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I would like to use a 30W RGB LED that is in common-anode configuration. The issue is the voltage ranges for the LEDs are different, 20-24V for red, and 30-36V for green and blue. Is it possible to run the red led at the same time as the blue or green? If using PWM w/mosfets you still get the full 34V across the red LED I'm assuming that's not going to work.

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One way is to use a single 36 V power supply, then essentially a small switching power supply for each LED to regulate the current thru each LED separately, but driven from PWM.

Here is what the circuit for each LED would be:

When the switch is closed, current builds up linearly thru L1 and therefore thru the LED. When the switch is opened, the current continues thru the LED, L1, but now thru D2. The current will now ramp down linearly.

With fast enough PWM there is little difference between the maximum current at the end of the switch closed time, and the minimum current at the end of the switch open time. The PWM duty cycle then controls the average current thru the LED with minor ripple at the PWM frequency.

You probably want to use a PWM frequency of a MHz or more. For extra filtering, add a ceramic capacitor from the LED cathode to ground.

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  • \$\begingroup\$ thanks this looks promising, I'll have to design something in a circuit simulator. Why do you suggest MHz frequency range? I suppose to get smooth output as well as fast response to brightness changes? I'm not sure if most microcontrollers can output 3 independent PWM channels at MHz frequencies are you aware of any info on this? Thanks. \$\endgroup\$ – perigalacticon Jan 8 '17 at 23:53
  • \$\begingroup\$ @peri: Higher frequency allows for a smaller inductor. The inductor current will go down rapidly when the switch is off due to 30 V or so across it in reverse. A cap from cathode to ground or across the will help a little. Some of dsPIC series have a special high speed PWM module that can do nearly ns resolution. \$\endgroup\$ – Olin Lathrop Jan 9 '17 at 11:44
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All LEDs can be driven from the same voltage source. Each LED should be driven by a separate constant current source LED Driver.

You did not specify the LED part number or the current which makes it difficult to help.

There are very few RGB LED driver chips with 36V out. You could take a triple output RGB LED driver and add 3 MOSFETs LIKE THIS.

This is a simple inexpensive ($15 off the shelf) triple output LED driver from SparkFun (1 channel shown).

enter image description here

Full Schematic

$15 Triple Output LED Driver

This would be a good solution as it has a separate Vin for the Green.
It has a Vin max of 30V and will not work with your LED.
RGB LED

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