. Efficacy/efficiency is (generally) lower at 600 mA, so the average output is less.

3. Both 1 and 2 mean higher heat generation.

4. Since efficacy changes are probably not the same in each color, combined color will likely not be the same.

Also, I don't know if colored LEDs have this issue, but pumped phosphor white LEDs show color changes with current changes and also with temperature changes.

How significant these differences are depends a lot on your LED and how it is cooled, not to mention how critical your color balances are.

That said, if you color balance at 600 mA/50%, then change the DC, the only variable affecting color is die temperature. This would yield much better results than balancing at 300mA/100% then changing to 600mA/PWM.

Hi! Thanks for your explanation!!

In this driver I've got, there were two smd resistors in paralel (R300 and R330) I removed the R330 one and the current lowered to 350 mA!

So now, pwm at 50% would give me the brightest of each color.