I have a power LED that requires 600mA at around 25 Volts. Since there are mainly 24 Volts and 36 Volts power supplies available on the market for a reasonable price, I needed to choose a 36 Volts model, because otherwise (at 24 Volts) the LED would be undervolted and probably draw a lot less than the nominal 600 mA. So this decision is already made and not part of the question.

But now for the constant current control: I found two models of constant current DC-DC converters, one rated for up to 36 Volts input, and the other for up to 56 Volts (for those interested, they are the Meanwell LDD-600L and LDD-600H, but that doesn't matter for the question).

I wonder if it is safe to choose the 36V input model and drive it with my 36V power supply, or if I should rather choose the 56V model.

In other words, can I generally assume that the manufacturer has already included a safety margin into the 36V input voltage rating, so that it can operate at this voltage for an indefinite amount of time. Or would prolonged operation so close to the upper boundary of the spec result in severely reduced life span?

At least as far as I can see, the datasheets seem to give no indication about that.

By the way: buying an already current limited power supply is not an option because I would need three of them (it is actually an RGB LED) because of cost and because I want PWM control.

  • \$\begingroup\$ I use a 3.3V to drive a 9V LED, as the constant current driver boosts the voltage. Its the capability of your driver that matters. Depending on your driver, some are designed to boost the voltage, controlling the voltage to control the current. Some won't work with a higher voltage in than out. \$\endgroup\$ – Puffafish Dec 2 '19 at 16:24
  • \$\begingroup\$ The mentioned models are step down converters, so no lower voltage, sorry. But anyway, the spec for the LDD-600L is unambiguous about input voltage: <=36 V. My question was, can I rely on the assumption that operating the driver at the upper limit does not severaly decrease its life span. \$\endgroup\$ – oliver Dec 2 '19 at 16:27
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    \$\begingroup\$ But are you operating at the upper limit? What's the tolerance on your 36V supply ? If it's 5% then you're output could be 37.8V, which clearly exceeds the 36V max input of your constant current supply. Even though it might work, it's never a good idea to exceed the max rating. (Assuming it is a max rating, not a nominal working voltage. If that's the case there should be a true max input voltage rating somewhere, but I don't see it on the datasheet.) If there's an output adjustment on the 36V supply you could trim it, then you would be fine. \$\endgroup\$ – John D Dec 2 '19 at 16:42
  • \$\begingroup\$ @JohnD: very good point. I looked up the adjustment range of the PSU, and it says 32.4 ~ 39.6 V. The regulation errors sum up to 2%, so if I adjust it to the lower bound, I am not going to have more than around 33V worst case. So I guess it is safe to choose the 36V drivers. \$\endgroup\$ – oliver Dec 2 '19 at 17:06

If a device is speced to run off of a common DC voltage such as 36v and does not specify a tolerance or other requirements for that common DC voltage, then it is going to have sufficient margin built in to run from typical power supplies. If it didn't, the vendor would have to deal with large numbers of returned units that failed when customers hooked them into poorly regulated smps.


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