Leds multi-voltage PWM control

[STARN]

Hi all!

First of all, sorry for my English.

What I got :
4 leds in serie with a resistor to limit current calculated for 12 VDC
--> no problem

What I would like :
Adapt this to 24 VDC or other voltage (maybe 32 for new vehicles) using PWM. (For example : 5 to 50 V)

What I tried :
I measure the "in voltage" with a DAC and pulse with proportional duty cycle.
For example: with a "in voltage" of 24V and a duty cycle of 50%, I got 12 volts but the leds and the resistors get too hot.

I understood that leds do not have to be driven by a voltage but well by a constant current.

So, is it possible to use PWM to drive leds without filtering the signal to make it constant?

If not, how do I have to filter the PWM signal?
Is it possible to do it like in the "buck" regulation? (diode coil capacitor)

Or how is it possible to give a constant current?
By pulsing, measuring the current and stop pulsing when the current is too high and wait that it comes down for pulsing again?

Thanks in advance for any information, links, ...

Arnaud Stoumont

 

[INRETECH]

What is the current of the LEDs ?

If they are low current, I would suggest using a LM317 in constant current mode; then you don't have to worry about input voltage - it will automatically make sure the current is always the same