TL;DR
Will a DC-DC step down (buck) happily supply strings of LED's that are being PWM controlled?

The project is for LED brightness control.
An Arduino is supplying the PWM signal to a MOSFET which is controlling the LED's to adjust for ambient light level.
Typically I'd just use resistors on each LED branch to limit the max current but I'm thinking it would be better to just limit the common voltage instead.

So the plan would be to have one buck converter reducing the voltage (14v down to 10v) which supplies the LED branches (in parallel), so there would be no need to use limiting resistors.
Main reason to avoid resistors is the heat they generate.

What do think, will the buck converter need an additional output capacitor to help it deal with the pulsed nature of the load?
These are the sort of cheap bucks I'm thinking of using.

https://www.ebay.co.uk/itm/115428999936

https://www.ebay.co.uk/itm/125810046176

Many thanks, Rich.

 

Putting a fixed voltage across an LED does not control the current.

Putting LEDs in parallel does not mean they will have the same current.

A current regulated buck converter driving a single string of LEDs without resistors is a good way to drive them.

 

Putting a fixed voltage across an LED does not control the current.

Agreed but I'm operating them so far below their max that it isn't an issue, they can't over current unless one of them goes short circuit (there's three in series).

Putting LEDs in parallel does not mean they will have the same current.

Agreed but it doesn't matter in this application. The light output from all looks uniform.

A current regulated buck converter driving a single string of LEDs without resistors is a good way to drive them.

Yes but means using multiple buck converters and more cables as it isn't desirable to site both close together.

 

Okay, then, the answer is yes, you can PWM it.

I certainly would not do it that way though. If you must, make up each string so that they all have the same voltage drop at the intended current, then use a buck converter with a current limit so they don’t blow up if they get too hot, and you will probably be alright.

 

If you must, make up each string so that they all have the same voltage drop at the intended current,

I'm not sure what you mean, are you just saying that all strings should be the same?

 

Yes. Try to match the total forward voltage in each string. This will make the currents as evenly distributed as possible.

 

A small value series resistor selected to set the current for each string is an acceptable way to deal with small differences.

 

A small value series resistor selected to set the current for each string is an acceptable way to deal with small differences.

That's a good suggestion, thank you.
Currently I'm using lots of small resistors (2 watt) to drop the voltage and spread the heat out.

I doing tests to get the maximum brightness but with acceptable heat at the LED's.
I also want to make it so that full brightness is set by the components and not by capping the PWM output.